Countermeasures to Protect Bridge Piers from Scour (2007)

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47 3.1 Introduction This chapter presents a summary of the research approach and results of laboratory testing of the following selected pier scour countermeasures: • Riprap • Partially grouted riprap and geotextile containers • Articulating concrete blocks • Gabion mattresses • Grout-filled mattresses The summary of the current state of practice in Chapter 2 is combined with an interpretation and appraisal of testing results to provide guidelines and specifications for design and construction, and guidelines for inspection, maintenance, and performance evaluation for the pier scour countermea- sures investigated in this study. Existing design equations for sizing the armor component of each countermeasure were used to develop the laboratory testing program. However, sizing the armor is only the first step in the comprehensive design, installation, inspection, and maintenance process required for a successful counter- measure. A countermeasure is an integrated system that in- cludes the armor layer, filter, and termination details. Suc- cessful performance depends on the response of each component of the system to hydraulic and environmental stresses throughout its service life. In this context, filter re- quirements, material and testing specifications, construction and installation guidelines, and inspection and quality con- trol procedures are also necessary. In addition, a countermeasure selection methodology was developed. It provides an assessment of the suitability of each of five specific countermeasure types based on a variety of fac- tors involving river environment, construction considera- tions, maintenance, performance, and estimated life-cycle cost of each countermeasure. The output from the selection method provides a quantitative ranking of countermeasure types by computing a Selection Index. The Selection Index in- cludes a fatal-flaw mechanism to identify situations where a particular countermeasure is unequivocally unsuitable due to one or more circumstances unique to the site being evaluated. The Selection Index is intended to identify the countermea- sure best suited for application at a particular site. To guide the practitioner in developing appropriate scour countermeasure designs and ensuring successful installation and performance of countermeasures at bridge piers, the findings of Chapter 2 and recommendations of Chapter 3 are combined to provide a detailed set of design guidelines for each countermeasure type as stand-alone appendices: • Appendix C, Guidelines for Pier Scour Countermeasures Using Rock Riprap • Appendix D, Guidelines for Pier Scour Countermeasures Using Partially Grouted Riprap • Appendix E, Guidelines for Pier Scour Countermeasures Using Articulating Concrete Block (ACB) Systems • Appendix F, Guidelines for Pier Scour Countermeasures Using Gabion Mattresses • Appendix G, Guidelines for Pier Scour Countermeasures Using Grout-Filled Mattresses 3.2 Laboratory Studies 3.2.1 Overview Laboratory research conducted for this study was per- formed at the Hydraulics Laboratory of CSU, located at the Engineering Research Center (ERC). CSU’s indoor Hy- draulics Laboratory is 280 ft (85 m) long by 120 ft (37 m) wide with a maximum ceiling clearance of 32 ft (9.8 m). Cov- ered laboratory space for testing and models exceeds 20,000 ft2 (1858 m2). Figure 3.1 is a photograph of the Hydraulics C H A P T E R 3 Testing, Interpretation, Appraisal, and Results

48 Laboratory, and Figure 3.2 shows a plan view of the Hy- draulics Laboratory with a listing of the available flumes and floor model space. As indicated in Figure 3.2, the Hydraulics Laboratory main- tains and operates a wide selection of flumes. Table 3.1 out- lines the dimensions and capacities of the flumes available. Testing conducted for NCHRP Project 24-07(2) utilized the largest of the laboratory recirculating flumes. The flume is 8 ft (2.4 m) wide by 4 ft (1.2 m) deep by 200 ft (61 m) long and capable of recirculating water and sediment over a range of slopes up to 2%. The maximum discharge in the flume is 100 cfs (2.8 m3/s) with a series of sediment pumps capable of transporting particle sizes up to 0.5 in. (2.7 mm). A mobile data acquisition cart traverses the flume and pro- vides flexibility in data collection. Any number of point gages or velocity probes can be mounted to the cart. The data ac- quisition cart can then be positioned to collect data at any given location in the flume. The cart also has the capacity to provide space and power for a personal computer for data Figure 3.1. Photograph of CSU’s Hydraulics Laboratory. 1 2 3 1. Sediment laboratory 4 2. Equipment room 3. Electrical room 4. Environmental laboratory 5. Laboratory office 6. Fall column 7. 8 ft x 200 ft sediment flume 8. 1 ft x 30 ft sediment flume 9. 4 ft x 60 ft sediment flume 11 12 13 10. 4 ft x 32 ft sediment flume 11. 20 ft x 100 ft river flume 12. Floor model space 13. Floor model space 14. 2 ft x 60 ft sediment flume 15. Floor model space 16. Calibration stand 17. Water treatment plant model 18. Storage room 19. Hardware supply room 20. Machine and fabrication shop 15 16 Dashed line represents outline of the 1-acre foot sump under the laboratory floor. * The laboratory measures120 ft wide by 280 ft long. 17 18 19 North 20 7 6 5 9 10 14 8 Figure 3.2. Schematic of CSU’s Hydraulics Laboratory.

49 collection. The flume is also equipped with a Plexiglas wall for flow and scour visualization. Plexiglas walls provide an ideal viewpoint for flow visualization using dyes or visual scour monitoring. Figure 3.3 provides a schematic of the flume, data cart, and ancillary components. 3.2.2 Research Approach Laboratory testing was structured to address the counter- measure deficiencies as identified in Section 2.3. Specifically, the design specifications and guidelines and the performance evaluation guidelines were used to design the testing pro- gram (see Section 2.3.1). For each of the five countermeasure types, a specific testing approach was developed addressing the deficiencies reported in Tables 2.4 through 2.9. Geotex- tile sand containers were tested as a filter for partially grouted riprap. To maximize the amount of testing within the available budget, the research team and the NCHRP Project 24-07(2) panel laboratory subgroup met in August 2002 to develop a prioritized plan of study. The plan was further modified by the laboratory subgroup in January 2003. From these discus- sions came the decision to place three piers along the center- line of the testing flume. Square piers 8 in. (200 mm) long by 8 in. (200 mm) wide were used. Spacing between the piers was approximately 40 ft (12 m) to ensure the formation of iden- tical flow lines upstream of each pier. Sand, with a d50 rang- ing from 0.7 to 0.9 mm, was placed in the flume to a depth of approximately 18 in. (460 mm). The flume layout is shown in Figure 3.3. A matrix of flume tests was completed for the research program. Each test consisted of a series of two discharges. Discharge rates were predetermined to correspond to flow velocities of Vcrit and 2Vcrit, where Vcrit is the calculated criti- cal velocity of the sediment size utilized throughout the research program. The Vcrit and 2Vcrit runs were performed without sediment recirculation. Separate runs on selected countermeasure configurations were performed at 2.5Vcrit with sediment recirculation, therefore, both clear-water and live-bed conditions were examined. Flumes/Models Width (ft) Length (ft) Depth (ft) Flow (cfs) Slope (%) Recirc. (Yes/No) Sediment Flume 7 8 200 4 100 Variable Yes Sediment Flume 10 4 32 4 12 Variable Yes Sediment Flume 9 4 60 4 20 Variable Yes Sediment Flume 14 2 60 2 10 Variable Yes Sediment Flume 8 1 30 1 3 Variable Yes Sediment Flume 11 20 100 3 40 Variable No Sediment Flumes 12, 13, 15 100 100 10 40 Variable Yes Table 3.1. Dimensions and capacities of flumes in the Hydraulics Laboratory. Point gauge assembly with velocity probe 1.3 m Tailgate sand bed 13 m (min) 13 m (min) 60 m 6 m6 m Mobile data acquisition cart 2.6 m Concrete cap PLAN VIEW PROFILE Figure 3.3. Schematic of flume and configuration.

50 During the live-bed runs, bed-form type, length, and height were recorded. Flow duration was sufficient to ensure that bed forms migrated through the system. One baseline flow was performed at 3Vcrit to determine the baseline per- formance of standard, loose riprap under conditions where particle dislodgement or entrainment is anticipated. Data collected during each test included pre-test surveys, approach flow velocity, local pier velocity, flow depth, and post-test surveys. In addition, non-professional photographic and video footage was recorded of each test. Water surface el- evations were collected every 4 ft (1.2 m) along the flume, and local and approach flow velocities collected at each pier. Water surface elevations were determined by a point gage accurate to ± 0.005 ft (1.5 mm). Velocities were collected with a three- dimensional (3-D) acoustic Doppler velocimeter, accurate to ± 2%. Where the flow depth was sufficient, approach veloci- ties were collected at 20%, 60%, and 80% of the flow depth. Local velocity profile measurements were collected at each pier. Pre- and post-test surveys were conducted with a point gage and a total station. Survey resolution was sufficient to accurately map each scour hole and document system performance. 3.2.3 Laboratory Test Plan Items identified as gaps in the current state of the practice (Section 2.3) were reviewed and a specific test, or series of tests, was designed to address each deficiency. The following sections detail the findings for each countermeasure type. Each test series was designed to permit one configuration to be carried forward to the next series. This design served to quantify the repeatability of the test program as well as iden- tify inconsistencies that could arise in the experimental set up. The laboratory tests were not designed to replicate any particular prototype-scale conditions. For example, the 2Vcrit run was not intended to represent specific scale ratio of a prototype pier or flow condition. However, in each case, the test countermeasure was “designed” to withstand the 2Vcrit hydraulic condition. For example, the riprap size was selected such that particle dislodgement or entrainment was not anticipated during the 2Vcrit run. This design did not mean that the riprap (or any other countermeasure) would not fail because of other factors, such as settling, edge un- dermining, or winnowing of substrate material. Runs uti- lizing an approach velocity of 2.5Vcrit were intended to take each system to failure by particle dislodgement. The performance of each countermeasure was compared with the benchmark performance of riprap. Criteria for rating performance were consistent between countermea- sures, but were not necessarily identical for all counter- measures. A countermeasure was considered to have failed if the countermeasure (or its component parts) was dis- lodged, lifted, or entrained. Relative performance was gauged by whether the countermeasure functioned as in- tended. Specifically, if settling along the countermeasure was expected, actual settlement was not considered poor performance. Maximum scour anywhere within the limits of the countermeasure or along the edge of the counter- measure was documented. The testing program also addressed stability and perform- ance issues associated with the extent of the countermeasure placement around the pier, and the termination details at the pier and around the periphery of the installation. Lastly, var- ious filter types and extents were investigated by varying this aspect for selected test runs. Sections 3.3 through 3.8 provide an overview of the labora- tory testing program including the materials used and the design intent for each test series for each countermeasure type. Typical configurations and test runs are illustrated. For each countermeasure, a “baseline” schematic is shown. These schematics are intended to illustrate the starting point for a test series, not a recommendation for design. The final rec- ommendations for design layout are presented in the design guideline appendix for each countermeasure type. Appen- dix H provides summary tables of the testing program, and the Reference Document (http://www.trb.org/TRBNet/ ProjectDisplay.asp?ProjectID=702) contains detailed labora- tory testing results. Section 3.9 summarizes design and specifi- cation guidance derived from the testing program as a basis for developing design guidelines for each countermeasure type. 3.3 Unprotected Runs 3.3.1 Materials Sand composing the bed material was characterized by a d50 grain size that ranged from approximately 0.7 to 0.9 mm. The coefficient of uniformity, Cu, defined as d60/d10, ranged from 4.1 to 5.2. A representative grain size distribution graph is shown in Figure 3.4. 3.3.2 Testing A conservative value of 1.0 ft/s (0.305 m/s) was adopted for establishing the target approach velocities. The intent was to create a condition for the initial run of each countermea- sure type that resulted in true clear-water conditions, with no movement of the bed material except for local scour in the immediate vicinity of the piers. Tests confirmed that an approach velocity of 1.0 ft/s (0.305 m/s) resulted in no bed material movement except for local scour; runs performed at 2.0 ft/s (0.61 m/s) or greater resulted in live-bed condi- tions and the formation of dunes throughout the entire length of the flume.

51 Classification of maximum scour was determined for unprotected square and rectangular piers, under clear-water and live-bed conditions. A live-bed test was run for a suffi- cient duration (8 hours) to permit bed forms to migrate through the system. Figure 3.5 shows the results of unpro- tected square pier tests under live-bed conditions in the CSU indoor flume. Figure 3.6 shows the results for unprotected rectangular piers with 0° skew under clear-water conditions (Figure 3.6a) and a rectangular pier with 15° skew under live- bed conditions (Figure 3.6b) in the indoor flume. A white arrow indicates direction of flow on test run photographs. 3.4 Riprap Most of the early work on the stability of pier riprap con- siders the size of the riprap stones and their ability to with- stand high approach velocities and buoyant forces. Secondary GRAIN SIZE DISTRIBUTION 3-in 2-in 1-in 0.5-in 4 8 704010 16 20 100 140 200 0 10 20 30 40 50 60 70 80 90 100 0.00010.0010.010.1110100 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size GRAVEL Coarse Fine SAND Coarse Medium Fine SILT CLAY (ASTM) Soil Sample 1 D50 = 0.9 mm Soil Sample 2 D50 = 0.85 mm Soil Sample 3 D50 = 0.72 mm Figure 3.4. Grain size distribution of bed material. Figure 3.5. Unprotected square piers.

52 currents induced by bridge piers cause high local boundary shear stresses, high local seepage gradients, and sediment ero- sion from the streambed surrounding the pier. The addition of riprap also changes the boundary stresses (see Section 2.1.3). Riprap failure at model bridge piers under clear-water con- ditions with gradually increasing approach flow velocities can be defined by three modes of failure: • Riprap shear failure, whereby the riprap stones cannot with- stand the down flow and horseshoe vortex associated with the pier scour mechanism • Winnowing failure, whereby the underlying finer bed mate- rial is removed through voids or interstices in the riprap layer • Edge failure, whereby instability at the edge of the coarse riprap layer initiates a scour hole beginning at the perime- ter and working inward until it ultimately destabilizes the entire layer Prior research has indicated that bed-form undermining is the controlling failure mechanism at bridge piers on rivers where mobile bed forms are present during high flows, espe- cially in sand bed rivers (see Section 2.1.3). Both clear-water and live-bed conditions were examined in this study, but the effects of contraction scour and long-term degradation were not investigated. Figure 3.7 shows the results of riprap tests under clear-water (Figure 3.7a) and live-bed (Figure 3.7b) conditions in the CSU indoor flume. a. Unprotected rectangular pier (0° skew) after 1.0Vcrit test. b. Unprotected rectangular pier (15° skew) after 2.0Vcrit test. Figure 3.6. Unprotected rectangular piers. b. Riprap after 2.5Vcrit test. Note particle displacement when areal extent is insufficient under live-bed conditions. a. Riprap after 1.0Vcrit test. Figure 3.7. Riprap tests under clear-water and live-bed conditions.

3.4.1 Materials Armor Stone Riprap is the most commonly used pier scour counter- measure and usually consists of large stones placed around a pier to armor the bed. This armoring prevents the strong vor- tex flow at the front of the pier from entraining bed sediment and forming a scour hole. The ability of the riprap layer to provide scour protection is, in part, a function of stone size, which is a critical factor in terms of shear failure (Lagasse et al. 2006). Riprap used for testing in the indoor flume was sized for stability at an approach velocity of 2Vcrit in accordance with the procedures outlined in HEC-23 (see Equation 2.3) (Lagasse et al. 2001). As recommended in HEC-23, the cross- sectional average velocity was multiplied by 1.7 for a square- nose pier shape and 1.2 to account for flow distribution across the flume, which yielded a design velocity of (1.2)(1.7)(2Vcrit) or 4.1 ft/s (1.25 m/s) for the riprap sizing calculations. Riprap d50 was determined using the standard Isbash for- mula for sizing riprap on a channel bed presented in HEC-23. The required d50 was 33 mm (1.3 in.). Two limiting gradation curves were developed given the riprap d50 of 33 mm, in accor- dance with guidelines presented in HEC-11 (Brown and Clyde 1989). See the Reference Document for computation details. Figure 3.8 shows the grain size distribution of riprap utilized in the testing program as well as the gradation limits. The riprap actually produced for the test runs had a d50 of 30 mm (1.2 in.) due to characteristics of the locally available supply. The riprap size was selected such that particle dislodgement or entrainment was not anticipated during the 2Vcrit run. However, the riprap could still fail due to other factors, such as settling, edge undermining, or winnowing of substrate 53 GRAIN SIZE DISTRIBUTION 200140100201610 40 70840.5-in1-in2-in3-in 0 10 20 30 40 50 60 70 80 90 100 0.010.11101001000 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size GRAVEL Coarse Fine SAND Coarse Medium Fine SILT CLAY (ASTM) Riprap D50 = 30 mm Lower Riprap Gradation Limit Upper Riprap Gradation Limit Figure 3.8. Riprap grain size distribution.

