To understand the effect of cold water injection on the mechanical properties and brittleness of rock, a series of mechanical tests were performed on granite samples. Firstly, granite samples were slowly heated to a series of target temperatures and then rapidly cooled down using flowing water. Thereafter, optical microscopic observation and uniaxial/triaxial compression tests were performed. Based on the experimental results, the thermal effect on brittleness was further analyzed. Experimental results show that numerous thermally-induced micro cracks were obviously generated when the thermal treatment levels were higher than 300 °C. The variation of mechanical parameters under compression showed a transition from thermal hardening to thermal cracking at 300 °C. The uniaxial compressive strength and Young's modulus significantly decreased only when the thermal treatment levels were higher than 300 °C. Meanwhile, significant energy dissipation was observed prior to the appearance of peak stress. The energy-based brittleness evaluation result shows that the brittleness of granite slightly increased before 300 °C, which followed by sharply decreasing due to the generation of thermally-induced micro cracks. These experimental results may provide references for the thermal effect on the mechanical properties of granite during the exploitation of HDR.

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