Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007)

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NCHRP 24-25 Page 43 Phase II Final Report 4. MITIGATING ACTIONS FOR SCOUR This section summarizes recent findings from States that have experience in mitigating scour for bridges with unknown foundations, and the three basic types of mitigating actions for scour: 1. Perform foundation reconnaissance. 2. Install automated scour monitoring. 3. Install scour countermeasures. 4.1. Pertinent Findings from Experience There are several options for mitigating the vulnerability of bridges with unknown foundations against sediment scour. One option is to monitor the scour and to close the bridge when the scour reaches some critical value. Since the penetration depth of the foundation is unknown it is, however, difficult to determine the “critical scour depth”. If the bridge has been in existence for a number of years and has experienced high velocity flows during its life, Webb et al. (7) show that it may be possible to measure the maximum scour depth experienced by the piers with the use of high frequency sonar or ground penetrating radar. Both of these techniques yield pictures of the sub-bottom with shaded lines at layers where there is a change in soil density. Relic scour holes can often be detected using these techniques and the depths quantified. If the structure did not experience any damage during the event that created the scour then it is safe to use this depth as a critical value for closing the bridge. Another way of dealing with the unknown foundation problem is to armor the bed where erosion and scour are anticipated. The FHWA HEC-23 manual (8) describes current countermeasures available for scour critical bridges and bridges with unknown foundations. In the FHWA HEC-23 manual scour countermeasures are divided into hydraulic,

NCHRP 24-25 Page 44 Phase II Final Report structural, and monitoring. The latest scour monitoring techniques and associated costs are described as well as the cost of armoring of the bed with riprap and a manmade product. The following summarizes the pertinent findings from a careful literature review and interviews (see Appendices B–C) regarding mitigating actions for scour. „ Significant investments are usually not made on bridges which need to be replaced within 5 years. „ Routine/Regular monitoring of bridges takes place once every two years. „ The most common countermeasures adopted for scour problems at bridges with unknown foundations include installing riprap or grout bags. 4.2. Foundation Reconnaissance Foundation reconnaissance will hereafter refer to using non-destructive methods to estimate unknown properties or dimensions of a bridge’s foundation. It is important to note that the methods summarized in this report provide a brief overview of the current state-of- the-art technology. Since some or all of this information will ultimately become obsolete, it is worth mentioning that the Central Federal Lands Highway Division web site (9) currently has a report on geophysical methods for determining bridge substructure, which they are likely to keep up-to-date. Foundation reconnaissance focuses on investigating buried man-made structures, but this is only a subset of the broader field of non-destructive testing (or evaluation) and geophysical methods. One important consideration in selecting an appropriate method for investigating a bridge’s foundation is to catalogue what is already known, and what can be inferred from design plans, material lists, and pertinent historical practice. Interviews with officials (see Appendix C) show that some States, like New Jersey, have inferred the pier or footing depth for most of their bridges with unknown foundations using inexpensive probing and soil cores. Soil cores and probing may yield a conservative estimate for the

NCHRP 24-25 Page 45 Phase II Final Report minimum depth of a pier or footing. For example, it may be known that historic practice entailed installing piers down to a specified depth below a certain bedrock (or fill) layer. Other bridge foundations are harder to infer because of uncertainties regarding the geologic setting of the bridge or the construction practice. Reducing the uncertainty, in this case, will entail using other geophysical methods, or what is generally termed non- destructive testing (or evaluation). The National Cooperative Highway Research Program (NCHRP) 21-5 project “Determination of Unknown Subsurface Bridge Foundations” (10) and the NCHRP 21-5(2) project “Unknown Subsurface Bridge Foundation Testing" (11) were performed to evaluate and develop existing and new technologies that can determine unknown subsurface bridge foundation depths. The NCHRP 21-5 Phase I research focused on the identification of potential NDE methods for determining depths of unknown bridge foundations at 7 bridges in Colorado, Texas and Alabama. The NCHRP 21-5 (2) Phase II research focused on evaluating the validity and accuracy of the identified NDE methods for determining depths of unknown bridge foundations. In this phase, 21 bridge sites were studied in North Carolina, Minnesota, New Jersey, Michigan, Oregon, Massachusetts and Colorado. Phase II research also involved the development of hardware and software needed to perform the NDE testing. A more detailed summary of the methods described in these NCHRP reports is given in Appendix E. This research generally showed that the borehole-based Parallel Seismic method was both the most accurate and most applicable NDE method for the determination of the depth of unknown bridge foundations that was considered. This suggests that it would be valuable to initially perform at least one Parallel Seismic test for each bridge to check the accuracy of depth predictions from any other less costly surface methods that may also be applicable for a given foundation type of the bridge being tested. Ultraseismic or

