Effects of non-uniform traction and specimen height in the direct shear test on stress and deformation in a rock fracture

In the direct shear test (DST), an internal moment is distributed within the rock specimen by non-coaxial shear loads applied to the specimen, which cause non-uniform distributions of both the traction on the loading planes and the stress and deformation in the specimen. To examine the validity of the DST for a rock fracture and to clarify the effect of specimen height, both the stress and deformation in a fracture in the DST were analyzed for specimens with three different heights using a three-dimensional finite element method with quadratic joint elements for a fracture model. The constitutive law of the fracture considers the dependence of the non-linear behavior of closure on shear displacement and that of shear stiffness on normal stress and was implemented in simulation code to give a conceptional fracture with uniform mechanical properties to extract only the effect of non-uniform traction on the stress and deformation in the fracture. The results showed that both normal and shear stresses are concentrated near the end edges of the fracture, and these stress concentrations decrease with a decrease in the specimen height according to the magnitude of the moment produced by the non-coaxial shear loads. Furthermore, although closure is greater near the end edges of the fracture, where normal stress is concentrated, this concentration of closure is not so significant within the range of this study because of the non-linear behavior of closure, that is, closure does not significantly increase with an increase in normal stress at large normal stresses.