UCLA

During earthquake ground shaking earth pressures on retaining structures can cyclically increase and decrease as a result of inertial forces applied to the walls and kinematic interactions between the stiff wall elements and surrounding soil. Limit equilibrium analysis imposes a pseudo-static inertial force to a soil wedge behind the wall (the mechanism behind the widely-used Mononobe-Okabe method), which is a poor analogy for either inertial or kinematic wall-soil interaction. Many basement walls and retaining structures are dominated by kinematic soil-structure interaction (SSI) effects arising from differences in displacement between the wall and the free-field soil. Kinematic SSI solutions are often formulated for uniform soil conditions, but the shear modulus of most soils is known to increase with mean effective stress, and therefore with depth. We examine the influence of vertical heterogeneity of shear modulus on kinematic SSI for rigid walls. An existing free-field displacement solution is presented first, followed by analysis of earth pressure increments using a Winkler assumption. Vertical heterogeneity is shown to reduce seismic earth pressures compared with a uniform soil case (for a given frequency and peak ground surface displacement) because free-field displacements are largest near the surface, where the soil is softest and Winkler stiffness is lowest. The proposed Winkler solution is then compared with an exact analytical solution for vertically heterogeneous soil over a rigid base and retained between two opposing rigid walls. The agreement is imperfect, but reasonable, with differences likely due to assumptions regarding the dynamic Winkler stiffness intensity.