Assessing the Long-Term Performance of Mechanically Stabilized Earth Walls (2012)

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47 PROBLEM TITLE Prediction of Remaining Service Life for Mechanically Stabi- lized Earth (MSE) Walls RESEARCH PROBLEM STATEMENT There are an estimated 16.3 million square meters of various types of walls along the nation’s highways (DiMaggio 2008), with an average of 850,000 square meters of mechanically stabi- lized earth (MSE) wall with precast facing now being built each year in the United States at a cost of $160 to $650 per square meter (Elias et al. 2004; Berg et al. 2009). However, unlike bridges and pavements, MSE walls and retaining walls in general are often overlooked as assets. While the U.S. federal government has fostered the development of the National Bridge Inventory System (NBIS) that involves inspection of the nation’s bridges every two years, there is no existing, dedicated management system addressing the whole of the nation’s retaining walls, MSE or otherwise. The long-term performance of MSE walls depends on various factors, and unfortunately there have been instances of adverse performance. Like every important class of assets, MSE walls need periodic inspection, assessment, and management. To date, some states have established MSE wall monitoring programs, while several others are looking for guidance, tools, and funding to establish their own monitoring program (Gerber 2012). During the development of NCHRP Project 20-05, Syn- thesis Topic 42-05, Assessing the Long-Term Performance of Mechanically Stabilized Earth (MSE) Walls, it was determined that less than a quarter of state-level transportation agencies in the United States have developed some type of MSE wall inventory beyond that which may be captured as part of their bridge inventories (Gerber 2012). Fewer still have the meth- ods and means to populate their inventories with data from on- going inspections from which assessments of wall performance could be made. The synthesis project determined that in order to “move beyond current inventorying activities and the data baselines now being established, repeated observations and per- formance predictions will be needed, as will rational decision- making methodologies” (Gerber 2012). To make this leap in asset management practice, research relative to the following topics is needed: • “Improved ability to evaluate the integrity of existing MSE reinforcement systems using methods that are economi- cally and logistically effective. • Standards for performance data baselines and data collec- tion activities. • Predictive models for remaining MSE wall service life. • Methods of risk assessment specifically for MSE walls and more generally for various types of retaining walls.” LITERATURE SEARCH SUMMARY As part of NCHRP Project 20-07, Task 259, Brutus and Tauber (2009), concluded that there was/is no data or methods available in technical literature for the estimation of design/service life of existing retaining walls. Based on a survey of transportation agencies, a similar conclusion was reached by Gerber (2012)—no transportation agency currently has a well-established methodol- ogy for predicting future MSE wall performance or remaining design life. Certainly some agencies are monitoring corrosion in some walls (see Fishman and Withiam 2011), but a systematic procedure for determining remaining wall life with consideration of all other parameters believed to be important to performance (such as drainage) was not identified. Additionally, methods for risk assessment for MSE walls were found to largely be absent, although nascent efforts can be found in work reported by Bern- hardt et al. (2003), Bay et al. (2009), and DeMarco et al. (2010). Consequently both methods for design life prediction and risk assessment are needed. Also needed are well-developed tools for gathering wall performance data that will be needed as input and/or calibration parameters for such methods. Again, some efforts in the area are underway (see Fishman and Withiam 2011 regarding corrosion monitoring, Lostumbo and Artieres 2011 regarding in-situ stress monitoring of reinforcement), but greater progress is needed. Recent technological advances in structural health monitoring present promising avenues of research and progress in asset management. References Bay, J.A., L.R. Anderson, T.M. Gerber, and R.B. Maw, An Inspection, Assessment, and Database of UDOT MSE Walls, Report Number UT- 09.21, Utah Department of Transporta- tion, Salt Lake City, 2009. Berg, R.R., B.R. Christopher, and N.C. Samtani, Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Volume 1, Report FHWA-NHI-10-024, Federal