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Understanding the deformation failure behavior of the composite rock strata has important implications for deep underground engineering construction. Based on the uniaxial compression laboratory test of the specimens of composite rock strata containing holes, the microscopic parameters in the particle discrete element simulation are firstly calibrated. Then, the mechanical properties and failure characteristics of the composite rock strata with holes under different confining pressures are studied. The results show that different dip angles and confining pressures have significant effects on the peak strength and elastic modulus of the specimens. Under the same confining pressure, the peak strength and elastic modulus decrease first and then increase with the increasing dip angle. As the dip angle is constant, both the peak strength and elastic modulus gradually increase with the increase in confining pressure. It shows that the first area to be damaged in composite rock strata transfers from soft rock to hard rock with the increase in dip angle. With the increase in confining pressure, the range of tensile stress concentration area decreases substantially, while the range of compressive stress concentration area changes less.

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The deformation and damage evolution of sandstone after heat treatment greatly influence the efficient and safe development of deep geothermal energy extraction. To investigate this issue, laboratory confined compression tests and numerical simulations were conducted on pre-holed sandstone specimens after heat treatment. The laboratory test results show that the failure modes are closely related to the heat treatment temperature, with increasing treatment temperature, the failure modes change from mixed and shear modes to a splitting mode. The cracks always initiate from the sidewalls of the hole and then propagate. The failure process inside the hole proceeds as follows: calm period, particle ejection period, rock fragment exfoliation period and rock failure period. The specimens have different final failure features for the entire rock after heat treatment, but have the same failure features inside the hole. These phenomena can be explained by numerical simulations. The numerical simulations reveal that the failure modes in the numerical results agree very well with those observed in the experimental results. The damage zone always occurs at sidewalls of the hole and then propagates to the entire rock affected by shear or tensile damage. From 20°C to 200°C, thermal effect may promote shear damage and restrain tensile damage, while from 200°C to 800°C, thermal effect promotes tensile damage and restrains shear damage. Notably, the damage zone near the sidewalls of the hole has the same distribution range and pattern. Finally, the differences in the mechanisms due to increasing heat temperature are evaluated using scanning electron microscope (SEM) observations.