# Experimental Study on Permeability Characteristics of
## 1 Introduction  
During the production and processing of coal resources, a large amount of coal gangue is generated [1, 2, 3]. Among these, only a limited amount of high-calorific-value coal gangue is utilized, while a significant quantity of low-calorific-value coal gangue is compelled to pile up. Consequently, land occupation and soil erosion are caused, and geological disasters such as landslides and mudslides can be triggered, which subsequently lead to the degradation of the ecological environment in mining areas. The large-scale disposal of coal gangue has become a major challenge for the environmental development of coal mines [3, 4, 5]. After coal seam mining, a substantial underground void is formed, that is, the mining space. The generation of the mining space inevitably induces the fracture of the overburden, damages the crack structure of the underground rock layer and the stability of the aquifer, and causes problems such as water resource loss. Consequently, water-preserved mining has emerged as a challenging issue in the development of coal resources [6, 7, 8]. The gangue grouting and backfilling technique refers to using filling pumps, filling pipelines, and other equipment to transport gangue slurry made of broken gangue and water to the underground mining space [9, 10]. Figure 1 shows the technical principle of gangue grouting and backfilling. Under the premise of meeting the critical speed of slurry and ensuring the stable and efficient conveying of gangue slurry, the timely filling of gangue slurry into the mining space can provide certain support for the overburden and suppress rock movement and surface deformation [11; 12; 13]. When the gangue slurry is injected into the mining space, it continuously flows in the voids and cracks of the fractured rock mass. The moisture of the gangue slurry is continuously absorbed by the rock layer, and the aggregates in the gangue slurry are gradually compacted by the gravity of the overburden, rock compression, and local stress concentration. In the separation zone, a whole CBB is formed, which plays a bearing and water-blocking role [14; 15; 16]. Therefore, the gangue grouting and backfilling technology offers a viable solution to the two major challenges associated with large-scale disposal and water conservation mining of gangue, and studying the permeability characteristics the CBB after gangue grouting and backfilling is of great significance. Extensive research on the permeability characteristics of loose rocks, cemented backfill materials, and other materials has been conducted through theoretical analysis, experimental testing, and other means. The patterns of change in material permeability and the main factors influencing it are investigated. To study the permeability of loose materials or broken rocks, [PERSON] et al. [17] tested the permeability characteristics of backfill materials mixed by coal gangue and fly ash using a self-made permeability testing system and introduced fractal characteristics to study the key factors affecting the complex pore permeability of broken coal gangue. The experimental results showed that adding a certain concentration of fly ash into the broken coal gangue can achieve good water resistance. [PERSON] et al. [18] studied the relationship between the permeability of carbonate rocks, porosity, and sound velocity. [PERSON] et al. [19] characterized the particle size distribution of saturated fractured sandstone under different axial stresses through the fractal dimension and studied the corresponding relationship between the particle size distribution of saturated fractured sandstone and permeability. To study the permeability characteristics of cemented backfill materials with different components, [PERSON] et al.