54 material. Riprap runs utilized approach velocities of 1Vcrit, 2Vcrit, and 2.5Vcrit. Runs utilizing an approach velocity of 2.5Vcrit were intended to take each system to failure by parti- cle dislodgement. Riprap used in the laboratory tests consisted of a hard, durable sandstone having a specific gravity of 2.55 to 2.60. Other types of rock materials having different densities were not tested during this study; long-term weathering potential also was not investigated. Filters Geotextile Filter. Selection of geotextile for filter fabric was made using the method outlined in Designing with Geosyn- thetics (Koerner 1998). The method establishes a maximum allowable aperture size and minimum allowable permeability to achieve compatibility with the bed material. According to this method, the geotextile for this application should exhibit a permeability that is more than four times greater than that of the bed material, i.e., Kg/Ks > 4.0. For particle retention, the effective aperture size of a geotextile filter must be less than the d90 of the bed material (approximately 2.0 mm) in this application. This method places more emphasis on perme- ability and less emphasis on particle retention compared to other procedures, such as HEC-11 or AASHTO M 288 (Lagasse et al. 2006). Table 3.2 summarizes the hydraulic and physical properties of the geotextile filters used in this study. The areal extent of filter placement around the pier was identified as a parameter to be investigated under this testing program. For geotextile filters, both full and two-thirds cov- erage were examined. The term “full coverage” indicates that the geotextile extended beneath the riprap all the way to the periphery of the installation, whereas “two-thirds” indicates that the geotextile extended only two-thirds of the distance from the pier face to the periphery of the riprap. The two- thirds geotextile coverage corresponds to recommendations developed in NCHRP Project 24-07 (Parker et al. 1998) and confirmed in this study. Granular Filter. Granular filter requirements were de- veloped using the criteria specified in HEC-11. The initial step establishes the compatibility of the filter with the sand bed material in terms of both particle retention and perme- ability by defining upper and lower limits of d15 for the filter. This determines the largest size allowable to maintain parti- cle retention and smallest size allowable to ensure the filter has greater permeability than the sand. The upper limit com- patibility criteria of the filter must be large enough so that the filter does not pass through the riprap (see the Reference Document for computation details). The material selected for use was a nominal 10-mm (3/8-in.) crushed rock from a local source. A grain size distribution graph for the granular filter layer is presented in Figure 3.9. Grain size distribution curves for the riprap stone, and the bed sand are included for comparison. Figure 3.10 shows the woven geotextile “W1” as well as the granular filter used in the testing program. Sand bed material and the 30-mm (1.2-in.) riprap are also shown in the photo- graph for comparison. 3.4.2 Testing Program The testing program addressed stability and performance issues associated with the extent of riprap placement around the pier, and the termination details at the pier and around the periphery of the installation. In addition, various filter types and extents were investigated by varying this aspect for selected test runs. Two 8-in. (200-mm) square piers (with no skew) and a 2-in. by 10-in. (51-mm by 254-mm) rectangular (wall) pier with 0°, 15°, and 30° skew angles were tested. (See Appendix H for details of the configurations tested.) Baseline Baseline riprap installation conditions were based on current HEC-23 layout guidelines, where the riprap is extended a min- imum of two pier widths in all directions and thickness of the riprap layer is a minimum of three times the d50 of the armor stone. The NCHRP Project 24-07 recommendation to extend the geotextile from the pier face to two-thirds of the distance to the periphery of the riprap was adopted for baseline runs. Fig- ure 3.11 presents the design layout for the baseline riprap tests. Filter Name Geotextile Type Mass/ Unit Area Apparent Opening Size (AOS) Permeability Trade Name Manufacturer Kg/Ks W1 Woven 205 g/m2 0.850 mm 0.20 cm/s Geotex® 117F SI Geosolutions (Propex) 5.0 NW1 Non-woven 163 g/m2 0.212 mm 0.21 cm/s Mirafi® 140 N Mirafi Construction Products 5.25 NW2 Non-woven 250 g/m2 ~ 0.10 mm 0.4 cm/s HaTe® B 250 K4 Huesker Synthetic GmbH 10.0 NW3 Non-woven 278 g/m2 0.18 mm 0.21 cm/s Mirafi® 180 N Mirafi Construction Products 5.25 Table 3.2. Hydraulic and physical characteristics of geotextile filters.

The design intent for the riprap baseline tests included examination of the following: • HEC-23 guidelines with recommended geotextile, no skew • HEC-23 guidelines with recommended geotextile, pier skewed to flow Representative results of the baseline conditions tests are shown in Figure 3.12. Extent of Coverage Typically, riprap used for pier scour protection is placed on the surface of the channel bed, in a pre-existing scour hole, or in a hole excavated around the pier (Figure 2.2). The FHWA recommends placing the top of the riprap layer flush with the channel bed for inspection purposes (Lagasse et al. 2001, Richardson and Davis 1995). 55 GRAIN SIZE DISTRIBUTION 200140100201610 40 70840.5-in1-in2-in3-in 0 10 20 30 40 50 60 70 80 90 100 0.010.11101001000 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size GRAVEL Coarse Fine SAND Coarse Medium Fine SILT CLAY (ASTM) Riprap d50 = 30mm Granular Filter d50 = 10mm Bed Sand d50 = 0.8 mm Figure 3.9. Granular filter, riprap stone, and bed sand grain size distributions. Bed material d50 = 0.7 to 0.9 mm Scale: inches Granular Filter D50 = 10 mm Geotextile AOS = 0.85 mm Riprap d50 = 30 mm Figure 3.10. Various materials used in the riprap testing program.

The design intent for the riprap coverage tests included ex- amination of the following: • Areal riprap coverage and edge treatment with recommended geotextile • Areal riprap coverage variation from HEC-23 with recom- mended geotextile • Areal riprap coverage and thickness variation from HEC- 23 with recommended geotextile • Scour hole extent with recommended geotextile • Scour hole extent without filter • HEC-18 guidelines • Thickness and filter variation from HEC-23 guidelines • Mounded riprap without filter Appendix H, Table H.2, provides details on the filter alter- natives tested, which included geotextile and granular filters with two-thirds and full coverage and, in some cases, no filter. Test results indicated that best performance was achieved when riprap extended at least two times the width of the pier (as measured perpendicular to the approach flow on all sides) in a flat pre-excavated hole with the top surface flush with the bed. Figure 3.13 shows the poor performance when the areal coverage was reduced to less than two pier widths on all sides. Riprap used for pier protection is often placed on the sur- face of the channel bed because of the ease and lower cost of placement and because it is more easily inspected. Test results indicated that, when the stable baseline riprap configuration was mounded on the surface without a filter, performance was poor. None of the mounded riprap in tests performed as well as the riprap in tests where it was level with the bed, given the 56 aFLOW 2a 2a Riprap placement = 2(a) from pier (all around) Pier width = “a” (normal to flow) t=3d50 Riprap thickness = 3d50 (minimum) Filter placement = 4/3(a) from pier (all around) Filter a. Baseline riprap installation after 2.5Vcrit live-bed test at square pier. b. Baseline riprap installation after 2.0Vcrit test at rectangular pier (no skew). Figure 3.11. Baseline riprap design. Figure 3.12. After baseline riprap installation testing.

same areal extent of riprap coverage. Figure 3.14 shows the results of a mounded riprap test. Numerous riprap studies (see Lagasse et al. 2006) suggest that thickness of the riprap layer placed around the bridge piers should be between two to three times median stone size (2d50 to 3d50) of the riprap. Testing results indicate that 3d50 is appropriate for specifying minimum thickness and that per- formance improved with increasing riprap layer thickness. Termination Detail The design intent for the riprap termination detail tests in- cluded examination of the following: • Areal coverage and edge treatment with recommended ge- otextile (two-thirds coverage) • HEC-18 guidelines with geotextile filter (full coverage) Four tests were performed to test conditions where the bottom of the riprap layer was not horizontal but instead sloped away from the pier while the surface of the riprap re- mained flush with the bed. Areal extent was decreased from the recommended 2a from the pier face in the installations, and thickness increased with distance from the pier. Particle launching and loss were observed after completion of several of the tests. Figure 3.15 shows the construction details for one of the termination tests. The results of two of the termination runs after 2Vcrit tests are shown in Figure 3.16. Filter NCHRP Project 24-07 (Parker et al. 1998) determined that placing a geotextile under a riprap layer with the same areal coverage as the riprap layer resulted in a relatively poor per- formance of the riprap. As a result of the effects of live-bed conditions, the rock at the edges tended to slide or be plucked off, exposing the underlying geotextile and ultimately result- ing in failure of the riprap layer as successive bed forms pass and pluck more stones from the riprap layer. Parker et al. sug- gest extending the geotextile from the pier to about two- thirds of the way to the periphery of the riprap would result in better performance. Additional test results for this study confirmed that riprap performance was best when a geotex- tile filter extended two-thirds the distance to the periphery of the riprap. 57 a. Areal extent decreased to 4a after 2.5Vcrit live-bed test. Note scour hole at nose of pier. b. Areal extent decreased to 4a, thickness increased to 4d50 after 2.5Vcrit live-bed test. Original limit of riprap Figure 3.13. Decreased areal coverage riprap tests. Figure 3.14. Mounded riprap after 2.0Vcrit test.

The design intent for the riprap filter tests included exam- ination of the following: • Thickness and filter variation from HEC-23 guidelines • HEC-23 guidelines and filter type variations • HEC-18 guidelines • Current practice and guidelines • Thickness variation from HEC-23 guidelines • Mounded riprap Appendix H, Table H.4, provides details on the filter alter- natives tested which included geotextile and granular filters with two-thirds and full coverage and, in some cases, no filter. Granular filters were found to perform poorly where bed forms are present. Specifically, where dune troughs that are 58 a. Riprap configuration from Figure 3.15 after 2.0Vcrit test. b. Increased depth of riprap at perimeter with insufficient areal extent after 2.0Vcrit test. Figure 3.16. Riprap termination tests. Pier B 8” Lateral extent = 12 inches Upstream extent = 16 inches Downstream extent = 8 inches Geotextile filter extended 2/3 the distance from the pier face to the perimeter of the riprap 4” 4” 2H:1V FLOW 16”8” Figure 3.15. Example riprap termination test configuration. deeper than the riprap armor pass the pier, the underlying finer particles of a granular filter are rapidly swept away. The result is that the entire installation became progressively desta- bilized beginning at the periphery and working toward the pier. Figure 3.17 shows two piers after testing: one pier had a geotextile filter that extended two-thirds the distance from the pier face to the periphery (Figure 3.17a) and the other pier had a granular filter that extended the full distance from the pier face to the periphery of the riprap (Figure 3.17b). 3.5 Partially Grouted Riprap Partial grouting of riprap with a cement slurry is presented as one of several standard design approaches for permeable revetments in a discussion of considerations regarding the ex-

perience and design of German inland waterways (BAW 1990). As with standard (loose) riprap, when partially grouted riprap is properly designed and installed for erosion protection, it has an advantage over rigid structures because it is flexible when under attack by river currents, it can remain functional even if some individual stones may be lost, and it can be repaired relatively easily. The grout is placed on the riprap leaving significant voids in the riprap matrix and con- siderable open space on the surface. For bridge pier protection, partially grouted riprap consists of rocks that are placed around a pier and grouted together with grout filling 50% or less of the total void space. The holes in the matrix allow for drainage of pore water; therefore, a fil- ter is required. The grout forms conglomerates of riprap so the stability against particle erosion is greatly improved and a smaller thickness of stone can be used (Lagasse et al. 2001). 3.5.1 Materials Grout For the indoor partially grouted riprap installations, vari- ous Portland cement grout mix designs were developed and tested in the dry using d50 riprap sizes of 0.58 in. (14.7 mm), 1 in. (25.4 mm), and 1.2 in. (30 mm). Consistency of the ce- mentitious grout mix was determined by trial and error. The initial mix design was based on pumpable fine aggregate con- crete mix used in the construction of grout-filled mattresses. Test pours were performed for all three riprap sizes and a grout mixture was chosen based on flowability. For testing installations, riprap was installed around a pier and then a measured volume of grout was hand poured into “spots” on the riprap in a stagger pattern. The target fill value of between 15% and 40% of the original void space volume was maintained for all installations tested. Conglomerate-like elements in the riprap were produced using the spot-by-spot grouting procedure. Figure 3.18 shows the conglomerates produced during a test pour. Filter Only geotextile filters were tested with partially grouted riprap. Geotextile selection for filter fabric was made using the method outlined in Koerner (1998), as summarized in Section 3.4.1. Table 3.3 summarizes the hydraulic and phys- ical properties of the geotextile filter used in the partially grouted riprap portion of this study. 59 a. Test 5d, riprap with two-thirds extent geotextile filter. b. Test 5d, riprap with full extent granular filter. Note displacement of riprap. Figure 3.17. Testing of granular and geotextile filters. Figure 3.18. Conglomerates produced by partial grouting of riprap.

3.5.2 Small-Scale Testing Program The partially grouted riprap testing program in the indoor flume addressed stone size, stability, and performance issues associated with the extent of partially grouted riprap place- ment around an 8-in. (200-mm) square pier, and the termi- nation details at the pier and around the periphery of the installation (see Appendix H for details on the configurations tested). Baseline Because limited information is available on the use of par- tially grouted riprap as a pier scour countermeasure, baseline partially grouted riprap installation conditions were based on observations from the riprap testing program (Section 3.4). All baseline partially grouted riprap tests incorporated the same design layout with variation in stone size (0.6, 1.0, and 1.2 in. d50) between the tests. Initial installation of partially grouted riprap extended a horizontal distance of one and a half pier widths on all sides for a total areal coverage of four pier widths; the geotextile filter extended two-thirds the coverage of the countermeasure, and thickness was 3d50 of the largest stone examined. Figure 3.19 presents the design layout for the base- line partially grouted riprap tests. The design intent for par- tially grouted riprap baseline tests was to examine rock size performance with a two-thirds extent geotextile filter. Initial partially grouted riprap test results indicated that armor stone size could be reduced from the design riprap size for standard (loose) riprap without compromising stability when exposed to 2Vcrit flow conditions with 4- to 6-in. (100- to 152-mm) dunes. The partially grouted smaller stones pro- duced the desired conglomerates. Extent of Coverage The design intent of the extent of coverage tests for par- tially grouted riprap included examination of the following: • Layer thickness and termination detail • Areal coverage and thickness • Areal coverage, layer thickness, and termination detail All tests were performed with a two-thirds extent geotex- tile filter. 60 Filter Name Geotextile Type Mass/ Unit Area AOS Permeability Trade Name Manufacturer Kg/Ks NW2 Nonwoven 250 g/m2 ~ 0.10 mm 0.4 cm/s HaTe® B 250 K4 Huesker Synthetic GmbH 10.0 Table 3.3. Hydraulic and physical characteristics of geotextile filter used for partially grouted riprap tests. Pier width = “a” (normal to flow) Extend partially grouted riprap a distance of 1.5(a) from pier (minimum, all around) a 1.5a 1.5a Filter placement = 1.0(a) from pier (all around) t = 4” Filter Pier FLOW a 1.5a 1.5a grout grout Figure 3.19. Partially grouted riprap baseline design layout. When thickness was increased from the baseline installa- tion design and grout quantity remained constant, under- mining on the sides and downstream of the countermeasure was observed. Grout quantity was reduced by a third and the desired flexibility of the countermeasure was achieved. With the grout quantity reduced, stable conditions resulted for all partially grouted riprap tests. Termination Detail Four tests were performed to test conditions where the par- tially grouted riprap layer was not horizontal but sloped away from the pier. All tests were performed with a two-thirds extent geotextile filter. The design intent for these tests in- cluded examination of the following: • Layer thickness and termination detail • Areal coverage, layer thickness, and termination detail

