Seismic bearing capacity of footings

Abstract

Design of foundations to withstand seismic loads has received considerable attention in recent years. Seismic loading significantly reduces bearing capacity of foundations due to induced stresses. This effect has not been considered in the design standards adequately. In this paper, an analysis has been developed for a dry c-ø soil, considering the effect of the vertical and horizontal coefficients of seismic acceleration. Seismic accelerations are considered both on the foundation and within the soil mass beneath the foundation. Limit equilibrium method is used assuming failure surface, comprising three zones: an unsymmetrical triangular zone, a log-spiral transition zone, and a triangular passive zone, on the side of the footing in the direction of horizontal seismic acceleration, on the other side soil is considered having partially mobilized shear strength. The problem has been solved for the three cases namely i) <i>c</i> = <i>q</i> = 0, ii) <i>c</i> = γ = 0 and iii) γ = <i>q</i> = 0, separately. <i>c, q</i> and γ are the cohesion, surcharge intensity (γ <i>D_f</i>) and unit weight of soil respectively, <i>D_f</i> being depth of the base of footing below ground surface. Finally the bearing capacity of footing (<i>q_uE)</i> is expressed in terms of seismic bearing capacity factors (i.e. <i>N_yE</i>, <i>N_qE</i> and <i>N_cE</i>). Values of these factors were obtained for maximum value of <i>m</i> which is obtained by trial satisfying all equilibrium conditions. It was noted that the value of mobilization factor ‘m’ decrease with the increase in horizontal seismic acceleration coefficient (<i>k_h</i>). In computations, vertical seismic acceleration coefficient (<i>k_v</i>) is considered equal to 2/3 of horizontal seismic acceleration coefficient (<i>k_h</i>).