NCHRP 24-25 Page 46 Phase II Final Report other surface methods that are subsequently proven to be accurate based on a comparison with the Parallel Seismic results may then be used with greater confidence to evaluate unknown foundation depths of other abutments and/or piers on a bridge. It should be noted that as local experience is gained with the use of any of the borehole or surface NDE methods for typical local bridge substructure types and subsurface conditions, the accuracy and applicability of the methods will become much better known to DOT engineers. This local knowledge can then be used to further optimize the selection of NDE methods from technical and cost perspectives. Knowledge of unknown foundation bridge substructure will range from knowing only what is visible to having design drawings and subsurface geology information without as-built plans. Table 15 shows the ranges of effectiveness of the various methods available for nondestructive evaluation of bridge foundations in the NCHRP study.

NCHRP 24-25 Page 47 Phase II Final Report Table 15 Effectiveness of NDT Methods Ability to Identify Foundation Parameters Sonic Echo (SE)/Impulse Response (IR) Test (Compressional Echo) Bending Wave (BW) Test (Flexural Echo) Ultraseismic (US) Test (Compression al and Flexural Echo) Spectral Analysis of Surface Wave (SASW) Test Surface Ground Penetrating Radar (GPR) Test Parallel Seismic (PS) Test Borehole Radar (BHR) Test Induction Field (IF) Test Foundation Parameters Depth of Exposed Piles Fair to Good Poor to Good Fair to Excellent Good to Excellent Poor to Excellent None to Excellent Depth of Footing/Cap Poor to Good Poor to Fair Fair to Excellent Fair to Good Poor Good Poor to Good Piles Exist Under Cap? Fair to Poor Good Fair to Good None to Excellent Depth of Pile below Cap? Poor Good to Excellent Fair to Good Geometry of Substructure Fair Poor to Good Poor to Good Fair Fair to Excellent Poor to Fair Material Identification Good Poor to Fair Poor to Fair Poor to Fair Access Requirements Bridge Substructure Yes Yes Yes Yes Yes Yes No Yes Borehole No No No No No Yes Yes Yes Subsurface Complications Low to High Medium to High Low to High Low High Medium High Medium to High Operational Cost $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 $2,000 to $2,500 Equipment Cost $10,000 to $20,000 $15,000 to $20,000 $20,000 $20,000 >$30,000 $15,000 to $25,000 >$35,000 $20,000 Required Expertise Field Acquisition Technician Technician Technician Technician- Engineer Technician- Engineer Technician- Engineer Engineer Engineer Data Analysis Engineer Engineer Engineer Engineer Engineer Engineer Engineer Engineer

