Countermeasures to Protect Bridge Piers from Scour (2007)

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Introduction, C-2 1 Design and Specification, C-2 2 Construction, C-13 3 Inspection, Maintenance, and Performance Evaluation, C-18 References, C-24 C-1 A P P E N D I X C Guidelines for Pier Scour Countermeasures Using Rock Riprap

Introduction When properly designed and used for erosion protection, riprap 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. Properly con- structed riprap can provide long-term protection if it is inspected and maintained on a periodic basis as well as after flood events. This design guideline considers the application of riprap as a pier scour countermeasure. Design of a pier scour countermeasure system using riprap requires knowledge of river bed and foundation material; flow conditions including velocity, depth and orientation; pier size, shape, and skew with respect to flow direction; riprap characteristics of size, density, durability, and availability; and the type of interface material between the riprap and underlying founda- tion. 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. The guidance provided in this document for pier protection applications of riprap has been developed primarily from FHWA Hydraulic Engineering Circular No. 23 (HEC-23) (Lagasse et al. 2001) and the results of NCHRP Project 24-07(2) (Lagasse et al. 2007), NCHRP Project 24- 23 (Lagasse et al. 2006), and NCHRP Project 24-07 (Parker et al.,1998). This document is organized into three parts: • Part 1 provides design and specification guidelines for riprap systems. • Part 2 presents construction guidelines. • Part 3 provides guidance for inspection, maintenance, and performance evaluation of riprap used as a pier scour countermeasure. Part 1: Design and Specification 1.1 Materials 1.1.1 Size, Shape, and Density Riprap design methods typically yield a required size of stone that will result in stable per- formance under the design loadings. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum allowable represen- tative size. For pier scour protection, the designer specifies a minimum allowable d50 for the rock composing the riprap, thus indicating the size for which 50% (by weight) of the particles are smaller. Stone sizes can also be specified in terms of weight (e.g., W50) using an accepted rela- tionship between size and volume, and the known (or assumed) density of the particle. Shape. The shape of a stone can be generally described by designating three axes of measurement: major, intermediate, and minor, also known as the “A, B, and C” axes, as shown in Figure C1.1. C-2 Figure C1.1. Riprap shape described by three axes.

Riprap stones should not be thin and platy, nor should they be long and needle-like. There- fore, specifying a maximum allowable value for the ratio A/C, also known as the shape factor, provides a suitable measure of particle shape, since the B axis is intermediate between the two extremes of length A and thickness C. A maximum allowable value of 3.0 is recommended: (C1.1) For riprap applications, stones tending toward subangular to angular are preferred, due to the higher degree of interlocking, hence greater stability, compared to rounded particles of the same weight. Density. A measure of density of natural rock is the specific gravity Sg, which is the ratio of the density of a single (solid) rock particle γs to the density of water γw: (C1.2) Usually, a minimum allowable specific gravity of 2.5 is required for riprap applications. Where quarry sources uniformly produce rock with a specific gravity significantly greater than 2.5 (such as dolomite, Sg = 2.7 to 2.8), the equivalent stone size can be substantially reduced and still achieve the same particle weight gradation. Size and weight. Based on field studies, the recommended relationship between size and weight is given by W = 0.85(γsd3) (C1.3) where W = Weight of stone, lb (kg) γs = Density of stone, lb/ft3 (kg/m3) d = Size of intermediate (“B”) axis, ft (m) Table C1.1 provides recommended gradations for ten standard classes of riprap based on the median particle diameter d50 as determined by the dimension of the intermediate (“B”) axis. These gradations were developed under NCHRP Project 24-23, “Riprap Design Criteria, Specifications, and Quality Control” (Lagasse et al. 2006). The proposed gradation criteria are based on a nominal or “target” d50 and a uniformity ratio d85/d15 that results in riprap that is well graded. The target uniformity ratio is 2.0 and the allowable range is from 1.5 to 2.5. Sg s w = γ γ A C ≤ 3 0. C-3 Nominal Riprap Class by Median Particle Diameter d15 d50 d85 d100 Class Size Min Max Min Max Min Max Max I 6 in 3.7 5.2 5.7 6.9 7.8 9.2 12.0 II 9 in 5.5 7.8 8.5 10.5 11.5 14.0 18.0 III 12 in 7.3 10.5 11.5 14.0 15.5 18.5 24.0 IV 15 in 9.2 13.0 14.5 17.5 19.5 23.0 30.0 V 18 in 11.0 15.5 17.0 20.5 23.5 27.5 36.0 VI 21 in 13.0 18.5 20.0 24.0 27.5 32.5 42.0 VII 24 in 14.5 21.0 23.0 27.5 31.0 37.0 48.0 VIII 30 in 18.5 26.0 28.5 34.5 39.0 46.0 60.0 IX 36 in 22.0 31.5 34.0 41.5 47.0 55.5 72.0 X 42 in 25.5 36.5 40.0 48.5 54.5 64.5 84.0 Table C1.1. Size gradations for 10 standard classes of riprap.

Based on Equation C1.3, which assumes the volume of the stone is 85% of a cube, Table C1.2 provides the equivalent particle weights for the same ten classes, using a specific gravity of 2.65 for the particle density. 1.1.2 Recommended Tests for Rock Quality Standard test methods relating to material type, characteristics, and testing of rock and aggre- gates typically associated with riprap installations (e.g., filter stone and bedding layers) are provided in this section and are recommended for specifying the quality of the riprap stone. In general, the test methods recommended in this section are intended to ensure that the stone is dense and durable, and will not degrade significantly over time. Rocks used for riprap should break only with difficulty, have no earthy odor, not have closely spaced discontinuities (joints or bedding planes), and not absorb water easily. Rocks composed of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as riprap. Table C1.3 summarizes the recommended tests and allowable values for rock and aggregate. 1.2 Hydraulic Stability Design Procedure To determine the required size of stone for riprap at bridge piers, NCHRP Project 24-23 rec- ommends using the rearranged Isbash equation from HEC-23 to solve for the median stone diameter: (C1.4) where d50 = Particle size for which 50% is finer by weight, ft (m) Vdes = Design velocity for local conditions at the pier, ft/s (m/s) Sg = Specific gravity of riprap (usually taken as 2.65) g = Acceleration due to gravity, 32.2 ft/s2 (9.81 m/s2) It is important to note that the design conditions in the immediate vicinity of a bridge pier are more severe than the approach conditions upstream. Therefore, the local velocity should be used in Equation C1.4. As recommended in HEC-23, the section-average approach velocity Vavg must be multiplied by factors that are a function of the shape of the pier and its location in the channel: Vdes = K1K2Vavg (C1.5) d V S g des g 50 20 692 1 2 = − . ( ) ( ) C-4 Nominal Riprap Class by Median Particle Weight W15 W50 W85 W100 Class Weight Min Max Min Max Min Max Max I 20 lb 4 12 15 27 39 64 140 II 60 lb 13 39 51 90 130 220 470 III 150 lb 32 93 120 210 310 510 1100 IV 300 lb 62 180 240 420 600 1000 2200 V 1/4 ton 110 310 410 720 1050 1750 3800 VI 3/8 ton 170 500 650 1150 1650 2800 6000 VII 1/2 ton 260 740 950 1700 2500 4100 9000 VIII 1 ton 500 1450 1900 3300 4800 8000 17600 IX 2 ton 860 2500 3300 5800 8300 13900 30400 X 3 ton 1350 4000 5200 9200 13200 22000 48200 Table C1.2. Weight gradations for 10 standard classes of riprap.