Good performance was observed for all termination tests; some particle launching was observed when the areal extent was decreased. The countermeasure was stable when exposed to live-bed 2.5Vcrit flow conditions when the areal extent and thick- ness were increased from baseline conditions and a turndown to the depth of passing dune troughs was added. The results of two of the termination tests at 2Vcrit are shown in Figure 3.20. Filter Results from the riprap portion of the testing program (Section 3.4) confirmed that a filter should not be extended fully beneath a riprap layer; instead, it should be terminated two-thirds the distance from the pier face to the edge of the riprap. All partially grouted riprap tests incorporated this partial coverage filter recommendation (Figure 3.19). 3.5.3 Prototype-Scale Tests of Partially Grouted Riprap Tarbela Flume The Tarbela flume at CSU measures 108 ft (33 m) long by 20 ft (6 m) wide and 8 ft (2.4 m) deep. Flow enters the flume by a 36-in. (900-mm) diameter pipe fed by a nearby reservoir. Flow enters the headbox and is discharged into the flume through a sluice gate with dimensions 6.25 ft (2 m) by 3.9 ft (1.2 m). A rock baffle 5.25 ft (1.6 m) tall and spanning the width of the flume was installed 15 ft (4.6 m) downstream of the headbox. The baffle was intended to uniformly distribute the flow across the width of the flume. Tail water depths were controlled by four sluice gates at the downstream end of the flume. Bed slope of the flume was 0.003 m/m (0.3%). A test section was created 30 ft (9 m) downstream of the rock baffle. The test section was 30.7 ft (9 m) long and spanned the width of the flume. It was filled with sand level with the approach section. Upstream and downstream of the test section the flume bed consists of smooth concrete floors. A rectangular pier measuring 1.5 ft (0.5 m) by 4.5 ft (1.5 m) was installed in the center of the test section. Figure 3.21 is a layout diagram for the prototype partially grouted riprap test- ing program. Surrounding the pier, a scour hole measuring 12 ft by 16 ft (4m × 5 m) was pre-formed into the sand bed to a maximum depth of 3 ft (0.4 m) as shown in Figure 3.22. Partially grouted riprap was tested at prototype scale to demonstrate constructability and to examine water quality is- sues during installation and performance in high-velocity flow conditions. In addition, partially grouted riprap was com- pared side by side to loose riprap under high-velocity flow conditions. Materials Geocontainers. Sand-filled geotextile containers were constructed using a geotextile fabric with the characteristics presented in Table 3.4. The geotextile containers measured 4 ft by 1.5 ft by 0.33 ft (1.2 m by 0.5 m by 0.1 m) with a typical volume of 2 ft3 (0.6 m3). Approximately 220 lbs (100 kg) of sand were placed in each container. Commercial concrete sand meeting appropriate filter criteria was used to fill the geotextile containers. Figure 3.23 shows the geotextile con- tainers before being placed around the pier. Bed Material. Commercial concrete sand with a d50 of approximately 0.7 mm was used for the sand bed (see Figure 3.4 for a graph of grain size distribution of the bed material). 61 a. Partially grouted riprap installation with no turndown detail after 2.0Vcrit live-bed (d50 = 0.6 in.). b. Partially grouted riprap installation with a 4H:1V turndown after 2.0Vcrit test (d50 = 0.6 in.). Figure 3.20. Partially grouted riprap termination detail test results.

62 FLOW 0.4 m Pier 9 m PLAN VIEW 6 m Partially grouted riprap Sand geocontainers 0.5 m/s PARTIALLY GROUTED RIPRAP: d50 = 15 cm Thickness of layer = 45 cm Area covered = 19.8 m2 1.5 m concrete sand concrete Pier 0.5 m Pier FLOW 0.5 m Sand - filled geocontainers Partially grouted riprap Scour hole: 5m long x 4m wide Pier 0.5 m Figure 3.21. Schematic layout for prototype partially grouted riprap tests (dimensions approximate). Figure 3.22. Tarbela installation.

Armor Stone. Durable sandstone riprap for testing had a d50 of 6 in. (152 mm). Figure 3.24 shows the grain size distri- bution of riprap used in the prototype portion of the testing program. Grout. A grout mixture created for underwater applica- tion was used in the testing program. A proprietary admixture was included in the grout to prevent dilution and dissipation of the grout into the water. Table 3.5 presents the approximate grout component quantities. Grout was mixed at a commercial batch plant. During the mixing process, water was added to the mixture in order to achieve the desired consistency and slump characteristics. Figure 3.25 shows the grain size distribution curve for the coarse aggregate in the grout mix. Testing Program The design intent for the prototype-scale partially grouted riprap tests included examination of the following: • Constructability • Environmental issues 63 Trade Name Mass/ Unit Area AOS Permeability Geotextile Type Kg/Ks Mirafi® 180 N 278 g/m2 0.18 mm 0.21 cm/s Non-woven needle punched 5.25 Table 3.4. Characteristics of geotextile. GRAIN SIZE DISTRIBUTION 0.5-in1-in.2-in.3-in.9-in. 0 10 20 30 40 50 60 70 80 90 100 101001000 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size Colorado Lien Type VL riprap with stones greater than 9" and less than 3" removed Allowable distribution Figure 3.23. Geocontainers before installation around the pier. Figure 3.24. Six-inch riprap grain size distribution.

• Performance at high velocity • Comparison to loose riprap at high velocity Appendix H, Table H.8, provides details on these tests. Constructability. An approach flow 1 ft (0.305 m) deep at approximately 1.5 ft/s (0.5 m/s) was established. A total of 32 geotextile containers were placed around the pier by drop- ping them from a height of about 5 ft (1.5 m) above the water surface. Installation was facilitated by a backhoe fitted with a special grapple attached to the bucket, which enabled the backhoe to pick up the geotextile containers, maneuver around the pier to a specified location, and release the con- tainers. Figure 3.26 is a photograph of a geotextile container being dropped near the pier; note the grapple plate attach- ment to the backhoe. Figure 3.27 shows the geotextile con- tainers after installation in approximately 1 ft (0.305 m) of flowing water. Next, riprap was positioned on top of the geotextile con- tainers using the backhoe with the grapple removed. Figure 3.28 shows riprap being dropped near the pier, and Figure 3.29 shows the riprap after installation, but prior to grouting. These tests confirmed that geotextile containers can be fabri- cated locally; that the containers and riprap can be placed with standard equipment; and that the grout mix can be batched, transported, and placed with commercially available equipment. Water Quality Grouting Procedure. Prior to underwater application of the grout in the flume, a preliminary grout application was performed in the dry on a pile of riprap about 1.5 ft (0.5 m) thick. The trial application was performed to determine if the equipment could supply and control the grout pumping rate as needed for the underwater installation conditions. Grout was dispensed from a flexible hose attached to a boom on a concrete pump truck. Grout was supplied to the pump truck from a standard concrete mixer truck, as shown in Figure 3.30. 64 Material Weight (lb) Proportion by Weight Ordinary Portland Cement 753 0.600 Water 450 0.400 Concrete Sand (d50 = 0.7 mm) 1191 1.000 Coarse Aggregate (d50 = 3.3 mm) 1191 1.000 Sicotan® Additive 6.7 0.006 Table 3.5. Grout mix for outdoor testing program. GRAIN SIZE DISTRIBUTION 9-in. 3-in. 2-in. 1-in. 0.5-in 0 10 20 30 40 50 60 70 80 90 100 0.1110100 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size LaFarge 1/4" Chips Figure 3.25. Grain size distribution for coarse aggregate in grout mix.

how the grout bridges riprap stones forming larger conglom- erate particles. In Figure 3.33, note that less than 50% of the total void space has been filled with grout. The preliminary application confirmed that the equipment planned for the underwater partial grout application was satisfactory. Grout placement in the flume was performed by an experi- enced underwater grout installation specialist from Germany. The specialist was located in the flume and placed the grout directly on the riprap in 1 ft (0.305 m) of water with a velocity 1 ft/s (0.305 m/s), as illustrated in Figure 3.34. Application of grout on the riprap lasted approximately 20 min. Approximately 1.4 yd3 (1.1 m3) of grout was placed, resulting in an application of 1.6 ft3/yd2 (56 L/m2). Typical grout application rates in German practice are 60 L/m2, so this test was representative of standard practice for this counter- measure type. Water Quality Monitoring. Water quality was moni- tored before, during, and after the grout placement. Water quality parameters monitored continuously were pH, con- ductivity, temperature, and turbidity. Based on research per- formed by the Virginia DOT (VDOT), pH is the only water quality parameter that is expected to change significantly dur- 65 Figure 3.26. Installation of geotextile containers (pier is on the left). Figure 3.27. Geotextile containers after installation. Figure 3.28. Installation of riprap around pier. Figure 3.31 shows the preliminary trial grout application in the dry. Figure 3.32 shows the surface of the riprap after par- tial grouting, and Figure 3.33 shows the interior of the dry riprap pile after several exterior stones had been removed to display penetration of the grout. Note in Figures 3.32 and 3.33

Water quality monitors, placed in stream at the seven loca- tions depicted in Figure 3.35, continually recorded measure- ments of pH, conductivity, turbidity, and temperature. Baseline conditions were established prior to initiation of the grout placement 12 ft (3.7 m) upstream of the pier along the center- line of the flume (Station A in Figure 3.35). During the test, the water discharge was 20 cfs (0.6 m3/s) and the average rate of grout placement was 0.032 cfs (0.001 m3/s); therefore, the water:grout dilution ratio was 20:0.032, or 625:1. Three grab samples were selected for analysis: a baseline sample taken at Station A when testing commenced, a sample taken at Station C 5 minutes after grout application began, and a sample taken at Station F when grout application finished. Grab sam- ples were collected in 250 mL polyethylene bottles that had been washed and rinsed with distilled water. Bottles were filled by dip- ping the bottle into the water upstream of where the sampling 66 Figure 3.30. Concrete mixer truck and pump truck with boom. Figure 3.31. Preliminary trial grout application in the dry. Figure 3.32. Surface of the riprap after partial grouting. Figure 3.29. Riprap prior to grouting. ing grout placement (Fitch 2003). In the VDOT study, per- mit conditions required that pH levels remain below a value of 9.0, otherwise grouting activities were to be stopped, and mitigation measures such as silt curtains were to be em- ployed. VDOT did not monitor turbidity during their study.

personnel were standing in the flume. The grab samples were an- alyzed for selected inorganics and metals. The laboratory results for the samples are presented in Table 3.6. Continuous water quality data were calibrated to back- ground data collected at Station A prior to grout placement. Results from the water quality monitoring program are pre- sented in the following paragraphs. pH. Background pH was 7.0 at all stations located in the flume itself. Downstream of the flume, Station J (located in the natural channel 150 ft (46 m) downstream of the flume tailgates) exhibited a background pH of 7.4. A spike in pH was observed at the locations directly downstream of the pier during grout pumping. A maximum pH of 9.9 was recorded by the continuous monitor located 12 ft (3.7 m) directly downstream of the pier 3 minutes after pumping began. After grout pumping was completed, pH values dropped off quickly and typically returned to base- line conditions within 30 minutes. The one exception was the probe at Station C, which was directly in the wake of the pier and at the downstream edge of the grouted area. At this location, the pH returned to background levels after about 4 hours. Considering its location, this probe was in position to record the cumulative effect of the entire grouted area for the duration required for it to cure. At Station F, located 12 ft (3.7 m) directly downstream of Station C, a much less pronounced pH profile and more rapid decay of concentra- tion was observed. Results of monitoring are presented in Table 3.7, and Figure 3.36 shows the pH measurements at all stations. Figure 3.37 shows the maximum pH values at any time during the test as a function of distance from the pier. 67 Figure 3.33. Interior of the dry riprap pile (some surface rocks removed). Figure 3.34. Underwater partial grouting of riprap. A B C D E F G PLAN VIEW Note: Stations H, I, and J are located further downstream and are not shown in this illustration. FLOW 24 ft 12 ft 12 ft Figure 3.35. Location of water quality monitoring stations.

Turbidity. Background turbidity was about 3 to 4 neph- elometric turbidity units (NTUs). Turbidity peaked at 53.9 NTUs immediately after grout application began. This peak was maintained for less than 30 seconds, after which turbid- ity measurements ranged from about 30 to 35 NTUs for approximately 5 minutes. Turbidity returned to pregrouting levels almost immediately after grout application was com- pleted. Results of monitoring are presented in Table 3.8, and Figure 3.38 is a plot of turbidity measurements. Note in Figure 3.38 an increase in turbidity can be seen prior to grout application, corresponding to personnel walking around the test section in preparation for grout application. Temperature. Temperature remained nearly constant, ranging from 44.5˚F to 44.7˚F (6.9˚C to 7.1˚C) throughout the testing period, indicating the grout application process did not adversely affect water temperature. Results of monitoring are presented in Table 3.9, and Figure 3.39 shows a plot of temperature measurements. Conductivity. Background conductivity was 45 to 50 μmhos/cm prior to the test. Contrary to the findings of the VDOT study, conductivity values did appear to follow the pat- tern of grout installation. A notable increase in conductivity was observed at the two monitoring stations immediately downstream of the pier beginning at 10:17, 3 minutes after grouting application commenced. Results of monitoring are presented in Table 3.10, and Figure 3.40 is a plot of conduc- tivity measurements. After pH values returned to pre-grouting levels, as indi- cated by the grab sample monitoring, the tailwater control gates were shut and water was backed up in the flume. The in- stallation remained submerged for 96 hours to allow the grout to cure. After 96 hours the tail gates were opened, the flume was drained, and the installation was allowed to dry. High-Velocity Performance Test. Loose riprap around the surface perimeter of the installation that was not firmly se- cured during the grouting process was removed and replaced with sand. To prevent degradation of the sand bed during high-velocity testing, the upper 4 in. (100 mm) were stabilized by tilling 4% Portland cement by dry weight (of the sand) into the sand bed. The material was compressed with a vibrating plate compactor after addition of the Portland cement. The high-velocity test ran for 2 hours and was terminated when the soil cement bed began to visibly fail. Approach ve- locities at 60% of depth during the high-velocity test ranged from 4.2 to 5.6 ft/s (1.3 to 1.7 m/s). After draining the flume, several scour holes were observed in the soil cement bed, and a significant scour hole was observed downstream of the riprap installation. The soil cement in these areas had been 68 STATION A STATION C STATION F 10:14 am 10:19 am 10:34 am LABORATORY VALUES mg/L meq/L mg/L meq/L mg/L meq/L Sodium Na+ 2.78 0.12 3.06 0.13 2.94 0.13 Potassium K+ 1.00 0.03 2.40 0.06 1.60 0.04 Calcium Ca2+ 9.93 0.50 23.60 1.18 16.40 0.82 Magnesium Mg2+ 1.77 0.15 1.80 0.15 1.77 0.15 Carbonate CO32- 0.00 0.00 44.00 1.47 14.00 0.47 Bicarbonate HCO3- 32.00 0.52 23.00 0.38 34.00 0.56 Chloride Cl- 2.00 0.06 3.00 0.08 2.00 0.06 Sulfate SO42- 3.30 0.07 7.80 0.16 5.40 0.11 TDS (lab ROE) < 25 < 25 < 25 FIELD MEASUREMENTS Conductivity, µmhos/cm 83 142 101 pH, Standard Units 7.1 10.0 Turbidity, NTU 2.6 8.4 7.1 9.4 Table 3.6. Detailed water quality analyses of selected grab samples. Station Initial Condition End Condition Maximum value Average During Grout Placement A 6.9 7.1 7.1 7.0 B 6.9 7.1 9.4 8.4 C 6.9 7.3 9.9 9.7 D 6.9 7.0 8.6 7.8 E 6.9 7.1 9.2 7.9 F 6.9 7.1 9.5 9.0 G 6.9 6.9 8.5 7.8 H 7.0 7.0 8.3 7.1 I 7.0 7.2 8.6 7.3 J 7.4 7.5 8.4 7.7 Note: Data at Stations A through G from continuous monitors Data at Stations H through J from grab samples Table 3.7. Summary of pH measurements.