NCHRP 24-25 Page 48 Phase II Final Report Ability to Identify Foundation Parameters Sonic Echo (SE) / Impulse Response (IR) Test (Compressional Echo) Bending Wave (BW) Test (Flexural Echo) Ultraseismic (US) Test (Compression al and Flexural Echo) Spectral Analysis of Surface Wave (SASW) Test Surface Ground Penetrating Radar (GPR) Test Parallel Seismic (PS) Test Borehole Radar (BHR) Test Induction Field (IF) Test Limitations Most useful for columnar or tabular structures. Response complicated by bridge superstructure elements. Stiff soils and rock limit penetration. Only useful for purely columnar substructur e, softer soils, and shorter piles. Response complicated by various bridge superstruct ure elements, and stiff soils may show only depth to stiff soil layer. Cannot image piles below cap. Difficult to obtain foundation bottom reflections in stiff soils. Cannot image piles below cap. Use restricted to bridges with flat, longer access for testing. Signal quality is highly controlled by environmental factors. Adjacent substructure reflections complicate data analysis. Higher cost equipment. Difficult to transmit large amount of seismic energy from pile caps to smaller (area) piles. Radar response is highly site dependent (very limited response in conductive, clayey, salt- water saturated soils). It requires the reinforcement in the columns to be electrically connected to the piles underneath the footing. Only applicable to steel or reinforced substructure. Advantages Lower cost equipment and inexpensive testing. Data interpretation for pile foundations may be able to be automated using neural network. Theoretical modeling should be used to plan field tests. Lower cost equipment and inexpensive testing. Theoretical modeling should be used to plan field tests. The horizontal impacts are easy to apply. Lower equipment and testing costs. Can identify the bottom depth of foundation inexpensively for a large class of bridges. Combines compressional and flexural wave reflection tests for complex substructures. Lower equipment and testing costs. Also shows variation of bridge material and subsurface velocities (stiffnesses) vs. depth and thicknesses of accessible elements. Fast testing times. Can indicate geometry of accessible elements and bedrock depths. Lower testing costs. Lower equipment and testing costs. Can detect foundation depths for largest class of bridges and subsurface conditions. Commercial testing equipment is now becoming available for this purpose. Relatively easy to identify reflections from the foundation; however, imaging requires careful processing. Low equipment costs and easy to test. Could work well to complement PS tests and help determine pile type.

NCHRP 24-25 Page 49 Phase II Final Report 4.3. Scour Monitoring Scour monitoring provides early identification of potential scour problems to reduce the potential for bridge failure. The FHWA HEC-23 manual (8) identifies three types of scour monitoring: fixed instrumentation, portable instrumentation, visual monitoring, and geophysical instrumentation. Fixed instrumentation continuously monitors scour from a secured location on the bridge structure. As such, multiple sensors are required to monitor multiple piers. These instruments connect to a data logger which can be configured to communicate remotely through telemetry. Table 16 lists the currently employed fixed instrumentation with their capabilities and limitations. Table 16 Fixed Scour Monitoring Methods Suitable River Environments Method Velocity* Bed Material† Ice/Debris Load‡ Estimated Allocation of Maintenance Resources‡ Installation Experience by State Fixed Instrumentation Sonar All All L M CO, FL, IN, NY, VA TX Magnetic Sliding Collar All S, F All M CO, FL, IN, MI, MN, NM, NY, TX Float Out Device All S, F All L AZ, CA, NV Sounding Rods M, S C M, L H AR, IA, NY Portable Instrumentation Physical Probes M, S All M, L L Widely Used Sonar Probes M, S All L L Widely Used Geophysical Instrumentation Reflection Seismic Profiles All All M H Special Circumstances Ground Penetrating Radar All S,F M, L H Special Circumstances * F=Fast; M = Moderate; S = Slow. † C = Coarse; S = Sand; F= Fine. ‡ H = High; M = Moderate; L = Low. Of the devices listed in the table, sonar and magnetic sliding collars have shown the most promise during deployments, according to many studies (8, 12–14). Prices for these two instruments are similar — the sonar cost approximately $4,000 and the magnetic sliding collar is also approximately $4,000, according to the FHWA HEC-23 manual. These

NCHRP 24-25 Page 50 Phase II Final Report costs include the basic instrument mounting hardware, power supply data logger, and instrument shelter/enclosure. Adding a cell-phone based telemetry link to the system adds approximately $3,000 to the cost. Installation costs for these instruments are dependent on the complexity of the situation. These complexities include bridge deck height, foundation geometry, and the bridge deck overhang distance. The FHWA HEC-23 manual reports the level of effort required for installation of an instrument system typically exceeds 5 person days. Fixed instrumentation is not feasible for all bridges. For example, the number of piers may deem placing fixed instruments at each bridge cost prohibitive. Under such conditions, portable instrumentation — capable of monitoring multiple piers and bridges — presents a cost-effective solution. Portable instruments provide flexibility to quickly respond to flood conditions at multiple bridges. The previous table lists the currently employed portable instrumentation with their capabilities and limitations based on the FHWA HEC-23 manual. The physical and sonar probes are widely used with methods that range from a simple lead lines for physical probes to 75,000 sonar probes deployed from a truck mounted articulated crane, according to Schall and Price (15). Geophysical instrumentation is a portable instrument that provides a forensic tool to evaluate scour conditions experienced during previous floods, according to Webb et al. (7). The two commonly used instruments are the reflection seismic profilers and ground penetrating radar. Both instruments provide detailed images of sub-bottom profiles for identifying/mapping in-filled scour holes. This equipment is expensive and requires specialized training to operate and interpret the data. Table 17 summarizes the advantages and limitations of the instrumentation presented above, according to the FHWA HEC-23 manual. In general, fixed instrumentation is used when continuous monitoring is required, portable instruments are

