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

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24 As MSE wall performance is monitored, assessments can be made regarding the adequacy of the wall’s design, construc- tion, and maintenance. These assessments can in turn be used to change practices and policies with the intent of improving wall performance, particularly for future walls. The feedback loop thus established becomes a means of continual improve- ment. One example of this process is the development and recent release of NCHRP Report 675, LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems (Fishman and Withiam 2011), in which the accumu- lation of reinforcement corrosion data over time has led to the development of more accurate metal loss models. This chap- ter discusses actions taken by survey respondents to improve the long-term performance of their MSE walls. These actions reflect lessons learned relative to design, construction, and maintenance. Ideally, these actions will lead to a decreased likelihood of failure or adverse performance of MSE walls in the long term. POLICIES AND PRACTICES DEVELOPED TO IMPROVE PERFORMANCE OF MECHANICALLY STABILIZED EARTH WALLS Survey participants were asked to respond regarding any approaches, besides monitoring, that their agency may have developed or implemented to improve the long-term perfor- mance of their MSE walls. Specific responses were sought relative to the following categories: • Regularly scheduled cleanout/maintenance of catch basins • Different requirements for backfill immediately behind wall face as compared with remainder of reinforced backfill • Developed special drainage details at ends of MSE walls • Developed special drainage details behind MSE walls • Specified vertical and horizontal distances for discharge points and water sources • Increased wall embedment • Other design specifications • Contractor/installer qualifications • Construction inspection • Post-construction inspection • Other. Typically, fewer than half of survey respondents provided feedback in any one category. The responses provided are generally summarized in the following paragraphs. With respect to regularly scheduled cleanout and mainte- nance of catch basins, respondents reported no special actions being taken in this regard. The responses offered suggest that performance of this activity varies significantly between agen- cies, ranging from its being “done as a matter of course,” and being done routinely, to “hit and miss if they actually do it.” With respect to different requirements for backfill imme- diately behind wall face as compared with remainder of rein- forced backfill, seven agencies specifically specified use of open-graded, free-draining aggregate or rock immediately behind the wall face. With respect to developing special drainage details at ends of MSE walls, agency improvements included turn- ing the wall ends into the slope, concrete headwalls being used (presumably at culvert openings), “plating all drainage surfaces above and around wall; insuring drainage does not enter and saturate reinforced backfill,” and use of water- proofing membranes together with weep drains and dedicated drainage collection systems. In the related query regarding specification of vertical and horizontal distances for dis- charge points and water sources, one agency reports using 100-ft intervals and another emphasized assuring that drainage below and above wall is on concrete inverts and concrete aprons. With respect to developing special drainage details behind MSE walls, multiple respondents indicated they require some type of underdrain located at the wall face and/or in back of the reinforced soil zone. One respondent emphasized that non-frost-susceptible aggregate and drain pipes should be extended to a depth below frost penetration. Other practices include using a drain gutter, lined swale, or concrete plating at the top of the wall. Most responses referred to needs for direct water away from the wall and to a lower elevation. A couple of respondents indicated that they had added weep drains and/or strip drains at the wall–soil interface rather than relying on drainage through panel or block joints. Texas reports that it has developed an inlet standard to “best accom- modate inlets . . . and also convey the water out of the wall in the quickest fashion.” With respect to increased wall embedment, most partici- pants who provided a response in this category indicated that their practice involves embedding the wall foundation below the frost line or at least some minimum depth (the value of chapter five OUTCOMES AND LESSONS LEARNED

