Interaction between Retaining Walls and Unsaturated Soils in Experiments and Using Slip Line Theory

Retaining walls are widely used for shoring excavations and stabilizing earth slopes. They often support naturally formed soils above the groundwater table and compacted soils, which are in an unsaturated state. To perform reliable designs, it is important to be able to model how retaining walls interact with unsaturated soils and, in particular, be able to estimate the earth pressures that act on the walls. In this paper, soil responses observed in three retaining wall model tests involving unsaturated soil samples are compared with computations made using slip line theory. Each test consisted of a rigid retaining wall rotating about its toe into a silty sand sample. The soil in the theory was treated as rigid perfectly plastic continuum, with strength controlled by the Mohr-Coulomb criterion. The effective stress was used to incorporate the effects of suction. The suctions inside the unsaturated soil samples made them stiff and brittle. Extensive rupturing occurred in the soil samples adjacent to the wall as rotations occurred. The rupturing caused the soil behavior to divert from that of a continuum, thus the slip line theory could not provide an accurate match to the experimental measurements. Most notably, the measured earth pressure profiles exhibited localized spikes that could not be modeled by the slip line theory. Theoretically computed vertical soil displacements also diverted from those measured experimentally, although the theoretical and measured horizontal displacements were in reasonable agreement. Although these results are negative, they will guide researchers and practitioners on what level of accuracy can be expected when using a rigid perfectly plastic continuum assumption to model highly brittle soils interacting with structures, where improvements are needed, and whether continuum-based theories should be overlooked in favor of others.