Surface residual subsidence and coal pillar stability influenced by coal creep in deep strip mining

Surface residual subsidence and coal pillar stability are two key factors for construction over strip mining gob. The prediction parameters of the probability integral method，widely used for residual subsidence，can hardly re⁃ flect the creep property of coal. The numerical simulation method can use the creep constitutive models，but it is hard to obtain the creep parameters of creep models. A creep and seepage test is carried out with coal samples from the Daizhuang Coal Mine. The creep curves of coal samples are classified into two types，that is，stable creep and unstable creep. With the test data，four creep bodies including Kelvin，Generalized⁃Kelvin，Burgers，and Modified⁃Burgers are used to obtain four groups of creep constitutive equations and their creep parameters K，G0，G1，η1 and η2 by the curve inversion method. A FLAC3D numerical simulation model is set up to simulate deep strip mining in the Daizhuang Coal Mine and the results show that the coal pillar will enter an unstable creep state after strip mining. Using the Bur⁃ gers creep constitutive model and creep parameters from inversion，the surface subsidence values within 13 years after strip mining are obtained. The mechanism of “strip mining⁃unstable creep of coal pillar⁃overlying strata movement⁃sur⁃ face residual subsidence” is obtained. The accumulated maximum surface subsidence is 434 mm and the subsidence coefficient is 0.167. Among annual subsidence velocity values，a maximum value 91 mm / a occurs at the end of the first year，other values are below 50 mm / a，and subsidence velocity values take on a downtrend generally and will be stable at last. According to the Mohr⁃Coulomb failure criteria and tension failure criteria， the Yield Approach Index (YAI) of coal body unit is set up to be a quantitative evaluation index of coal pillar stability. Using the CVISC creep constitutive model，the influence of shear strength (cohesion and inner friction angle) and mining⁃pillar⁃width on coal pillar stability at the end of the first year after mining is analyzed with the YAI. The results show that the coal pillar stability of deep strip mining is mainly influenced by mining，time，and strength. Deep strip mining will result in the stress concentration of coal pillar，if stress in the core of coal pillar reaches the threshold value of unstable creep， coal strain will increase continually，and the shear strength of coal body will decrease increasingly under the influence of water flowing to the gob，which will shrink the Mohr⁃Coulomb yield surface. Once the shear stress of coal body rea⁃ ches the yield surface，the range of coal failure area will increase and the core area will decrease further，till entirely unstable.