Evaluation of Seismic Active Earth Pressure Using Horizontal Slice Method and Log-spiral Failure Surface

Evaluation of active earth pressure in seismic condition is one of the important problems of geotechnical earthquake engineering. In this study the static and seismic active earth pressure exerted by a homogeneous backfill behind a rigid retaining wall is evaluated using Horizontal Slice Method (HSM). Both cohesionless and cohesive soils are considered using c- soil in the formulation. The failure surface is assumed to be a logarithmic spiral which represents a more realistic failure surface compared to the commonly used planar surface. The seismic inertia force is considered by using pseudo-static horizontal acceleration coefficient, leading to simple formulation of an otherwise complex phenomenon. Important parameters in the seismic earth pressure problem viz., depth of tension crack, surcharge over the backfill, seismic horizontal acceleration, batter angle of the wall, and, the shear strength parameters of the backfill are included in the study. Earth pressure is expressed in terms of dimensionless factors termed as Earth Pressure Factors (EPF) which makes it possible to express both static and seismic earth pressure by simple equation resembling the familiar bearing capacity equation. The efficacy of HSM is first verified using a planar wedge failure for which results are known from previous studies. The method is then extended for logarithmic spiral failure surface. The critical failure surfaces are identified by using different initial radii of the spiral in order to optimize the EPFs producing maximum active earth-pressure. The advantage of HSM is exploited to find the variation of seismic earth pressure along the height of the retaining wall. The results are presented in the form of user friendly non-dimensional design charts. Seismic Earth-Pressure, Horizontal Slice Method, Pseudostatic