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

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NCHRP 24-25 Page 56 Phase II Final Report 5. SCOUR RISK MANAGEMENT GUIDELINES If we consider the fact that there are approximately 400,000 bridges over water, that over 60,000 of these have unknown foundations, and that research shows that – on average –approximately 80 fail due to scour every year, these facts strongly indicate that the strategies employed by bridge owners to prevent scour failure are working, even for bridges with unknown foundations. Several States have guidelines for managing bridges with unknown foundations. These guidelines are often not formally documented, but exist nonetheless as informal operating procedures. These guidelines benefit from information collected through literature searches, formal and informal surveys, and various interviews with experts across the country. Thus, States are encouraged to assess the effectiveness of their current guidelines to determine whether or not the guidelines included herein offer any benefits over their own. Given the infrequency of scour failure, many States might reasonably choose to stay the course with existing procedures. For those who do not have guidelines (formal or informal) in place, these guidelines should be selected or used to develop a pertinent management plan. A single bridge failure can have significant economic and political consequences, and these potential consequences should drive the implementation of reasonable management guidelines. The general flow of the guidelines to be presented in this section is illustrated in Figure 4. While the decisions in this figure apply to individual bridges, the schedule of work orders should correlate with the functional priority and/or estimated risk of failure of the pertinent bridges.

NCHRP 24-25 Page 57 Phase II Final Report No No No No Yes No Are scour countermeasures warranted? Are foundation reconnaissance and scour analysis warranted? Include risk of death in lifetime risk of failure calculation. Neglect risk of death in lifetime risk of failure calculation. Develop a bridge closure plan. 1. Install automated scour monitoring. 2. Develop a bridge closure plan. 1. Install countermeasures without field reconnaissance or standard scour analysis, or close or replace the bridge. 2. Consider developing a bridge closure plan. 3. Monitor scour during significant events. 1. Determine pier and footer depths (field reconnaissance). 2. Treat as a known foundation and perform standard scour analysis. 3. Consider scour countermeasures, bridge replacement, or bridge closure. 1. Treat as a known foundation and perform standard scour analysis. 2. Consider scour countermeasures, bridge replacement, or bridge closure. Is it a high priority structure? Does the bridge meet the minimum performance level? Is automated scour monitoring (ASM) warranted? Look for foundation records (e.g. pile driving, test pile, or material quantity records). Can the foundation be inferred? Is significant scour occurring? Yes Yes Yes Yes Yes No Yes No Calculate risk of failure. Monitor bridge scour. Yes No Was ASM warranted? Figure 4 Scour risk management guidelines flow chart

NCHRP 24-25 Page 58 Phase II Final Report 5.1. Can the Foundation Be Inferred? The introduction shows that there are over 3,700 bridges built in the past 10 years (i.e. 1995 – 2005) for which foundation information is not available. In fact, 69 principal arterials have been built between 2000 and 2005 for which foundation information is not available. Perhaps transportation agencies are not devoting enough effort toward finding these plan sets, especially those developed over the past decade. Every effort should be made to find construction records before going any further with these guidelines. These guidelines suffer from gross assumptions and significant uncertainties. Efforts expended to locate foundation information will be repaid with greater confidence in future management activities. Foundation information to be collected would include as-built plans that might include pile driving records, material-use records, and other pertinent footing or abutment records. The following summarizes the pertinent findings from a careful literature review and interviews (see Appendices B–C) regarding common assumptions for unknown foundations. „ Older structures (built before 1960) were usually built on timber piling. „ Depth of piles can be assumed as at least 10 feet for bridges with unknown foundations. „ If rock is near the surface, spread foundations can be assumed to support bridges with unknown foundations. „ The top of a typical spread footing can be assumed to be 3 feet below the top of the soil and the bottom 7 feet below the top of the soil. If foundation records are located, take the following steps: 1. Assume that the foundation information from any identified plan set is accurate and use this information to determine/estimate the necessary parameters for a scour evaluation. In other words, continue as if the foundation is known.

NCHRP 24-25 Page 59 Phase II Final Report 2. Evaluate scour using FHWA HEC-18 (2). 3. If scour analysis indicates that countermeasures are warranted, countermeasures should be designed using FHWA HEC-23 (8) – or consider replacing or closing the bridge. 5.2. Is the Bridge a High Priority Structure? High priority structures are bridges that are so important that every possible effort should be made to determine the foundation and protect it as necessary. In other words, the ramifications of failure are so devastating that investment is warranted even if a cost- benefit analysis doesn’t justify such action. Each State Transportation Agency can set its own definition for these high priority structures, with the following suggestion provided herein: „ Principal arterials „ Evacuation routes „ Bridges that provide access to local emergency services such as hospitals „ Bridges that are defined as critical by a local emergency plan (e.g., bridges that enable immediate emergency response to disasters) Principal arterials have importance beyond the simple measure of ADT. Oftentimes these are critical economic links that have national economic importance. On a regional level, principal arterials are the major (and in some rural cases, only) link between towns, cities, and other developed areas. Failure of a principal arterial will affect far more than just the traffic that normally travels across the bridge. As traffic is rerouted, the traffic that normally travels the minor arterials and collector roads may be caught in severe delays resulting from extreme overcapacity. Evacuation routes are suggested in this category since these routes are oftentimes the only practical means of evading natural disasters (e.g., hurricanes). The risk of injury

