Estimation of earth surface deformations during pillar mining

Abstract

Stoping of potash seams during pillar mining is accompanied by deformations in underworked rock mass and the destruction of roof rocks and floors of mine workings in the extraction pillars of longwall faces. This type of mining can also cause sudden caving of the roof and seam floor in an extraction pillar of a longwall face, thus posing a threat to miners, leading to unplanned stoppages of mining operations and reducing the efficiency of mining. In addition, during the advance of a longwall face, subsidence of the soil surface occurs. It is therefore important to study the deformation of the soil surface and find ways of predicting this during the underground mining of potash seams. To evaluate the stress-strain state of the rock mass during longwall mining, a geomechanical model of potash seam mining was constructed under conditions of slice mining. This model took into account the main structural features of the underworked mass, and the caving of the roof of an underworked seam with filling of the worked-out longwall face and the opening of clay contacts in the salt stratum during mining operations. The results of mathematical modelling showed that a localised area of plastic deformation is formed in the soil, which is associated with the formation of opening mode fractures. At the same time, caving of the roof rock and simultaneous filling of the worked-out space take place, preventing further development of the rock jointing zones. The properties of the rocks from the caved roof determine the nature of this mass deformation. By varying these properties, we were able to achieve acceptable agreement between the estimated subsidence and field observations of the displacement of the earth surface. A comparative analysis of the estimated and actual subsidence shows that in order to yield an accurate description of the mass deformation, softening of the caved rocks should not take place immediately to give the final values of the deformation properties, but should change over time.