Effective height based interaction surface approach for the seismic design of shallow foundations resting on homogeneous slopes

Despite being seismically vulnerable, foundations on slopes continue to be designed using the bearing capacity equation meant for shallow foundations resting on flat ground. While some bridge design specifications specify reduction factors for the estimation of bearing capacity of shallow foundations on slopes, the loading condition assumed in arriving at these reduction factors is not consistent with the actual field conditions. The present study highlights the limitations of the current code-based approaches for the seismic design of shallow foundations resting on slopes. Triaxial (V-H_x-H_y) seismic capacity surfaces are developed by performing three-dimensional finite element limit analyses, using pseudo-static approach, on surface and embedded square footings resting on dry and homogeneous sandy slopes, using Optum G3. The relationship between the horizontal force and the moment acting on a foundation, governed by the effective height of the supported column, is considered in developing the capacity surfaces. The vertical load capacity of a foundation on a slope is significantly lower than that of a similar foundation on flat ground. Additionally, the difference in horizontal capacities in the down-slope and up-slope directions increases with the axial load level. Foundation embedment significantly enhances both vertical and horizontal load capacities. A foundation located at the minimum edge distance from the slope face has a much higher vertical load capacity than a surface foundation placed at the crest. Failure mechanisms governed by horizontal load result in lower foundation capacity compared to those governed by moment. The seismic coefficient in the down-slope direction negatively affects foundation capacity.