25 now requiring production testing of MSE backfill stockpiles on-site rather than just at the material source. With respect to post-construction inspection, no new devel- opments were reported beyond a few agencies that now make a complete inspection of the wall at the end of construction routine. One responding agency indicated having a three-year warranty period for its MSE walls. Two agencies (Kansas and New York) reported that they retain construction quality control/quality assurance data and point out that retaining such data has the potential to diagnose future problems if they arise. While not being a practice unique to agencies responding to this survey, use of an impervious membrane above the entire reinforced soil mass to prevent the migration of aggressive materials (such as salts used to de-ice the overlying pavement) was cited by several respondents as a means of protecting the reinforcement from corrosion/degradation. MOST IMPORTANT “LESSON LEARNED” As part of the survey conducted for this synthesis project, recipients were asked to give their opinion as to what is the most significant lesson learned by their agency with respect to the long-term performance of MSE walls. Responses varied from design and backfill specification to construction practices and post-construction drainage maintenance. Given the poten- tial significance of these responses—being the most important thing(s) learned—all responses are presented in Appendix C in their entirety. Although the scope of the responses was broad, certain topics appeared more frequently than others. The four most frequent topics (in order of decreasing frequency) mentioned were drainage, construction, backfill, and modular block issues. Approximately one-fourth of respondents indicated that the most important lesson learned by their agency was drain- age-related—as two respondents put it: “Drainage; drainage; drainage,” and “W-a-t-e-r: from any and all directions and sources.” Although these particular responses lack specific- ity, it is readily apparent that the two respondents believe that drainage is essential to the successful performance of MSE walls. Another respondent suggested that the most impor- tant lesson was “providing a sound and firm foundation for support of the wall; and providing proper drainage within the wall system and adjacent to the wall geometry.” Approximately one-fifth of respondents reported that the most important lesson they learned was construction-related. One pointed out that “the systems can last forever but must be designed and built correctly.” Similarly, another noted, “For the most part [my agency] has had very few problems with MSE structures. We do know that great care must be taken in constructing these structures. If you start wrong in the beginning you’ll always be seeing problems in the walls.” which is most frequently 0.6 m, but appears to range up to 1.2 and 1.5 m in northern states such as Minnesota and New Hampshire). Some agencies reported using increased embed- ment for walls founded on slope, with minimum depths con- forming to AASHTO design specifications or as needed to satisfy global stability requirements. Although not reported in the survey, recent inspection of Utah DOT MSE walls indicates that the 1.2-m-wide horizontal bench required by AASHTO to be placed at the base of MSE walls founded on slopes is frequently absent. A proposed alternative to the bench suggested that embedment depth be increased to pro- duce the same amount of distance from the buried base of the wall to the face of the slope had the bench been installed. New Brunswick reported that maintaining such benches was one of its most important lessons learned. Elsewhere in the survey, Texas reported that it strongly encourages that walls not be perched on slopes, and if a slope is to exist at the base of wall that the slope be limited to 6:1 or flatter in combina- tion with an increased wall embedment. With respect to other design specifications, responses var- ied greatly. Several respondents indicated that they were in the process of revising or had recently improved their speci- fications but did not provide details, although one respondent implied that the presence of regular specification and design manual updates in and of itself is a beneficial practice. The most frequently reported focus is on being more restric- tive in specifying backfill, particularly with respect to grada- tion, fines content, and physiochemical-electrical properties. (Interestingly, current research being performed by W.A. Marr as NCHRP Project 24-22, “Selecting Backfill Materials for MSE Retaining Walls,” aims to broaden current FHWA specifications for MSE wall backfills.) One respondent indi- cated an improved practice in using concrete level pads at the base of the wall. Although not reported in the survey, owing to some instances of adverse wall performance, some states (e.g., Ohio) discourage the use of acute corners for its MSE walls. Nearly all responses to the question of contractor/installer qualifications (11 out of 12) indicate that agencies use an approved (or pre-approved) list of products and/or vendors. However, only two respondents (Colorado and Oregon) explic- itly indicated that their specifications require wall system vendors to provide contractor training or that the contractor possess some type of previous training. Sixteen agencies responded with comments regarding con- struction inspection; only one indicated that it does not do con- struction inspection on a regular basis. Four of the responding agencies (Colorado, Minnesota, New York, and Texas) have developed manuals and/or provide specific training for MSE wall construction. Four agencies (Massachusetts, Michi- gan, Montana, and Nova Scotia) indicated that they require wall supplier/vendor/manufacturer personnel on-site at least some time during construction. One agency (Nevada) reports

26 Lessons involving either MSE wall backfill or modu- lar blocks accounted for about 14% and 10% of responses, respectively. With respect to backfill, one respondent replied, “Use of fine grained select fill has resulted in the migration of material out from behind walls. We have thousands of square foot of wall that was backfilled with this type of material. Many walls have shown distress as a result. We have coars- ened up the gradation of select fill to lessen the potential of fill migration.” When modular blocks were mentioned, it was usually in the context of durability and degradation because of roadway de-icing activities. According to one respondent, “By having a formal wall approval process we have limited the use of modular block wall systems and the deterioration of these facing elements due to deicing chemicals.” One of the more extensive commentaries provided by a survey respondent related to the deformation-tolerance of MSE walls, and has bearing on wall inspection activities: The outside may get ugly [but] it’s the inside that matters. We had an MSE ride a landslide downslope 32 ft back in the 1970s. It deformed significantly, but is still in service today. We have had several lose foundation support, but as long as they were able to move and readjust the stresses through deformation, with no loss of backfill, they have all been able to stay in service— some for decades. However, excessive consolidation settlement and internal drainage failures have led to issues with cavities and retainment loss. These MSE failed within months and had to be replaced. Amazing[ly] flexible, but only up to a limit. It’s what’s inside that counts. Although different agencies appear to have had varying experiences with MSE walls, the “most important lessons learned” do tend to focus on the topics of drainage, con- struction, backfill, and modular block issues. Considering the importance given these topics by survey respondents, those issues could be important focal points in the develop- ment of future MSE wall assessment and/or management activities.