NCHRP 24-25 Page 60 Phase II Final Report and death – not from the bridge failure, but from the natural disaster - may be too great to bear if such a route is not available due to failure. For high priority structures, the following steps should be taken. 1. Perform field reconnaissance to determine foundation type and depth. If the foundation is a spread footing, drill through the footing to determine elevation of the footing bottom. If the foundation is piles, use foundation reconnaissance to determine depth of piles. The parallel seismic test is generally the most effective reconnaissance method. Assume that the foundation information from the field evaluation is accurate. If field reconnaissance is unsuccessful (no access for testing, poor signal from NDT, etc.), assume a foundation depth. For piles, assume a 10 foot depth or use local knowledge. This should be a conservative assumption. Spread footing depths are easily discovered and an assumption should not be necessary for this type of foundation. In other words, continue as if the foundation is known. 2. Evaluate scour using FHWA HEC-18 (2). 3. If scour analysis indicates that countermeasures are warranted, countermeasures should be designed using FHWA HEC-23 (8) – or consider replacing or closing the bridge. 5.3. Screening Bridges According to Risk For those bridges foundations not discovered through research or field evaluations (the previous steps), a screening analysis should be performed. The screening tool utilizes the annual probability of failure, which can be estimated from NBI items 26, 60, 61, and 71 using Tables 12–14.

NCHRP 24-25 Page 61 Phase II Final Report 5.3.1. Does the Bridge Meet Minimum Performance Level? The minimum performance level (MPL), as shown below in Table 27, is the probability of failure that a bridge with a certain functional classification (NBI item 26) must outperform. For example, an urban minor arterial must have an annual probability of failure less than 0.0002 to meet the MPL. This is based on the finding that bridges have an average annual probability of failure due to scour of approximately 0.0002 and this results in a total number of scour failures that is low (probably on the order of 100 bridges per year). Given this average target, the performance level is adjusted higher or lower depending upon roadway functional classification (see Appendix D). As clearly stated earlier in the report, the performance level does NOT correspond to a design standard. Design standards have many conservative assumptions and factors of safety that result in performance that is perhaps an order of magnitude (or more) more conservative than the design return period would indicate. Table 27 Minimum Performance Levels for Bridges NBI Item 26 Description Minimum Performance Level (Threshold Probability of Failure) Rural 01, 02 Principal Arterial – All 0.0001 06, 07 Minor Arterial or Major Collector 0.0005 08 Minor Collector 0.001 09 Local 0.002 Urban 11, 12, 14 Principal Arterial – All 0.0001 16 Minor Arterial 0.0002 17 Collector 0.0005 19 Local 0.002 First, compare the annual probability of failure (from Table 12) to the pertinent MPL in Table 27. If the annual probability of failure is greater than or equal to the MPL, the following steps should be taken. 1. Perform field reconnaissance to determine foundation type and depth. If the foundation is a spread footing, drill through the footing to determine elevation of

NCHRP 24-25 Page 62 Phase II Final Report the footing bottom. If the foundation is piles, use foundation reconnaissance to determine depth of piles. The parallel seismic test is generally the most effective reconnaissance method. Assume that the foundation information from the field evaluation is accurate. If field reconnaissance is unsuccessful (no access for testing, poor signal from NDT, etc.), assume a foundation depth. For piles, assume a 10 foot depth or use local knowledge. This should be a conservative assumption. Spread footing depths are easily discovered and an assumption should not be necessary for this type of foundation. In other words, continue as if the foundation is known. 2. Evaluate scour using FHWA HEC-18 (2). 3. If scour analysis indicates that countermeasures are warranted, countermeasures should be designed using FHWA HEC-23 (8) – or consider replacing or closing the bridge. If the MPL is met, compute K and the lifetime probability of failure (PL) using Equations 5 through 7, and continue. 5.3.2. Is Automated Scour Monitoring Warranted? The lifetime risk of death is the product of the adjusted lifetime probability of scour failure, the number of deaths, and the cost of each death (i.e. Rdeath = K*PL*X*C6). This cost should be compared to the cost of installing automated scour monitoring (ASM) since ASM will reduce the likelihood of death if failure occurs to a negligible level. With ASM, a bridge is constantly monitored for scour and can be closed if scour levels are deemed threatening to structural stability. The cost of installing ASM can be estimated from information reported in and references cited in Section 4.3, entitled “Scour Monitoring”. If the lifetime risk of death is greater than the cost of installing ASM, then ASM is provisionally recommended. However, if the next step in these guidelines recommends

