Effect of Bedrock Level on Dynamic Bearing Capacity of Shallow Foundation Under Earthquake Load

Abstract

Prevailing state of the art in estimating bearing capacity of a foundation under earthquake load is to multiply the static bearing capacity by a factor of 1.25. This practice is in vogue in number of international codes including the IS code for design of such foundations, though it has been shown by Richard et al. (J Geotech Div ASCE 105(GT4):662–674, 1993) that the bearing capacity factors reduce significantly when the ground is subjected to earthquake loads. The analysis is essentially pseudostatic in nature. In this chapter, Budhu and Richards’ method has been modified to cater to fluctuating response of the ground due to passage of vertically propagating SV and P waves to estimate the free field time period of the site which in turn is used to estimate the design spectral acceleration (based on modal analysis) that affects the bearing capacity factors N _c , N _q , and N _γ of the foundation. It is shown that bearing capacity factors fluctuate with modes and are also affected profoundly by the bedrock level prevailing at site. Finally, a comparison with static bearing capacity is made with various dynamic parameters that affect the bearing capacity of foundation.