Analysis and Application of Hydraulic Fracturing to Control Hard and Stable Roof in Initial Mining Stage

Abstract

Previously conducted studies have established that the sudden collapse of large areas of overhanging roofs in longwall working faces can cause shock loads and wind blast. In order to solve the problem of the hard and stable roof (HSR) being difficult to collapse in the initial mining stage, a combination of theoretical analysis and numerical simulation is used, taking Dongqu coal mine as the engineering background. The mechanical model of the initial fracture of the main roof is established, the relationship between the thickness-to-span ratio of main roof, the internal stress of the roof is analyzed, and the relationship between the thickness of the roof and the initial weighting step is revealed. The method of hydraulic fracturing (HF) is proposed to pre-crack the main roof in order to achieve controllability of the roof. The effect of HF on the control of the HSR is analyzed. The main conclusions are as follows: under the condition of a certain span of the rock beam, a smaller thickness of the rock beam makes for a larger tensile stress inside of it, increasing the likelihood that the rock beam will fracture. It is possible to reduce the initial weighting interval of the main roof by decreasing the thickness of the HSR and increasing the thickness of the load layer. The abutment pressure in the initial mining stage is obviously reduced after HF. The technical scheme of HF was proposed and applied in the field, and the field observations show that after HF, the HSR can collapse in time at the initial mining stage, leading to remarkable results being achieved.