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

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27 Combining a literature review with a survey and interviews, this synthesis project has attempted to determine: • The current state of practice in assessing the performance of mechanically stabilized earth (MSE) walls, particu- larly in the long term; • The direction of the state of practice; • What the current and effective practices are; and • What areas need improvement and/or research. Key findings and conclusions regarding each of these items have been summarized and are presented below. CURRENT STATE OF PRACTICE MSE walls are important infrastructure assets. However, unlike bridges and pavements, they are often overlooked. The current state of practice with respect to the management of MSE walls as assets can be characterized thusly: • There is no widely used, consistently applied system for managing MSE wall assets. • Fewer than one-quarter of state-level transportation agen- cies have developed any type of MSE wall inventory data beyond that which may be captured as part of their bridge inventories. • Still fewer agencies have the methods and/or means to support their inventories with data from ongoing inspec- tions from which assessments of wall performance can be made. • Some previously established wall inventory and inspec- tion activities have ceased because of lack of resources and funding. Regarding the inventory and gathering of MSE wall-related data once the walls are constructed and accepted, current prac- tice can be generally described as follows: • Responsibility for MSE walls after their construction usually rests with maintenance personnel operating in a decentralized structure, while most inventories are managed by a geotechnical engineer or similar person at an agency-wide level. However, in 20% of agencies, no one has end responsibility for MSE walls. • Various types of data are collected and maintained in order to assess wall performance. Most frequently, the data consist of ratings that describe the observed condi- tion of wall features. • The manner in which wall features are observed and assessed varies between agencies, as do the rating criteria themselves. • Rating criteria are usually more subjective than objective. • When scheduled, the frequency of data collection var- ies from two to 15 years, although wall performance monitoring activities are most often (i.e., two-thirds of the time) simply reactive to reported incidents of adverse performance. Once asset data have been collected, they must be assessed to predict future performance and determine maintenance and management activities. With respect to MSE walls, current practice in the area of assessment can be basically described this way: • Agencies believe that drainage, global stability, and corrosion/degradation of internal reinforcement are the most important issues affecting the long-term perfor- mance of MSE walls. • Wall performance is sometimes only one factor used in making asset-management decisions. • No state transportation agency has a specific method- ology for assessing long-term performance of existing MSE walls. • Similarly, there appears to be no specific methodology for accurately predicting the remaining service life of an MSE wall. DIRECTION OF STATE OF PRACTICE As walls have aged and adverse performance (whether age- related or not) has occurred, more agencies are becoming aware of a need for long-term performance monitoring of MSE walls. An opinion voiced by some survey respondents is that there is insufficient attention given to long-term performance of MSE walls despite the potential for poor performance of this important asset. One reason is that, while other assets such as pavements and bridge structures are subject to formal inspec- tion and reporting requirements, there are no such requirements for retaining walls, and in particular MSE walls. Without such requirements, respondents noted difficulty in obtaining fund- ing for wall inspection and management. Consequently, it appears that the direction of practice is largely limited to the status quo, with relatively few agencies performing inspections or conducting assessments. However, it is anticipated that as experience with MSE walls accumulates, those that are able to secure funding and resources will continue to develop, refine, chapter six CONCLUSIONS

28 In summary, current effective practices for inventorying and assessing the performance of MSE walls include: • Use of inventory and assessment systems with features that are as simple to use and as objective as reasonably possible • Use of rating criteria that are specific to particular wall elements and/or conditions • Use of numeric rating scales that correspond to other scales already in use for other asset classes such as bridges • Incorporation of MSE wall inventory and assessment systems together with systems for other asset classes. Current effective practices for improving the performance of MSE walls include: • Use of pre-approval process for wall design and/or wall supplier • Provision of adequate internal and external drainage. AREAS NEEDING IMPROVEMENT AND/OR RESEARCH Today there are many millions of square meters of MSE walls with typical design lives of 75 to 100 years. The oldest of these walls are about 40 years old. Instances of MSE wall failures and poor performance are expected to increase as walls age. To better assess the performance of MSE walls, the following practices would be beneficial: • Greater recognition of MSE walls and retaining walls in general as important infrastructure assets • Increased availability of funding and other resources for inventory and assessment activities • Active involvement of a larger number of agencies in MSE wall inventory and assessment activities • Greater consistency across agencies relative to the way that inventory and assessment activities are performed • Greater use of bridge and other existing asset inventory data for MSE wall inventories. To move beyond current inventory and the data baselines now being established, repeated observations and performance predictions will be needed, as will specific decision-making methodologies. To this end, research relative to the following topics would be helpful: • 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 col- lection 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. A potential research problem statement for predictive models for remaining MSE wall service life is presented in Appen- dix D. The statement is adaptable to the other identified research needs. and better calibrate procedures regarding design, construction, condition assessment, and asset management decision making. EFFECTIVE PRACTICES Although wall inventory and monitoring practices vary between agencies, effective practices can be extracted from systems currently in use. The most well-implemented and developed wall inventory and assessment system in the United States appears to be the Wall Inventory Program developed by the FHWA for the National Park Service. The system uses “conditions narratives” (the preparation of which is illustrated by only general guidance, thus making them fairly subjective) to describe the conditions of certain wall elements, and then these narratives are converted to a numeric rating. Although the multiple steps in the rating process increase the effort required to use the system, an inherent strength is that it can be applied to many wall types (not just MSE walls). Other wall inventory and assessment systems such as those used by Pennsylvania and Nebraska are relatively simple to use and appear to be less interpretive. Such characteristics typically lead to greater consistency in data interpretation and broader use. Without consistency in collected datasets, broadly applica- ble conclusions are more difficult to reach, and methodologies developed from inconsistent data are inherently less robust. The numeric ratings associated with these two particular systems are also compatible with the 0 (worst) to 9 (best) scale already used by many in the assessment of bridges, thus facilitating the development of readily accessible MSE wall assessment tools and methods within the domain of asset management already occupied by other asset types. Other desirable practices include that reflected in the Nebraska system, in which rating criteria are specific to each element or wall condition rather than being generic. This specificity avoids vagueness and contributes to greater consis- tency. For example, a rating of 6 is assigned “when less than 25% of the wall area shows deterioration,” and a rating of 5 is assigned “when wall panels have bowed outward to where connectors between panels are visible and deforming.” This would be in contrast to a system in which a rating of 3 is assigned if “the wall exhibits ‘extensive’ distress.” The wall inventory and assessment system employed in Pennsylvania reflects another apparently effective practice, in that it actively and regularly inspects all of its retaining walls (inclusive of MSE types) and manages its inventory within the same framework as it does its bridges. In this man- ner, overlaps and gaps in inventory are minimized, and data and assessments are kept current. Although individual experiences and beliefs regarding prac- tices that improve wall performance vary, most agencies agree that the use of a pre-approved wall design and/or wall supplier helps ensure better wall performance. Similarly, based on the “most important lessons learned,” many agencies believe that providing adequate drainage, both internal and external, is an essential practice in realizing good MSE wall performance.