Numerical Simulation of the De-stressed Deformation of Mining Coal Strata at Different Pore Pressures

Gas extraction from mining induced de-stressed areas is one of the most efficient and cost-effective methods for eliminating coal and gas outbursts in China. In the paper, the characteristics of fracture induced pressure relief at different pore pressures and confining pressures on the de-stressed deformation of strata are analysed by using FLAC software. A strain-softening constitutive relation was used and the confining pressure was unloaded after the model was loaded to reach a static equilibrium state. The numerical results show that under the same confining pressure, and with the increase of pore pressure, the deformation of strata due to de-stressing as well as the fracture zones also increase significantly, and the characteristics of fissure induced pressure relief becomes more apparent. For seams with high gas pressure, the impact of pore pressure on the deformation of mining disturbed strata cannot be neglected. The numerical results are in good agreement with field observations, and improve the development of the coupled gas-solid theory in mining induced de-stressed strata. INTRODUCTION Coal seams characterized by low gas permeability usually less than 2.5 10 md (0.01 m/ (MPa·d)), are regarded as the hard-to-drain coal seams in China. In order to drain coal gas efficiently, the coal seam permeability must be improved significantly. Various workers (Yu, 1986; Yu, 2005; Cheng, et al., 2003; 2004; Yu and Cheng, 2007) and practices proved that pressure-relief gas drainage, especially the use of the protective seam and pressure-relief gas drainage from the protected layer, was one of the most effective and cost-effective regional methods to eliminate coal and gas outbursts and reduce gas content. The Rules of Coal and Gas Outburst Prevention (hereinafter referred to as The Rules were published in 2009 by the State Administration of Coal Mine Safety of China (SACMSC, 2009). The Rules require that the regional methods must be used as a priority for coal and gas outburst prevention, and local methods could be used as supplemental methods. The regional comprehensive methods, with the protected layer as priority selection, must be taken prior to the extraction of seams with outburst risk. For different geologic conditions, the effects of gas control methods based on de-stressing can be different in the same coal field. Gas control methods should be selected to meet the depressurized effect and the requirement of The Basic Indicator of Gas Drainage in Coalmine (hereinafter referred to as The Indicator). Yuan (2005) showed that dilatational deformation could be used as the indicator of the depressurized effect for the protected layer, which related to the mining height of protective seam, roof control method, the dimension of the coal face and mining depth. Much of the research work on the protective seam are based on physical-scale modelling or numerical calculations that are associated with geo-stress without due consideration of the pore pressure. When the gas pressure is high, the effective stress is much less than the total stress. Liang et al., (1995) showed gas pressure could affect the mechanical strength of the coal mass. Thus, pore pressure could directly influence the mechanical response of coal and rock mass, and consequently the simulations without taking pore pressure into consideration could not reflect the real depressurized effect on the protected layer. This makes it difficult for the use of the protective layer, especially when choosing the protective layer and designing the drainage holes and other projects for coal and gas outburst prevention, which also influences the effectiveness, safety and economical efficiency of pressure-relief gas drainage. Various workers (Liang, et al., 1995; Yao and Zhou, 1988; Zhao, 1992; Jin, et al., 1991; Xu, et al., 1993 and He, et al., 1996) found that the changing of mechanical strength, mechanical response and deterioration is influenced by pore pressure. Lu et al. (2001) updated the Karl Terzaghi effective stress formula of porous media, found that the regulation of effective pore pressure coefficient changes the National Engineering Research Centre of Coal Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China, ypcheng620924@163.com 2011 Underground Coal Operators’ Conference The AusIMM Illawarra Branch 362 10 – 11 February 2011 complete stress-strain process. By using shear strain gradient plasticity theory, Wang and Pan (2001) established the relationship of stress and strain under the confining pressure and pore pressure. Yin and Wang (2009) and Wang et al. (2010) set up a coupled elastic-plastic damage constitutive model, and described the mechanical characters and behaviour of gas coal and rock at different loads. Using FLAC, Wang et al. (2009) numerically modelled a rock specimen with random defects in uniaxial plane strain compression, and investigated the effects of pore pressure on the failure processes, overall deformational characteristics and precursors. In this paper, the characteristics of fissure induced pressure relief of mining coal mass at different pore pressures and confining pressures are analysed by using FLAC software. A strain-softening constitutive relationship is used and the confining pressure is unloaded after the model is loaded to reach a static equilibrium state based on the geological condition of Xinjing coal mine in Yangquan coal field.