## 5 Summary and Conclusions  
In this study, deep-mine sandstones with pore seepage problems were selected as the test samples. The permeability evolution of sandstones with different pore structures under high confining pressures, high water pressures and high temperatures was studied by single-factor and three-factor orthogonal tests. The main conclusions are as follows:  
1. The single-factor test of confining pressure showed that the permeability of the four subcategories of sandstone nonlinearly decreased with increasing confining pressure, and the permeability rapidly decreased in the low confining pressure range of 0-10 MPa. However, the rate and extent of permeability reduction for the four subcategories of sandstone were different, gradually increasing from subcategory I\({}_{1}\) to I\({}_{2}\) to II\({}_{1}\) to II\({}_{2}\). 2. The single-factor test of water pressure showed that under the three levels of confining pressure tested, the permeabilities of the four subcategories of sandstone varied with the water pressure difference, but the variation trends were different. The difference was shown in the following two aspects: (a) subcategory I\({}_{1}\), I\({}_{2}\) and II\({}_{1}\) sandstones increased linearly with increasing water pressure, but the rate of increase in the permeability gradually slowed from subcategory I\({}_{1}\) to I\({}_{2}\) to II\({}_{1}\). (b) Under different confining pressure levels, the variations in the permeability of subcategory II\({}_{2}\) sandstone were different; the permeability varied greatly and decreased as a power function under a low confining pressure (20 MPa), but the variation was small or basically stable under a high confining pressure (over 30 MPa). 3. The single-factor test of temperature showed that the permeability of the four subcategories of sandstone decreased with increasing temperature, but the variation trends were different. With increasing temperature, the permeability of subcategory I\({}_{1}\) sandstone decreased linearly and slowly; the permeability of subcategory I\({}_{2}\) and II\({}_{1}\) sandstone decreased as a cubic curve (decreasing slowly at the beginning, but the deceleration increased significantly when the temperature was higher than 45 \({}^{\circ}\)C); the permeability of subcategory II\({}_{2}\) sandstone decreased almost as a power function (decreasing rapidly at the beginning, but the deceleration slowed significantly, with little change when the temperature was higher than 45 \({}^{\circ}\)C). 4. The orthogonal test results showed that the confining pressure had the greatest influence on the permeability of deep-mine sandstone, followed by temperature and seepage pressure. Under the action of high water pressures and high temperatures, the variations in the permeability of sandstones with different pore structures were different. Sandstone with many pores and large pore throat radii corresponded to a more obvious expansion effect on the pore walls by the water pressure, while sandstone with fine pore throats was more sensitive to the microscale volume change caused by the temperature change. 5. High confining pressures, high water pressures and high temperatures can change the pore structure of deep-mine sandstone through the stress-strain relationship, leading to a change in the pore permeability of sandstone. The permeability evolution of sandstones with different pore structures varies under the same external conditions. Hence, the pore seepage of deep-mine sandstone was the result of the comprehensive influence of the pore structure of the sandstone itself and factors such as high confining pressure, high water pressure and high temperature. This conclusion can be used in guiding classified grouting of deep sandstone and optimizing grouting parameters. editing, supervision, funding acquisition. S.Z. : validation, data curation. X.L. : resources, investigation, software. All authors have read and agreed to the published version of the manuscript.