Pore structure characterization and nonlinear seepage characteristics of rock mass in fault fracture zones

In order to reveal the mechanism of the occurrence of water inrush hazards in fault fracture zones in coal mining, it is necessary to study the pore structure characteristics and permeability of rock mass in fault fracture zones. A test device for fractured rock mass under coupled seepage and stress was developed. A method was proposed for determining the pore structure and hydraulic properties of fault rock mass under compression stress. The pore size distribution and nonlinear seepage behavior of fault rock mass affected by particle size distribution and stress were investigated. A permeability prediction model based on the nuclear magnetic resonance(NMR) was proposed. The experimental results show that(1) the pore size distribution of both continuously graded and gap graded rock mass with a particle fractal dimension of 2.6 shows a three-peak structure, and the absence of small particles in graded rock mass leads to the increase of porosity and maximum pore size, which decreases the compressibility of the fault rock mass sample.(2) The permeability of graded fractured rocks is dominated by the permeable porosity, and the bound porosity is less influenced by the stress. The increased stress and the absence of large particles lead to the decrease of permeability of fault rock mass sample.(3) The nonlinear seepage behaviors of both continuously graded and gap graded fault rock mass can be described by the Forchheimer equation, and the permeability and the β-factor of non-Darcy flow of fault rock mass vary in opposite trends under the influence of stress. The absence of small particles leads to a reduction in the β factor of graded fractured rock mass.(4) A NMR-based permeability prediction model for graded fault rock mass was proposed, which can significantly improve the accuracy of permeability prediction for graded fractured rock mass by removing the influence of bound pores and considering the weighted contribution of seepage pores. The results provide a scientific basis for revealing the mechanism of water inrush disaster and the prediction and prevention of water inrush disaster in coal mining.