where Vdes = Design velocity for local conditions at the pier (ft/s) (m/s) K1 = Shape factor equal to 1.5 for round-nose piers and 1.7 for square-edged piers K2 = Velocity adjustment factor for location in the channel (ranges from 0.9 for pier near the bank in a straight reach to 1.7 for pier located in the main current of flow around a sharp bend) Vavg = Section average approach velocity (Q/A) upstream of bridge (ft/s) If the local velocity Vlocal is available from stream tube or flow distribution output from a 1-D model, or directly computed from a 2-D model, then only the pier shape coefficient should be used to determine the design velocity. The maximum local velocity is recommended since the channel could shift and the maximum velocity could impact any pier: Vdes = K1Vlocal (C1.6) Once a design size d50 for the riprap is established, a standard gradation class can be selected, if design criteria and economic considerations permit. Using standard sizes, the appropriate gradation can be achieved by selecting the next size larger size class, thereby creating a slightly over-designed structure, but economically a less expensive one. C-5 Test Designation Property Allowable value Frequency (1) Comments AASHTO TP 61 Percentage of Fracture < 5% 1 per 20,000 tons Percentage of pieces that have fewer than 50% fractured surfaces AASHTO T 85 Specific Gravity and Water Absorption Average of 10 pieces: Sg > 2.5 Absorption < 1.0% 1 per year If any individual piece exhibits an Sg less than 2.3 or water absorption greater than 3.0%, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. AASHTO T 103 Soundness by Freezing and Thawing Maximum of 10 pieces after 25 cycles: < 0.5% 1 per 2 years Recommended only if water absorption is greater than 0.5% and the freeze-thaw severity index is greater than 15 per ASTM D 5312. AASHTO T 104 Soundness by Use of Sodium Sulfate or Magnesium Sulfate Average of 10 pieces: < 17.5% 1 per year If any individual piece exhibits a value greater than 25%, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. AASHTO TP 58 Durability Index Using the Micro- Deval Apparatus Value > 90 > 80 > 70 Application Severe Moderate Mild 1 per year Severity of application per Section 5.4, CEN (2002). Most riverine applications are considered mild or moderate. ASTM D 3967 Splitting Tensile Strength of Intact Rock Core Specimens Average of 10 pieces: > 6 MPa 1 per year If any individual piece exhibits a value less than 4 MPa, an additional 10 pieces shall be tested. If the second series of tests also exhibits pieces that do not pass, the riprap shall be rejected. ASTM D 5873 Rock Hardness by Rebound Hammer See Note (2) 1 per 20,000 tons See Note (2) Shape Length to Thickness Ratio A/C < 10%, d50 < 24 in < 5%, d50 > 24 in 1 per 20,000 tons Percentage of pieces that exhibit A/C ratio greater than 3.0 using the Wolman count method (Lagasse et al., 2006) ASTM D 5519 Particle Size Analysis of Natural and Man-Made Riprap Materials 1 per year See Note (3) Gradation Particle Size Distribution Curve 1 per 20,000 tons Determined by the Wolman count method (Lagasse et al., 2006), where particle size, d, is based on the intermediate (B) axis. (1) Testing frequency for acceptance of riprap from certified quarries, unless otherwise noted. Project-specific tests exceeding quarry certification requirements, either in performance value or frequency of testing, must be specified by the Engineer. (2) Test results from D 5873 should be calibrated to D 3967 results before specifying quarry-specific minimum allowable values. (3) Test results from D 5519 should be calibrated to Wolman count (Lagasse et al., 2006) results before developing quarry-specific relationships between size and weight; otherwise, assume W = 85% that of a cube of dimension d having a specific gravity of Sg. Table C1.3. Recommended tests for rock quality.

1.3 Layout Dimensions Based on information derived primarily from NCHRP Project 24-07(2) (Lagasse et al. 2007), the optimum performance of riprap as a pier scour countermeasure was obtained when the riprap extended a distance of 2 times the pier width in all directions around the pier. In the case of wall piers or pile bents consisting of multiple columns where the axis of the structure is skewed to the flow direction, the lateral extent of the protection should be increased in proportion to the additional scour potential caused by the skew. While there is no definitive guidance for pier scour countermeasures, it is recommended that the extent of the armor layer should be multiplied by a factor Kα, which is a function of the width, a, and length, L, of the pier (or pile bents) and the skew angle, α, as given below (after Richardson and Davis 2001): (C1.7) Riprap should be placed in a pre-excavated hole around the pier so that the top of the riprap layer is level with the ambient 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. The riprap layer should have a minimum thickness of 3 times the d50 size of the rock. How- ever, when contraction scour through the bridge opening exceeds 3d50, the thickness of the riprap must be increased to the full depth of the contraction scour plus any long-term degra- dation. In river systems where dune-type bed forms are present during flood flows, the depth of the trough below the ambient bed elevation should be estimated using the methods of Karim (1999) and/or van Rijn (1984). In general, an upper limit on the crest-to-trough height, Δ, is provided by Bennett (1997) as Δ < 0.4y where y is the depth of flow. This limit suggests that the maximum depth of the bed-form trough below ambient bed elevation will not exceed 0.2 times the depth of flow. Additional riprap thickness due to any of these conditions may warrant an increase in the extent of riprap away from the pier faces, such that riprap launch- ing at a 2H:1V slope under water can be accommodated. When placement of the riprap must occur under water, the thickness should be increased by 50%. Recommended layout dimen- sions are provided in Figure C1.2. A filter layer is typically required for riprap at bridge piers. The filter should not be extended fully beneath the riprap; instead, it should be terminated two-thirds of the distance from the pier to the edge of the riprap. When a granular stone filter is used, the layer should have a minimum thickness of 4 times the d50 of the filter stone or 6 in. (15 cm), whichever is greater. As with riprap, the layer thickness should be increased by 50% when placing under water. Sand-filled geocon- tainers made of properly selected materials provide a convenient method for controlled place- ment of a filter in flowing water. This method can also be used to partially fill an existing scour hole when placement must occur under water, as illustrated in Figure C1.3. For more detail, see Lagasse et al. (2007). 1.4 Filter Requirements The importance of the filter component of a riprap installation should not be underestimated. Two kinds of filters are used in conjunction with riprap: granular filters and geotextile filters. Some situations call for a composite filter consisting of both a granular layer and a geotextile. The specific characteristics of the base soil determine the need for, and design considerations, of the filter layer. In cases where dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be considered. K a L a α α α = +⎛⎝⎜ ⎞⎠⎟ cos sin .0 65 C-6

