Seismic Bearing Capacity of Shallow Footing in Layered Soil

Abstract

Ultimate bearing capacity of a footing resting on a stratified deposit reduces under a seismic excitation as soil stiffness degrades during a shaking. Ultimate bearing capacity depends on the shear strength parameters (cohesion and angle of internal friction) of subsoil, along with shape and size of footings. An attempt has been made to study the bearing capacity of shallow circular and strip footings in a layered cohesionless soil under static and seismic conditions. The subsoil conditions resemble that of a site located at Rajarhat, Kolkata, West Bengal, India. The modeling has been done using finite element method. The analysis has been carried out for footing width (B) of 2.0 m and depth (Df)-to-width ratio (Df/B) of 0.5 and 1.0. For each Df/B ratio, the analysis has been repeated for three different ratios of layer thickness [top layer (weaker): bottom layer (stronger)] which are 0.33, 1 and 3. For seismic condition, pseudo-static analysis has been performed for horizontal seismic acceleration 0.1–0.3 g. It has also been confirmed from the initial evaluation of liquefaction potential based on the SPT data for the site that the cohesionless soil layers considered here are not prone to liquefaction. It has been observed that due to variation of layer thickness ratio from 0.33 to 3, the ultimate bearing capacity decreases up to 20.00% under static case, whereas for seismic case, decrement of bearing capacity is about 18.00% under similar condition. It has also been found that with the increase in horizontal seismic acceleration from 0.1–0.3 g, the seismic bearing capacity factors Nq and Nγ reduce appreciably by 30% and 60%, respectively. Further attempt has been made to find the effect of shape of footing on seismic bearing capacity. The paper presents the importance of seismic effect, on layered soil and shape and size of footing in terms of ultimate bearing capacity.