69 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 9:15 10:15 11:15 12:15 13:15 14:15 Time pH (s tan da rd un its ) Probe A Probe B Probe C Probe D Probe E Probe F Probe G Grout placement 10:14 to 10:34 Figure 3.36. pH vs. time. 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 -25 0 25 50 75 100 125 150 175 200 225 Distance from Centerline of Pier (ft) M ax im um p H (s tan da rd un its ) Pier Riprap FLOW Figure 3.37. Maximum observed pH vs. distance from pier.

left side of the pier and replaced with loose riprap of the same gradation and d50 shown in Figure 3.24. Because the soil ce- ment proved to be inadequate to stabilize the area around the partially grouted riprap, it was completely removed from the bed, exposing the underlying sand bed 4 in. (100 mm) lower than the surrounding flume floor and top surface of the riprap. A geotextile fabric, with the hydraulic and physical characteristics presented in Table 3.4, was installed over the exposed sand portion of the test section. Four-inch (100-mm) thick ACBs were installed on the geotextile fabric adjacent to the riprap. The ACBs were intended to prevent degradation of the bed in the test section as well as facilitate a smooth transition from the flume floor to the test section. Temporary walls were installed to reduce the width of the flow area and increase velocity in the test section. Walls were installed 2.5 ft (0.76 m) from the existing flume walls, transi- tioning the section from 20 ft (6 m) to 15 ft (4.6 m). Figure 3.42 shows the test section after the modifications were completed. The high-velocity comparison test ran for 4 hours, during which time the discharge was steadily increased to the full flow capacity. At maximum discharge, the approach velocity upstream of the pier reached a maximum of 6.4 ft/s (2 m/s). At the higher flows, the loose riprap began to displace. Figure 3.43 70 Station Initial Condition End Condition Maximum Value Average During Grout Placement A 3.6 3.3 8.3 3.7 B 3.6 3.6 27.9 C 4.0 3.7 51.3 22.7 D 3.8 3.4 20.6 E 3.1 2.9 19.1 F 7.1 4.0 53.9 19.5 G 3.7 3.7 9.1 4.9 H 3.2 2.6 3.2 I 2.5 2.6 3.0 J 3.3 3.4 4.6 7.1 7.1 6.1 3.3 2.7 3.6 Note: Data at Stations A through G from continuous monitors Data at Stations H through J from grab samples Table 3.8. Summary of turbidity measurements (NTUs). 0.0 10.0 20.0 30.0 40.0 50.0 60.0 9:15 10:15 11:15 12:15 13:15 14:15 Time Tu rb id ity (N TU ) Probe A Probe B Probe C Probe D Probe E Probe F Probe G Grout placement 10:14 to 10:34 Figure 3.38. Turbidity vs. time. destabilized and the underlying sand scoured to a depth of about 2.5 ft (0.8 m). The partially grouted riprap installation and underlying geotextile containers remained intact. Figure 3.41 shows the test section after the high-velocity test. High-Velocity Comparison Test. To facilitate a com- parison of the performance of loose riprap to partially grouted riprap, all riprap and grout were removed from the

shows the loose riprap side of the installation after completion of the second half of the high-velocity comparison test. Note the scour hole on the near side of the pier and the displaced riprap behind and downstream of the pier compared to the previous figure. The partially grouted side of the riprap instal- lation can be seen in this figure, and remained essentially undisturbed. Figure 3.44 shows the partially grouted side of the installation after the end of this test. 3.6 Articulating Concrete Block Systems There is limited experience with the use of ACB systems as a scour countermeasure for bridge piers alone. More fre- quently, these systems have been used for bank revetments and channel armoring where the mat is placed across the en- tire channel width and keyed into the abutments or bank pro- tection. For this reason, guidelines for placing ACB systems along bank lines and in channels are well documented (e.g., Ayres Associates 2001), but there are few published guidelines on the installation of these systems around bridge piers. There are two failure mechanisms for ACB systems: over- turning and rollup of the leading edge of the mat where it is not adequately anchored or toed in, and uplift at the center of the mat where the leading edge is adequately anchored. In the absence of a filter or geotextile, winnowing can still occur and can result in subsidence of all or a portion of the ACB mat. Studies conducted on the effectiveness of ACBs as a countermeasure have determined that the use of a filter fab- 71 Station Initial Condition End Condition Maximum Value Average During Grout Placement A 44.5 44.5 44.7 44.6 B 44.5 44.5 44.7 44.5 C 44.5 44.5 44.7 44.6 D 44.5 44.5 44.7 44.5 E 44.5 44.5 44.7 44.5 F 44.5 44.5 44.7 44.6 G 44.5 44.5 44.7 44.5 H 44.5 44.5 44.7 44.6 I 44.5 44.5 44.7 44.5 J 44.5 44.5 44.7 44.6 Note: Data at Stations A through G from continuous monitors No temperature data available at Stations H through J Table 3.9. Summary of temperature measurements (°F). 40.0 41.0 42.0 43.0 44.0 45.0 46.0 47.0 48.0 49.0 50.0 9:15 10:15 11:15 12:15 13:15 14:15 Time Te m pe ra tu re (F ) Probe A Probe B Probe C Probe D Probe E Probe F Probe G Grout placement 10:14 to 10:34 Figure 3.39. Temperature vs. time.

cabled and installed as mats. Cabling is primarily a construc- tion convenience, and while cables may prevent blocks from being lost entirely, they do not keep blocks from failing through loss of intimate contact with the subgrade, which is the criterion generally accepted for stability design. The test- ing procedure for this ACB examination did not incorporate any simulation of cabling. Miniature open-cell blocks measuring 1.6 in. (40 mm) long by 1.4 in. (36 mm) wide by 0.7 in. (17 mm) high were used in the testing program. The blocks were made from a sand-cement mortar having a specific gravity of 1.84 and a moisture absorption of 16% by weight. The critical shear stress for the blocks was determined in a smaller flume prior to placement around the test piers. The blocks were sized using the factor of safety method for hydraulic conditions representative of the CSU 8-ft flume. Results indicate that at a flow depth of 1.0 ft (0.305 m) and an approach velocity of 2Vcrit, a target factor of safety of 1.0 (incipient failure) was achieved under these conditions. Because the factor of safety method presented in HEC-23 (Lagasse et al. 2001) for ACB countermeasure design does not account for any added sta- bility that may be afforded by cables, the testing procedure re- flects HEC-23 philosophy. Table 3.11 provides a summary of the physical and hydraulic characteristics of the miniature 72 Station Initial Condition End Condition Maximum Value Average During Grout Placement A 48 48 48 48 B 48 48 62 51 C 48 49 74 61 D 48 49 57 50 E 48 48 56 50 F 48 49 76 62 G 48 48 58 50 H 45 44 46 44 I 47 44 49 47 J 51 43 53 49 Note: Data at Stations A through G from continuous monitors Data at Stations H through J from grab samples Table 3.10. Summary of conductivity measure- ments (mhos/cm). 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 9:15:00 10:15:00 11:15:00 12:15:00 13:15:00 14:15:00 Time Co nd uc tiv ity (µ m ho s/ cm ) Probe A Probe B Probe C Probe D Probe E Probe F Probe G Grout placement 10:14 to 10:34 Figure 3.40. Conductivity vs. time. ric or geotextile was important to the overall effectiveness and stability of the ACB system. 3.6.1 Materials Blocks ACBs were examined for their suitability as a pier scour countermeasure. Many ACB systems in use today are pre-

blocks used in this study. Figure 3.45 shows a close-up view of the blocks and their interlocking installation pattern. The blocks were placed directly on a non-woven, needle- punched geotextile but were not glued or otherwise affixed to the geotextile, and no cables were used in any of the tests. In all cases, a sand-cement grout seal was placed between the blocks and the pier. For some tests, grout seams were also used at the intersection of plane surfaces where typical field applications would normally call for saw-cut blocks and grout seams to be used. Filter Geotextile selection for filter fabric was made using the method outlined in Koerner (1998), as summarized in Sec- tion 3.4.1. Table 3.12 summarizes the hydraulic and physical 73 Displaced riprap Scour hole Note: Loose riprap is on the near side of the pier and partially grouted riprap on the far side Figure 3.41. Damage to the soil cement and scour at the downstream left corner after the high velocity performance test. Figure 3.42. Loose riprap, ACB, and contraction wall installation. Figure 3.43. Loose riprap after completion of the high-velocity comparison test. Figure 3.44. Partially grouted riprap after completion of the high-velocity comparison test.

pier, and the termination details at the pier and around the pe- riphery of the installation. In addition, various filter extents were investigated by varying this aspect for selected test runs (see Appendix H for details on the configurations tested). Baseline Because limited information is available on the use of ACBs as a pier scour countermeasure, baseline ACB installa- tion conditions were based on observations from the riprap testing program and experience with ACBs in other erosion control applications. Initial installation of ACBs extended a horizontal distance of two pier widths on all sides for a total areal coverage of five pier widths. ACBs were toed down into the bed at a 2H:1V slope. Figure 3.46 presents the design lay- out for the baseline ACB tests. For all ACB tests the interface of the blocks and pier was sealed with grout and a full cover- age geotextile filter was used. The design intent for ACB baseline tests included exami- nation of the following: • Standard ACB layout • Standard ACB layout with grouted interface Initially, the baseline ACB installation was tested without grouting the interface of the ACB planes. A loss of blocks was observed under these conditions, as shown in Figure 3.47. The baseline ACB installation was stable with the ACB inter- faces grouted when exposed to 1.9Vcrit flow velocity and 4- to 6-in. (100- to 152-mm) dune passage. After being exposed to 2.5Vcrit flow velocity and 8- to 10-in. (200- to 254-mm) dune passage, loss of blocks was observed. Results of two baseline conditions tests are shown in Figure 3.48. The ACBs were sta- ble at 1.9Vcrit (the design condition). Loss of blocks occurred at 2.5Vcrit (20% greater than the design condition). 74 Property U.S. Customary Units Metric Units Comments Length 1.5625 in. 40 mm Width 1.4375 in. 36 mm Height 0.6875 in. 17 mm Average weight/block 0.056 lb 25.30 g Saturated Average density 134 lb/ft3 2.15 g/cm3 Saturated Critical shear stress 0.305 lb/ft2 14.6 N/m2 Tested at horizontal Manning's n value 0.016 0.016 Filter Name Geotextile Type Mass/ Unit Area AOS Permeability Trade Name Manufacturer Kg/Ks NW1 Non-woven 163 g/m2 0.212 mm 0.21 cm/s Mirafi® 140 N Mirafi Construction Products 5.25 NW2 Non-woven 250 g/m2 ~ 0.10 mm 0.4 cm/s HaTe® B 250 K4 Huesker Synthetic GmbH 10.0 Table 3.11. ACB properties. Table 3.12. Hydraulic and physical characteristics of geotextile filters. Figure 3.45. ACBs used in testing program. properties of the geotextile filters used for the ACB portion of this study. 3.6.2 Testing Program The testing program addressed stability and performance issues associated with the extent of ACB placement around the

Extent of Coverage The design intent for ACB extent of coverage tests included examination of the following: • Areal coverage and termination detail • ACBs in conjunction with riprap For the tests with ACBs and riprap, the geotextile filter extended beyond the perimeter of the blocks under the riprap. The results of two of the ACB coverage tests after exposure to 1.9Vcrit flow conditions are shown in Figure 3.49. Termination Detail The design intent of the ACB termination detail tests in- cluded examination of the following: • Termination detail • Areal coverage and termination detail • ACB used in conjunction with riprap For the tests with ACBs and riprap, the geotextile filter extended beyond the perimeter of the blocks under the riprap. When the ACBs did not extend below the dune troughs, severe loss of blocks was observed regardless of the turndown detail at the periphery. Figure 3.50 shows the construction details for 75 FLOW 2a a 2a ACB placement = 2(a) from pier (all around) Pier width = “a” (normal to flow) Filter Filter extended to the edge of the block around the entire perimeter ACB Blocks sloped 1V:2H (50%) Figure 3.47. Loss of blocks during testing without the ACB interfaces grouted. Figure 3.46. Baseline ACB design. a. Baseline ACB installation after 1.9Vcrit test. b. Baseline ACB installation after 2.5Vcrit live-bed test. Figure 3.48. After baseline ACB with grouted interface testing.

one of the termination tests. The results of the two termina- tion detail tests without riprap after 7 hours of 1.9Vcrit tests are shown in Figure 3.51. Filter Studies conducted on the effectiveness of ACBs as a coun- termeasure to scour have determined that the use of a filter fab- ric or geotextile is essential to the overall effectiveness and sta- bility of the ACB system. In the absence of a filter or geotextile, winnowing can occur and result in subsidence of all or a por- tion of the ACB mat. One test was performed to examine two- thirds coverage of a geotextile. After being exposed to 2.5Vcrit flow velocity with sediment feed and 8- to 10-in. (200- to 254- mm) dune passage, catastrophic loss of blocks was observed. The design intent of ACB filter tests included examination of the following: • Standard ACB layout with grouted interface below the bed surface • ACB used in conjunction with riprap For the tests with ACBs and riprap, the geotextile filter ex- tended beyond the perimeter of the blocks under the riprap. Figure 3.52 shows two piers after testing: one pier had a ge- otextile filter that extended two-thirds the distance from the pier face to the periphery (failed) and the other pier had a ge- otextile filter that extended beyond the perimeter of the ACBs and extended two-thirds the distance of an overlying riprap layer (stable). 3.7 Gabion Mattresses There is limited experience with the use of gabion mattress systems as a scour counter-measure for bridge piers alone. More frequently, these systems have been used for structures such as dams or dikes, or for channel slope stabilization. For this reason, the gabion testing program was derived from results of the riprap and ACB testing programs. Typically, during gabion mattress installation in the field, the units are interconnected to form a single continuous layer. Testing pro- cedures for the gabion mattresses included tests of both un- connected and connected units. 76 a. ACB configuration with coverage increased downstream of pier, after 2 hours of exposure at 1.9Vcrit flow conditions. b. ACB configuration with blocks installed below ambient bed elevation, after 2 hours of exposure at 1.9Vcrit flow conditions 8” 4” Pier 4” 12” 12”16” Extend geotextile to edge of blocks around entire perimeter 4H:1V 3H:1V FLOW Figure 3.49. ACB coverage tests. Figure 3.50. Example ACB termination test configuration.

