Over the past 20 years, the use of mechanically stabilized earth (MSE) walls and slopes has become very common in a large number of construction applications in the U.S. and around the world. The technology used to build these structures is really quite simple: reinforcing, typically metal or synthetic grids or sheets, are layered in with compacted soils, adding shear strength and allowing the soils to stand at progressively steeper angles. Wall faces are typically constructed using concrete panels, split-face masonry blocks, or even vegetation that primarily provides erosion control and aesthetics. The faces provide little if any structural support to the retained soils.
The relatively low cost of MSE structures have made them quite prevalent in transportation and site development projects, and have also led to their use for waste management and environmental remediation projects. MSE walls can often be constructed for less than half the cost of comparable concrete or steel structures. This cost advantage increases as the height of the structure increases. This reduced cost has enabled the development of increasingly marginal projects, and pushed the limits of the technology, literally, to new “heights”. For example, several recent airport expansion projects in the U.S. have utilized MSE wall and slopes well in excess of 100 feet tall.
However, this lower cost and increased use of the technology has come at a price. While there are no specific published numbers available, the failure rate of these structures has been estimated by some to be as high as 5% to 7%, with 2% likely being a low-end estimate. “Failure” in this case encompasses not only large-scale collapse or movement, but also settlement and performance issues. In any case, the number of MSE walls and slopes exhibiting problems is alarmingly high for an engineered structure, and the cost to repair these problems can be many times the original construction cost.
So why do these failures occur? Over the past 10 years, CEC has been involved with the specification, design, construction monitoring, and failure investigation of a number of MSE walls and slopes. Published evaluations on MSE wall failures are also quite numerous. Many studies have shown that, particularly in the private site development sector, engineering site layout, surface and subsurface drainage features, geotechnical engineering evaluations, and construction monitoring are often inadequate. CEC’s experience investigating failures has identified a number of construction errors that have led to performance issues. One re-occurring construction factor leading to failure is inadequate backfill compaction when clayey soils are used in the wall construction.
The published studies and our experience also indicate that the contracting methods used for both design and construction of MSE walls and slopes may be contributing to the high failure rate. Most MSE walls are designed and constructed using a design-build contract where the contractor provides the detailed wall design and constructs the wall. This process results in highly competitive “cut-throat” bidding among vendors, encourages overly optimistic design assumptions, and often hampers communication and review by the design team. This process often places numerous risks unknowingly back on the owner.
How can you protect yourself and reduce the risk of failure for MSE walls and slopes on your project? First, hire civil and geotechnical engineers with experience in the investigation, design, and specification of these structures and ensure that their services are carried through into construction. If a design-build process is used, a detailed wall layout and performance specification must be prepared listing all wall design, testing, and construction requirements. Full-time, on-site construction monitoring should be provided by either the wall designer or geotechnical engineer to ensure that the proper testing and site inspections are done. The contractor should not provide the construction monitoring services. Finally, hire a contractor with experience and certification in MSE construction.
If you have any questions about the use, specification, design, or construction of MSE wall and slopes and how they may impact an upcoming project, contact Douglas Clark, P.E. (dclark@cecinc.com) or Jeffrey Woodcock, P.E. (jwoodcock@cecinc.com) at 800-365-2324.
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