Effect of thermal treatment on unloading-induced fracture activation of granites during triaxial shear slip experiments

Abstract

The thermal effect has a significant impact on the activation and slip characteristics of fractures. In this study, four pairs of granite fractures were treated by temperatures T ranging from 25 °C to 900 °C. The fractures were then employed to carry out triaxial unloading-induced shear slip experiments. The step unloading of confining pressure σ 3 was used as a disturbed stress to activate fractures that were in a near-critical stress state. The slip characteristics, frictional behaviors, as well as damage modes of fractures with different T , were systematically investigated. The results show that at T = 25 °C and 300 °C, no stick-slip events were observed, and the slipping process of the fractures was characterized by aseismic slip and creep, respectively. For T = 600 °C and 900 °C, the fractures slipped stably, with occasional interruptions by episodic stick-slip events. Ultimately, they entered the dynamic slip stage after a series of consecutive stick-slip episodes. With increasing T , the number of sheared-off asperities increases due to thermal damage, which in turn leads to an increase in the occurrence of stick-slip events. The slip modes of the fractures transited from friction strengthening to friction weakening. As T increased from 300 °C to 900 °C, a considerable quantity of generated gouge layer acted as a lubricant for the slipping of fractures. This resulted in a notable increase in the proportion of aseismic slip, which rose from 24% to 54%. As the temperature increased from 25 °C to 900 °C, the crack length increased exponentially from 2.975 mm to 45.349 mm. For T = 600 °C and 900 °C, the duration between stick-slip events decreased as stick-slip events occurred more frequently.

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