NCHRP 24-25 Page 51 Phase II Final Report used when a greater area coverage is required (multiple bridges and multiple piers), and geophysical instruments are used as a forensic tool. Table 17 Comparison of Instrument Types Instrument Category Advantage Limitation Fixed Continuous monitoring, low operational cost, ease of use May miss maximum scour, maintenance of equipment Portable Complete mapping, use at multiple bridges Labor intensive Geophysical Forensic investigations Labor intensive, specialized training Tables 18 through 20 provide a summary of the fixed and portable instrumentation, and an estimate of the cost of the instruments. Table 18 Comparison of Fixed Instrumentation Instrument Best Application Advantages Disadvantages Sonar Coastal regions Time history, built with off the shelf components Debris, high sediment or air entrainment Sounding Rod Coarse -bed channels Simple, mechanical device Unsupported length, binding, auguring Magnetic Sliding Collar Fine-bed channels Simple, mechanical device Unsupported length, binding, debris Float Out Ephemeral channels Lower cost, ease of installation Battery life

NCHRP 24-25 Page 52 Phase II Final Report Table 19 Comparison of Portable Instrumentation Instrument Best Application Advantages Disadvantages Physical Probes Small bridges and channels Simple technology Accuracy, high flow application Sonar Larger bridges and channels Point data or complete mapping, accurate High flow application Reflection Seismic Profilers Larger bridges and channels and coastal environments Accurate map of the bottom and sub- bottom in water depths on the order of hundreds of feet Expensive, must be submerged, data contamination by bridge piers etc. Ground Penetrating Radar Small to medium bridges and in freshwater channels Accurate map of the bottom and sub- bottom on sand bars and to water depths on the order of 30 feet, samples under good conditions Expensive, post processing, water depth exceed 30 ft, saline waters, clay Table 20 Estimated Instrument Cost Instrument Instrument Cost Cost for Installation or Use Operation Cost Physical Probes <$2,000 Varies by use Varies, minimum 2- person crew for safety Portable Sonar $500 (fish finder) - $75,000 (sonar on truck mounted articulated crane) Varies by use Varies, minimum 2- person crew for safety Fixed Sonar $5,000 - $15,000 Minimum 5-person days Typically <$1/hr per site visit Sounding Rod $7,500 - $10,000 Minimum 5-person days Typically <$1/hr per site visit Magnetic Sliding Collar $5,000 - $10,000 Minimum 5-person days Typically <$1/hr per site visit Float Out $3,000 + $500/float out Varies with number installed Typically <$1/hr per site visit Ground Penetrating Radar $15,000 - $50,000 Varies by site conditions Contractors costs range from $1,000 to $2,000 per day Reflection Seismic Profilers >$20,000 Dependent on required survey vessel Dependent on vessel costs

NCHRP 24-25 Page 53 Phase II Final Report 4.4. Scour Countermeasures Guidelines developed for management of bridges with unknown foundations will undoubtedly include some protocol to implement countermeasures against scour. There are a number of ways to armor a bed to minimize or prevent scour. Of the various materials that can be used, broken stone or riprap is the most common. In recent years, however, several manmade systems have been developed that are cost effective for many situations. The cost associated with any system is very site/location-specific. Some parts of the country have an abundant supply of dense stone while others have little or no stone. Transportation costs are expensive and it is in locations with little or no natural stone that locally manufactured products are most practical. For cost comparison purposes a particular rip- rap gradation and median diameter specifications have been selected. These specifications, which are used by the State of Florida for erosion mitigation, are presented in Tables 21 and 22. Also, since there is a significant difference in costs for different locations around the United States, average costs are given for three locations, Florida (Table 23), New York State (Table 24), and Colorado (Table 25). The costs are divided into: 1. Material cost at the source, 2. Cost per unit surface area (including filter material and bedding stone), 3. Transportation cost per mile from the source to the site, and 4. Installation cost. It should be noted that installation costs can vary significantly from one situation to the next (distance of barges and cranes from site, water depths, bridge heights, presence of environmentally sensitive flora and fauna, etc.). For local pier scour protection the FHWA HEC-23 recommends that the armor coverage extend horizontally at least two times the pier width, measured from the pier face.

