Analysis of triaxial compression deformation and strength characteristics of limestone after high temperature

Understanding the mechanical behavior of rock under conditions of high temperature and pressure is critical when it comes to implementing underground coal gasification, deep disposal of highly radioactive nuclear waste, development and utilization of deep underground space, development and utilization of geothermal resources, etc. To understand the effect of confining pressure on limestone at high temperature, uniaxial and triaxial compression testing was conducted on limestone under high temperature (20–800 °C) using the MTS 815 rock mechanics testing system. Using this system, the change in the strength and deformation parameters of the limestone at varying temperature and confining pressure conditions were studied. The results show that the triaxial compression stress-strain curve of limestone at high temperatures is divided into five stages: compaction stage, elastic stage, plastic deformation stage, post-peak failure stage, and residual stage. In addition, the rock brittleness and ductility decrease as the temperature increases. The peak stress and residual strength of the limestone at high temperatures increase as the confining pressure increases, and the strength of the rock specimens decreases as the temperature increases. The internal friction angle (φ) first increases and then decreases as the temperature increases, but the relationship of the cohesive force (c) to temperature is opposite that of φ, indicating that the shear strength of limestone at high temperatures is determined by both c and φ. The elastic modulus of limestone at high temperatures increases with the increase in confining pressure and decreases as the temperature increases, and the peak strain of limestone at high temperatures increases as the confining pressure and temperature increase. The effect of temperature on the failure of limestone is not obvious, and the failure of rock specimens during uniaxial compression testing was mostly through axial splitting, while the failure of rock specimens under triaxial compression testing was mostly through shear failure. As the confining pressure increases, the fracture type of the rock specimens gradually changes from brittle tensile fracture to shear fracture, and the instability type of the rock specimen changed from the sudden instability type to the progressive failure type. Our research results are especially useful in areas where engineering is performed on rocks under high temperature.