3.7.1 Materials Gabions Gabion mattresses used in the laboratory tests consisted of wire mesh boxes filled with small gravel. Each gabion mattress was hand constructed with nominal dimensions of 6 in. (152 mm) long by 4 in. (102 mm) wide by 0.5 in. (12.5 mm) high. The wire mesh had a grid aperture size of 0.125 in. (3.2 mm) and was relatively flexible, but could be bent to obtain and hold a rectangular shape. Strips of plastic mesh typically used for craft projects were inserted as dividers in the gabion mattress to create three compartments in each mattress. Gabion stone requirements were developed using the filter criteria specified in HEC-11 (Brown and Clyde 1989). The initial step establishes the compatibility of the filter (gabion stone) with the sand bed material in terms of both particle retention and permeability by defining upper and lower lim- its of d15 for the filter. This definition determines the largest size allowable to maintain particle retention and smallest size allowable to ensure the filter has greater permeability than the sand. The fill material for the gabion mattresses was obtained from a local gravel supplier. It consisted of fine gravel having a d50 of 0.23 in. (5.8 mm) and a uniformity ratio d85/d15 of 2.1. The specific gravity of the gravel ranged from 2.55 to 2.60. A 77 a. ACB configuration with increased areal extent and depth after 7 hours of exposure at 1.9Vcrit flow conditions. b. ACB configuration from Figure 3.49a after 7 hours of exposure at 1.9Vcrit flow conditions. a. ACBs with two-thirds extent geotextile filter. b. Riprap with ACBs. Riprap and geotextile filter extended beyond the ACBs. Figure 3.51. ACB termination tests. Figure 3.52. ACB filter tests.

grain size distribution graph for the gabion stone is presented in Figure 3.53. Figure 3.54 shows a typical gabion mattress used in the testing program. Tests to determine the critical shear stress for the gabion mattresses were performed in a smaller flume prior to place- ment around the test piers. The gabion mattresses did not fail under the prescribed hydraulic conditions. The maximum cross section averaged velocity observed during shear stress testing was 8.1 ft/s (2.5 m/s) and the maximum applied shear stress was 2.3 lb/ft2 (110 N/m2), which occurred during a flow of 4.1 cfs (0.12 m3/s). Table 3.13 provides a summary of the physical and hydraulic characteristics of the gabion mat- tresses used in this study. No attempt was made to scale the strength characteris- tics of the wire mesh and compartment divider material to the small size of the mattresses used in the testing program. Guidance for material strength and properties for field- scale applications are derived from relevant ASTM stan- dards for these products and are described in detail in Appendix F. Filter A non-woven, needle-punched geotextile compatible with the bed material in the flume was used as the filter for all the gabion mattress tests. Geotextile selection for filter fabric was made using the method outlined in Koerner (1998), as sum- marized in Section 3.4.1. Table 3.14 summarizes the hydraulic and physical properties of the geotextile filter used in the gabion mattress portion of this study. 3.7.2 Testing Program The testing program addressed stability and performance issues associated with the extent of gabion mattress place- ment around the pier and the termination details at the pier and around the periphery of the installation. In addition, var- ious filter extents were investigated by varying this aspect for selected test runs (see Appendix H for details on the configu- rations tested). After testing coverage extent, filter extent, and termination detail, three installation designs were retested with the gabions sewn together at the edges. 78 GRAIN SIZE DISTRIBUTION 3-in 2-in 1-in 0.5-in 4 8 704010 16 20 140 200100 0 10 20 30 40 50 60 70 80 90 100 0.00010.0010.010.1110100 Grain Size in millimeters Pe rc en t F in er b y w ei gh t - % Sieve Size Gabion Mattress Fill D50 = 5.8 mm Figure 3.53. Grain size distribution for gabion mattress stone.

Baseline Because limited information is available on the use of gabion mattresses as a pier scour countermeasure, baseline gabion mat- tress installation conditions were not available. Observations from the ACB and riprap testing programs and experience with gabion mattresses in other erosion control applications were used to determine an initial starting point. First, gabion mat- tresses were examined for areal extent when installed flush with the ambient bed elevation, then extent of a geotextile filter and periphery turndown detail was examined. Extent of Coverage In the coverage tests, unconnected gabion mattresses extended a minimum horizontal distance of three pier widths normal to flow and four pier widths (1 gabion width = 0.5 a) parallel to flow; a geotextile filter extended from the pier face to the periphery of the gabion mattresses on all tests. Figure 3.55 presents the typical design layout for the gabion mattress coverage tests. All areal coverage tests of gabion mattresses in- cluded a full extent geotextile filter. Dimensions parallel and normal to the flow were varied. Initial conditions were satisfactory when the areal extent was a minimum of four pier widths normal to flow and four pier widths parallel to flow; significant gabion displacement and loss was observed when areal coverage was less. Results of unconnected gabion mattress tests after 2Vcrit flow condi- tions are shown in Figure 3.56. Termination Detail The design intent of the gabion mattress termination de- tail tests included examination of the following: • Termination detail with full geotextile filter • Filter extent and termination detail • Termination detail with two-thirds geotextile filter Appendix H, Table H.14, provides details on these tests. Figure 3.57 shows the construction details for one of the ter- mination tests. Termination detail tests examined conditions where the unconnected gabions were not installed horizontal but exhibited some form of turndown at the periphery. Filter Riprap test results confirmed that riprap performance was best when a geotextile filter extended two-thirds the distance to the periphery of the riprap. In the gabion mattress testing, filter extent was tested in conjunction with termination de- tail, thereby making results for performance of filter coverage difficult to isolate. Unconnected gabion mattresses tended to slide off the edge when the geotextile extended the full dis- tance from the pier face to the periphery. When the geotex- tile coverage was two-thirds the distance of the gabions, the gabion mattresses were observed to displace into the troughs on the side of the installation or were lost completely. See the following section for results of connected gabion mattresses tested with two-thirds geotextile filter coverage. Connected Gabion Mattresses Gabion mattresses are often joined together to form a large mattress that when undermined or unstable can mold itself to the underlying subsurface, thus restoring stability to the unit. A series of tests was run with installation designs identical to previous tests, including a two-thirds geotextile filter extent, 79 Figure 3.54. Typical gabion mattress with wire mesh and three compartments filled with stone. Property U.S. Customary Units Metric Units Length 6.0 in. 152 mm Width 4.0 in. 102 mm Height 0.5 in. 12.5 mm d50 of stone fill 0.23 in. 5.8 mm Maximum applied velocity 8.1 ft/s 2.5 m/s Maximum applied shear stress 2.3 lb/ft2 110 N/m2 Table 3.13. Gabion mattress properties. Filter Name Geotextile Type Mass/ Unit Area AOS Permeability Trade Name Manufacturer Kg/Ks NW2 Non-woven 250 g/m2 ~ 0.10 mm 0.4 cm/s HaTe® B 250 K4 Huesker Synthetic GmbH 10.0 Table 3.14. Hydraulic and physical characteristics of geotextile filter.

with the gabions connected along the edges. The gabions were observed to be stable and conforming to changes in the bed; however, these tests underscored the importance of providing an effective seal at the pier. Figure 3.58 shows a comparison between identical installa- tion designs with connected (Figure 3.58a) and unconnected (Figure 3.58b) gabions. Figure 3.59 shows the results of two connected gabion mattress tests. 3.8 Grout-Filled Mattresses There is limited field experience with the use of grout-filled mattress systems as a scour countermeasure for bridge piers. More frequently, these systems have been used for shoreline protection, underwater pipelines, and channel armoring where the mattress is placed across the entire channel width and keyed into the abutments or bank protection. For this reason, the grout-filled mattress testing program was derived from results of the gabion mattress and ACB testing pro- grams. Both rigid and articulating configurations were tested. The primary failure mechanisms for grout-filled mat- tresses consist of rolling, undercutting, and scouring at gaps. Rolling, the most severe form of failure, is related to uplift forces created by flow over the mattress. This flow allows the mattress at midsection to be “lifted up” slightly and then pushed loose by the force of the current or allows the edge of the mattress to be rolled back. Undercutting is a gradual process arising from local scour at the mattress edges and from the main horseshoe vortex. Scouring at the gaps be- tween mattress and the pier wall allows the horseshoe vortex to generate a scour hole beneath the mattress. 3.8.1 Materials Mattresses Rigid fabric-formed grout-filled mattresses were modeled by soaking a synthetic batting, typically used in quilting, in a cement-rich concrete grout. Mattresses were cut to fit the in- stallation design, but typically a mattress was 8 in. x 12 in. (200 mm × 300 mm). The grout-filled mattresses were placed while wet on top of a geotextile filter around each pier. Relief of pore water pressure from beneath the mattress was allowed through weep holes cut into the center of each mattress and at corners where two mattresses joined. After the grout cured, 80 Pier a Geotextile extends full length of the gabions for baseline tests FLOW Top of gabion mattress flush with ambient bed elev. a. Greatest areal extent gabion mattress testing after 2.0Vcrit test. b. Gabion mattresses with insufficient areal extent after 2.0Vcrit test. Figure 3.55. Baseline gabion mattress coverage test layout. Figure 3.56. After unconnected gabion mattress coverage testing.

the thickness of each mattress was approximately 0.25 in. (6 mm). Figure 3.60 shows a rigid grout-filled mattress being placed while wet around a pier. The flexible grout-filled mattresses were modeled using sheets of 1-in. (25-mm) square mosaic tile. The synthetic ad- hesive that connects the tiles together was scored with a razor blade to allow maximum flexibility while still maintaining block-to-block connection. The sheets were cut to fit each installation; abutting sheets were connected using several layers of cheesecloth, which also acted as a filter. Although each sheet exhibited excellent flexibility in the x- and y- directions separately, flexibility was quite limited when the sheet needed to flex in both planes simultaneously, for example, when wrapping around a corner or warped transi- tion area. Figure 3.61 shows an articulating grout mattress being installed. 81 4H:1V Pier a1.5a 1.5a Geotextile extends 2/3 the length of the gabions FLOW Figure 3.57. Example gabion mattress termination test configuration. a. Connected gabion mattresses after 2 hours of exposure at 2.0Vcrit flow conditions. b. Unconnected gabion mattresses after 2 hours of exposure at 2.0Vcrit flow conditions. a. Connected gabion mattresses after 2 hours of exposure at 2.0Vcrit flow conditions. b. Connected gabion mattresses after 2 hours of exposure at 2.0Vcrit flow conditions. Figure 3.58. Comparison of results of connected and unconnected gabions of the same installation design. Figure 3.59. Connected gabion mattress test results.

Filter Geotextile selection for filter fabric was made using the method outlined in Koerner (1998), as summarized in Section 3.4.1. Table 3.15 summarizes the hydraulic and physical prop- erties of the geotextile filter used in the rigid grout-filled mat- tress portion of this study. For the flexible grout mattress portion of the testing pro- gram, several layers of cheesecloth served as substitute for the geotextile filter. Hydraulic and physical properties were not available for this material. 3.8.2 Testing Program The testing program addressed stability and performance issues associated with the extent of grout-filled mattress placement around the pier and the termination details at the pier and around the periphery of the installation (see Appen- dix H for details on the configurations tested). Baseline Because limited information is available on the use of grout- filled mattresses as a pier scour countermeasure, baseline grout-filled mattress installation conditions were not available. Observations from the ACB and gabion mattress testing pro- gram, as well as experience with grout-filled mattresses in other erosion control applications, were used to determine an initial starting point. First grout-filled mattresses were examined for areal extent when installed flush with the ambient bed eleva- tion and then periphery turndown details were examined. The results of these tests are described in the following sections. Extent of Coverage In the coverage tests, grout-filled mattresses extended a minimum horizontal distance of three pier widths normal to flow and four pier widths parallel to flow; a geotextile filter ex- tended from the pier face to the periphery of the mattress on all tests. Figure 3.62 presents the typical design layout for the rigid grout-filled mattress coverage tests. Figure 3.63 shows the results of two rigid grout-filled mattress areal coverage tests. The rigid grout-filled mattresses did not articulate with passing bed forms and as a result a significant amount of ma- terial was removed from beneath the mattresses in each test. Test results were unsatisfactory for all areal coverage tests. Termination Detail Termination detail tests examined conditions where the rigid grout-filled mattresses were not installed horizontal but exhibited some form of turndown at the periphery. Stable conditions were observed when the grout-filled mattresses sloped away from the pier in all directions, but further inves- tigation revealed that material had been removed from under the mattress, leaving pockets of empty space beneath the 82 Figure 3.60. Placement of rigid grout-filled mattress. Figure 3.61. Installation of flexible grout mattress. Filter Name Geotextile Type Mass/ Unit Area AOS Permeability Trade Name Manufacturer Kg/Ks NW2 Non-woven 250 g/m2 ~ 0.10 mm 0.4 cm/s HaTe® B 250 K4 Huesker Synthetic GmbH 10.0 Table 3.15. Hydraulic and physical characteristics of geotextile filter.

83 Top of grout filled mattress flush with ambient bed elev. Geotextile extends full length of the mattresses Pier a FLOW a. Grout-filled mattress coverage test with greatest areal extent, after 2.0Vcrit test. b. Grout-filled mattress coverage test with smallest areal extent, after 2.0Vcrit test. Figure 3.62. Typical grout-filled mattress coverage test layout. Figure 3.63. Grout-filled mattress coverage test results. countermeasure. Figure 3.64 shows the construction details for one of the termination tests. Filter Filter extent was not examined in this portion of the test- ing program. All rigid grout-filled mattress tests incorporated a geotextile that extended from the pier face to the periphery of the mattress. Flexible Grout Mattresses A series of tests was run with installation designs identical to previous tests except the rigid grout-filled mattresses were re- placed with a flexible grout mattress. When the grout mattress was installed horizontal with a turndown on the periphery, test results reveal a significant amount of material removed from under the grout mattress behind the pier. For the case where the grout mattresses sloped away from the pier, further investiga- tion revealed that material had been removed from under the mattresses, leaving pockets of empty space beneath the coun- termeasure. Figure 3.65 shows the results of two flexible grout mattress tests. 3.9 Design and Specification 3.9.1 Riprap When properly designed and used for pier scour protec- tion, riprap has an advantage over rigid structures because it is flexible when under attack by river currents, it can re- main functional even if some individual stones may be lost,

and it can be repaired relatively easily. Properly constructed riprap can provide long-term protection if it is inspected and maintained on a periodic basis as well as after flood events. Riprap tests conducted under NCHRP Project 24-07(2) in- cluded a wide variety of layout configurations at both square and rectangular (wall-type) piers, including piers skewed to the flow direction. Various filter extents and toedown meth- ods were also investigated. An interpretation and appraisal of riprap as a pier scour countermeasure is presented in this sec- tion, drawing from the results and observations of the NCHRP Project 24-07(2) testing program, as well as from the literature review conducted for this project and guidance from existing practice. Design Tests of riprap at piers were conducted using the pier riprap sizing method recommended in HEC-23 (Lagasse et al. 2001). The results of the tests confirmed that this velocity- based procedure is appropriate for sizing riprap at piers, provided that the extent and thickness of the armor layer, the gradation, and the design of the filter, also follow recom- mended guidelines, as discussed in the following paragraphs. 84 Top of grout filled mattress flush with ambient bed elev. Geotextile extends full length of the mattresses Pier a0.75a 0.75a 3H:1V FLOW a. Flexible grout mattress after 2 hours of exposure at 2.0Vcrit flow conditions. A significant amount of material was removed from under the grout mattress. b. Flexible grout mattress with 2:1 turndown from pier face, after 2 hours of exposure at 2.0Vcrit flow conditions. Further investigation revealed voids beneath the grout mattress. Figure 3.64. Example grout-filled mattress termination test configuration. Figure 3.65. Flexible grout mattress test results.

