PSI - Issue 32

A.K. Fedoseev et al. / Procedia Structural Integrity 32 (2021) 202–208 A. K. Fedoseev/ StructuralIntegrity Procedia 00 (2021) 000 – 000

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Though in 2015 mined out space in layers KrII and AB were backfilled with ratio 0.6 and 0.3 correspondingly and from May 2015 visible decrease in subsidence rate were observed, high gradient at south and north edges of subsidence trough was already formed. In 2020 subsidence increment per year stayed around 4-5 cm and maximum subsidence reached 2.90 m. High gradient of vertical displacements maintained at south and north borders of the problem area indicates high possibility of WPS integrity violation. Naturally, during all those years yearly geomechanical researches in 2D and 3D problem statements have been held (Baryakh et al., 2017). Main geological structure features of mined rock mass and gravity forces were taken into account during mathematical modelling. To analyze mined rock mass deformation with time rheological approach based on mathematical description of earth surface subsidence increase prediction diagrams was used (Amusin, 1973; Baryakh et al., 2005; Baryakh and Samodelkina, 2012). Possibility for subvertical cracks formation in rock was estimated using Mohr-Coulumb criterion. Numerical implementation was performed by the finite element method in displacements (Zienkiewicz, 2013). According to the results of mathematical modelling, at beginning of 2020 due to tensile stress a crack zone in the upper part of WPS is already formed at the border between east block 4 and well 492 safety pillar (Fig.2). Annual geophysical researches confirm the calculated results. As one of the possible methods of preventing water inflow into mine openings of the BKRU-4 mine pumping of grouting materials and creation of antifiltration screen in salt-marl overburden (SMO), providing waterproofing of fault zone in WPS, was suggested. FORTIS WN-AM100S was chosen as the most suitable grouting material for SMO rocks. Dependence of pressure distribution on distance from tamping wellbore under different pumping pressures is shown on Fig.3.

Fig.3. Pressure distribution within range of pumped grouting material Fortis after 45 minutes from the start of pumping (moment of jellification beginning) 4. Calculation results When pumping grouting solution through a single borehole over the center of fault zone in WPS top with pressure even or less than 2 MPa only insignificant downward propagation (1-2 m) of fault zone occurs. Grouting material infiltration into crumbling part of WPS doesn’t change situation significantly. Only slight growth of fault zone is observed mostly for pumping pressure around 2 MPa. But as it’s obvious from figure 2 and figure 3 pumping through a single borehole won’t provide enough screen over fault zone which diameter at the border with overburden aquifer exceeds 40 m. Hence the problem statements from Fig.1b and Fig.1d should carry more validity.