Shear characteristics of a rock discontinuity under various thermal, hydraulic and mechanical conditions

Abstract

Discontinuities in rock mass behave as weak planes, and thus, understanding the behaviors of discontinuities is crucial to assess the stability of underground structures. At great depth, shear characteristics of discontinuities are more affected by the interaction among overburden depending on the depth, tectonic stresses, water pressure by ground water level and high temperature than at the near surface environment. Many researchers have studied the combined effect on shear characteristics, however, the results were varied with rock types, surface roughness, normal stress, and the degree of temperature and water pressure. Furthermore, most studies examined earth crustal behavior and hence, testing conditions such as normal stress, temperature and water pressure were extremely high. Thus, it is necessary to evaluate the variations of the frictional properties of various rock types which can be suitable for the engineering problems. A series of shear tests were performed under various thermal, hydraulic and mechanical conditions. Three types of rocks including a single saw-cut surface were used, Daejeon granite, Goheung diorite, and Linyi sandstone. The testing conditions were determined considering the situation in the vicinity of underground structures such as a disposal facility for radioactive waste, enhanced geothermal system, and oil reservoir. The friction angle was analyzed based on Mohr-Coulomb failure criterion, which remains the most widely used one in geotechnical practice. The friction angle of the three different types of rock decreased when water pressure was applied on the saw-cut surfaces. Different mechanisms affect the reduction of the friction angle. In XRD analysis and SEM observation, layer lattice minerals were found in Daejeon granite and Goheung diorite, which can reduce the friction angle by readily adsorption of water. For Linyi sandstone, in SEM observation, a large amount of pulverized fine grain was observed after the tests when water pressure was applied. Thus, it can be deduced that detached soft powdered grains may reduce the shear strength of the saw-cut surfaces. On the contrary, the effect of temperature at 80˚C was not significant on the shear behavior. In addition to the tests on saw-cut specimens, the effect of surface roughness on shear characteristics was examined. Cement-mortar was used to reproduce identical rough discontinuities, and two kinds of rough discontinuities were prepared (JRC=2.05 and 11.63). Due to asperity degradation, friction angles were analyzed at low and high normal stress separately based on Pattons failure criterion. When water pressure was applied, the friction angle of rough discontinuities showed a larger decrease at low normal stress below than 10 MPa. The decrease might be caused by the weakening of tensile strength under wet conditions. Thus, it can be concluded that the adverse effect of water on friction angle and shear strength increases on the rougher discontinuities. Whereas at high normal stress greater than 10 MPa, friction angle showed almost similar values independent on roughness, because asperities are sheared off. At an elevated temperature of 80˚C, shear characteristics were not also varied in the same way as in the saw-cut specimens. Numerical simulation using two-dimensional particle flow code was performed for the description of the shear behavior of cement-mortar specimens. Smooth joint model was used to generate a single discontinuity in the numerical model. The results showed a good agreement with the experimental results at room temperature and 80˚C conditions. However, in the HM simulation, there was some discrepancy between experimental and numerical one in friction angle and dilation at low normal stress which should be resolved in the further study. From this study, shear characteristics of different rock types and the effect of roughness were investigated along with numerical simulation. The presence of water reduced the shear strength of a rock discontinuity depending on rock type and normal stress, but temperature of 80˚C did not show any clear effect. This study can contribute to the understanding of the mechanisms of the variations of shear characteristics under various thermal, hydraulic and mechanical conditions. Also, this result can be used to assess and design the stability of rock and underground rock structures, where the effect of T-H-M interaction should be considered.