The HEC-23 pier riprap sizing procedure is recommended for use in designing pier scour countermeasures, and is pre- sented in detail in Appendix C. Layout Results from riprap testing indicated that riprap areal cov- erage should be a minimum of two pier widths in all direc- tions. Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the am- bient channel bed elevation. Placing the top of the riprap flush with the bed is ideal for inspection purposes and does not create additional obstruction to the flow. The riprap layer should have a minimum thickness of three times the d50 size of the rock. Poor results were observed when riprap was mounded on top of the bed. Mounding riprap around a pier is not accept- able for design in most cases, because it obstructs flow, captures debris, and increases scour at the periphery of the installation. Tests confirmed that the lateral extent of riprap protection at rectangular piers must be increased when the longitudinal axis of the pier is skewed to the flow direction. At the outset of this study, no quantitative guidance existed to address this issue adequately. However, the required extent of protection at skewed piers can be inferred from the testing conducted under this project. Guidance for skewed piers is provided in the design guidelines appended to this report. The research team recommends that this topic be considered as an area needing future research. Tests also confirmed that a filter should not be extended fully beneath the riprap; instead, the filter should be termi- nated two-thirds the distance from the pier to the edge of the riprap. When using a granular filter, the layer should have a minimum thickness of four times the d50 of the filter stone or 6 in. (152 mm), whichever is greater. Placing the filter and riprap under water was not investigated during the NCHRP Project 24-07(2) tests; therefore, guidance for this aspect comes from existing practice, which recommends that the layer thickness of both riprap and granular filter should be in- creased by 50% when placing under water. Granular filters are not recommended when dune-type bed forms are pres- ent. In addition, the riprap thickness should be increased if the depth of the bed form trough is greater than the recom- mended thickness of three times the d50 size of the riprap. Materials Riprap used in the laboratory tests consisted of a hard, durable sandstone having a specific gravity of 2.55 to 2.60. Other types of rock materials having different densities were not tested during this study; long-term weathering potential also was not investigated. Recommendations for rock riprap quality, durability, and gradation are therefore derived from guidance developed using sources from both the United States and Europe. These recommendations, as well as conformance testing requirements, are provided in detail in Appendix C. With respect to filter materials, both granular and geotextile filters were tested under clear-water and live-bed scour condi- tions. The effects of contraction scour and long-term degrada- tion were not investigated in this study. Existing guidelines for the required engineering properties of these materials were found to be adequate under the conditions tested. 3.9.2 Partially Grouted Riprap Partially grouted riprap consists of appropriately sized rocks that are placed around a pier and grouted together with grout filling 50% or less of the total void space. In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. It also allows for the use of smaller rock com- pared to standard riprap, resulting in decreased layer thickness. The system typically includes a filter layer, either a geotextile fabric or a filter of sand and/or gravel, specifically selected for compatibility with the subsoil. The filter allows infiltration and exfiltration to occur while providing particle retention. Tests of partially grouted riprap were conducted for NCHRP Project 24-07(2) around 8-in. (200-mm) square piers in an indoor flume using angular stone. Three different sizes of stone were investigated, with d50 values of 14.7, 25.4, and 30.0 mm. Partially grouted riprap was also tested at pro- totype scale in a large outdoor flume around a rectangular pier measuring 1.5 ft (0.5 m) wide by 4.5 ft (1.5 m) long. The riprap used in the outdoor tests had a d50 of 6 in. (152 mm). That stone size was somewhat smaller than the minimum rec- ommended d50 of 9 in. (230 mm) used for field-scale partial grouting applications. For the prototype-scale tests, a filter composed of sand-filled geocontainers was placed under flowing water in a pre-existing scour hole around the pier. These tests confirmed the applicability of partially grouted riprap as a scour countermeasure for bridge piers. Design Design guidance for partially grouted riprap comes from the BAW in Germany. The intent of partial grouting is to “glue” stones together to create a conglomerate of particles. Each conglomerate is therefore significantly greater than the d50 stone size and typically is larger than the d100 size of the in- dividual stones in the riprap matrix. For practical placement in the field, riprap having a d50 smaller than 9 in. (230 mm) exhibits voids that are too small for grout to effectively penetrate to the required depth within 85

the rock matrix. At the other extreme, riprap having a d50 greater than 15 in. (380 mm) has voids that are too large to retain the grout, and does not have enough contact area between stones to effectively glue them together. With par- tially grouted riprap, there are no relationships per se for selecting the size of rock, other than the practical considera- tions of proper void size and adequate stone-to-stone contact area. Specific recommendations for design and specification are presented in detail in Appendix D. Layout The optimum performance of partially grouted riprap as a pier scour countermeasure was obtained when the riprap installment extended a minimum distance of one and a half times the pier width in all directions around the pier. Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the am- bient channel bed elevation. Placing the top of the riprap flush with the bed is ideal for inspection purposes and does not create any added obstruction to the flow. Mounding riprap around a pier is not acceptable for design in most cases, because it obstructs flow, captures debris, and increases scour at the periphery of the installation. When used in a par- tially grouted application, the riprap layer should have a min- imum thickness of two times the d50 size of the design riprap. When placement must occur under water, the thickness should be increased by 50%. A filter layer is typically required for riprap at bridge piers. The filter should not be extended fully beneath the riprap; in- stead, it should be terminated two-thirds of the distance from the pier to the edge of the riprap. Materials Riprap used in the laboratory tests consisted of a hard, durable sandstone having a specific gravity of 2.55 to 2.60. Other types of rock materials having different densities were not tested during this study; long-term weathering potential also was not investigated. Recommendations for rock riprap quality, durability, and gradation are therefore derived from guidance developed using sources from both the United States and Europe. These recommendations, as well as conformance testing requirements, are provided in detail in Appendix D. With respect to filter materials, only geotextile filters were tested with the partially grouted riprap. Existing guidelines for the required engineering properties of these materials were found to be adequate under the conditions tested. The use of sand-filled geocontainers composed of non-woven, needle-punched geotextile was confirmed to be an appropri- ate means of establishing a filter layer around a pier when placement must occur under water. Standard Portland cement–based grout was used in the tests. For tests where the grout was placed under water, the recommended amount of Sicotan® admixture was included in the mix to minimize segregation and improve the “stickiness.” Specific recommendations for grout design and specification, including application quantities, are presented in detail in Appendix D. 3.9.3 Articulating Concrete Blocks ACB systems provide a flexible armor for use as a pier scour countermeasure. These systems consist of preformed concrete units that either interlock, are held together by cables, or both. After installation is complete, the units form a continuous blanket or mat. The term “articulating” implies the ability of individual blocks of the system to conform to changes in the subgrade while remaining interconnected. Block systems are typically available in both open-cell and closed-cell varieties. There is little field experience with the use of ACB systems as a scour countermeasure for bridge piers alone. More fre- quently, these systems have been used for bank revetment and channel armoring where the mat is placed across the entire channel width and keyed into the abutments or bank protection. Tests conducted under NCHRP Project 24-07(2) confirm the applicability of ACB systems as a scour counter- measure for bridge piers. Design Tests of ACBs at piers were conducted using the factor of safety method recommended in HEC-23 (Lagasse et al. 2001). The results of the tests confirmed that this procedure is appro- priate for designing ACBs for hydraulic stability at piers, pro- vided that the extent of the armor layer, as well as the design of the filter, also follows recommended guidelines, as discussed in the following paragraphs. The HEC-23 procedure is therefore recommended for use in designing pier scour countermeasures using ACBs and is presented in detail in Appendix E. Testing also confirmed the importance of including block placement tolerance in the factor of safety calculations. Layout Results from the ACB testing indicated that the optimum performance of ACBs as a pier scour countermeasure was obtained when the blocks were extended a distance of at least two times the pier width in all directions around the pier. Because ACBs are essentially an erosion-resistant veneer that is one particle thick, the system edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. Blocks should not be placed on slopes greater than 2H:1V. When placed as pre-assembled mats, 86

they should never be placed such that a portion of one mat lies on top of another mat. When dune-type bed forms were present, it was found that the armor must be sloped away from the pier in all directions such that the depth of the ACB system at its periphery is greater than the depth of the bed-form troughs. Although contraction scour and long-term degradation were not tested in this study, it is presumed that this same guidance applies in cases where these conditions may be present at the bridge crossing. In some cases, this requirement may result in blocks being placed fur- ther than two pier widths away from the pier. Test results con- firmed that a filter should be extended fully beneath the ACBs. Materials Potential issues associated with long-term durability of the ACBs and variations in subgrade preparation were not inves- tigated in this study. Therefore, guidance for concrete prop- erties and construction techniques are derived from ASTM D 6684 and D 6884, respectively. This guidance is described in detail in Appendix E. 3.9.4 Gabion Mattresses Gabion mattresses are containers constructed of wire mesh and filled with rocks. The length of a gabion mattress is greater than its width, and the width is greater than its thick- ness. Diaphragms are inserted widthwise into the mattress to create compartments. Wire is typically galvanized or coated with polyvinyl chloride to resist corrosion, and either welded or twisted into a lattice. Stones used to fill the containers can be either angular rock or rounded cobbles; however, angular rock is preferred because of the higher degree of natural in- terlocking of the stone fill. During installation, individual mattresses are connected together by lacing wire or other connectors to form a continuous armor layer. The wire mesh allows the gabions to deform and adapt to changes in the bed while maintaining stability. Additionally, when compared to riprap, less excavation of the bed is required and smaller, more economical stone can be used. The obvious benefit of gabion mattresses is that the size of the individual stones used to fill the mattress can be smaller than stone that would otherwise be required to withstand the hydraulic forces at a pier. Tests conducted under NCHRP Project 24-07(2) con- firm the applicability of these systems as a scour countermea- sure for bridge piers. Design There is limited field experience with the use of gabion mattresses as a scour countermeasure for bridge piers alone. More frequently, these systems have been used for structures such as in-channel weirs or drop structures, or for channel slope stabilization. The guidance for pier scour applications provided in this document has been developed primarily from the results of this study. Initial guidance to develop the testing program for gabion mattresses at piers was developed from the second edition of FHWA HEC-15 (Chen and Cotton 1988). The suitability of the basic design method, which is based on the concept of permissible shear stress, was subsequently confirmed for use at bridge piers by comparing the results of this testing program with the latest version of HEC-15 (Kilgore and Cotton 2005). It should be noted that durability of the wire mesh under long-term exposure to flow conditions specific to bridge piers has not been demonstrated; therefore, the use of gabion mat- tresses as a bridge pier scour countermeasure has an element of uncertainty (Parker et al. 1998). Layout Results from the gabion mattress testing indicated that the optimum performance as a pier scour countermeasure was obtained when the mattresses were extended a distance of at least two times the pier width in all directions around the pier. Because gabion mattresses are essentially an erosion-resistant veneer that behaves as a unit that is one layer thick, the system edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. Gabion mat- tresses should not be placed on slopes greater than 2H:1V, nor should they be placed in a manner that causes them to lie on top of adjacent mattresses. When dune-type bed forms were present, it was found that the armor must be sloped away from the pier in all di- rections such that the depth of the gabion mattress system at its periphery is greater than the depth of the bed-form troughs. Although contraction scour and long-term degra- dation were not tested during this study, it is presumed that this same guidance applies in cases where these conditions may be present at the bridge crossing. In some cases, this re- quirement may result in mattresses being placed further than two pier widths away from the pier. Similar to riprap, results confirmed that the filter should only be extended two-thirds of the distance from the pier to the periphery of the gabion mattress installation. Testing under NCHRP Project 24-07(2) also confirmed that the gabion mattresses must be tied together using lac- ing wire or other types of mattress-to-mattress connectors. When mattresses were simply placed against one another without tying them together, scour around the perimeter of the installation caused individual mattresses to slide out of position. Additional scour and/or mattress displacement subsequently occurred between these gaps. From a practical standpoint, this observation indicates that field installations 87

must use mattress-to-mattress connection materials that are at least as strong as the wire mesh of the mattresses. Materials No attempt was made to scale the strength characteristics of the wire mesh and compartment divider material to the small size of the mattresses used in the testing program. Guid- ance for material strength and properties for field-scale applications are derived from relevant ASTM standards for these products and are described in detail in Appendix F. 3.9.5 Grout-Filled Mattresses Grout-filled mattresses are composed of a double layer of strong synthetic fabric, typically woven nylon or polyester, sewn into a series of pillow-shaped compartments (blocks) that are connected internally by ducts. The compartments are filled with a concrete grout that flows from compartment to compartment via the ducts. Adjacent mattresses are typically sewn together or otherwise connected (less commonly) by special zips, straps, or ties prior to filling with grout. The benefits of grout-filled mattresses are that the fabric installation can be completed quickly, without the need for de- watering. Because of the flexibility of the fabric prior to filling, laying out the fabric forms and pumping them with concrete grout can be performed in areas where room for construction equipment is limited. When set, the grout forms a single-layer veneer made up of a grid of interconnected blocks. The blocks are interconnected by cables laced through the mattress before the grout is pumped into the fabric form, thus creating what is often called an articulating block mat (ABM). Flexibility and permeability are important functions for pier scour counter- measures. Therefore, systems that incorporate filter points or weep holes (allowing for pressure relief through the mat) com- bined with relatively small-diameter ducts (to allow grout breakage and articulation between blocks) are the preferred products. There is limited field experience with the use of grout-filled mattresses as a scour countermeasure for bridge piers. More frequently, these systems have been used for shoreline protec- tion, protective covers for underwater pipelines, and channel armoring where the mattresses are placed across the entire channel width and keyed into the abutments or banks. The guidance for pier scour applications provided in this docu- ment has been developed primarily from HEC-23 (Lagasse et al. 2001) and the results of NCHRP Project 24-07(2). Design Initial guidance for sizing the grout-filled mattresses for this study was developed from HEC-23, which recommends a design method based on sliding stability. Both rigid and flex- ible materials were used to simulate grout-filled mattresses around 8-in. (200-mm) square piers. The tests confirmed that grout-filled mattresses can be effective scour counter- measures for piers under clear-water conditions. However, when dune-type bed forms were present, the mattresses were subject to both undermining and uplift, even when they were toed down below the depth of the bed-form troughs. Therefore, the study cannot support the use of these products as pier scour countermeasures under live- bed conditions when dunes may be present. Layout The optimum performance of grout-filled mattresses as a pier scour countermeasure was obtained when the mattresses were extended at least two times the pier width in all directions around the pier. Because these products are essentially an erosion-resistant veneer that behaves as a unit that is one layer thick, the system edges must be toed down into a termination trench to prevent undermining and uplift around its periphery. Although not specifically tested in this study, it is inferred that where long-term degradation and/or contrac- tion scour is expected at a bridge crossing, grout-filled mat- tresses must be sloped away from the pier in all directions such that the depth of the mattress system at its periphery is greater than the depth of anticipated scour. The grout- filled mattresses should not be laid on a slope steeper than 2H:1V (50%). In some cases, this limitation may result in grout-filled mattresses being placed further than two pier widths away from the pier. Also, mattresses should not be placed such that a portion of one mattress lies on top of an adjacent mattress. A filter layer is typically required for grout-filled mattresses at bridge piers. The filter should be extended fully beneath the system to its periphery. Materials Flexibility and permeability are important functions for pier scour countermeasures. Therefore, grout-filled mattress systems that incorporate filter points or weep holes (allowing for pressure relief through the mat) combined with relatively small-diameter ducts (to allow grout breakage and articula- tion between blocks) are the preferred products. No attempt was made to scale the strength characteristics of the fabric, grout, or block-to-block flexibility for the grout mattresses tested in this study. Guidance for material strength and prop- erties for field-scale applications are derived from relevant ASTM standards for these products and are described in detail in Appendix G. 88

3.10 Construction 3.10.1 Overview While construction techniques vary for each of the pier scour countermeasure systems tested, certain guidelines are common to all countermeasures. These are summarized in this section. More detailed, system-specific construction guidelines are provided in the appendixes. The guidelines are derived from several standard reference works including the Manual on the Use of Rock in Hydraulic Engineering (CUR 1995) published in the Netherlands; California Department of Transportation publications (e.g., Racin et al. 2000); the “Code of Practice: Use of Standard Construction Methods for Bank and Bottom Protection on Waterways” (MAR) (BAW 1993b) published in Germany; and manuals prepared by the U.S. Army Corps of Engineers (e.g., USACE 1987 and 1990). 3.10.2 General Guidelines For any pier scour countermeasure system, the contractor is responsible for constructing the project according to the plans and specifications; however, ensuring conformance with the project plans and specifications is the responsibility of the owner. Conformance to plans and specifications is typically ensured through the owner’s engineer and inspectors. Inspec- tors observe and document the construction progress and performance of the contractor. Prior to construction, the con- tractor should provide a quality control plan to the owner (for example, see USACE 1995) and provide labor and equipment to perform tests as required by the project specifications. Construction requirements for countermeasure placement are included in the project plans and specifications. Inspec- tion and quality assurance must be carefully organized to ensure that materials delivered to the job site meet specifica- tions. Acceptance should not be made until measurement for payment has been completed. The engineer and inspectors reserve the right to reject incorrect or unsuitable materials at the job site and have them removed from the project site. Material that has been improperly placed should also be rejected throughout the duration of the contract. Construction techniques can vary tremendously because of the following factors: • Size and scope of the overall project • Size and weight of the riprap particles or armor units • Placement under water or in the dry • Physical constraints to access and/or staging areas • Noise limitations • Traffic management and road weight restrictions • Environmental restrictions • Type of construction equipment available Competency in construction techniques and management in all their aspects cannot be acquired from a book. Training on a variety of job sites and project types under the guidance of experienced senior personnel is required. The following sections provide some general information regarding pier scour countermeasure installations that are common for all countermeasure types. 3.10.3 Filters All pier scour countermeasures require a filter of some type. Generally, both geotextiles and granular filters can be used; however, some restrictions apply as noted in the indi- vidual design guidelines in the appendixes. For riverine ap- plications where dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be con- sidered for pier scour countermeasures. Geotextile Filters Either woven or non-woven, needle-punched fabrics can be used. If a non-woven fabric is used, it should have a mass density greater than 12 oz/yd2 (400 g/m2). Under no circum- stances may spun-bond or slit-film fabrics be allowed. Each roll of geotextile should be labeled with the manufacturer’s name, product identification, roll dimensions, lot number, and date of manufacture. Geotextiles should not be exposed to sunlight prior to placement. Granular Filters Samples of granular filter material should be tested for grain size distribution to ensure compliance with the gradation specification used in design. Sampling and testing frequency should be in accordance with requirements established by the owner or owner’s authorized representative. Subgrade Soils When the countermeasure and filter is placed in the dry, they should be placed on undisturbed native soil, on an exca- vated and prepared subgrade, or on acceptably placed and compacted fill. Unsatisfactory soils include soils having excessive in-place moisture content; soils containing roots, sod, brush, or other organic materials; soils containing turf clods or rocks; or frozen soil. These soils should be removed, and the site backfilled with approved material and compacted prior to placement of the riprap. Unsatisfactory soils may also be defined as soils such as very fine non-cohesive soils with uniform particle size, gap-graded soils, laminated soils, and dispersive clays. 89

