Cover Stress Re-establishsment And Its Permeability Evolution In Mining-induced Fracture Rock Mass

This paper focused on cover stress re-establishment and its influence onpermeability evolution in the mining-induced fracture rock mass. The cover stressre-establishment was studied by theoretical analysis and scale model test, there-establishment characteristics of complexity and irregularity and a calculationformula for stress re-establishment distance were proposed.The characteristics of overburden failure were concluded in this paper. Thebulking factor followed logarithmic distribution with the distance from the gob in themining-induced fractured zone through scale model test and in-situ monitoring results.A simple fracture model for fractured zone was proposed based on the bulking factordistribution. The mechanism of cover stress re-establishment influence onmining-induced fractures evolution was analyzed compared with in-situ monitoringresults. A lateral restraint and axial loading permeability system was designed anddeveloped to investigate the permeability rule for fractures in test cubes. At last theresearch results were applied to analyze the water inflow variation in a water inrushaccident induced by coal mining in the Taiping Coalmine. The main research resultsare as follows:(1) A stress re-establishment calculation model was proposed based on loadconservation principle. According to the characteristics of stress distribution, thestress re-establishment distance in the gob was calculated through the increment ofabutment stress. The elastic region was not definite in the bearing stress area made theresult small in a case analysis.(2) The internal structure in the gob made stress re-establishment with thenonuniform characteristics. Scale model tests showed that, the stress re-establishmentcoefficients were different at different positions, which stated that the internalstructures performed in the gob, and to increase the mining thickness enhanced thedegree of fragmentation and decreased the nonuniformity of the gob.(3) The fracture ratio or bulk coefficient followed the negative logarithmicdistribution in the mining induced fractured rock mass. The rock mass disturbingdegree decreased with the distance increasing above the coal seam, scale model testsand in-situ monitoring results showed that the fracture ratio or bulk coefficientfollowed the negative logarithmic distribution in the mining induced fractured rockmass.(4) A fracture model was proposed for mining induced fractured rock mass. Based on the rule that the fracture ratio or bulk coefficient followed the negativelogarithmic distribution in the mining induced fractured rock mass and theBlake-Kozeny formula, a porosity model was proposed and then it was converted to afracture model according to the seepage flow equivalence relations.(5) The permeability of the mining induced fractured rock mass decreasedremarkably and the top of the fractured zone with small aperture fractures lose thepermeability under the stress re-establishment. The height of water-flow fracturedzone decrease nearly40%after15years mining which was monitored by colorborehole television, mud slurry leakage and RQD(Rock Quality Designation). Scalemodel test results show that the fracture ratio in fractured rock mass induced bymining reduced40%after cover stress re-establishment, the water flowing fracturedreduced by10%after cover stress re-establishment using the fracture model analysis,the rock mass softening with water and creep deformation in long time made theheight reduce further.(6) The formula for calculating the re-disturbed height of fractured rock masswas modified. In consideration of the closure of fractures with small aperture understress re-establishment, the formula for calculating the re-disturbed height of fracturedrock mass was modified, which provided an effective basis for more reasonableretainment of waterproof pillar.(7) A lateral restraint and axial loading fractured sample permeable tests systemwas developed. In order to analyse the influence of aperture and angle on fractureclosure, a lateral restraint and axial loading fractured sample permeable tests systemwhich can provide the maximum permeable pressure difference of0.62MPa,maximum seepage flow of1000ml/s, and maximum axial loading of1000kN wasdeveloped. The test results showed that turbulent flow occurred in the permeabilitytest under high water head, seepage flow could be calculated by Lomize turbulenceformula, and the seepage flow decreased following the quadratic polynomial withaxial loading increase; the seepage flow turned into laminar flow when the fractureaperture decreased to a certain degree under large axial loading, at this stage theseepage flow decreased following exponential function with axial loading increase;under the same loading path, the fracture with a small aperture and angle closedeasily.(8) The water inflow decreased remarkably with stress re-establishment.Combination with the accident of water inrush in the Taiping Coalmine andapplication of the fracture model to study the water inflow decrease mechanism in the process of stress re-establishment, based on the permeability evolution under stressre-establishment in the water inrush area, the water inflow was calculated, thecalculation results were in good agreement with monitoring results. This case studyalso proved that the stress re-establishment could reduce the permeable height offractured rock mass.