Stress and displacement discontinuity element method for undrained analysis

Stress (SD) and displacement (DD) discontinuity methods have been developed for problems where seepage and deformation are not coupled. This is an important limitation in geomechanics where the ground is usually saturated. Of particular importance are problems that fall within the “short term analysis” concept, where a phenomenon occurs over a period of time that is very short compared to the permeability of the medium. In this paper we present an analytical formulation to solve undrained conditions and deformation problems in a poroelastic saturated medium in plane strain conditions, with the assumption of short term analysis (i.e. no change in volume of the medium). Explicit functions for SD and DD elements are obtained and are incorporated into the code FROCK. The new code is verified by a number of comparisons with theoretical solutions, also presented in the paper, and with solutions from the Finite Element Method. In undrained compression, the compression field around the tip of an open flaw decreases and the tension field increases compared to drained compression loading, while the shear stresses remain unchanged. For a closed frictional flaw undrained compression decreases the compression and tension fields around the tip. The shear stresses however have the same distribution as in drained loading but are larger. This is caused by the decrease of the effective normal stresses due to the excess pore pressures which in turn decrease the shear resistance of the flaw.