PSI - Issue 32

S. Yu. Devyatkov et al. / Procedia Structural Integrity 32 (2021) 56–63 S. Yu. Devyatkov / StructuralIntegrity Procedia 00 (2019) 000 – 000

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1. Introduction The pillar mining of potash seams is accompanied by the destruction of roof rocks and floors of mine workings in the extraction pillars of longwall faces and by the deformation of underworked rock masses, including the soil surface. This type of mining therefore entails a constant risk of sudden caving of the roof and the discontinuity of a waterblocking stratum, leading to a breakthrough of oversaline water into mine workings (Prugger and Prugger 1991; Shiman 1992) and the transition of static subsidence into a dynamic form. This poses a real threat to both the wider mining operation and safe operation in underworked territories, and can cause significant financial losses and negative social, economic and environmental consequences (Ponomarenko 2012). This study focuses on the displacement of the earth surface and the changes in the stress-strain state of the underworked rock mass during the advance of a longwall face. Calculations were carried out based on a two dimensional elastoplastic formulation, using the finite element method. The relationship between the deformation and stresses at the sublimit stage was described by Hooke's law, and the ultimate stresses in the compression region were determined based on the Mohr-Coulomb yield criterion. The ultimate tensile strength was used to analyse the rock discontinuity in the region of tensile stresses. 2. Geomechanical Model To estimate the changes in the stress-strain state (SSS) of the rock mass during longwall mining, geomechanical modelling of mining layers II, II-III and III was carried out under conditions of slice mining of the seam. Our two-dimensional scheme was designed to evaluate changes in the SSS of the rock mass. It was built to represent the advance in a longwall face and corresponds to the typical geological section of the Starobinskoye field (Fig. 1). The main structural features of the underworked mass were taken into account in the mathematical model of the investigated area. The boundary conditions were formulated as follows: the upper boundary (daylight surface) was free; at the bottom boundary, the vertical displacements were equal to zero; and at the lateral boundaries, the horizontal displacements were also equal to zero. The computational domain was under the action of mass forces with intensity (where is the specific weight of the rocks of the -th element of the geological section). A model of an ideal elastoplastic medium with internal friction was used to determine the SSS of the rocks. A parabolic Mohr's envelope was used as the plasticity condition in the compression region (Kuznetsov 1947). Localisation of the plastic deformation takes place when the following relation is fulfilled: ൌ ∗ ൌ ൅ ʹ − ʹ ൅ ൅ Ǥ In the tensile region, ͳ ൌ ǡ where ൌ ͳ − ͵ ʹ is the maximum tangential stress, ൌ ͳ ൅ ͵ ʹ is the normal stress, is the ultimate compressive strength, is the ultimate tensile strength, and ͳ , ͵ are the principal stresses, which are determined based on the results of mathematical modelling. We note that the localisation zones of plastic deformation in the areas of compression and tension are identified with rock jointing processes,due to the development of shear and opening mode fractures, respectively. The problem was solved numerically using the standard finite element method (Zienkiewicz 1971). To take into account the plastic nature of the rock deformation, the secant matrix method was used (Fadeev 1987). The ultimate tensile strength of the rocks is much lower than their compressive strength. In view of this, the localisation of tensile tractions in some regions of the mass is a prerequisite for the destruction of the rocks. At the same time, it is obvious that the rocks destroyed by tensile stresses will not cave into the worked-out space if they are surrounded by a material that has not lost its bearing capacity. The emergence of the action zone of tensile stresses on daylighting was therefore taken as the first condition of the rock caving. This criterion was implemented