Fracture Slip Behavior in Granite Under High-Temperature True Triaxial Loading Tests

Precise knowledge of the thermal and mechanical properties of polycrystalline rocks is vital for the assessment of geothermal targets. However, although the slip behavior of rough fractures in granite under high temperatures and true triaxial stress is closely related to that corresponding to hydroshearing and induced earthquakes during geothermal recovery, it has rarely been investigated. In this study, true triaxial loading tests are conducted on intact and macrofractured granite under various high temperatures. The influences of high temperature on the thermal expansion, strength, frictional stick-slip, and failure mode are compared. The initiation and propagation of fracture slip are specifically investigated using the monitored deformation. The results indicate that the intact granite expands uniformly along sigma 1, sigma 2, and sigma 3. In contrast, the thermal expansion of the confined prefractured granite is anisotropic (varies in different directions), especially at higher temperatures. In addition, the peak strength of the confined intact granite tends to increase with increasing temperature above 100 degrees C. The fracture activation strength, residual strength, and frictional stick-slip amplitude of the prefractured granite tend to increase with increasing temperature, and the increase in peak strength becomes much more significant above 300 degrees C. Fracture strengthening is closely associated with a decrease in the fracture aperture and an increase in the asperity contact area between two macroscopically mated fracture surfaces at elevated temperatures. Moreover, a long quasi-static slip stage with a gradually increasing slip rate (0 - 1.6 mu m/s) precedes the final seismic slip event (at a rate of 1.1 -2.6 mm/s). This study simultaneously considers the (1) irregular fracture surface, (2) true triaxial stresses, (3) high temperatures, and (4) thermal deformation and related mechanical behavior of the specimens to interpret the true triaxial loading test results and thus contributes to a better understanding of the dynamic slip process of rough fractures under thermal-mechanical coupling conditions in deep rock masses. Prefractured granite specimens were sheared under different high temperatures using a true triaxial loading system to closely simulate the real thermal-mechanical coupling conditions of a deep rock mass.The thermal expansion of prefractured granite along the directions of 1, 2 and 3 is anisotropic due to the presence of fractures, while intact granite undergoes isotropic thermal expansion.The fracture activation strength, residual strength and stick-slip amplitude tend to increase with increasing temperature, which is related to the decreasing fracture aperture and increasing asperity contact area at higher temperatures.Long preseismic slip precedes abrupt seismic slip, and the seismic slip rate is three orders of magnitude greater than the loading rate.