Static and Seismic Bearing Capacity of Strip Footings on Sand Overlying Clay Soils

This paper examined the ultimate bearing capacity of a strip footing located on a two-layer substrate composed of a sandy soil on a clay layer. The analysis was conducted through discontinuity layout optimization, which is an upper bound limit analysis. This study focused on various factors contributing to the problem, i.e. sand layer internal friction angle, sand layer thickness, clay layer undrained shear strength (c(u)), unit weight of soil layers (gamma), footing width (B) and surcharge pressure (q). Failure in both soil layers was governed by the Mohr-Coulomb criterion. The bearing capacity improves when there are two layers of sand-clay soil in comparison to the homogeneous clay soil. Such improvement is intensified by increasing the internal friction angle and thickness of the sand layer and surcharge pressure. The maximum improvement of the bearing capacity was found in a scenario where the entire failure zone was within the sand layer. The pseudo-static method was used to examine the seismic mode. According to the results, the higher the sand internal friction angle and lower c(u)/(gamma B), the lower the decrease in the seismic bearing capacity compared to the static mode. Furthermore, the variations of failure surface in static and seismic modes were investigated. The results obtained in this study were compared against the results of other methods, indicating good consistency.