The filter must retain the coarser particles of the subgrade while remaining permeable enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass through the fil- ter, leaving a coarser substrate behind. 1.4.1 Geotextile Filter Properties Either woven or non-woven, needle-punched fabrics may be used. If a non-woven fabric is used, it must have a mass density greater than 12 oz/yd2 (400 g/m2). Under no circumstances may spun-bond or slit-film fabrics be allowed. 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 percent open area). In addition, geotextiles must be sufficiently strong to withstand stresses during instal- lation. These properties are readily available from manufacturers. The following list briefly describes the most relevant properties: • Permeability. The permeability, K, of a geotextile is a calculated value that indicates the abil- ity of a geotextile to transmit water across its thickness. It is typically reported in units of cen- timeters per second (cm/s). This property is directly related to the filtration function that a C-7 a 2a 2a t Riprap placement = 2 (a) from pier (all around) Pier width = “a” (normal to flow) F L O W Minimum riprap thickness t= 3d50, depth of contraction scour, or depth of bedform trough, whichever is greatest Filter placement = 4/3(a) from pier (all around) FilterPier Figure C1.2. Riprap layout diagram for pier scour protection.

geotextile must perform, where water flows across the plane of 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. Per- mittivity, ψ, is defined as K divided by the geotextile thickness, t, in centimeters; therefore, permittivity has a value of (s)−1. Permeability (and permittivity) is extremely important in fil- ter design. For pier riprap installations, the permeability of the geotextile should be at least 10 times greater than that of the underlying material: Kg > 10Ks (C1.8) where Kg = Permeability of geotextile (cm/s) Ks = Permeability of subgrade soil (cm/s) • Transmissivity. The transmissivity, θ, of a geotextile is a calculated value that indicates the abil- ity 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 directly related to the drainage func- tion and is most often used for high-flow drainage nets and geocomposites, not geotextiles. Woven monofilament geotextiles have very little capacity 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 (3-D) microstructure. Transmissivity is not particularly relevant to filter design. • Apparent opening size (AOS). Also known as equivalent opening size, this measure is gener- ally 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 geotex- tile. This measure is applicable to non-woven geotextiles only. Porosity is used to estimate the potential for long-term clogging and is typically reported as a percentage. C-8 FLOW Rock riprap placed flush with Sand - filled geocontainers channel bed Pier Minimum riprap thickness t = 3d50, depth of contraction scour, or depth of bedform trough, whichever is greatest Filter placement = 4/3(a) from pier (all around) Figure C1.3. Schematic diagram of sand-filled geocontainers beneath pier riprap.

• Percent open area (POA). POA is a comparison of the total open area to the total geotextile area. This measure is applicable to woven geotextiles only. POA is used to estimate the poten- tial for long-term clogging and is typically reported as a percentage. • Thickness. As mentioned above, thickness is used to calculate traditional 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 Newtons 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 typ- ically reported in Newtons or pounds. • Puncture strength. Puncture strength is the force required to puncture a geotextile using a standard penetration apparatus. It is typically reported in Newtons or pounds. Table C1.4 provides the recommended characteristics for geotextile filters. There are many other tests to determine various characteristics of geotextiles; only those deemed most rele- vant to applications involving pier riprap have been discussed here. Geotextiles should be able to withstand the rigors of installation without suffering degradation of any kind. Long-term C-9 Allowable value (1)Test Designation Property Elongation < 50%(2) Elongation > 50%(2) Comments ASTM D 4632 Grab Strength > 315 lbs (Class 1) > 250 lbs (Class 2) > 180 lbs (Class 3) > 200 lbs (Class 1) > 160 lbs (Class 2) > 110 lbs (Class 3) From AASHTO M 288 ASTM D 4632 Sewn Seam Strength (3) > 270 lbs (Class 1) > 220 lbs (Class 2) > 160 lbs (Class 3) > 180 lbs (Class 1) > 140 lbs (Class 2) > 100 lbs (Class 3) From AASHTO M 288 ASTM D 4533 Tear Strength (4) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4833 Puncture Strength > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4751 Apparent Opening Size Per design criteria (Section 1.4 of this design guide) Maximum allowable value ASTM D 4491 Permittivity and Permeability Per design criteria (Section 1.4 of this design guide) Minimum allowable value ASTM D 4355 Degradation by Ultraviolet Light > 50% strength retained after 500 hours of exposure Minimum allowable value ASTM D 4873 Guide for Identification, Storage, and Handling Provides information on identification, storage, and handling of geotextiles. ASTM D 4759 Practice for the Specification Conformance of Geosynthetics Provides information on procedures for ensuring that geotextiles at the jobsite meet the design specifications. (1) Required geotextile class for permanent erosion control design is designated below for the indicated application. The severity of installation conditions generally dictates the required geotextile class. The following descriptions have been modified from AASHTO M 288: • Class 1 is recommended for harsh or severe installation conditions where there is a greater potential for geotextile damage, including when placement of riprap must occur in multiple lifts, when drop heights may exceed 1 ft (0.3 m) or when repeated vehicular traffic on the installation is anticipated. • Class 2 is recommended for installation conditions where placement in regular, single lifts are expected and little or no vehicular traffic on the installation will occur, or when placing individual rocks by clamshell, orange-peel grapple or specially equipped hydraulic excavator with drop heights less than 1 ft. • Class 3 is specified for the least severe installation environments, with drop heights less than 1 ft onto a bedding layer of select sand, gravel or other select imported material. (2) As measured in accordance with ASTM D 4632. (3) When seams are required. (4) The required Minimum Average Roll Value (MARV) tear strength for woven monofilament geotextiles is 55 lbs. The MARV corresponds to a statistical measure whereby 2.5% of the tested values are less than the mean value minus two standard deviations (Koerner 1998). Table C1.4. Recommended requirements for geotextile properties.

endurance to stresses such as ultraviolet solar radiation or continual abrasion are considered of secondary importance, because once the geotextile has been installed and covered by the riprap, these stresses do not represent the environment that the geotextile will experience in the long term. 1.4.2 Geotextile Filter Design Procedure Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of permeability, and the plasticity index (PI is required only if the base soil is more than 20% clay). Step 2. Determine Particle Retention Criterion. A decision tree is provided as Figure C1.4 to assist in determining the appropriate soil retention criterion for the geotextile. The figure includes guidance when a granular transition layer (i.e., composite filter) is necessary. A com- posite filter is typically required when the base soil is greater than 30% clay or is predominantly fine-grained soil (more than 50% passing the #200 sieve). If a granular transition layer is required, the geotextile should be designed to be compatible with the properties of the granular layer. If the required AOS is smaller than that of available geo- textiles, then a granular transition layer is required. However, this requirement can be waived if the base soil exhibits the following conditions for hydraulic conductivity, K; plasticity index, PI; and undrained shear strength, c: K < 1 × 10−7 cm/s PI > 15 c > 10 kPa Under these soil conditions there is sufficient cohesion to prevent soil loss through the geo- textile. A geotextile with an AOS less than a #70 sieve (approximately 0.2 mm) can be used with soils meeting these conditions and essentially functions more as a separation layer than a filter. Step 3. Determine Permeability Criterion. The permeability criterion requires that the filter exhibit a permeability at least 4 times greater than that of the base soil (Koerner 1998) and for critical or severe applications, at least 10 times greater (Holtz et al. 1995). Generally speaking, if the permeability of the base soil or granular filter has been determined from laboratory testing, that value should be used. If laboratory testing was not conducted, then an estimate of perme- ability based on the particle size distribution should be used. To obtain the permeability of a geotextile in cm/s, multiply the thickness of the geotextile in cm by its permittivity in s−1. Typically, the designer will need to contact the geotextile manufac- turer to obtain values of permeability, permittivity, and thickness. Step 4. Select a Geotextile that Meets the Required Strength Criteria. Strength and durabil- ity requirements depend on the installation environment and the construction equipment that is being used. See Table C1.4 for recommended values based on AASHTO M 288, “Geotextile Specification for Highway Construction,” which provides guidance on allowable strength and elongation values for three categories of installation severity. For additional guidelines regard- ing the selection of durability test methods, refer to ASTM D 5819, “Standard Guide for Select- ing Test Methods for Experimental Evaluation of Geosynthetic Durability.” Step 5. Minimize Long-Term Clogging Potential. When a woven geotextile is used, its POA should be greater than 4% by area. If a non-woven geotextile is used, its porosity should be greater than 30% by volume. A good rule of thumb suggests that the geotextile having the largest AOS that satisfies the particle retention criteria should be used (provided of course that all other minimum allowable values described in this section are met as well). C-10

