Experimental study on thermo-mechanical properties of Beishan granite under mild temperature

Temperature is believed to have an important influence on the mechanics behavior of granite. To investigate thermo-mechanical properties of Beishan granite, the host rock of China's preselected underground laboratory of high-level radioactive waste (HLW) geological repository, mechanical tests including uniaxial compression test and incremental cyclic loading-unloading test under mild temperature range (mom temperature to 300 degrees C) were carried out with Acoustic emission (AE) activity monitoring. Results show: (1) The temperature threshold for Beishan granite is 150 degrees C. The UCS (uniaxial compressive strength) is increasing and pore is healing with increasing temperature in the range of room temperature to 150 degrees C, while the UCS is decreasing and pore is enlarging with increasing temperature in the range of 150 degrees C-300 degrees C. (2) The cracking activity in the compression process is significantly influenced by the ambient temperature. The AE activity of Beishan granite under different temperature exhibits three patterns (inactive-active for granite under room temperature, active-inactive-active under 60 degrees C-150 degrees C, active-inactive for granite under 200 degrees C and 300 degrees C). (3) Results of incremental cyclic loading-unloading compression tests show that the peak stress, peak strain and AE energy of granite increase with the elevated temperature. The strength and ductility are enhanced with increasing temperature in the mild range. (4) AE activity during the incremental cyclic loading-unloading test is used for loading-unloading response ratio (LURR) and felicity ratio (FR) analysis. The LURR value keeps increasing before pre-failure stress under lower temperature, but shows decrease trend before failure under higher temperature. The stress corresponding to the sharp drop on FR is identified as the stress threshold for rock damage, which are 0.57.7, to 0.66.7, for Beishan granite under the mild temperature conditions. This work will provide support for the long-term safety assessment for the HLW geological disposal engineering.