3.10.4 Installation Subgrade Preparation The subgrade soil conditions should meet or exceed the re- quired material properties described in Section 3.10.3 prior to placement of the countermeasure. Soils not meeting the re- quirements should be removed and replaced with acceptable material. When the countermeasure is placed in the dry, the areas re- ceiving the countermeasure should be graded to establish a smooth surface and ensure that intimate contact is achieved between the subgrade surface and the filter, and between the filter and the countermeasure. Stable and compacted subgrade soil should be prepared to the lines, grades, and cross sections shown on the contract drawings. Termination trenches and transitions between slopes, embankment crests, benches, berms, and toes should be compacted, shaped, and uniformly graded. The subgrade should be uniformly compacted to the geotechnical engineer’s site-specific requirements. When the countermeasure is placed under water, divers should be used to ensure that the bed is free of logs, large rocks, construction materials, or other blocky materials that would create voids beneath the system. Immediately prior to placement of the filter and countermeasure system, the pre- pared subgrade must be inspected. Filter Placement Whether the filter comprises one or more layers of granu- lar material or is made of geotextile, its placement should re- sult in a continuous installation that maintains intimate con- tact with the soil beneath. Voids, gaps, tears, or other holes in the filter must be avoided to the extent practicable, and re- placed or repaired when they occur. Placement of Geotextile. The geotextile should be placed directly on the prepared area, in intimate contact with the subgrade. When a geotextile is placed, it should be rolled or spread out directly on the prepared area and be free of folds or wrinkles. The rolls should not be dragged, lifted by one end, or dropped. The geotextile should be placed in such a manner that placement of the overlying materials (counter- measure armor layer) will not excessively stretch or tear the geotextile. After geotextile placement, the work area should not be trafficked or disturbed in a manner that might result in a loss of intimate contact between the countermeasure, the geotex- tile, and the subgrade. The geotextile should not be left ex- posed longer than the manufacturer’s recommendation to minimize potential damage due to ultraviolet radiation; therefore, the overlying materials should be placed as soon as practicable. The geotextile should be placed so that upstream strips overlap downstream strips. Overlaps should be in the direc- tion of flow wherever possible. The longitudinal and trans- verse joints should be overlapped at least 1.5 ft (0.46 m) for dry installations and at least 3 ft (0.91 m) for underwater installa- tions. If the seam of the geotextile is to be sewn, the thread to be used should consist of high-strength polypropylene or polyester and should be resistant to ultraviolet radiation. If necessary to expedite construction and to maintain the rec- ommended overlaps, anchoring pins, U-staples, or weights such as sandbags should be used. Placement of Geotextiles Under Water. Placing geotex- tiles under water can be problematic for a number of reasons. Several techniques for placing geotextiles under water are presented in Section 3.12.5. Placement of Granular Filter. For placing a granular filter, front-end loaders are the preferred method for dumping and spreading the material on slopes milder than approximately 4H:1V. A typical minimum thickness for granular filters is 0.5 to 1.0 ft (0.15 to 0.3 m), depending on the size of the overlying armor layer and whether a layer of bedding stone is to be used between the filter and the countermeasure. For a granular filter placed under water, the thickness should be increased by 50%. Underwater placement of granular media around a bridge pier is best accomplished using a large-diameter tremie pipe to con- trol the placement location and thickness, while minimizing the potential for segregation. Countermeasure Placement Most countermeasure systems may be placed from either land-based or water-based operations and can be placed under water or in the dry. The necessary equipment and tech- niques are specific to each countermeasure type. These equip- ment and techniques are covered in the individual design guidelines in the appendixes. 3.11 Inspection, Maintenance, and Performance Evaluation 3.11.1 Inspection During Construction Inspection during construction should be conducted by qualified personnel who are independent of the contractor. Underwater inspection of pier scour countermeasures should only be performed by divers specifically trained and certified for such work. Subgrade Inspection of the subgrade should be performed immedi- ately prior to geotextile or granular filter placement. The sub- 90

grade should be clean and free of projections, debris, construction materials, or other foreign objects that would prevent the filter from being properly placed. Likewise, there should be no potholes, rills, or other voids that the filter material might bridge over. The subgrade material itself should not be muddy or frozen, and should not contain organic material or other deleterious substances. Variations in subgrade characteristics over the project area should be noted and photographed. Observations of such should be brought to the attention of the project engi- neer as they may represent conditions that are different than those used for design. It is generally recommended that com- paction testing be performed at a frequency of one test per 2,000 ft2 (186 m2) of surface area, unless project specifications require otherwise. Geotextile Each roll of geotextile delivered to the job site must have a label with the manufacturer’s name and product identification. The inspector must check the labels to ensure that the geotex- tile is the same as that specified in the design. It is a good idea for inspectors to familiarize themselves with the different kinds of geotextiles on the market. Spun-bond fabrics and slit-film geotextiles should never be used in pier scour countermeasure applications. The geotextile must be stored so that it is out of direct sun- light, as damage can occur from exposure to ultraviolet radi- ation. When placed, it must be free of wrinkles, folds, or tears. Sandbags, rocks, anchoring pins, or U-shaped soil staples may be used to hold the geotextile in position while the counter- measure is being placed. The countermeasure should be placed within 48 hours after the geotextile is placed unless un- usual circumstances warrant otherwise. Countermeasure Inspection requirements for the countermeasure differ for each countermeasure type. These requirements are covered in detail in the individual design guidelines in the appendixes. In general, the subgrade preparation, geotextile placement, coun- termeasure system, and overall finished condition including termination trenches, if any, should be inspected before accepting the work. 3.11.2 Periodic and Post-Flood Inspection Pier scour countermeasures will typically be inspected dur- ing the biennial bridge inspection program. However, more frequent inspection might be required by the Plan of Action for a particular bridge or group of bridges. In some cases, inspection may be required after every flood that exceeds a specified magnitude. Underwater inspection should only be performed by divers specifically trained and certified for such work. Specific inspection requirements for each pier scour countermeasure system are provided in the design guidelines in the appendixes. 3.11.3 Maintenance Deficiencies noted during the inspection should be corrected as soon as possible. As with any armor system, pro- gressive failure of a pier scour countermeasure from succes- sive flows must be avoided by providing timely maintenance intervention for most countermeasure systems. Where localized areas are limited to loss of individual armor ele- ments, there may be opportunities to repair the area by adding additional armor elements and tying them into the original armor layer. Voids or undermining underneath the armor system should be filled with material that meets the specifications of the original design. Guidance specific to each counter- measure type is provided in the design guidelines in the appendixes. 3.11.4 Performance Evaluation The evaluation of any countermeasure’s performance should be based on its design parameters as compared to actual field experience, longevity, and inspection/mainte- nance history. For proper performance assessment of a pier scour countermeasure, the history of hydraulic loading on the installation, in terms of flood magnitudes and frequen- cies, must also be considered and compared to the design loading. Changes in channel morphology may have occurred over time subsequent to the installation of the pier scour counter- measure. Present-day channel cross-section geometry and planform should be compared to those at the time of coun- termeasure installation. Both lateral and vertical instability of the channel in the vicinity of the bridge can significantly alter hydraulic conditions at the piers. Approach flows may become skewed to the pier alignment, causing greater local and con- traction scour. Although the person making the performance evaluation will probably not be the inspector, inspection records will be fundamental to the evaluation. Maintenance records must also be consulted so that costs can be documented and reported as a percentage of the initial capital improvement cost. To guide the performance evaluation for each pier scour countermeasure, a rating system is presented in each of the design guidelines in the appendixes. Numerical ratings from 0 (worst) to 6 (best) are established for each of three topical areas: 91

• Hydraulic history: Has the countermeasure been subjected to severe hydraulic loading since it was constructed? • Maintenance history: Has the installation required a lot of attention and repair over its installed life to date? • Current condition: What is the current condition of the countermeasure? Recommended actions corresponding to the current con- dition rating codes are also provided. For several counter- measures, a case history or example of a field performance evaluation is provided. 3.12 Filter Requirements 3.12.1 Filter Design The importance of the filter component of a pier scour countermeasure installation should not be underestimated. Emphasis must be given to compatibility criteria between the filter (granular or geotextile) and the soil. Correct filter de- sign reduces the effects of piping by limiting the loss of fines, while simultaneously maintaining a permeable, free-flowing interface. Figures 3.66a and 3.66b illustrate the basic differ- ence between stable and unstable soil structures. 92 a) Stable soil structure b) Unstable soil structure STABLE SOIL UNSTABLE SOIL Intermediate size particles Large particles Fines c) Filter with large openings over a stable soil Fines escaping Newly created voids Geotextile Weakened soil structure d) Filter with large openings over an unstable soil (piping) FinesLarge particles Geotextile Clogged zone f) Filter with small openings over an unstable soil (clogging) e) Filter with small openings over a stable soil Figure 3.66. Examples of soil and filter compatibility processes.

Figures 3.66c through f illustrate several common filtering processes that can occur in stable and unstable base soils (modified from Geosyntech Consultants 1991). The large ar- rows indicate the direction of water flow in the base soil. In Figure 3.66c, the fine particles immediately adjacent to the fil- ter are initially washed away (through the filter). The large and intermediate particles are retained by the filter; they in turn prevent any further loss of fines. This soil matrix will continue to remain stable over time. In Figure 3.66d, an unstable soil is covered by a filter with large pores. Piping of the fine particles will continue un- abated, because there are no particles of intermediate size to prevent their movement by the forces of seepage flow and turbulence at the interface. In Figure 3.66e, a stable soil is covered by a filter with small pores. This filter will retain most of the fines, but the presence of intermediate-sized particles prevents the continued mi- gration of fines from lower in the matrix. Thus a clogging layer is prevented from forming to any significant extent. This is contrasted with the condition shown in Figure 3.66f, where no particles of intermediate size are present to mitigate the buildup of an impermeable barrier of plugged void spaces and clogging at the interface. Filters must be sufficiently permeable to allow unimpeded flow from the base soil through the filter material for two rea- sons: (1) to regulate the filtration process at the base soil-filter interface, as illustrated in Figure 3.66, and (2) to minimize hydrostatic pressure buildup from local groundwater fluctu- ations in the vicinity of the channel bed and banks (e.g., sea- sonal water level changes or storm events). The permeability of the filter should never be less than the material below it (whether base soil or another filter layer). Figures 3.67a through c illustrate the typical process that oc- curs during and after a flood event. Seepage forces can result in piping of the base soil through the countermeasure armor layer (e.g., riprap). If a less permeable material underlies the riprap, an increase of hydrostatic pressure can build beneath the riprap. A permeable filter material, properly designed, will alleviate problems associated with fluctuating surface water levels. 93 Seepage flow Seepage flow Normal water level a) Normal (baseflow) conditions Groundwater table Elevated groundwater after flood Normal water level Area of high seepage gradients and uplift pressure c) After flood recession Seepage flow Flood water level b) During flood peak Figure 3.67. Changes in water levels and seepage patterns during a flood.

3.12.2 Base Soil Properties Base soil is defined here as the subgrade material upon which the countermeasure and filter will be placed. Base soil can be native in-place material, or imported and recom- pacted fill. The following properties of the base soil should be obtained for proper design of the filter, when using either a geotextile or a layer of aggregate. General Soil Classification Soils are classified based on laboratory determinations of particle size characteristics and the physical effects of varying water content on soil consistency. Typically, soils are de- scribed as coarse grained if more than 50% by weight of the particles is larger than a #200 sieve (0.075-mm mesh) and fine grained if more than 50% by weight is smaller than this size. Sands and gravels are examples of coarse-grained soils, while silts and clays are examples of fine-grained soils. The fine-grained fraction of a soil is further described by changes in its consistency caused by varying water content and by the percentage of organic matter present. Soil clas- sification procedures are described in ASTM D 2487, “Standard Practice for Classification of Soils for Engineer- ing Purposes: Unified Soil Classification System” (ASTM 2003a). Particle Size Distribution The single most important soil property for design pur- poses is the range of particle sizes in the soil. Particle size is a simple and convenient way to assess soil properties. Also, par- ticle size tends to be an indication of other properties such as permeability. Characterizing soil particle size involves deter- mining the relative proportions of gravel, sand, silt, and clay in the soil. This characterization is usually done by sieve analysis for coarse-grained soils or sedimentation (hydrom- eter) analysis for fine-grained soils. ASTM D 422, “Standard Test Method for Particle-Size Analysis of Soils,” outlines the specific procedure (ASTM 2003a). Plasticity Plasticity is defined as the property of a material that allows it to be deformed rapidly, without rupture, without elastic re- bound, and without volume change. A standard measure of plasticity is the plasticity index (PI), which should be deter- mined for soils with a significant percentage of clay. The results associated with plasticity testing are referred to as the Atterberg Limits. ASTM D 4318, “Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils,” defines the testing procedure (ASTM 2003a). Porosity Porosity is that portion of a representative volume of soil that is interconnected void space. It is typically reported as a dimensionless fraction or a percentage. The porosity of soils is affected by the particle size distribution, the particle shape (e.g., round vs. angular), and degree of compaction and/or cementation. Permeability Permeability, also known as hydraulic conductivity, is a measure of the ability of soil to transmit water. ASTM pro- vides two standard laboratory test methods for determining permeability. They are ASTM D 2434, “Standard Test Method for Permeability of Granular Soils (Constant Head)” or ASTM D 5084, “Standard Test Method for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter” (ASTM 2003b). In these tests, the amount of water passing through a saturated soil sample is measured over a specified time interval, along with the sample’s cross-sectional area and the hydraulic head at specific locations. The soil’s permeability is then calculated from these measured values. Permeability is related more to particle size distribution than to porosity, as water moves through large and interconnected voids more easily than small or isolated voids. Various equations are available to estimate permeability based on the grain size distribution. Table 3.16 lists average values of porosity and permeability for alluvial soils. 3.12.3 Geotextile Filter Properties For compatibility with site-specific soils, geotextiles must exhibit the appropriate values of permeability, pore size (otherwise known as apparent opening size), and porosity (or percentage of open area). In addition, geotex- tiles must be sufficiently strong to withstand the stresses during installation. These values are available from manu- facturers. The following paragraphs briefly describe the most relevant properties. 94 Type of Material Porosity (vol/vol) Permeability (cm/s) Gravel, coarse 0.28 Gravel, fine 0.34 4 x 10 -1 Sand, coarse 0.39 5 x 10-2 Sand, fine 0.43 3 x 10-3 Silt 0.46 3 x 10-5 Clay 0.42 9 x 10-8 Source: modified from McWhorter and Sunada (1977) Table 3.16. Typical values of porosity and permeability of alluvial soils.

