Design Charts to Estimate the Post-seismic Bearing Capacity of Shallow Foundations

In several major earthquakes (e.g. Niigata 1964, Kocaeli 1999, Christchurch 2011, Golbasi 2023), the reduction in soil shear strength caused by the build-up of excess pore water pressure (Δu) resulted in a corresponding reduc-tion of the bearing capacity of shallow foundations, even when soil liquefaction was not fully triggered (i.e., pore pressure ratio ru = Δu/σ’v0 < 1), eventually lead-ing to enormous settlements or complete tilt of the structures. In this framework, the paper proposes a simplified method to quantify the post-seismic reduction in bearing capacity of shallow foundations in saturated, potentially liquefiable soils. The problem is approached as a static one, considering excess pore pressures that have not yet dissipated at the end of seismic shaking. Traditional methods estimate seismic bearing capacity reduction based on the residual undrained shear strength of fully liquefied soils, overlooking cases in which liquefaction is only partially triggered. Data on undrained mobilized shear strength for sands experiencing par-tial pore pressure buildup are scarce. To address this gap, new laboratory tests using a non-conventional simple shear apparatus have been performed. Our experimen-tal results demonstrate that the undrained mobilized shear strength depends on excess pore pressure during cyclic loading, void ratio, fines content, and initial effective vertical stress. Based on these findings, a relatively simple expression for post-seismic bearing capacity is proposed and design charts are developed with immediate practical relevance for engineering applications.