Numerical Investigation of the Mechanical and Damage Behaviors of Veined Gneiss During True-Triaxial Stress Path Loading by Simulation of In Situ Conditions

Investigations on the Posiva Olkiluoto spalling experiment (POSE) at the nuclear waste disposal site in Olkiluoto, western Finland, have revealed that the presence of anisotropic veined gneiss (VGN) could affect the evaluation of spalling/damage strength of the host rock. In this study, the mechanical and damage behaviors of anisotropic VGN during complex true-triaxial in situ stress path loading is numerically investigated, considering the lifetime stress evolution of POSE. Overall, the discrete element numerical results show reasonable agreement with those of laboratory experiments for both strength and damage development. Numerical simulations at the microscale showed that the damage is concentrated within the discontinuous foliation bands and at the interfaces between weak and strong bonds when the mineral composition of the foliation bands is heterogeneous. It is possible that the varying stiffnesses of the two materials cause strain concentration at strong segments within the weak foliation bands or high strain gradients at the interfaces between the weak and strong bands, leading to tensile failure of the strong bonds. However, if continuous foliation bonds are employed, fractures mainly propagate within the weak bonds, indicating that the heterogeneity of the foliation bonds is a predominant factor affecting the failure of VGN. After weak bonds and interfaces failure, shear movements occur along the weak bonds, forcing the strong bonds to bear most of the load. The simulations also show that the unloading of σ₂ and σ₃ triggers microcracks in the early stages, revealing the role of unloading in rock damage. The change in the secant modulus in relation to the foliation angle is generally found to follow a U-shaped anisotropy and obtains the lowest value at 45°. For each test, the modulus gets the minimum value in the damage stage and recovers in the unloading stage, but it cannot reach its original value. Sample strength increases as the foliation orientation angle increases from 0° to 45° in relation to σ₂, because the increase in normal stress from σ₂ causes the role of the strong foliation bonds to become significant as the bonds strengthen. The damage to the strong bonds leads to a larger reduction in the modulus at the damage stage.