Dynamic Passive Pressures on a Foundation with a Dense Sand Backfill

While the ultimate passive earth pressure acting on a foundation under static loading conditions can readily be calculated using several different theories, the effects of cyclic and dynamic loading conditions are less well understood. A full-scale pile cap with a densely compacted sand backfill was tested under static, cyclic, and dynamic lateral loading conditions and soil pressures were measured using pressure plates. During dynamic loading, it was observed that the foundation gradually developed a residual offset from its initial position, accompanied by a reduction in backfill force. While the foundation and backfill appeared to move integrally during static and cyclic loadings, the backfill exhibited out-of-phase movement relative to the pile cap during dynamic loading. Observed losses in backfill contact force were associated with both cyclic softening and dynamic out-of-phase effects. Losses due to dynamic loading were partially offset by increases in peak force due to damping. When quantifying passive earth forces with cyclic/dynamic losses without damping, the Mononobe-Okabe (M-O) equation with a 0.75 or 0.85 multiplier applied to the peak acceleration can be used to obtain a reasonable estimate of the force. When including increases in resistance due to damping, a 0.6 to 0.7 multiplier can similarly be used.