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Mechanically stabilized earth (MSE) walls are an important class of infrastructure assets whose long-term performance depends on various factors. As with most all other classes of assets, MSE walls need periodic inspection and assessment of performance. To date, some agencies have established MSE wall monitoring programs, whereas others are looking for guidance, tools, and funding to establish their own monitoring programs. The objective of this synthesis project is to determine how transportation agencies monitor, assess, and predict the long-term performance of MSE walls. The information used to develop this synthesis came from a literature review together with a survey and interviews. Of the 52 U.S. and 12 Canadian targeted survey recipients, 39 and five, respectively, responded. This synthesis reveals that unlike bridges and pavements, MSE walls and retaining walls in general are often overlooked as assets. Fewer than one-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. Fewer still have the methods and means to populate their inventories with data from ongoing inspections from which assess- ments of wall performance can be made. In the United States, there is no widely used, consistently applied system for managing MSE walls. Wall inventory and monitoring practices vary between agencies. This synthesis examines existing practices concerning the nature, scope, and extent of existing MSE wall inventories. It also examines the collection of MSE wall data, including the types of perfor- mance data collected, how they are maintained in wall inventories and databases, the fre- quency of inventory activities, and assessment practices relevant to reinforcement corrosion and degradation. Later parts of this synthesis discuss how MSE wall performance data are assessed, interpreted, and used in asset management decisions. This synthesis finds that the most well-implemented wall inventory and assessment sys- tem in the United States is the Wall Inventory Program developed by FHWA for the National Park Service. However, this system, like some others, uses âcondition narrativesâ in a process that can be somewhat cumbersome and subjective. Other systems use more direct numeric scales to describe wall conditions, and an advantage of such systems is that they are often compatible with those used in assessments of bridges. As experience with MSE walls accumulates, agencies will likely continue to develop, refine, and better calibrate procedures affecting design, construction, condition assessment, and asset management decisions. One portion of this synthesis is dedicated to summarizing the actions taken thus far by survey respondents to improve the long-term performance of their MSE walls. Many agencies prescribe the use of a pre-approved wall design and/or wall supplier. Other actions or policies frequently focus on drainage-related issues. Also included as part of this synthesis are statements from survey respondents as to what the most important lesson learned by their agency has been. Although the scope of the SUMMARY ASSESSING THE LONG-TERM PERFORMANCE OF MECHANICALLY STABILIZED EARTH WALLS
2 responses is broad, certain topics appear more frequently than others, with the four most fre- quent being (in order of decreasing frequency) drainage, construction, backfill, and modular block issues. In examining various reported practices for inventorying and assessing the performance of MSE walls, those appearing to be more effective are: (1) use of inventory and assessment systems with features that are simple to use and as objective as possible; (2) use of rating cri- teria that are specific to particular wall elements and/or conditions; (3) use of numeric rating scales that correspond to other scales already in use for other asset classes such as bridges; and (4) the incorporation of MSE wall inventory and assessment systems into systems for other asset classes. An important conclusion of this synthesis is that there exists a need for greater recogni- tion of MSE walls (and retaining walls in general) as important infrastructure assets. In the same vein, a greater number of agencies need to be actively involved in MSE wall inventory and assessment activities, and for greatest benefit there should be greater consistency across agencies relative to the way that these activities are performed. The synthesis also finds that performance assessment methodologies need to be more fully developed; similarly, service life prediction and risk assessment methodologies need to be developed. To realize such goals, it appears that greater funding and allocation of other resources is needed. In follow-up discussions regarding the synthesis survey, multiple participants expressed a hope that such increased awareness and resource allocation can be realized without significant, adverse- performance events such as those that led to the legislative creation and ongoing funding of the nationâs bridge inspection and assessment programs.
