Design of MSE retaining walls placed in front of a stable face

Widening of existing highways often results in new mechanically stabilized earth (MSE) walls being placed in front of existing stable walls. The design of these walls is unique because limited right-of-way often forces the length of the reinforcing strips to be shorter than normally used. As MSE walls are placed in more confined space, the stresses behind the wall are also different from those behind conventional walls. The Plaxis finite element software was used to study the vertical and horizontal stresses in the backfill behind nondeformable walls constructed in a confined space. The vertical stresses were found to be less than the computed overburden pressure. A vertical stress influence factor was introduced to describe the reduced vertical stresses which also are influenced both by the depth below the top of the wall and by the wall aspect ratio. Horizontal earth pressures were found to be less than those calculated using the current design guidelines for nondeformable walls constructed in a confined space. A chart which may be used to adjust the horizontal earth pressures for these walls was developed. Limit equilibrium analyses were also performed to evaluate the effect of wall aspect ratio on the stability of "narrow" MSE walls. Based on the analyses, factors of safety for noncircular slip surfaces were found to be significantly less than factors of safety for circular slip surfaces. Accordingly, analyses employing noncircular slip surfaces are recommended for all MSE walls placed in front of existing stable faces.