An experimental study of a ultimate bearing capacity of a foundation in anisotropic rock masses

Abstract

Rock Masses encountered beneath the foundations of heavy civil engineering structures are generally jointed and anisotropic in mechanical behavior. General approach to estimate the bearing capacity of shallow foundations in such rock masses is to consider the rock mass an isotropic continuum. Very few studies are reported in literature which consider the anisotropy in bearing capacity computations. Probably, the only methodologies suggested for anisotropic rock masses are those proposed by Prakoso and Kulhaway (2004), and, Singh and Rao (2005). Singh and Rao (2005) approach was derived from the outcome of experimental studies comprising uniaxial and triaxial tests on specimens of jointed model of plaster of Paris (POP). The approach considers four fold failure mechanism namely rotation of blocks, shearing of blocks, tensile splitting of intact rocks and sliding along the existing joint. However, no experimental or field investigation is available to validate the four fold mechanism assumed by Singh and Rao (2005). In the present study, an attempt has been made to investigate the failure mechanism of jointed blocky rock mass below shallow foundations through bearing capacity tests under plane strain conditions. Plaster of Paris has been used as a model material to simulate the intact rock material. Specimens of rock mass, having various joint configurations were assembled using plaster of Paris blocks. Broadly the jointed rock mass specimens were divided into two categories. Type-A specimens comprised of a set of continuous joints orthogonal to each other. The type-B specimens used staggered joints. The orientation of joints was varied from 0ο to 90ο for both the specimen types. A footing plate was placed on the top of the mass and was loaded up to the bearing capacity failure. The results on failure modes, anisotropy in bearing capacity and settlements are presented in this paper.