NCHRP 24-25 Page 63 Phase II Final Report installing countermeasures, then ASM is probably not warranted. If ASM is warranted, then the lifetime risk of failure (PL) in the next step should be revised by subtracting from it the risk of death. If, on the other hand, the lifetime risk of death is less than the cost of automated scour monitoring, then biennial scour monitoring or countermeasures might be warranted instead of ASM. 5.3.3. Are Scour Countermeasure Warranted? Countermeasure costs can be estimated based on local experience (preferable) or information provided in the “Scour Countermeasures” subsection. Every effort should be made to use local information for estimating countermeasure cost and environmental permitting requirements should be considered since these requirements may dictate countermeasure selection and design. Use the “Lifetime Risk of Scour Failure” subsection to compute the lifetime risk of failure in accordance with the “Is Automated Scour Monitoring Warranted?” section. If the lifetime risk of failure is greater than the estimated cost of countermeasures, countermeasures are warranted (proceed to “Is Foundation Reconnaissance and Scour Analysis Warranted?”). If the lifetime risk of failure is less than the estimated cost of countermeasures, countermeasures are not warranted (proceed to ”Has Bed Elevation Significantly Lowered?”). 5.3.4. Is Foundation Reconnaissance and Scour Analysis Warranted? Typically, engineering costs represent approximately 10 to 20% of total project costs. If engineering costs are high relative to construction costs, a reasonable course of action might be to construct without detailed engineering. This is the course selected by Maryland State Highway Administration (MSHA) for small bridges (see Appendix C). They have found that scour analysis (and all the data collection associated with it) typically costs on the order of $50,000, while installing countermeasures might cost $10,000 for a small bridge. The MSHA decision to forego analysis in such a case is reasonable. Given the

NCHRP 24-25 Page 64 Phase II Final Report criticality of bridge structures and the potential for loss of life, analysis to illuminate proper countermeasure design is probably worth more than 20% of total cost. If the cost of foundation reconnaissance and scour analysis is less than 50% of the estimated cost of countermeasures, the following steps should be taken. 1. Perform field reconnaissance to determine foundation type and depth. If the foundation is a spread footing, drill through the footing to determine elevation of the footing bottom. If the foundation is piles, use foundation reconnaissance to determine depth of piles. The parallel seismic test is generally the most effective reconnaissance method. Assume that the foundation information from the field evaluation is accurate. If field reconnaissance is unsuccessful (no access for testing, poor signal from NDT, etc.), assume a foundation depth. For piles, assume a 10 foot depth or use local knowledge. This should be a conservative assumption. Spread footing depths are easily discovered and an assumption should not be necessary for this type of foundation. In other words, continue as if the foundation is known. 2. Evaluate scour using FHWA HEC-18 (2). 3. If scour analysis indicates that countermeasures are warranted, countermeasures should be designed using FHWA HEC-23 (8) – or consider replacing or closing the bridge. This test assumes that it is reasonable to spend up to 50% of countermeasure costs on field reconnaissance and scour analysis. This can be adjusted based on local willingness to accept the uncertainty involved with installing countermeasures without field reconnaissance and scour analysis.

NCHRP 24-25 Page 65 Phase II Final Report If the cost of foundation reconnaissance and scour analysis is greater than the estimated cost of countermeasures, then proceed to Section 5.4, entitled “Install Countermeasures without Field Reconnaissance and Scour Analysis”. 5.4. Install Countermeasures without Field Reconnaissance and Scour Analysis Use local experience to install grout bags, rip rap, or other countermeasures without detailed field reconnaissance of the foundation and scour analysis. Maryland State Highway Administration (MSHA; see Appendix C) often uses grout bags or riprap without detailed scour analysis. The grout bags used are usually class 3 grout bags that are 3 feet by 4 feet by one foot. A grout bag installation for a small two lane bridge might cost $10,000. This is inexpensive relative to surveying and modeling required to analyze scour, estimated at approximately $50,000 by MSHA. If countermeasures are installed without analysis, the uncertainty involved with the adequacy of the countermeasure warrants more rigorous monitoring than the standard 2-yr frequency. These bridges should be monitored during the first significant event (perhaps a rainfall of a few inches) to check on the stability of the installation during high flow conditions. Thereafter, it should be monitored during events that are more intense than those it has already withstood. For example, if the countermeasure has withstood a 5 year event and a 10-year event is predicted, then monitoring during the event is suggested. If the countermeasure has already withstood a 25 year event, then monitoring may not be warranted if a 10-year event is predicted. The bridge closure plan (see Section 5.5, entitled “Develop a Bridge Closure Plan”) should be followed to guide actions to be taken depending upon monitoring findings. If the bridge owner is not confident that a countermeasure can be designed for the site without doing field analysis, then the bridge owner should consider foundation

