Discontinuities in rock mass behave as weak planes, and thus, understanding the behaviors of discontinuities is crucial to assess the stability of underground structures. It should be noted that shear characteristics of discontinuities are usually affected by the interaction among overburden depending on the depth, tectonic stresses, water pressure by ground water level and elevated temperature at depth. Thus, it is necessary to evaluate the variations of the frictional properties of rock discontinuities which can be suitable for the engineering problems. In this study, a series of shear tests were carried out on three types of rocks (Daejeon granite, Goheung diorite, and Linyi sandstone) which have a single saw-cut surface to investigate the shear characteristics of a rock discontinuity under various thermal -hydro-mechanical (T-H-M) 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 value of 2.05 and 11.63. The testing conditions were determined considering in situ THM conditions at the vicinity of an underground structure such as a disposal facility for radioactive waste, enhanced geothermal system, and oil reservoir. The experimental results were analyzed based on Coulomb's and Patton's failure criterion. It was observed that the shear characteristics of the discontinuity were sensitive to confining pressure and water pressure variations but not to temperature change below 80°C. XRD analysis and SEM observation were made to figure out the mechanism which causes the decrease of the friction angle. Clay minerals having layer lattice structure and soft powder grains may have reduced the friction angle.
Understanding the behaviors of rock discontinuities is of great importance to ensure the stability of an underground structure such as a disposal facility for radioactive waste, enhanced geothermal system, and oil/gas reservoir. In particular, the frictional behavior tends to change depending on the complex interaction among thermal, hydraulic and mechanical (T-H-M) characteristics and their coupled effects. Numerous studies including Lockner et al. (1982), Tembe et al. (2000) and Kim and Jeon (2016) have been performed to predict shear strength of discontinuities under T-H-M coupled conditions.