Seismic bearing capacity of soils

The Authors present a limit equilibrium method to calculate the reduction in the bearing capacity coeflicients in the case when seismic loadings are present. Seismic bearing capacity has been investigated by Soubra & Reynolds (1992) who used a variational limit equilibrium method to calculate the seismic bearing capacity of strip footings on slopes, and Soubra (1992) who used an upper-bound method in limit analysis and gave upper-bound solutions for the seismic bearing capacity problem. Soubra & Reynolds showed that the variational limit equilibrium method is equivalent to the upper-bound method in limit analysis for a rotational log-spiral mechanism. However, the mechanism considered by Soubra (1992) is a translational one: it consists of a triangular active wedge and a log-spiral radial shear zone. By applying the upper-bound theorem of limit analysis, the bearing capacity of strip foundations in seismic areas was found for both these mechanisms. In our opinion, a rigorous method for calculating the seismic bearing capacity coefficients is one which gives directly the critical seismic factors and not one using static factors which will be affected by reduction coefficients determined by certain methods such as those presented by the Authors. The model used to calculate the seismic coefficients must give directly acceptable seismic bearing capacity factors and be in good agreement with the results of other authors in both the static and seismic cases. Our results for values of N, are now compared with those of the Authors, Sarma & Iossifelis (1990) and Richards, Elms & Budhu (1990b).