Influence of roadway under coal seam floor on the water resistance of coal seam floor strata with far field stresses and the optimization of control technology

The construction of floor pumping roadway in flooding mining area can affect the water resistance of floor strata, increase the risk of coal mining and bring challenges to the floor grouting reinforcement technology. So, the study on the influence of roadway on the water resistance of floor strata under far field stresses and the optimization of control technology was carried out. According to the axial and cross section characteristics of the floor pumping roadway, an analysis model of the bending resistance of key rock strata under the far field principal stresses was established and the theoretical solutions of the deformation and the bending moment of the siltstone under the floor failure area with and without a floor pumping roadway were obtained. Furthermore, the application effect of grout filled casing pipes on the reinforcement of rock strata is studied and verified numerically. First, according to the far field principal stresses, the stress component σ′ acting on siltstone II is obtained in the local coordinate system. According to Levy solution of elastic rectangular thin plate single trigonometric series, the precise solutions of rock deformation and bending moment are obtained by using the generalized simply supported edge theory and superposition method. The exact solution of the deflection equation for the thin plate of strata penetrated by the gas roadway with two adjacent edges free and two adjacent edges fixed is got, while the ratio of mining distance to working face length is 0.50, 0.75, 1.00, 1.50 and 2.00, respectively. The calculation results show that the displacement of siltstone II at the intersection of roadway under the influence of roadway under the elastic condition is the largest, which is larger than the maximum displacement of the four sides fixed with the same geometric size. The maximum bending moment of the fixed side of siltstone II under the influence of floor pumping roadway is one order of magnitude higher than that of the fixed side. It illustrates that the siltstone II influenced by gas roadway is easier to be deformed than that without roadway, and its resistance to deformation is reduced to some extent. Based on this, an optimization method for grouting reinforcement of floor rock strata is proposed. The vertical depth of grout filling casing pipes entering into siltstone III is designed to be greater than 2 m, which can play the role of fixed end and increase constraints on free edge of siltstone II. The long slurry casing pipes can improve the boundary conditions of siltstone II and increase the resistance strength due to the deformation difference within the reinforcement range.The numerical results verify the actual effect of grout filling casing on the siltstone deformation.