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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3 chapter one INTRODUCTION BACKGROUND AND OBJECTIVE Mechanically stabilized earth (MSE) walls were introduced in the United States about 40 years ago (see Elias et al. 2001). As the technology has improved and gained wider recogni- tion, the number of MSE walls designed and constructed has increased dramatically; however, the long-term performance of these structures depends on various factors, and unfortunately there have been instances of adverse performance. Like every important class of assets, MSE walls need periodic inspection and assessment of performance. To date, some states have established MSE wall monitoring programs, while several others are looking for guidance, tools, and funding to estab- lish their own monitoring programs. This synthesis project is undertaken to determine how state transportation agencies monitor, assess, and predict the long-term performance of MSE walls. This project provides information regarding current methodologies and procedures relating to the following topics: • Inspection and evaluation of the condition of existing MSE walls along the states’ highways; • Maintenance of design and construction information; • Recording and applying the results of inspections in each department’s centralized database; • Monitoring corrosion in MSE walls with inextensible steel reinforcement; • Monitoring degradation of geosynthetics; • Maintenance of internal and external drainage; • Assessment of wall performance and evaluation of the consequences of failure based on these inspection and monitoring programs; • Identification of preservation strategies that can reduce the likelihood of failure of MSE walls; • Assessment of the key causal factors that affect perfor- mance; and • Use of wall data to make programming decisions. It is anticipated that this information will lead to better design, construction, monitoring, and maintenance of these important structures. This project can benefit many state agen- cies by combining the lessons learned from experienced states with the experience and innovative practices of academicians, MSE wall designers, and contractors as presented in technical literature. For the purposes of this synthesis, the following defini- tions are used: • MSE wall: Retaining walls that rely on internal rein- forcement embedded in the backfill for stability. The reinforcement is attached to the wall’s face, which confines the backfill. The reinforcement can be either metallic (strips or meshes) or geosynthetic (fabrics or grids). Soil nail or anchor walls are not considered to be MSE walls for the purposes of this synthesis. • Panel MSE wall: Either one- or two-stage MSE walls that have concrete facing panels; internal soil reinforce- ment is usually metallic. • One-stage MSE wall: A MSE wall that uses a concrete panel attached to the internal reinforcement to retain the backfill. The panel is in direct contact with the backfill. • Two-stage MSE wall: A MSE wall that uses a metal- lic mesh or grid and geosynthetic liner attached to the internal reinforcement to retain the backfill. A concrete panel is subsequently attached to the vertical mesh. The panel is not in direct contact with the retained backfill. This wall type is typically used where settlements are expected to be relatively large. • Block MSE wall: A MSE wall that uses a modular block facing attached to the internal soil reinforcement (which is often geosynthetic), and is often referred to as a segmental block wall. The focus of this synthesis document is the long-term performance of MSE walls, where the term “long-term” nominally refers to the period of time from shortly after con- struction and acceptance of the MSE wall until the end of the design life, which is typically 75 or 100 years. The term “performance” is used in this report to refer to the behavior as well as the functionality and serviceability of a wall. Poor or adverse performance includes any performance that is less than that intended (e.g., serviceability limits are exceeded) and can structurally be manifest as small to large distortions, cracking, and even collapse. METHODS OF STUDY This synthesis project has gathered relevant information through (1) a literature review; (2) a survey of U.S. state and Canadian provincial transportation agencies, as well as other select entities (e.g., FHWA); and (3) interviews with select agencies. The scope of information collected addresses both permanent block and panel types of MSE walls, the latter of which consists of both one- and two-stage varieties. Both

4 extensible and inextensible internal wall reinforcements are also considered. Although the current body of literature contains many descriptions and references to the monitoring and assessment of MSE walls, much of that literature relates to conditions existing during and immediately after construction. For exam- ple, case histories are sometimes presented for particular MSE walls where foundation or geometric conditions are perceived as being particularly adverse or even unique and thus neces- sitating analytical and/or field studies to validate the adequacy of current design or construction processes (e.g., Reddy et al. 2003; Stuedlein et al. 2010). In other instances, MSE wall performance literature is simply the result of the “observa- tion method” (see Peck 1969) being applied and documented for ordinary wall conditions. One also finds case histories and/or forensic assessments of walls that failed (e.g., Bay et al., 2009; Koerner and Koerner 2009; Holtz 2010). Although indirectly related to long-term performance of MSE walls, the literature also contains construction/inspector manuals (e.g., Passe 2000) as well as guidance for the use and deploy- ment of instrumentation for assessing performance during and soon after construction (e.g., Koerner and Koerner 2011). In examining the literature, one also finds academic studies in which walls are monitored throughout the construction process and immediately thereafter (perhaps a year) with the goal of improving design techniques (e.g., Allen and Bathurst 2001). As stated previously, the focus of this synthesis document is the longer-term performance of MSE walls; hence, discussion of this previously referenced portion of literature is minimal. In addition to the literature review, U.S. state and Canadian provincial level transportation agencies were surveyed. The survey questionnaire is presented in Appendix A. The sur- vey was web-based and administered through the Internet. The questionnaire was designed to balance comprehensive- ness with conciseness to maximize benefit while minimiz- ing response effort, which is essential in achieving a high response rate. Thirty-nine of the 52 U.S. and five of the 12 Canadian targeted recipients responded; they are listed in Appendix B. Follow-up interviews with select agencies were undertaken to provide additional details and insights into survey responses. ORGANIZATION OF REPORT This report is organized into six chapters and four appen- dices. Chapter one presents the background and objectives of this synthesis project, explains the methods used, and out- lines the remainder of this document. Drawing on the results of the literature review, survey questionnaire, and select inter- views, chapter two describes the state of MSE wall inven- tory practice with particular emphasis on the nature, scope, and extent of existing inventories. Chapter three discusses the collection of MSE wall data, including the types of performance data collected and maintained in wall inven- tories and databases, the frequency of inventory activities, and aspects relating to reinforcement corrosion and degra- dation. Chapter four reviews how MSE wall performance data are assessed, interpreted, and used in asset management decisions. The chapter also discusses practices of estimating design life and risk assessment for MSE walls. Chapter five presents actions reported by transportation agencies and others to improve the long-term performance of MSE walls. This chapter also presents what survey respondents believe is their greatest lesson concerning long-term performance of MSE walls. Finally, in chapter six, a summary of the key findings of this synthesis project is presented, including the state of practice relative to the long-term performance of MSE walls. Other items presented include the direction of the states of practice, effective practices inferred from the literature review and survey respondents, and areas needing improvement and/or research. The appendices include a copy of the survey questionnaire, a list of survey respondents, and examples of existing methodology and tools developed and provided by agencies (e.g., inspection forms, rating or scor- ing worksheets, and assessment guidelines).