An estimation method for cover pressure re-establishment distance and pressure distribution in the goaf of longwall coal mines

Measurement of deformations and stresses in the goaf of longwall panels is difficult due to the inaccessibility of the waste area. This paper proposes a method for estimating the distance to return of the cover pressure and the stress distribution in the goaf of flat-lying longwall panels where the caving is bulking controlled. In the development of the method, it has been considered that the final subsidence on the surface is a result of deformation of the caved material and deformation of the strata over the caved zone, i.e. during both the caving and the compaction of the caved material. In this respect, the stress-strain behaviour of the caved rock pile, bending of the lowest uncaved strata depending on seam thickness, and residual expansion of fractured strata vertically over the caved zone have been described by using previous field and laboratory investigations. From the relations established between these parameters and surface subsidence, the re-establishment of cover pressure distance has been characterised with a three-parameter power function in which the independent variables are depth, excavation height, bulking factor and compressive strength of the rock fragments. It has been found that the cover pressure distance increases exponentially by increasing depth, unlike previous findings of investigations in which the cover pressure distance increases as a linear function of depth. On the other hand, the cover pressure distance for a constant depth and mining height increases if the bulking factor of the rock pile increases and/or the strength of rock fragments decreases. When the mining height increases, the bulking factor of the caved rock pile increases, and hence this result with increase in cover pressure distance. An estimation of the stress distribution curve for panels which are wide enough for the development of cover pressure has been expressed with a six-parameter Taylor series polynomial function. The general form of the stress development curves found from the proposed method has been verified with curves obtained through numerical models. (C) 2003 Elsevier Ltd. All rights reserved.