## 1 Introduction  
In the 21 st century, due to a large number of deep underground engineering projects, such as deep coal mining, deep tunnel construction and deep nuclear waste storage [1, 2, 3, 4], the impacts of groundwater seepage through pores on these projects cannot be ignored [5]. For example, most of the strata that are crossed by deep mining shafts are sedimentary sandstones with abundant water and developed pores [6, 7, 8, 9, 10]. Years of experience in grouting sandstone aquifers to plug water seepage in deep mining shafts have shown that even when cementing and chemical grouting are carried out in the rock mass behind the wall and the main water inflow channels (fractures) in the rock mass are effectively blocked, residual water can still keep through sandstone pores in the shaft wall. This seepage cannot be ignored because the total water leakage exceeds the upper limit of the accepted specification (\(\leq\)10 m\({}^{3}\)/h, see the relevant Code for acceptable shaft sinking and drifting for coal mines in China GB 50213-2010) [11]. The difficulty of grouting sandstone pores has become a bottleneck that restricts the rapid excavation and normal handover of shafts [12]. Unlike fracture seepage, the underground water that seeps from sandstone pores has no visible seepage outlet, so it manifests as \"large-area sweating\" rather than \"local spraying\". A small area of \"sweating\" in shallow shafts generally produces a small seepage discharge, so the prevention and control of sandstone pore water in shallow shafts is often ignored. However, deep mine shafts in sandstones have two remarkable characteristics--alarge water seepage area and a high pore water pressure on the shaft wall--resulting in a high degree of pore seepage discharge compared with shallow shafts. Because the pore seepage channels of sandstone are generally narrow and structurally complex, they are difficult to grout. The traditional prevention and control methods for fracture water are not ideal for pore water. For example, the freezing method is not ideal for the deep parts of mining shafts, and ground pregrouting technology generally involves clay cement slurry, which is mainly suitable for fractured strata but unsatisfactory for pore water in cemented sandstone. At present, there is no ideal method for preventing pore water seepage in sandstone, and research on sandstone permeation grouting is limited; only a few scholars have carried out laboratory porous seepage experiments with porous materials similar to rocks [13]. According to past engineering practice [14; 15; 16; 17; 18], the main technical problems involved with the use of permeation grouting to plug seepage water from sandstone pores include two aspects: groutability and reduced permeability. Groutability is the primary problem to be solved and mainly depends on the sandstone pore structure. Therefore, by analyzing various types of data, such as casting thin slice observations, scanning electron microscopy (SEM) results and mercury injection capillary pressure (MICP) test results, the sandstone pore structure and its influential factors were studied in this paper to establish an effective classification and evaluation method of the pore permeability for sandstone grouting and to explore a classified permeation grouting method. An overview of the flow of this study is shown in Figure A1 of Appendix A. First, taking the three most widely distributed layers of sandstone aquifers in the Southwest Shandong coalfield as experimental samples, four appropriate experimental methods for studying sandstone pore structure and permeability are selected, which are widely used in petroleum fields. Next, based on the analysis of the experimental results, the pore structure of sandstone is determined, various influencing factors of sandstone permeability are analyzed, and the key evaluation indicators of sandstone permeability are selected.