Combined seismic bearing capacity factor using modified pseudo-dynamic approach.

Abstract

The seismic bearing capacity of a shallow foundation is often estimated using empirical equations or design charts. These are generally developed using pseudo-static approach which is an approximate method of estimating seismic forces. The modified pseudo-dynamic method has recently been developed to estimate seismic acceleration in vertical and horizontal direction. It takes into account the variation of these accelerations with time and depth, inherently considers dynamic soil properties and also the amplification caused due to soil, thus making it more realistic. The present study uses this method to estimate the static (Ncγ) and seismic bearing capacity factor (Ncγe) by combining the effects of cohesion, depth of embedment and unit weight for a strip footing. Assuming a composite failure surface consisting of planar and log-spiral surfaces, limit equilibrium equations have been used in the present study. An iterative search is made to locate the focus of the log-spiral to estimate the critical value of Ncγe. The variation of Ncγe is greatly affected by soil properties such as cohesion (c) and angle of internal friction (ϕ), seismic parameters such as seismic acceleration coefficients (kh and kv) and normalised frequency (ωh/Vs) as well as embedment depth (Df) of the footing. Ncγe was found to reduce by 84% when (2c/γB) was changed from 1 to 0 for ϕ = 35°, Df/B = 0.5, kh = 0.2, kv = 0.1 where γ represents the unit weight of soil and B is the width of the footing. The results are presented graphically for practical use.