# Pressure-Relief Gas Cooperative Drainage Technology in a Short-Distance Coal Seam Group
## 2 Fracture Evolution Mechanism of the First Mining Layer in the Short-Distance Coal Seam Group  
In the mining conditions of the short-distance coal seam group, the roof and floor of coal seam are displaced and deformed, generating cracks and forming gas migration channels. Based on the slip-line field theory, the coal and rock mass in a certain range under the stope floor lies in the stress cross area [22; 23]. Under the action of shear stress, the floor coal and rock mass in the stress reduction area is easily further damaged in the edge area of the coal pillar and produces mining fissures. The supporting pressure range moves continuously with the advance of the working face. The damaged coal and rock mass moves to the goat under the action of extrusion pressure, and the goat floor expands to form a continuous sliding surface, as shown in Figure 1. At the same time, under the influence of ore pressure, the floor coal and rock mass are periodically destroyed, and the permeability is further increased. The pressure-relief gas of the underlying coal seam rises to the stopping layer along the fracture network through diffusion and seepage.  
In the short-distance coal seam group, the underlying coal seam close to the goat expands and deforms due to full pressure-relief, and the underlying coal seam fracture is fully developed. The horizontal bed separation fractures and vertical broken fractures are connected with the primary fractures of the coal and rock mass to form the main channel of pressure-relief gas migration. Under the actions of rising and the pressure gradient, the relieved gas migrates to the mining seam through the fracture network, which increases the difficulty of gas management at the working face of the first mining layer.  
In the short-distance coal seam group, there is an obvious inflection point in the deformation and displacement of the roof strata. The upper and lower bearing surfaces of the rock stratum in front of the inflection point bear tensile and compressive stresses, respectively, and the upper and lower bearing surfaces of the rock stratum at the rear bear compressive and tensile stresses, respectively. Due to the heterogeneity of the rock strata, the ultimate tensile and compressive strengths of the rock masses within and between the rock strata are different. When the tensile and compressive stresses exceed the ultimate tensile and compression strengths of rock masses, there must be a transverse and longitudinal fracture network, as shown in Figure 2. Based on the co-mining theory of coal and gas, combined with the migration characteristics of pressure-relieved gas in the \"three-vertical zones\", the overlying coal strata of the first mining seam in the short-distance coal seam group is divided into a \"gas-conducting fracture zone\", \"pressure-relieved desorption zone\", and \"difficult desorption zone\" [24]. There is a fracture network consisting of fractures along the seam and broken fractures in the gas-conducting fracture zone [25]. The gas in the adjacent seam can be fully relieved and flows into the mining seam. The gas in this area can be fully exploited, and its height is related to the location of the key seam [26,27].  
Figure 1: Stress field and fracture distribution of mining floor in the first mining layer of the short-distance coal seam group.  
Figure 2: Stress and deformation of the roof in the short-distance seam group.  
The spacing of overlying strata on the first mining layer of the short-distance coal seam group is small, and the occurrence conditions of coal strata are complicated. Under the disturbance of mining, the distribution of surrounding rock stress and overlying rock fracture is more complex, the deformation and failure of the coal and rock mass are more serious, the pressure-relief of mining is more sufficient, and the difficulty of gas management is increased.  
Gas Migration and Enrichment Characteristics of Pressure-relief Gas in the First Mining Seam of the Short-Distance Coal Seam Group