High- way Administration, Washington, D.C., 2009, 306 pp. Bernhardt, K.L.S., J.E. Loehr, and D. Huaco, “Asset Manage- ment Framework for Geotechnical Infrastructure,” Journal of Infrastructure Systems, Vol. 9, No. 3, 2003, pp. 107–116. Brutus, O. and G. Tauber, Guide to Asset Management of Earth Retaining Structures, prepared as part of NCHRP Project 20-07, Task 259, Transportation Research Board of the National Academies, Washington, D.C., Oct. 2009, 120 pp. DeMarco, M.J., R.J. Barrows, and S. Lewis, “NPS Retaining Wall Inventory and Assessment Program (WIP): 3,500 Walls Later,” Proceeding of Earth Retention Conference 3, Bellevue, Wash., Aug. 1–4, 2010a, pp. 870–877. DiMaggio, J.A., “Geotechnical Engineering Assets and Liabilities on Surface Transportation Facilities,” presented at National Workshop on Highway Asset Management and Data Collec- tion, Durham, N.C., Sep. 25, 2008. APPENDIX D Research Problem Statement

48 Task 3: Apply the method in order to both calibrate and verify it against case histories and/or known performance data for par- ticular groups of MSE walls. It is recognized that a rigorous assessment of the method’s predictive ability by comparison with existing wall inventories will be limited by the availability of performance data as well as the ages of walls in our existing MSE wall inventories. Task 4: Publish and disseminate results. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD Recommended Funding: $XXX,XXX.XX Research Period: XX Months URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION MSE walls are being constructed at an ever-increasing rate. The oldest walls in the U.S. inventory are about 40 years old, and most walls have an intended design life of 75 to 100 years. However, the age-related performance of the technology has not been fully assessed, and more instances of adverse performance are expected with time. Some agencies are now gathering per- formance data, but predictive models for remaining MSE wall service life are needed so that appropriate management and main- tenance and/or replacement decisions can be made. The initial availability of predictive tools would assist agencies in determin- ing whether and/or how much to invest in MSE wall inventory and assessment systems. By facilitating broader participation and greater consistency in methods and practice, greater improve- ments in asset management and service-life predictive models will be realized. Without the initial investment represented by this development of a remaining service life model, needed prog- ress will continue to go unrealized. Elias, V., J. Welsh, J. Warren, R. Lukas, J.G. Collin, and R.R. Berg, Ground Improvement Methods, Participant Note- book, NHI Course 132034, FHWA NHI-04-001, National Highway Institute, Federal Highway Administration, Wash- ington, D.C., 2004, 1,022 pp. Fishman, K.L. and J.L.Withiam, NCHRP Report 675: LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems, Transportation Research Board of the National Academies, Washington, D.C., 2011, 116 pp. Gerber, T.M., Assessing the Long-term Performance of Mechan- ically Stabilized Earth (MSE) Walls, NCHRP Project 20-05, Synthesis Topic 42-05, Transportation Research Board of the National Academies, Washington, D.C., 2012. Lostumbo, J.M. and O. Artieres, “Geosynthetic Enabled with Fiber Optic Sensors for MSE Bridge Abutment Supporting Shallow Bridge Foundations,” Proceedings of GeoFrontiers 2011, Dallas, Tex., Mar. 13–16, 2011, pp. 3497–3504. RESEARCH OBJECTIVE The primary objective of this research effort is to establish a meth- odology for predicting the remaining service life of MSE walls. To meet this objective, the following tasks are proposed. Task 1: Review literature for information regarding methods for predicting service life of engineered structures other than retaining walls (such as pavements and bridges). From this review, identify key parameters and/or approach concepts that can be applied to MSE walls. Also part of this task will be the collection of case history data for subsequent calibration and verification activities. Task 2: Develop an initial methodology based on the results of Task 1. While corrosion rate is anticipated to play a major role in the method, other parameters such as drainage are also anticipated to be important. It is anticipated that the method will tie wall features and performance observations to par- ticular distress mechanisms. Because of this, particular con- sideration will be given to the nature and robustness of the analytical model’s input parameters. The parameters selected for the model will influence future standards for MSE wall performance data baselines and data collection activities.