1.4.3 Granular Filter Properties Generally speaking, most required granular filter properties can be obtained from the parti- cle size distribution curve for the material. Granular filters can be used alone or can serve as a transitional layer between a predominantly fine-grained base soil and a geotextile. The follow- ing list briefly describes the most relevant properties: • Particle Size Distribution. As a rule of thumb, the gradation curve of the granular filter 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. Heibaum (2004) presents C-11 FROM SOIL PROPERTY TESTS MORE THAN 30% CLAY (D30 < 0.002 mm) LESS THAN 30% CLAY AND MORE THAN 50% FINES (d30 > 0.002 mm, AND d50 < 0.075 mm) LESS THAN 50% FINES AND LESS THAN 90% GRAVEL (d50 > 0.075 mm, AND d90 < 4.8 mm) MORE THAN 90% GRAVEL (d90 > 4.8 mm) USE CISTIN – ZIEMS METHOD TO DESIGN A GRANULAR TRANSITION LAYER, THEN DESIGN GEOTEXTILE AS A FILTER FOR THE GRANULAR LAYER O95 < d50WIDELY GRADED (CU > 5) O95 < 2.5d50 and O95 < d90 UNIFORMLY GRADED (CU ≤5) d50 < O95 < d90 WAVE ATTACK OPEN CHANNEL FLOW Definition of Terms dx = particle size for which x percent is smaller PI = plasticity index of the base soil K = permeability of the base soil O95 = the AOS of the geotextile c = Undrained shear strength Cu = Coefficient of Uniformity, d60/d10 Note If the required O95 is smaller than that of available geotextiles, then a granular transition layer is needed. O95 ≤ #70 SIEVE (0.2 mm) YES NO PI > 5 ? YES NO K < 10-7 cm/s, and c > 10 kPa, and PI > 15 ? Source: modified from Koerner (1998) Figure C1.4. Geotextile selection based on soil retention.

a summary of a procedure originally 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 C1.5 provides a design chart based on the Cistin–Ziems approach. • Permeability. Permeability of a granular filter material is determined by laboratory test or estimated using relationships relating permeability to the particle size distribution. The per- meability of a granular layer is used to select a geotextile when designing a composite filter. For pier riprap, the permeability of the granular filter should be at least 10 times greater than that 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 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. • Thickness. Practical issues of placement indicate that a typical minimum thickness of 6 to 8 in. is specified. For placement under water, thickness should be increased by 50%. • Quality and durability. Aggregate used for a granular filter should be hard, dense, and durable. 1.4.4 Granular Filter Design Procedure Numerous texts and handbooks provide details on the well-known Terzaghi approach to designing a granular filter. That approach was developed for subsoils consisting of well-graded sands and may not be widely applicable to other soil types. An alternative approach that is con- sidered more robust in this regard is the Cistin–Ziems method. The suggested steps for proper design of a granular filter using this method are outlined below. Note that the subscript “s” is used to represent the base (finer) soil, and “f” is used to represent the filter (coarser) layer. Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of permeability, and the plasticity index (PI is required only if the base soil is more than 20% clay). C-12 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 C1.5. Granular filter design chart according to Cistin and Ziems.

Step 2. Determine Key Indices for Base Soil. From the grain size information, determine the median grain size, d50, and the coefficient of uniformity, Cus = d60/d10, of the base soil. Step 3. Determine Key Indices for Granular Filter. One or more locally available aggregates should be identified as potential candidates for use as a filter material. The d50 and coefficient of uniformity, Cuf = d60/d10, should be determined for each candidate filter material. Step 4. Determine Maximum Allowable d50 for Filter. Enter the Cistin–Ziems design chart (Figure C1.5) with the coefficient of uniformity, Cus, for the base soil on the x-axis. Find the curve that corresponds to the coefficient of uniformity, Cuf, for the filter in the body of the chart and, from that point, determine the maximum allowable A50 from the y-axis. Compute the max- imum allowable d50f of the filter using d50fmax equals A50max times d50s. Check to see if the candi- date filter material conforms to this requirement. If it does not, continue checking alternative candidates until a suitable material is identified. Step 5. Check for Permeability. From laboratory permeameter tests or the grain size distri- bution of the candidate filter material, determine whether the hydraulic conductivity of the fil- ter is at least 10 times greater than that of the subsoil. Step 6. Check for Compatibility with Riprap Rock. Repeat steps 1 through 4 above, con- sidering that the filter material is now the “finer” soil and the riprap is the “coarser” material. If the Cistin–Ziems criterion is not met, then multiple layers of granular filter materials should be considered. Step 7. Filter Layer Thickness. For practicality of placement, the nominal thickness of a sin- gle filter layer should not be less than 6 in. (15 cm). Single-layer thicknesses up to 15 in. (38 cm) may be warranted where large rock fill particle sizes are used. When multiple filter layers are required, each individual layer should range from 4 to 8 in. (10 to 20 cm) in thickness (Brown and Clyde 1989)). NOTE: In cases where dune-type bed forms may be present or of underwater installation, it is strongly recommended that only a geotextile filter be considered. Part 2: Construction Riprap is placed in a riverine or coastal environment to prevent scour or erosion of the bed, banks, shoreline, or near structures such as bridge piers and abutments. Riprap construction involves placement of rock and stone in layers on top of a bedding or filter layer composed of sand, gravel, and/or geotechnical fabric. The basis of the protection afforded by the riprap is the mass and interlocking of the individual rocks. Factors to consider when designing riprap structures begin with the source for the rock; the method to obtain or manufacture the rock; competence of the rock; and the methods and equip- ment to collect, transport, and place the riprap. Rock for riprap may be obtained from quarries, by screening oversized rock from earth borrow pits, by collecting rock from fields, or from talus deposits. Screening borrow pit material and collecting field rocks present problems such as rocks that are too large or that have unsatisfactory length-to-width ratios for riprap. Quarries are generally the best source for obtaining large rock specified for riprap. However, not all quarries can produce large rock because of the characteristics or limited volume of the rock formation. Because quarrying generally uses blasting to fracture the rock formation into material suitable for riprap, cracking of the large rocks may only become evident after loading, transporting, and dumping the material at the quarry or after moving the material from quarry to stockpile at the job site or from the stockpile to the final placement location. C-13

