Seismic Design of Mechanically Stabilized Earth Walls Built in Confined Spaces Considering Strain Localization and Bilinear Failure Surfaces

Mechanically stabilized earth (MSE) walls are constructed in proximity to existing stable structures or rock faces due to urbanization and significant transportation demand during the widening of existing highways. Presently, there is no well-established design approach for earth-retaining structures in front of stable slopes or rock faces with confined spaces. The design and construction of narrow backfill width mechanically stabilized earth walls are not addressed within the scope of the Federal Highway Administration guidelines. Unlike conventional gravity walls, the design of MSE walls with narrow backfill widths is unique due to differing earth pressures caused by wall geometry and insufficient development of the active thrust (failure) wedge. This study introduces an analytical method to calculate the seismic active earth pressure and its point of application on MSE walls with narrow backfill widths. This method utilizes the limit equilibrium approach and assumes the failure surfaces as planar, employing a three-wedge mechanism and accounting for strain softening behavior (strain localization) during significant earthquake (kh > 0.4) loading. It considers the transition in shear strength from the peak friction angle to the residual friction angle along the bilinear failure surfaces in the backfill soil. The resulting seismic active earth pressure and its application point (seismic active pressure and additional dynamic thrust) are used to optimize the proportions of MSE walls by considering three failure modes: sliding mode of failure, eccentricity mode of failure, and bearing mode of failure. A notable reduction of 71.02% in seismic active earth pressure coefficient is observed when b/h = 0.1, kh = 1.0, and z/h = 0.95. Conventional methods have been found to overestimate the optimal reinforcement length by 45% for b/h = 0.1 and kh = 0.6. Design charts are provided to illustrate the effect on the optimal dimensions of MSE walls with narrow backfill widths due to various parameters.