EFFECTS OF HORIZONTAL AND VERTICAL SEISMIC LOADS ON THE BEARING CAPACITY OF A SURFACE FOOTING ADJACENT TO A SLOPE

The correction factor (eta(ie)) for the ultimate bearing capacity of a footing placed adjacent to a slope, taking into account the combined effect of horizontal and vertical seismic loads, represented by horizontal and vertical seismic coefficients, k(h), and k(v), respectively, was derived using a modified Janbu's slice method. The influence of slope angles 'alpha' on the values of 'eta(ie)' are studied here. It was found that the values of eta(ie) can be expressed as an exponential function of 'k(h)/(1 - k(v))' and 'alpha', with a measurable interdependency between 'alpha' and 'eta(ie)'. The influence of 'alpha' on the value of 'eta(ie)' increases as the input value of 'k(h)/(1 - k(v))' increases. Equations derived based on the analytical results are proposed to account for this effect. Based on the analyses of 11 near-fault seismographers obtained in the 1999 Chi-Chi earthquake in Taiwan, a ratio between the vertical and the horizontal seismic coefficients, lambda, of between +/- 0.25 is suggested for including the combined effect of vertical and horizontal seismic forces in evaluating the seismic bearing capacity of footings located in near-fault areas.