In most cases, the production of the rock material will occur at a source that is relatively remote from the construction area. Therefore, this discussion assumes that the rock is hauled to the site of the installation, where it is either dumped directly, stockpiled, or loaded onto water- borne equipment. The objectives of construction of a good riprap structure are (1) to obtain a rock mixture from the source that meets the design specifications and (2) to place that mixture in a well-knit, compact, and uniform layer without segregation of the mixture. The guidance in this section has been devel- oped to facilitate the proper installation of riprap systems to achieve suitable hydraulic performance and maintain stability against hydraulic loading. The proper installation of riprap systems is essen- tial to the adequate functioning and performance of the system during the design hydrologic event. Guidelines are provided herein for maximizing the correspondence between the design intent and the actual field-finished conditions of the project. This section addresses the preparation of the sub- grade, placement of the filter, riprap placement, and measurement and payment. 2.1 General Guidelines The contractor is responsible for constructing the project according to the plans and specifi- cations; however, ensuring conformance with the project plans and specifications is the respon- sibility of the owner. This responsibility is typically performed by the owner’s engineer and inspectors. Inspectors observe and document the construction progress and performance of the contractor. Prior to construction, the contractor should provide a quality control plan to the owner (for example, see ER 1180-1-6 [U.S. Army Corps of Engineers 1995]) and provide labor and equipment to perform tests as required by the project specifications. Construction requirements for riprap placement are included in the project plans and speci- fications. Standard riprap specifications and layout guidance are found in Part 1 of this appen- dix. Recommended requirements for the stone, including the tests necessary to ensure that the physical and mechanical properties meet the requirements of the project specifications, are pro- vided. Field tests can be performed at the quarry and/or on the job site, or representative sam- ples can be obtained for laboratory testing. Typically, one or more standard riprap gradations are specified and plan sheets show loca- tions, grades, and dimensions of rock layers for the countermeasure. The stone shape is impor- tant and riprap should be blocky rather than elongated, platy, or round. In addition, the stone should have sharp, angular, clean edges at the intersections of relatively flat surfaces. Segregation of material during transportation, dumping, or off-loading is not acceptable. Inspection of riprap placement consists of visual inspection of the operation and the finished surface. Inspection must ensure that a dense, rough surface of well-keyed graded rock of the specified quality and sizes is obtained, that the layers are placed such that voids are minimized, and that the layers are the specified thickness. Inspection and quality assurance must be carefully organized and conducted in case potential problems or questions arise over acceptance of stone material. Acceptance should not be made until measurement for payment has been completed. The engineer and inspectors reserve the right to reject stone at the quarry, at the job site or stockpile, and in place in the structures throughout the duration of the contract. Stone rejected at the job site should be removed from the project site. Stone rejected at the quarry should be disposed of or otherwise prevented from mixing with satisfactory stone. Construction techniques can vary tremendously because of the following factors: • Size and scope of the overall project • Size and weight of the riprap particles • Placement under water or in the dry C-14

• 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 infor- mation regarding construction of riprap installations and some basic information and descrip- tion of techniques and processes involved. 2.2 Materials 2.2.1 Stone The best time to control the gradation of the riprap mixture is during the quarrying opera- tion. Generally, sorting and mixing later in stockpiles or at the construction site is not recom- mended. Inspection of the riprap gradation at the job site is usually carried out visually. There- fore, it is helpful to have a pile of rocks with the required gradation at a convenient location where inspectors can see and develop a reference to judge by eye the suitability of the rock being placed. On-site inspection of riprap is necessary both at the quarry and at the job site to ensure proper gradation and material that does not contain excessive amounts of fines. Breakage dur- ing handling and transportation should be taken into account. The Wolman count method (Wolman 1954) as described in NCHRP Report 568 (Lagasse et al. 2006) may be used as a field test to determine a size distribution based on a random sampling of individual stones within a matrix. This method relies on samples taken from the surface of the matrix to make the method practical for use in the field. The procedure determines frequency by size of a surface material rather than using a bulk sample. The middle dimension (B axis) is measured for 100 randomly selected particles on the surface. The Wolman count method can be done by stretching a survey tape over the material and measuring each particle located at equal intervals along the tape. The interval should be at least 1 ft for small riprap and increased for larger riprap. The longer and shorter axes (A and C) can also be measured to determine particle shape. One rule that must be followed is that if a single particle is large enough to fall under two interval points along the tape, then it should be included in the count twice. It is best to select an interval large enough that this does not occur frequently. 2.2.2 Filter Geotextile. Either woven or non-woven, needle-punched fabrics may be used. If a non- woven fabric is used, it must have a mass density greater than 12 oz/yd2 (400 g/m2). Under no circumstances may spun-bond or slit-film fabrics be allowed. Each roll of geotextile shall be labeled with the manufacturer’s name, product identification, roll dimensions, lot number, and date of manufacture. Geotextiles shall not be exposed to sunlight prior to placement. Granular filters. Samples of granular filter material shall be tested for grain size distribution to ensure compliance with the gradation specification used in design. Sampling and testing fre- quency shall be in accordance with the owner or owner’s authorized representative. 2.2.3 Subgrade Soils When the riprap and filter are placed in the dry, they shall be placed on undisturbed native soil, on an excavated and prepared subgrade, or on acceptably placed and compacted fill. Unsatisfactory soils shall be considered those soils having excessive in-place moisture content; soils containing C-15

roots, sod, brush, or other organic materials; soils containing turf clods or rocks; or frozen soil. These soils shall be removed, and the excavation backfilled with approved material that is 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, per the geotechnical engineer’s recommendations. 2.3 Installation 2.3.1 Subgrade Preparation The subgrade soil conditions shall meet or exceed the required material properties described in Section 2.2.3 prior to placement of the riprap. Soils not meeting the requirements shall be removed and replaced with acceptable material. When riprap is placed in the dry, the areas to receive the riprap shall 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 riprap. Stable and compacted subgrade soil shall be pre- pared to the lines, grades, and cross sections shown on the contract drawings. Termination trenches and transitions between slopes, embankment crests, benches, berms, and toes shall be compacted, shaped, and uniformly graded. The subgrade should be uniformly compacted to the geotechnical engineer’s site-specific requirements. When riprap is placed under water, divers shall 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 riprap system, the prepared sub- grade must be inspected. 2.3.2 Placing the Filter Whether the filter comprises one or more layers of granular material or is made of geotextile, its placement should result in a continuous installation that maintains intimate contact with the soil beneath. Voids, gaps, tears, or other holes in the filter must be avoided to the extent practi- cable, and repaired or the filter replaced when they occur. Placement of Geotextile. The geotextile shall 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 shall not be dragged, lifted by one end, or dropped. The geotextile should be placed in such a manner that placement of the overlying materials (riprap and/or bedding stone) will not excessively stretch or tear the geotextile. After geotextile placement, the work area shall not be trafficked or disturbed in a manner that might result in a loss of intimate contact between the riprap stone, the geotextile, and the sub- grade. The geotextile shall not be left exposed 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 shall be placed so that upstream strips overlap downstream strips. Overlaps shall be in the direction of flow wherever possible. The longitudinal and transverse joints shall be overlapped at least 1.5 ft (46 cm) for dry installations and at least 3 ft (91 cm) for below-water installations. If a sewn seam is to be used for the seaming of the geotextile, the thread to be used shall consist of high-strength polypropylene or polyester and shall be resistant to ultraviolet radi- ation. If necessary to expedite construction and to maintain the recommended overlaps, anchor- ing pins, U-staples, or weights such as sandbags shall be used. Placing Geotextiles Under Water. Placing geotextiles under water can be problematic for a number of reasons. Most geotextiles that are used as filters beneath riprap are made of polyeth- ylene or polypropylene. These materials have specific gravities ranging from 0.90 to 0.96, mean- C-16