Permeability The permeability, K, of a geotextile is a calculated value that indicates the ability of a geotextile to transmit water across its thickness. It is typically reported in units of centimeters per second (cm/s). This property is directly related to the filtration function that a geotextile must perform, where water flows perpendicularly through the geotextile into a crushed stone bedding layer, perforated pipe, or other more permeable medium. The geotextile must allow this flow to occur without being impeded. A value known as the permittivity, ψ, is used by the geotextile industry to more readily compare geotextiles of different thicknesses. Permittivity, ψ, is defined as K divided by the geotextile thickness, t, in centimeters; therefore, per- mittivity has a value of (s)−1. Permeability (and permittivity) is extremely important in riprap filter design. For scour coun- termeasure installations, the permeability of the geotextile should be at least 10 times that of the underlying material. Transmissivity The transmissivity, θ, of a geotextile is a calculated value that indicates the ability of a geotextile to transmit water within the plane of the fabric. It is typically reported in units of square centimeters per second (cm2/s). This property is di- rectly related to the drainage function and is most often used for high-flow drainage nets and geocomposites, not geotex- tiles. Woven monofilament geotextiles have very little capac- ity to transmit water in the plane of the fabric, whereas non- woven, needle-punched fabrics have a much greater capacity due to their three-dimensional microstructure. Transmissiv- ity is not particularly relevant to filter design. Apparent Opening Size (AOS) Also known as equivalent opening size, this measure is generally reported as O95, which represents the aperture size such that 95% of the openings are smaller. In similar fashion to a soil gradation curve, a geotextile hole distribution curve can be derived. The AOS is typically reported in millimeters, or in equivalent U.S. standard sieve size. Porosity Porosity is a comparison of the total volume of voids to the total volume of geotextile. This measure is applicable to non- woven geotextiles only. Porosity is used to estimate the po- tential for long-term clogging and is typically reported as a percentage. Percent Open Area (POA) POA is a comparison of the total open area to the total geo- textile area. This measure is applicable to woven geotextiles only. POA is used to estimate the potential for long-term clogging and is typically reported as a percentage. Thickness As mentioned above, thickness is used to calculate tradi- tional permeability. It is typically reported in millimeters or mils (thousandths of an inch). Grab Strength and Elongation Grab strength is the force required to initiate a tear in the fabric when pulled in tension. It is typically reported in New- tons or pounds as measured in a testing apparatus having standardized dimensions. The elongation measures the amount the material stretches before it tears and is reported as a percentage of its original (unstretched) length. Tear Strength Tear strength is the force required to propagate a tear once initiated. It is typically reported in Newtons or pounds. Puncture Strength Puncture strength is the force required to puncture a geo- textile using a standard penetration apparatus. It is typically reported in Newtons or pounds. There are many other tests to determine various charac- teristics of geotextiles; only those deemed most relevant to applications involving pier scour countermeasure installation have been discussed here. Geotextiles should be able to with- stand the rigors of installation without suffering degradation of any kind. Long-term endurance to stresses such as ultra- violet solar radiation or continual abrasion are considered of secondary importance, because once the geotextile has been installed and covered by the countermeasure, these stresses do not represent the environment that the geotextile will experience in the long term. 3.12.4 Granular Filter Properties Generally speaking, most required granular filter proper- ties can be obtained from the particle size distribution curve for the material. Granular filters may be used alone or as a transitional layer between a predominantly fine-grained base soil and a geotextile. Particle Size Distribution As a rule of thumb, the gradation curve of the granular fil- ter material should be approximately parallel to that of the base soil. Parallel gradation curves minimize the migration of particles from the finer material into the coarser material. 95

Heibaum (2004) presents a summary of a procedure origi- nally developed by Cistin and Ziems whereby the d50 size of the filter is selected based on the coefficients of uniformity (d60/d10) of both the base soil and the filter material. With this method, the grain size distribution curves do not necessarily need to be approximately parallel. Figure 3.68 provides a de- sign chart based on the Cistin–Ziems approach. Permeability Permeability of a granular filter material is determined by laboratory test, or estimated using relationships relating per- meability to the particle size distribution. The permeability of a granular layer is used to select a geotextile when designing a composite filter. For countermeasure installations, the per- meability of the filter should be at least 10 times the perme- ability of the underlying material. Porosity Porosity is that portion of a representative volume of soil that is interconnected void space. It is typically reported as a dimen- sionless fraction or a percentage. The porosity of soils is affected by the particle size distribution, the particle shape (e.g., round vs. angular), and degree of compaction and/or cementation. Thickness Practical issues of placement indicate that a typical mini- mum thickness of 6 to 8 in. should be specified. For place- ment under water, thickness should be increased by 50%. Quality and Durability Aggregate used for a granular filter should be hard, dense, and durable. 3.12.5 Placing Geotextiles Under Water Placing geotextiles under water is problematic for a num- ber of reasons. Most geotextiles that are used as filters beneath riprap are made of polyethylene or polypropylene. These materials have specific gravities ranging from 0.90 to 0.96, meaning that they will float unless weighted down or other- wise anchored to the subgrade prior to placement of the riprap (Koerner 1998). In addition, unless the work area is isolated from river currents by a cofferdam, flow velocities greater than about 1.0 ft/s (0.3 m/s) create large forces on the geotextile. These forces cause the geotextile to act like a sail, often resulting in wavelike undulations of the fabric (a con- dition that contractors refer to as “galloping”) that are extremely difficult to control. In mild currents, geotextiles (precut to length) have been placed using a roller assembly, with sandbags to hold the fabric temporarily. To overcome these problems, engineers in Germany have developed a product that consists of two non-woven geotex- tiles (or a woven and a non-woven geotextile) with sand in between. This blanket-like product, known as SandMatTM, has layers that are stitch-bonded or sewn together to form a heavy, filtering geocomposite. The composite blanket ex- hibits an overall specific gravity ranging from approximately 1.5 to 2.0, so it sinks readily. According to Heibaum (2002), this composite geotextile has sufficient stability to be handled even when loaded by currents up to approximately 3.3 ft/s (1 m/s). At the geotextile-subsoil interface, a non-woven fabric should be used because of the higher angle of friction compared to woven geotextiles. Figure 3.69 shows a close-up photo of the SandMatTM material. Figure 3.70 shows the SandMatTM blan- ket being rolled out using conventional geotextile placement equipment. In deep water or in currents greater than 3.3 ft/s (1 m/s), German practice calls for the use of sand-filled geocontain- ers. For specific project conditions, geosynthetic containers can be chosen that combine the resistance against hydraulic loads with the filtration capacity demanded by the applica- tion. Geosynthetic containers have proven to give sufficient stability against erosive forces in many applications, includ- ing wave-attack environments. The size of the geocontainer must be chosen such that the expected hydraulic load will not transport the container during placement (Heibaum 2002). Once placed, the geocontainers are overlaid with the final ar- moring material. Figure 3.71 shows a geotextile container being filled with sand. Figure 3.72 shows the sand-filled geocontainer being 96 M ax im u m A 50 = d 50 f/d 50 s Coefficient of Uniformity (filter) Cuf = d60f/d10f Coefficient of Uniformity (soil) Cus = d60s/d10s Source: Heibaum (2004) Cuf = 18 Cuf = 14 Cuf = 4 Cuf = 2 Cuf = 1 Cuf = 10 Cuf = 6 Figure 3.68. Filter design chart according to Cistin-Ziems.

97 Source: Colcrete–Von Essen Inc. Figure 3.69. Close-up photo of SandMatTM geocomposite blanket. Source: Colcrete–Von Essen Inc. Source: Colcrete–Von Essen Inc. Figure 3.70. SandMatTM geocomposite blanket being unrolled. Figure 3.71. Filling geocontainer with sand. Source: Colcrete–Von Essen Inc. Figure 3.72. Handling a 1-tonne sand-filled geocontainer. handled with an articulated-arm clam grapple. The filled geo- container in the photograph is a nominal 1-tonne (1,000-kg or 2,200-lb) unit. The preferred geotextile for these applica- tions is always a non-woven, needle-punched fabric, with a minimum mass per unit area of 500 g/m2. Smaller geocon- tainers can be fabricated and handled by one or two people for smaller-sized applications. As a practical minimum, a 200-lb (90.7 kg) geocontainer cov- ering a surface area of about 6 to 8 ft2 (0.56 to 0.74 m2) can be fashioned from non-woven, needle-punched geotextile having a minimum mass per unit area of 200 g/m2, filled at the job site, and field-stitched with a hand-held machine. Figures 3.73 and

3.74 illustrate the smaller geocontainers being installed at a pro- totype-scale test installation (for more detail see Section 3.5.3). 3.13 Pier Scour Countermeasure Selection Selecting the most appropriate pier scour countermeasure for a particular bridge site requires knowledge of not only the bridge characteristics and riverine conditions that have com- bined to create a potential scour-critical situation, but also the strengths and vulnerabilities of the countermeasures being considered. In addition, the costs associated with the installa- tion and maintenance of the countermeasure throughout the remaining life of the bridge must be considered. A methodology was developed under NCHRP Project 24- 07(2) to assist practitioners in the selection of appropriate pier scour countermeasures for a given set of site-specific conditions. The method has been incorporated into a Microsoft® Excel workbook that is available on the TRB web site (http://www.trb.org/news/blurb_detail.asp?id=7998). The user-friendly Excel workbook allows the practitioner to customize the selection process to determine the relative suitability of six different pier scour countermeasure alter- natives at a given site. The methodology provides a quanti- tative ranking of armoring countermeasure types and incorporates a fatal-flaw mechanism that identifies situa- tions where a particular countermeasure is unequivocally unsuitable because of one or more circumstances unique to the site. The methodology is presented in detail in Appen- dix B of this report. The selection methodology is intended to identify the countermeasure type best suited for application at a particu- lar site. It is not intended to be used as a tool for comparing between different sites and would not be useful for prioritiz- ing among various bridge sites where pier scour countermea- sures are being considered. Alaska, California, and Virginia DOTs participated in beta testing the selection methodology. Comments and recom- mendations from the beta testers, as well as review comments received from members of the NCHRP Project 24-07(2) panel, were incorporated into the methodology. Appendix B describes the final methodology that resulted from this process. 3.14 Implementation Plan 3.14.1 The Product As described in more detail in the preceding sections, the product of this research was practical selection criteria for bridge pier scour countermeasures; guidelines and recom- mended specifications for design and construction; and 98 a. Demonstrating puncture resistance of geocontainers b. Placing geocontainers with small front-end loader into scour hole FLOW Countermeasure armor placed flush with channel bed Pier Sand-filled geocontainers placed in pre-existing scour hole Figure 3.73. Small (200-lb [90.7-kg]) sand-filled geocontainers for prototype-scale test. Figure 3.74. Schematic diagram of sand-filled geocontainers beneath riprap armor.

guidelines for inspection, maintenance, and performance evaluation. The following countermeasures were considered: • Riprap • Partially grouted riprap and geotextile containers • ACB systems • Gabion mattresses • Grout-filled mattresses 3.14.2 The Market The market or audience for the results of this research will be hydraulic engineers and maintenance and inspection per- sonnel in state, federal, and local agencies with a bridge-re- lated responsibility. These would include the following: • State highway agencies • Federal Highway Administration • City/county bridge engineers • Railroad bridge engineers • U.S. Army Corps of Engineers • Bureau of Land Management • National Park Service • Forest Service • Bureau of Indian Affairs • Any other governmental agency with bridges under its ju- risdiction • Consultants to the agencies above 3.14.3 Impediments to Implementation A serious impediment to successful implementation of re- sults of this research will be difficulties involved in reaching a diverse audience scattered among numerous agencies and in- stitutions; however, this can be countered by a well-planned technology transfer program. Because of the complexity and geographic scope of the bridge scour problem and the diversity of bridge foundation geometries, a major challenge was to pres- ent the results in a format that can be applied by agencies with varying levels of engineering design capabilities and mainte- nance resources. Presenting the selection criteria and guidelines in a format familiar to bridge owners, who are the target audi- ence, will facilitate their use of the results of this research. The standard format adopted for this study will help ensure suc- cessful implementation. 3.14.4 Leadership in Application Through the National Highway Institute (NHI) and its training courses, FHWA has the program in place to reach a diverse and decentralized target audience. For example, rec- ommendations from this study could be considered for the next edition of HEC-23, “Bridge Scour and Stream Instabil- ity Countermeasures,” and NHI Course No. 135048, “Coun- termeasure Design for Scour and Stream Instability.” TRB—through its annual meetings and committee activities, publications such as the Transportation Research Record, and periodic bridge conferences—can also play a leading role in dis- seminating the results of this research to the target audience. AASHTO is the developer and sanctioning agency for stan- dards, methods, and specifications. Thus, research results can be formally adopted through the AASHTO process. As a col- lective representation of individual state DOTs, AASHTO can also suggest any needed training to be developed by FHWA or others. The AASHTO Subcommittee on Bridges and Structures could provide centralized leadership through the involvement of all state DOT bridge engineers. ASTM is a recognized leader in the development of standard specifications for the testing and documentation of material quality and performance. In 1997, Subcommittee D18-25 on Erosion and Sediment Control Technology was created. This subcommittee consists of 11 sections that are developing stan- dards for a variety of erosion control products and applications, including articulating concrete blocks (D18-25.04), gabions (D18-25.05), and grout-filled fabric mattresses (D18-25.07). Obviously, material quality standards for manufactured prod- ucts are essential for durability and longevity in their applica- tion as scour countermeasures. Similarly, performance testing is essential for the development of design procedures. ASTM standards development can provide a valuable linkage between the proposed research activities and the engineering commu- nity involved in design and specification. Professional societies such as ASCE host conferences and publish peer-reviewed journals through which the latest advances in engineering research and applications reach a wide audience, including many state, federal, and local hydraulic engineers. For example, the Environmental & Water Resources Institute (EWRI)/ASCE Task Committee on Bridge Scour can play an important role in disseminating the results of this research. Regional bridge conferences, such as the Western Bridge Engineer Conference or the International Bridge Engineering Conferences, reach a wide audience of bridge engineers, man- ufacturers, consultants, and contractors. The groups would have an obvious interest in pier scour countermeasures and their acceptance of the results of this research will be key to implementation by bridge owners. 3.14.5 Activities for Implementation The activities necessary for successful implementation of the results of this research relate to technology transfer activ- ities, as discussed in the previous section, and the activities of appropriate AASHTO and ASTM committees. 99

“Ownership” of the guidelines and specifications by AASHTO will be key to successful implementation. Although the guidelines and specifications that result from this research will be considered and possibly adopted by AASHTO, it is essential that the various technical committees in AASHTO accept and support these results and use the committee struc- ture to improve them in the future. Standards development activities within ASTM’s Erosion and Sediment Control Technology subcommittee include “Standard Guidelines for Design” and “Standard Practices for Installation” associated with various erosion and sediment control products, techniques, and areas of application. The design and installation guidance developed for selected pier scour countermeasures under this study could be formatted and published as ASTM standards documents. Such publica- tion would unquestionably further the dissemination of information and enhance the usefulness of this work for the professional design community as well as for installation con- tractors and owners. 3.14.6 Criteria for Success The best criteria for judging the success of this implementa- tion plan will be acceptance and use of the guidelines and spec- ifications that result from this research by state highway agency engineers and others with responsibility for design, mainte- nance, rehabilitation, or inspection of highway facilities. Progress can be gaged by peer reviews of technical presenta- tions and publications and by the reaction of state DOT per- sonnel during presentation of results at NHI courses. A sup- plemental critique sheet could be used during NHI courses to provide feedback on the applicability of the guidelines and sug- gestions for improvement. The desirable consequences of this project, when imple- mented, will be more efficient design, maintenance, and in- spection of highway facilities considering the threat from pier scour, and more effective use of countermeasures against that threat. The ultimate result will be a reduction in the number of bridge failures and reduction in damage to highway facili- ties attributable to pier scour. 100