Evolution of Mechanically Stabilized Earth Wall Design to Incorporate Permanent Shoring

Abstract

Mechanically stabilized earth (MSE) walls are often constructed to accommodate widening of existing roads or construction of new roadways. In steep terrain, substantial excavation must be performed to establish a flat bench upon which to construct the wall, and unshored construction may not be practical, particularly if room for traffic must be maintained during construction. Soil nail walls are often constructed to stabilize the backslope excavation with an MSE wall being designed and constructed in front of it. Where the two wall types are used together, termed a shored mechanically stabilized earth (SMSE) wall system, a design procedure has been developed by the Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) that considers both the stabilizing effect of the shoring wall with regard to reduction of lateral loads acting on the MSE wall mass as well as contributions to global stability. Existing public sector guidelines for MSE wall design suggest a minimum MSE reinforcement length equivalent to seventy percent of the design height of the MSE wall (i.e., 0.7H). However, if permanent shoring is included, a minimum SMSE reinforcement length equivalent to 30 percent of the wall height (0.3H) is recommended. This paper, written as a summary of the CFLHD design guidelines, discusses the evolution of MSE walls through the development of this innovative type of wall construction and provides guidance on when SMSE wall construction may be viable.