ing that they will float unless weighted down or otherwise anchored to the subgrade prior to placement of the riprap (Koerner 1998). 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 condition that contractors refer to as “galloping”) that are extremely difficult to control. The preferred method of controlling geotextile placement is to isolate the work area from river cur- rents by a temporary cofferdam. In mild currents, geotextiles precut to length can be placed by divers, using sandbags to hold the filter temporarily. For riprap at piers, sand-filled geocontainers made of non-woven, needle-punched fabric are particularly effective for placement under water as shown in Figure C1.3. The geotextile fabric and sand fill that compose the geocontainers should be selected in accordance with appropriate filter design criteria presented in Part 1 and placed such that they overlap to cover the required area. Geocontainers can be fabricated in a variety of dimensions and weights. Each geocontainer should be filled with sand to no more than 80% its total volume so that it remains flexible and “floppy.” The geocontainers can also serve to fill a pre-existing scour hole around a pier prior to riprap placement, as shown in Figure C1.3. For more information, see Lagasse et al. (2006, 2007). 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 riprap and whether a layer of bedding stone is to be used between the filter and the riprap. When a granular filter is placed under water, the thickness should be increased by 50%. Placing granular media under water around a bridge pier is best accomplished using a large-diameter tremie pipe to control the placement location and thickness, while minimizing the potential for segregation. NOTE: For riverine applications where dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be considered. 2.3.3 Placing the Riprap Riprap may be placed from either land-based or water-based operations and can be placed under water or in the dry. Special-purpose equipment such as clamshells, orange-peel grapples, or hydraulic excavators (often equipped with a “thumb”) is preferred for placing riprap. Unless the riprap can be placed to the required thickness in one lift using dump trucks or front-end loaders, tracked or wheeled vehicles are discouraged from use because they can destroy the inter- locking integrity of the rocks when driven over previously placed riprap. Water-based operations may require specialized equipment for deep-water placement or can use land-based equipment loaded onto barges for near-shore placement. In all cases, riprap should be placed from the bottom working toward the top of the slope so that rolling and/or seg- regation does not occur. Riprap Placement on Geotextiles. Riprap should be placed over the geotextile by methods that do not stretch, tear, puncture, or reposition the fabric. Equipment should be operated to minimize the drop height of the stone without the equipment contacting and damaging the geo- textile. Generally, this will be about 1 ft of drop from the bucket to the placement surface (ASTM D 6825). Further guidance on recommended strength properties of geotextiles as related to the severity of stresses during installation are provided in Part 1 of this appendix. When the preferred equipment cannot be utilized, a bedding layer of coarse granular material on top of the geotex- tile can serve as a cushion to protect the geotextile. Material composing the bedding layer must be more permeable than the geotextile to prevent uplift pressures from developing. Riprap Placement Under Water. Riprap placed in water requires close observation and increased quality control to ensure a continuous well-graded uniform rock layer of the required thickness (ASTM D 6825). A systematic process for placing and continuous monitoring to ver- C-17

ify the quantity and layer thickness is important. Typically, riprap thickness is increased by 50% when placement must occur under water. Excavation, grading, and placement of riprap and filter under water require additional meas- ures. For installations of a relatively small scale, the stream around the work area can be diverted during the low-flow season. For installations on larger rivers or in deeper water, the area can be temporarily enclosed by a cofferdam, which allows for construction dewatering if necessary. Alternatively, a silt curtain made of plastic sheeting may be suspended by buoys around the work area to minimize environmental degradation during construction. Depending on the depth and velocity of the water, sounding surveys using a sounding pole or sounding basket on a lead line, divers, sonar bottom profiles, and remote operated vehicles (ROVs) can provide some information about the riprap placement under water. 2.3.4 Inspection The subgrade preparation, geotextile placement, riprap system installation, and overall finished condition including termination trenches shall be inspected before work acceptance. Inspection guidelines for the riprap installation are presented in detail in Part 3 of this appendix. 2.4 Measurement and Payment Riprap satisfactorily placed can be paid for based on either volume or weight. When a weight basis is used, commercial truck scales capable of printing a weight ticket including time, date, truck num- ber, and weight should be used. When a volumetric basis is used, the in-place volume should be deter- mined by multiplying the area, as measured in the field, of the surface on which the riprap was placed by the thickness of the riprap measured perpendicular as dimensioned on the contract drawings. In either case, the finished surface of the riprap should be surveyed to ensure that the as-built lines and grades meet the design plans within the specified tolerance. Survey cross sections per- pendicular to the axis of the structure are usually taken at specified intervals. All stone outside the limits and tolerances of the cross sections of the structure, except variations so minor as not to be measurable, is deducted from the quantity of new stone for which payment is to be made. In certain cases, excess stone may be hazardous or otherwise detrimental; in this circumstance, the contractor must remove the excess stone at its own expense. Payment will be full compen- sation for all material, labor, and equipment to complete the work. Part 3: Inspection, Maintenance, and Performance Evaluation 3.1 Inspection During Construction Inspection during construction shall be conducted by qualified personnel who are independ- ent of the contractor. Underwater inspection of riprap scour countermeasures at piers shall be performed only by divers specifically trained and certified for such work. 3.1.1 Subgrade Inspection of the subgrade shall be performed immediately prior to geotextile placement. The subgrade 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 shall be noted and photographed; observations of such should be brought to the attention C-18

