# Experimental Study on the Mechanical Properties of Deep Granite under Gradient-Confining Pressure
## 1 Introduction  
The deformation properties of rocks are extremely important in geological engineering and geology research [1; 2; 3]. In particular, under diverse confining pressure conditions, the mechanical behavior and deformation mechanisms of rocks may undergo substantial alterations [4; 5]. For underground engineering, especially mining engineering, development towards greater depths is an inescapable trend [6]. Various countries have different definitions for depth, and most countries consider 600 to 1200 m underground as the demarcation line between deep and shallow depths [7; 8]. Granite, as a crucial component of the deep earth, possesses unique geological features and has a complex formation process [9; 10; 11]. In-depth research is of tremendous significance for unveiling the evolutionary laws of the inner earth and ensuring the safety and stability of underground engineering [12; 13]. There are apparent differences between deep granite and shallow granite in many aspects. Deep granite exhibits some distinctive characteristics. It is formed in the relatively deep part of the crust and is subjected to a high-pressure and high-temperature environment. This results in its crystal size generally being small, with fine and tightly packed grains. The rock structure is compact, the rock strength is high, the rock has outstanding resistance to weathering, and the rock can maintain stability under complex geological conditions [14]. The confining pressure exerts multiple influences on the mechanical properties of rocks. It can notably enhance the strength of rocks, enabling them to endure pressure and shear force more effectively [15]. Moreover, it can modify the deformation characteristics of rocks, transitioning from brittle to ductile and increasing their toughness to better withstanddeformation and destruction. Additionally, the confining pressure affects the fracture mode and permeability of rocks, and variations in confining pressure may lead to changes in the fracture mode and a reduction in permeability [16; 17]. In previous studies, a multitude of scholars have conducted extensive and in-depth discussions regarding the properties of granite through a substantial number of tests and theoretical analyses, and these studies have revealed the substantial influence of confining pressure on the strength of the rocks, the deformation modulus, and the failure mode. These research findings have provided a solid foundation for a better understanding the failure mechanism of deep granite. [PERSON] et al. investigated the mechanical behavior of granite after heat treatment via conventional triaxial compression tests, and the results indicated that, with increasing temperature, the brittleness of the granite specimen decreased, while the ductility and plasticity increased [18]. [PERSON] et al. investigated the thermodynamic properties of granite under triaxial stress and discovered that the elastic modulus of coarse-grained granite exhibited an initial slow increase followed by a sharp decrease with increasing temperature [19]. [PERSON] et al. carried out cooling and heating cycles on granite at diverse temperatures and reported that, at a given heat treatment temperature, the thermal diffusivity of the granite decreased as the number of cycles increased [20]. [PERSON] et al. 's research revealed the presence of anisotropy in the long-term strength of granite [21]. [PERSON] et al. examined the mechanical properties of jointed granite under high-temperature conditions and analyzed the failure patterns of jointed rock masses in conjunction with increases in temperature, decreases in temperature, and variations in joint type. Research has indicated that high-temperature conditions modify the mechanical properties of granite, particularly when the temperature is higher than 400 \({}^{\circ}\)C, at which point the peak stress sharply decreases [22]. [PERSON] et al. quantitatively investigated the damage to granite through mineral analysis, uniaxial tensile tests, uniaxial compression tests, and acoustic wave tests [23].