NCHRP 24-25 Page 66 Phase II Final Report reconnaissance and scour analysis warranted and follow the recommendations in the “Is Foundation Reconnaissance and Scour Analysis Warranted?” subsection. 5.5. Develop a Bridge Closure Plan If countermeasures are not installed or if countermeasures are installed without detailed surveys and analysis, then it is strongly recommended that the bridge owner develop a detailed closure plan to mitigate the risk of loss of life during and after scour- critical events. The Plans of Action for Scour Critical Bridges Office Manual published by the Idaho Transportation Department in 1994 (see Appendix B) has several examples of such plans, part of which are included in these guidelines. This document should be consulted for detailed guidance on developing and implementing a bridge closure plan. Each bridge closure plan should have two basic components: „ Closure Criteria: critical water surface elevation markers, critical scour depths, damage assessments, etc. „ Traffic Control Plans: detour routes, sign placement, public announcements, personnel lists, emergency contacts, etc. Due to the uncertainty regarding bridges with scour around an unknown foundation, it is acknowledged that it will be difficult to select critical water surface elevations for these bridges with any certainty. In these cases, the local bridge engineer should examine the inspection data and use their best judgment to set closure markers (e.g. easy-to-see lines on a pier or abutment) to indicate the maximum water level that they feel the bridge can safely endure. Note that California DOT officials recommend installing a remote stage sensor in lieu of just paint on the substructure. These sensors are fairly simple and reliable instruments, which can monitor numerous trigger elevations and do not require the physical presence of personnel until conditions warrant. If the closure water level is uncertain, then the local engineer should establish another marker to indicate when

NCHRP 24-25 Page 67 Phase II Final Report frequent scour measurements should be initiated. These markers or triggers should be reviewed and/or updated after each scour inspection. A bridge should be closed if the water surface elevation (WSEL) exceeds the designated closure marker or if scour measurements exceed a predetermined depth. A bridge should also be closed if any other evidence of bridge distress is noted. Evidence of bridge distress includes, but is not limited to: „ Bridge movement under load „ Joint deflection „ Bridge deck sagging „ Pressure flow conditions „ Excessive debris buildup „ Bridge or approach embankment overtopping „ High-velocity flow impinging directly on abutments or unarmored embankments „ Abutment armor failure Furthermore, if, at any time, monitoring personnel do not feel the bridge is safe or if they are uncomfortable working on the bridge due to flood conditions at the bridge, they should close the bridge to traffic and stay off of the structure until it has been inspected for stability. The bridge monitoring team should be given sufficient information, training, and equipment to perform scour monitoring, observe the WSEL marks, take measure-down readings to the WSEL with a weighted tape measure, use any other monitoring equipment, and perform an emergency closure of the bridge – if necessary. Each closure monitoring team should have an information card with necessary bridge data, detour route(s), emergency contact information for traffic enforcement and the district engineer, and closure instructions for each bridge.

NCHRP 24-25 Page 68 Phase II Final Report Closure instructions might include load restrictions, lane closures and total closure criteria. The method of closure should also be described (e.g. barricades, law enforcement officers detour routes, etc.). The method of closure should also consider the scour vulnerability of any bridges along the detour route(s). Instructions for re-opening the bridge or lanes should also be provided. The closure plan should clearly state the notification protocol when a bridge closure may be required. Bridge inspectors who detect a problem at a bridge need to know who to contact in order to initiate the decision to close or limit a bridge, and how to implement the closure plan. A different notification protocol may be needed for situations where emergency remediation is required but closure is not. 5.6. Is Significant Scour Occurring? If both automated scour monitoring (ASM) and scour countermeasures are not warranted, then scour monitoring should be performed with every 2-yr routine bridge inspection for all bridges with unknown foundations. However, if ASM was warranted then ASM should be used to monitor scour continuously. See Section 4.3, entitled “Scour Monitoring, for detail on monitoring options. If the scour depth increases significantly from baseline conditions (as-built drawings or initial scour survey), action should be taken. The first action is to follow the bridge closure plan (see Section 5.5, entitled “Develop a Bridge Closure Plan”) to take any necessary immediate action. Countermeasures should then be considered for this site (return to Section 5.3.4, entitled “Is Foundation Reconnaissance and Scour Analysis Warranted?”). The scour depth trigger elevation can be adjusted based on local geotechnical and engineering considerations and should represent the depth of scour that the bridge engineer thinks the individual bridge can withstand (i.e. based on experience and relevant event histories).