of the project engineer as they may represent conditions that are different than those used for design. It is generally recommended that compaction testing be performed at a frequency of one test per 2,000 ft2 (186 m2) of surface area, unless project specifications require otherwise. 3.1.2 Geotextile Each roll of geotextile delivered to the job site must have a label with the manufacturer’s name and product designation. The inspector must check the labels to ensure that the geotextile 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 riprap applications. The geotextile must be stored so that it is out of direct sunlight, as damage can occur from expo- sure to ultraviolet radiation. When placed, it must be free of wrinkles, folds, or tears. Sandbags, extra concrete blocks, or U-shaped soil staples may be used to hold the geotextile in position while the blocks are being placed. The riprap should be placed within 48 hours after the geotextile is placed unless unusual circumstances warrant otherwise. 3.1.3 Riprap Inspection of riprap placement typically consists of visual inspection of the operation and the finished surface. Inspection must ensure that a dense, rough surface of well-keyed graded rock of the specified quality and sizes is obtained, that the layers are placed such that voids are mini- mized, and that the layers are the specified thickness. 3.2 Periodic and Post-Flood Inspection Pier riprap is typically inspected during 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 spec- ified magnitude. Underwater inspection of a riprap system shall be performed only by divers specifically trained and certified for such work. The following guidance for inspecting riprap is presented in the National Highway Institute (NHI) training course 135047, “Stream Stability and Scour at High- way Bridges for Bridge Inspectors”: 1. Riprap should be angular and interlocking. (Old bowling balls would not make good riprap. Flat sections of broken concrete paving do not make good riprap.) 2. Riprap should have a granular or synthetic geotextile filter between the riprap and the sub- grade material. 3. Riprap should be well graded (a wide range of rock sizes). The maximum rock size should be no greater than about twice the median (d50) size. 4. For bridge piers, riprap should generally extend up to the bed elevation so that the top of the riprap is visible to the inspector during and after floods. 5. When riprap is inspected, affirmative answers to the following questions are strong indica- tors of problems: • Has riprap been displaced downstream? • Has angular riprap blanket slumped down slope? • Has angular riprap material been replaced over time by smoother river run material? • Has riprap material physically deteriorated, disintegrated, or been abraded over time? • Are there holes in the riprap blanket where the filter has been exposed or breached? 3.3 Inspection Coding Guide To guide the inspection of a riprap installation, a coding system was developed under NCHRP Project 24-23 (Lagasse et al. 2006). Similar to the National Bridge Inspection Standards (NBIS) C-19

(U.S.DOT 2004) Item 113, it establishes numerical ratings from 0 (worst) to 9 (best). Recom- mended action items based on the numerical rating are also provided. This coding system is applicable to any riprap installation, including bridge pier protection. 3.4 Maintenance Deficiencies noted during the inspection should be corrected as soon as possible. As with any armor system, progressive failure from successive flows must be avoided by providing timely maintenance intervention. 3.5 Performance Evaluation The evaluation of any riprap system’s performance should be based on its design parameters as compared to actual field experience, longevity, and inspection/maintenance history. To properly assess the performance of pier riprap, the history of hydraulic loading on the installation, in terms of flood magnitudes and frequencies, 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 riprap. Present-day channel cross-section geometry and planform should be compared to those at the time of installation. Both lateral and vertical instability of the channel can signifi- cantly alter hydraulic conditions at the site. Approach flows may exhibit an increasingly severe angle of attack (impinging flow) over time, increasing the hydraulic loading on the riprap. It is recognized that the person making the performance evaluation will probably not be the inspector; however, 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. 3.5.1 Performance Rating Guide To guide the performance evaluation for riprap as a pier scour countermeasure, a rating sys- tem is presented in this section. It establishes numerical ratings from 0 (worst) to 6 (best) for each of three topical areas: • 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 present physical condition of the countermeasure? Tables C3.1 through C3.3 present a rating system for riprap used as a pier scour countermea- sure. A single numerical score is not intended; rather, an independent rating (0-6 or U) is given C-20 Code Hydraulic History Code Hydraulic History U N/A 3 Moderate: The countermeasure has experienced one or more flows greater than the 10-year event. 6 Extreme: The countermeasure has experienced one or more flows greater than the 100-year event. 2 Low: The countermeasure has experienced one or more flows greater than the 5-year event. 5 Severe: The countermeasure has experienced one or more flows greater than the 50-year event. 1 Very Low: The countermeasure has experienced one or more flows greater than the 2-year event. 4 High: The countermeasure has experienced one or more flows greater than the 25-year event. 0 Negligible: The countermeasure has not experienced any flows greater than a 2-year event. Table C3.1. Rating system for riprap: hydraulic history.

for each of the three topical areas. Recommended actions corresponding to the current condi- tion rating are also provided. 3.5.2 Pier Riprap Failure Modes Schoharie Creek Case Study. FHWA’s HEC-18 (Richardson and Davis 2001) and HEC-23 (Lagasse et al. 2001) document the catastrophic bridge failure at Schoharie Creek attributed to inadequate pier riprap. The failure of the I-90 bridge over Schoharie Creek near Albany, New York, on April 5, 1987, which cost 10 lives, was investigated by the National Transportation Safety Board (NTSB). The peak flow was 64,900 cfs (1,838 m3/s) with a 70- to 100-year return period. The foundations of the four bridge piers were large spread footings 82 ft (25 m) long, 18 ft (5.5 m) wide, and 5 ft (1.5 m) deep without piles. The footings were set 5 ft (1.5 m) into the stream bed in very dense ice contact C-21 Code Maintenance History Code Maintenance History U N/A 3 Moderate: The system has required occasional maintenance since installation. 6 None Required: No maintenance has been needed since installation. 2 High: Frequent maintenance has been required. 5 Very Low: The system has required maintenance for very small, local areas once or twice. 1 Very High: Significant maintenance is usually required after flood events. 4 Low: The system has required minor maintenance. 0 Excessive: The system typically requires maintenance every year. Table C3.2. Rating system for riprap: maintenance history. Code Description of Current Condition Code Description of Current Condition U The system is uninspectable, due to burial by sediment, debris, or other circumstance. 3 Fair: Some missing particles as evidenced by irregular armor surface; localized areas exhibit decreased layer thickness. 6 Excellent: The system is in excellent condition, with no displacement of particles and no undermining. System is well abutted to pier with no gaps. 2 Poor: Obvious deterioration of the system has occurred. Gaps or holes are present that have exposed the underlying filter. 5 Very Good: The system exhibits only minimal evidence of settlement or particle movement around the periphery. 1 Badly Damaged: The system has experienced substantial deterioration in terms of particle displacement. The armor layer has separated from the pier, leaving gaps. 4 Good: Some minor settlement and/or particle displacement observed. 0 Severe: The system has suffered damage such that it is no longer providing scour protection. The only recourse is to remove the remains of the installation and replace it with a redesigned countermeasure. Recommended actions based on current condition rating: Code U: The riprap cannot be inspected. A plan of action should be developed to determine the condition of the installation. Possible remedies may include removal of debris, excavation during low flow, probing, or non-destructive testing using ground penetrating radar or seismic methods. Codes 6 or 5: Continue periodic inspection program at the specified interval. Codes 4 or 3: Increase inspection frequency. The rating history of the installation should be tracked to determine if a downward trend in the rating is evident. Depending on the nature of the riprap application, the installation of monitoring instruments might be considered. Code 2: The maintenance engineer’s office should be notified and maintenance should be scheduled. The cause of the low rating should be determined, and consideration given to redesign and replacement. Larger stone size or alternative scour countermeasure systems should be considered as a replacement. Codes 1 or 0: The maintenance engineer’s office should be notified immediately. Depending upon the nature of the riprap application, other local officials and/or law enforcement agencies identified in the Plan of Action for the bridge may also need to be notified. Table C3.3. Rating system for riprap: current condition.

