# Experimental Study on Permeability Evolution of Sandstone during Triaxial Compression Damage
## 1 Introduction  
The expansion of various engineering activities in China, such as transport tunnelling, water conservation, hydropower projects, and deep mining, has led to a transition from shallow to deeper underground projects. This shift has introduced new challenges as rocks in the deep underground are subjected to complex geo-stress and seepage pressure conditions, making the determination of their mechanical parameters and seepage prediction more intricate. Ensuring stability and safety in deep rock engineering projects requires a complete knowledge of the mechanical and permeability attributes of rocks under different seepage pressures and confining pressures. As a result, investigating the coupling between rock stress and permeability holds significant theoretical and practical importance for deep rock engineering [1; 2; 3; 4]. Therefore, the study of rock under stress-permeability coupling conditions has important theoretical and applied value for deep rock engineering. In the past few years, many experimental studies have studied the permeability of various types of rocks during the triaxial seepage stress process, including high-strength granite [5; 6], low-strength sedimentary rock [7], and sandstone with high porosity [8; 9; 10]. These results provide an important reference for further understanding the mechanisms by which strength and porosity affect rock permeability. [PERSON] et al. [11] carried outtriaxial hydromechanical tests for three different lithologies and analyzed their permeability evolution by combining thin section image analysis and micro-focus X-ray computed tomography of structural changes. [PERSON] et al. [12] explored the evolution of axial strain, volume strain, and permeability in rock specimens during the failure process by conducted a hydro-mechanical coupling test on limestone and sandstone. In addition, [PERSON] et al. [13] formulated a model for the permeability evolution of rocks in the volume compression stage and volume shearing stage. These results provide an important reference for understanding the rock deformation and fracture mechanism. [PERSON] et al. [14] studied the variation of the Biot coefficient and permeability by conducting a hydro-mechanical test. [PERSON] et al. [15] and [PERSON] et al. [1] investigated the seepage characteristics of rocks under low pressure and high pressure, and the correlation between deformation characteristics and permeability was revealed. This means that the fracture and deformation characteristics of rock have an important effect on permeability. In addition, some scholars have carried out hydro-mechanical coupling experiments for fracture rocks, such as [PERSON] et al. [16], who investigated the influence of stress paths on the mechanical properties and permeability evolution of quartz-filled fracture rock. [PERSON] et al. [17] conducted a series of tests on the shear-seepage coupling characteristics of a single fractured rock mass under cyclic loading and unloading. During the hydromechanical test, energy accumulation, evolution, and dissipation were measured. [PERSON] et al. [18] conducted a series of compaction-permeability tests with in situ nuclear magnetic resonance (NMR) measurement; the permeability variation mechanism and the influence of stress damage on permeability were investigated during the complete stress-strain process. [PERSON] et al. [19] investigated the damage characteristics of sandstone during hydromechanical tests by conducting an acoustic emission test, and the power law relationship between permeability and porosity were determined. [PERSON] et al. [20] revealed the relationship between damage properties and the permeability of granite by conducting hydro-mechanical tests. [PERSON] et al. [21] studied the effects of pore volume change and seepage pressure on sandstone deformation and defined a damage variable and established a semi-analytical permeability change model. In addition, [PERSON] et al.