PSI - Issue 32

A.A. Baryakh et al. / Procedia Structural Integrity 32 (2021) 109–116 A.A. Baryakhet al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction An increase in the depth of mining to 1,000 metres or more causes not only an increase in rock pressure but also complicates the structure of the rock mass. When mining mineral salts, there is a significant difference in the strength and deformation properties of rocks developed in the geological section, which affects the nature of deformation and destruction of underworked strata. The development of water-soluble ore deposits is associated with the need to preserve the integrity of the water resistant strata separating water-bearing horizons from mined-out spaces (Prugger et al. 1991; Baryakh et al. 2013; Laptev 2009). If they are damaged, fresh water can break through into mine workings, which, due to flooding and the dissolution of salt rocks, often leads to mine loss, intense deformations of the earth’s surface up to the formation of dips in a dynamic form (Baryakh et al. 2018; Andreichuk et al. 2000; Whyatt et al. 2008; Baryakh, Devyatkov 2018). The development of mineral salt deposits at great depths is a relatively new concept. Therefore, the deformation of underworked rock masses has remained understudied, primarily due to the lack of representative experimental data. The Saskatchewan field (Canada), which is the world's largest potash ore producer, is probably the only example of long-term potash mining at depths exceeding 1,000 metres (Ong et al. 2007; Gendzwill et al. 1992). At present, several mines with mine potash seams at great depths are being or have already been built in Russia (Gremyachinskoe and Nivenskoe deposits). The analysis of changes in the stress-strain state of the rock mass is a critical issue for designing safe parameters for mining operations. 2. Methods of mathematical modelling The design conditions of mining the Gremyachinskoe potash salt deposit have been considered. The depth of the mining operations is about 1,200 metres. Like in the Saskatchewan field, the use of a pillar mining system including rigid barrier pillars has been planned. The mathematical modelling of changes in the stress-strain state of the underworked rock mass during mining operations was based on the elastoplastic approach. Using an ideal elastoplastic model, the relationship between deformations and stresses during the prelimiting stage was described by Hooke's law, and the limiting stresses in the compression region were determined by the Mohr-Coulomb criterion, represented as a parabolic Mohr's envelope (Kuznetsov 1947): ൌ ൌ ሺ ൅ ሻ ʹ − ʹ ∙ ൅ ൅ (1) where , are the ultimate one-axial compressive and tensile strengths, respectively. The tangential ሺ ሻ and normal ሺ ሻ stresses are found in the regions where the relation reaches the maximum value. The use of the plasticity criterion in form (1) leads to significant computational difficulties. In this regard, the parabolic Mohr's envelope (1) was approximated by a two-link straight line. Its description is reduced to the classic form of the Mohr-Coulomb criterion: ൌ ൌ ൅ (2) with an adhesion coefficient and inner friction coefficient . In expression (2), stresses and are calculated through values of the principal stresses: ൌ ͳ − ͵ ʹ ൌ ͳ ൅ ͵ ʹ (3) In the tensile region, the ultimate stress was limited by the ultimate tensile strength. The deformation of contacts between the layers was described by Goodman contact elements (Goodman 1974; Groth 1980), adapted for the analysis of their lamination (Baryakh et al. 1992). The numerical implementation was carried out with the finite