stratified glacial drift, which was considered non-erodible by the designers (Figure C3.1). However, flume studies of samples of the stratified drift showed that some material would be eroded at a velocity of 4 ft/s (1.5 m/s), and, at a velocity of 8 ft/s (2.4 m/s), the erosion rates were high. A 1:50-scale, 3-D model study established a flow velocity of 10.8 ft/s (3.3 m/s) at the pier that failed. Also, the 1:50-scale, 3-D model and a 1:15-scale, 2-D model study gave 15 ft (4.6 m) of max- imum scour depth. The scour depth of the prototype pier (pier 3) at failure was 14 ft (4.3 m) (Figure C3.2). Design plans called for the footings to be protected with riprap. Over time (1953 to 1987), much of the riprap was removed by high flows. NTSB gave as the probable cause “. . . . the fail- ure of the New York State Thruway authority [NYSTA] to maintain adequate riprap around the bridge piers, which led to severe erosion in the soil beneath the spread footings. Contributing to the severity of the accident was the lack of structural redundancy in the bridge.” C-22 Figure C3.1. South elevation of Schoharie Creek bridge showing key structural features and a schematic geological section. Note: Pier 2 is in the foreground with Pier 3 in the background. Figure C3.2. Pier scour holes at Schoharie Creek bridge in 1987.

The NYSTA inspected the bridge annually or biennially with the last inspection on April 1, 1986. A 1979 inspection by a consultant hired by the New York State Department of Transportation indi- cated that most of the riprap around the piers was missing (Figures C3.3 and C3.4); however, the 1986 inspection failed to detect any problems with the condition of the riprap at the piers. Based on the NTSB findings, the conclusions from this failure are that inspectors and their supervisors must recognize that riprap does not necessarily make a bridge safe from scour, and inspectors must be trained to recognize when riprap is missing and the significance of this condition. Summary. Examples of the most common modes of riprap failure at piers provide guidance for post-flood and post-construction performance evaluation. Inspectors need to be aware of, and understand, the causes of riprap inadequacies that they see in the field. While the specific mechanism causing failure of the riprap is difficult to determine, and a number of factors, act- ing either individually or combined, may be involved, the reasons for riprap failures at bridge piers can be summarized as follows: • Particle size was too small because – Shear stress was underestimated C-23 Note: Flow is from right to left. Figure C3.3. Photograph of riprap at pier 2, October 1956. Figure C3.4. Photograph of riprap at pier 2, August 1977.

– Velocity was underestimated – Inadequate allowance was made for channel curvature – Design channel capacity was too low – Design discharge was too low – Inadequate assessment was made of abrasive forces – Inadequate allowance was made for effect of obstructions (such as debris) • Channel changes caused – Increased angle of attack (skew) – Decreased channel capacity or increased depth – Scour • Riprap material had improper gradation • Material was placed improperly • No filter blanket was installed or blanket was inadequate or damaged References American Association of State Highway and Transportation Officials (AASHTO) (2003). “Geotextile Specifica- tion for Highway Construction,” AASHTO M 288, Washington, D.C. American Association of State Highway and Transportation Officials (AASHTO) (2003). “Standard Specifica- tions for Transportation Materials and Methods of Sampling and Testing,” Washington, D.C. American Society for Testing of Materials (ASTM) (2003). “Annual Book of ASTM Standards,” Volume 4.08, West Conshohocken, PA. American Society for Testing of Materials (ASTM) (2003). “Annual Book of ASTM Standards,” Volume 4.09, West Conshohocken, PA. ASTM International (2005). “Standard Guide for Selecting Test Methods for Experimental Evaluation of Geosynthetic Durability,” ASTM D 5819-05, West Conshohocken, PA. Bennett, J.P. (1997). “Resistance, Sediment Transport, and Bedform Geometry Relationships in Sand-Bed Chan- nels,” In: Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4–7. Brown, S.A., and Clyde, E.S. (1989). “Design of Riprap Revetment,” Hydraulic Engineering Circular No. 11 (HEC-11), Report FHWA-IP-89-016, Federal Highway Administration, Washington, D.C. Comité Européen de Normalisation (CEN) (2002). “European Standard for Armourstone,” Report prEN 13383-1, Technical Committee 154, Brussels, Belgium. Heibaum, M.H. (2004). “Geotechnical Filters – the Important Link in Scour Protection,” Federal Waterways Engineering and Research Institute, Karlsruhe, Germany, 2nd International Conference on Scour and Ero- sion, Singapore. Holtz, D.H., Christopher, B.A., and Berg, R.E. (1995). “Geosynthetic Design and Construction Guidelines,” Fed- eral Highway Administration, FHWA-HI-95-038, Washington, D.C. Karim, F. (1999). “Bed-Form Geometry in Sand-Bed Flows,” Journal of Hydraulic Engineering, Vol. 125, No. 12. Koerner, R.M. (1998). Designing with Geosynthetics, Fourth Edition, Prentice-Hall, Inc., Englewood Cliffs, NJ, 761 p. Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E. (2001). “Bridge Scour and Stream Instability Countermeasures,” Hydraulic Engineering Circular No. 23 (HEC-23), Second Edition, FHWA NHI -01- 003, Federal Highway Administration, Washington, D.C. Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Ruff, J.F. (2006). NCHRP Report 568: Riprap Design Crite- ria, Recommended Specifications, and Quality Control, Transportation Research Board of the National Acad- emies, Washington, D.C. Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Gerard, L.G. (2007). NCHRP Report 593: Countermeasures to Protect Bridge Piers from Scour, Transportation Research Board of the National Academies, Washington, D.C. Parker, G., Toro-Escobar, C., and Voight, R.L., Jr. (1998). “Countermeasures to Protect Bridge Piers from Scour,” Users Guide (revised 1999) and Final Report, NCHRP Project 24-7, prepared for National Coop- erative Highway Research Program, Transportation Research Board by St. Anthony Falls Hydraulic Labo- ratory, University of Minnesota, MN. Richardson, E.V., and Davis, S.R. (2001). “Evaluating Scour at Bridges,” Hydraulic Engineering Circular No. 18 (HEC-18), Fourth Edition, FHWA NHI 01-004, Federal Highway Administration, Washington, D.C. U.S. Army Corps of Engineers (1995). “Construction Quality Management,” Engineering Regulation No. 1180-1-6, Washington, D.C. C-24

U.S. Department of Transportation, Federal Highway Administration (2004). “National Bridge Inspection Stan- dards,” Federal Register, Volume 69, No. 239, 23 CFR Part 650, FHWA Docket No. FHWA-2001-8954, Final Rule, December 14, 2004, effective January 13, 2005, Washington, D.C. van Rijn, L.C. (1984). “Sediment Transport, Part III: Bed Forms and Alluvial Roughness,” Journal of Hydraulic Engineering, Vol. 110, No. 12. Wolman, M.G. (1954). “A Method of Sampling Coarse Bed Material,” American Geophysical Union, Transac- tions, 35, pp. 951–956. C-25