Experimental Investigation On The Sandstone Permeability Under Plastic Flow

The plastic zone of surrounding rock in goaf continues to expand after the coal seam is exploited.When the mining panel is pushed forward to a certain distance,the water in the aquifer will flow along the crack to goaf.A water inrush disaster will occur once the penetrating cracks develop from the aquifer to the goaf.Investigation of rock permeability under plastic flow is a basic topic for the prevention and control of water inrush disaster in coal mines.In this paper,the plastic flow and permeability experiments on sandstone were conducted using the MTS816 rock mechanics testing system and self-developed permeation system(triaxial chamber),confining pressure system,and percolation circuit.Based on the theory and method of rock plastic mechanics and seepage mechanics,the flow law of sandstone after shear yield and the evolution law of permeability parameters(permeability,the non-Darcy flow b factor and acceleration coefficient)under plastic flow are studied.Major achievements are provided as follows:(1)The effect of plastic work,plastic equivalent strain and plastic volumetric strain on subsequent yield cohesion and internal friction angle of sandstone are obtained by the conventional triaxial compression tests.(2)The plastic factor expression in Coulomb-Mohr and Drucker-Prager non-associated flow rules was modified,where the plastic work,plastic equivalent strain and plastic volumetric strain were used as plastic internal variables.The modified Coulomb-Mohr non-associated flow rule is applied to the single-step and multi-step predictions of principal strain under the three paths.Results verify the applicability of the modified Coulomb-Mohr non-associated flow rule.(3)Considering the influence of yield criterion,the Prandtl-Reuss flow rule was revised,and then used for the single-step and multi-step predictions of principal strain under three paths.It is applied for the simulation of axial strain-axial stress hysteresis curve and the radial strain axial stress hysteretic curve under the conventional triaxial compression.The results approve the wide applicability of the modified Prandtl-Reuss flow rule.(4)Based on the analysis of the influence of volume and shear deformation on permeability parameters,three models of permeability parameters containing six influence coefficients are constructed.The volume strain and equivalent strain under Coulomb-Mohr criterion,the volumetric strain and shear strain on shear surface,the positive and shear strain on the shear plane are respectively independent variables.Through permeability test under simple loading path,the best estimation of three permeability coefficient sets was determined,which were subsequently applied to predict the permeability parameter under a complicated loading path.The applicability of the model is verified;the permeability parameter influence coefficient is proved as a material constant.(5)The permeability parameter calculation model of the principal stress increment was established by combining Coulomb-Mohr non-associated flow rule,Prandtl-Reuss flow rule and the permeability parameter calculation model.The model is applied to the single-step prediction of permeability parameters under the complex stress path,and the results verify the applicability of the model.(6)The Bouc-Wen model was adopted in the simulation of the axial strain-axial stress hysteretic curve and the radial strain-axial stress hysteretic curve under the triaxial compression.(7)Based on the relationship between the three principal stresses and the six stress components,and upon the Mohr-Coulomb flow rule expressed in principal stress,the flow rule expressed by stress component was constructed.