NCHRP 24-25 Page 54 Phase II Final Report Table 21 Stone Riprap weights Weight Maximum a kg b [lbs] Weight 50% c kg [lbs] Weight Minimum d kg [lbs] Minimum Blanket Thickness m [ft] 320 [700] 135 [300] 25 [60] 0.75 [2.5] a Ensure that at least 97% of the material by weight is smaller than weight maximum. b Bulk specific gravity not less than 2.3. c Ensure that at least 50% of the material by weight is greater than weight 50%. d Ensure that at least 85% of the material by weight is greater than weight minimum. Table 22 Bedding stone sizes Standard Sieve Sizes (inches) (mm) Individual Percentage by Weight Passing 12 305 100 10 254 70 to 100 6 152 60 to 80 3 75 30 to 50 1 25 0 to 15 Note: Minimum blanket thickness of 1 ft and bulk specific gravity of not less than 2.3 Table 23 Material costs (Florida) Item Material a ($/ton) Material b ($/m2) Transportation c ($/m2) Installation ($/m2) Rip-Rap d 16.50 34 26 91 Cabled Block e 140.30 - 180.40 48 - 57 2.70 - 8.00 21.50 - 43.00 a Metric ton = 1.102 short tons = 2204.6 lbs. b Costs include filter material and bedding stone. c Based on 440 miles haul (distance from Atlanta, Ga. To Orlando, FL). d Rip-rap specifications shown in Tables 21 and 22. e Cost information based on one manufacturer’s estimates. Table 24 Material costs (New York State) Item Material a ($/ton) Material b ($/m2) Transportation c ($/m2) Installation ($/m2) Rip-Rap d 9.90 22 10 59 Cabled Block e 140.30 – 180.40 48 - 57 2.70 - 8.00 21.50 - 43.00 a Metric ton = 1.102 short tons = 2204.6 lbs. b Costs include filter material and bedding stone. c Based on 40 mile haul distance, as documented by Kuennen (16). d Rip-rap specifications shown in Tables 21 and 22. e Cost information based on one manufacturer’s estimates. Table 25 Material costs (Colorado) Item Material a ($/ton) Material b ($/m2) Transportation c ($/m2) Installation ($/m2) Rip-Rap d 9.90 21 10 70 Cabled Block e 140.30 - 180.40 48 - 57 2.70 - 8.00 21.50 - 43.00 a Metric ton = 1.102 short tons = 2204.6 lbs. b Costs include filter material and bedding stone. c Based on 40 mile haul distance, as documented by Kuennen (16). d Rip-rap specifications shown in Tables 21 and 22. e Cost information based on one manufacturer’s estimates. The spatial extent of armoring for a given pier depends on the size of the pier. The FHWA HEC-18 manual (2) recommends that pier scour protection extend two pier widths

NCHRP 24-25 Page 55 Phase II Final Report out from the pier in all directions. Based on the costs presented in Table 23, the costs of rip- rap and cabled block for various pier widths and lengths are presented in Table 26. Table 26 Average Total Armor Costs per Pier (Florida) Pier width/length ratio (m) Material (2/6) (3/8) (4/10) (5/15) Rip-Rap 19,328 41,676 72,480 120,800 Cabled Block 9,242 - 13,830 19,927 - 29,822 34,656 - 51,864 57,760 - 86,440 The local scour protections outlined in this document are based on the assumption that the effective bed shear stress near a pier is twice that on a flat bed upstream of the pier. The cost estimates presented here represent averages as indicated and are only valid for this point in time. Local conditions and circumstances can and will alter these values.