A Tale of Two Very Different MSE Wall Structures

Mechanically stabilized earth (MSE) walls are composed of three main elements: soil, reinforcement and a facing. If any of these elements are not functioning, its internal performance would be unacceptable. In extreme cases, it will result in a collapse. How­ever, if material characterization, design and construction are properly done, MSE walls will exhibit excellent long-term performance while being economical. In fact, when using multiple reinforce­ments, redundancy is rendered, i.e., 'overstressing' loads can be alleviated by shedding it to understressed layers. Common design of MSE walls with respect to reinforcement and facing is generally conservative. However, soil is often poorly characterized. The result is either an overly conservative structure or worse, unacceptable structural per­formance. Underestimation of backfill strength is typical as often default shear strength is used. For example, the American Association of State Highway and Transportation Officials (AASHTO) specifies select reinforced soil, sug­gesting a default friction angle of q>=34° with an upper limit of 40°. For the select backfill specified in AASHTO, this angle is likely to be 45-55° implying that the actual strength of the compacted backfill could be as much as 50% larger. However, if in construction the rein­forced soil is poorly compacted or drained, conservatism due to underes­timated soil strength may vanish, potentially leading to poor wall per­formance. This is not different from other geotechnical structures. The objective of this article is to report failures of two dissimilar walls, ending with costly remediations that changed the characteristics of original MSE structures. The cases reported are not unusual and reflect similar prob­lems in other MSE wall failures the author has investigated. The reported cases are based on forensic studies and, in a general sense, are structured as such: A known outcome, e.g., excessive settlement or collapse. An investigation process to separate the chaff from the wheat, i.e., distin­guish between the "noise" typically promoted by lawyers who are ad­vocating for a client.