Modelling rock mass behaviour during direct shear stress test using damage plasticity model

One of the most important in situ experiments for determining rock mass fracture is a direct shear test. During this test, rock mass is loaded in two phases: in the first phase, a direct normal pressure is applied and consolidation is performed, and in the second phase, a shearing stress is applied, which leads to fracture in rock mass. After fracture is reached (peak shear strength), stress decreases and shear deformation increases [1].In this paper a simulation of rock mass behavior during direct shear test using damage plasticity model [2,3,4] is presented. For this analysis, a three-dimensional FEM model of direct shear test with dimensions 130x80x60 m was formed. In the first phase of the FEM test normal stress corresponding to measured pressure during in situ test was applied, and in the second phase of the FEM test shear displacement corresponding to recorded displacement during in situ test was applied. In the first phase normal displacements are recorded based on calculations from the FEM model, and in the second phase shear stresses are recorded.The damage plasticity model [2,3,4] is defined by the following parameters: E0-Modulus of elasticity, v0 - Poisson's ratio, fc - uniaxial compressive strength, ft - uniaxial tensile strength, ac - pressure curve parameter, Dc - degradation value for maximum compressive stress, at - tensile curve parameter, Dt - degradation value for tensile stress, Gc - fracture energy for uniaxial pressure, Gt - fracture energy for uniaxial tension, αp - dilatancy parameter, α - constant defining uniaxial and biaxial compressive yield stress ratio, γ - parameter for triaxial compression, lch - characteristic size of elements, s0 - stiffness recovery parameter, Dcr - critical degradation value.Numerical simulation of direct shear test (consolidation phase and shear phase) was performed for the estimated parameters of damage plasticity model. For each step of normal stress loading, corresponding value of normal displacement was calculated, and for each step of shear displacement, corresponding value of shear stress was calculated.Based on the obtained results, it is shown that damage plasticity model can successfully simulate a complex stress state during direct shear test in rock mass. Over the entire range of applied shear displacements, shear stress increases until there is a fracture in the material (peak shear strength), and afterwards stress drop is recorded (residual shear strength).