Experimental Study on Shear Behavior of a Rock Discontinuity Under Various Thermal, Hydraulic and Mechanical Conditions

Discontinuities in rock mass behave as weak planes, and it is crucial to understand their behavior when assessing the stability of underground structures. Shear characteristics of discontinuities are usually affected by the interaction among overburden stress, tectonic stresses, water pressure by groundwater level and temperature at depth. Thus, it is necessary to evaluate the variation of the frictional properties of rock discontinuities at different conditions. In this study, a series of shear tests were carried out for three types of rocks (Daejeon granite, Goheung diorite, and Linyi sandstone) having a saw-cut surface to investigate the shear characteristics of a rock discontinuity under various thermal-hydro-mechanical conditions in a triaxial compression chamber. 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 having JRC values of 2 and 12. The testing conditions were determined considering in situ conditions at the vicinity of underground opening such as radioactive waste disposal facilities, enhanced geothermal systems, and oil reservoirs. The experimental results were analyzed based on Coulomb’s and Patton’s failure criterion. It was observed that the shear characteristics of rock discontinuities were sensitive to confining and water pressure but not to a temperature below 80 °C. XRD analysis and SEM observation were performed to identify the mechanism responsible for the change of friction angle. Clay minerals having a layer lattice structure that results in weakening of the bonding of minerals may have reduced the friction angle.