Evolution Of Fracture Self-healing In Water-proof Overburden Rock And Limitation Mechanism Of Seepage After Coal Mining

The ecological environment of Northwest China is fragile where the growth of vegetation depends on the maintenance of surface water and shallow groundwater.Coal mining will cause the deterioration of ground ecology.The two hinge geological components for maintain surface water and shallow groundwater are soil and water-barriers rock mass(WBRM).Vegetation grows in soil,which is the main storage and flow space of surface water and shallow groundwater.In the process of coal mining,the water content of the overlying mudstone increases to saturation.This makes the fractures in weakly cemented mudstone layer close gradually.In the meantime,the WBRM can still prevent the water.According to the point of surface water and shallow groundwater needed for vegetation growth,we use experimental testing,theoretical analysis and numerical simulation to complete the research.The research work includes: the tolerance limits of surface water and shallow groundwater leakage in Yili area have been calculated and determined;the mechanical characteristics of overlying mudstone after immersion are obtained;the closure mechanism and influencing factors of mudstone fractures are analyzed;the seepage-stress coupling model of fractured mudstone is established.The effects of self-healing evolution of mining fissures in mudstone strata on vegetation growth are analyzed,and the following research results are obtained.(1)The seepage experimental system considering fractures closure has the characteristics of wide measuring range,long time,good stability,not easy to block,and allowing experiments with solid particle fluids,which meets the needs of experiments.Fracture permeability of red sandstone is basically unchanged with constant confining pressure for a long time.The higher the water content of mudstone,the uniaxial strength and elastic modulus of mudstone will be smaller,and the more obvious the viscoelasticity of mudstone.The larger the water content of mudstone and the normal stress of fracture,the faster the decay rate of the permeability coefficient of the fractured mudstone sample.The permeability coefficient of fractured mudstone samples decreases exponentially with time,and the decrease is more than two orders of magnitude.Stress is a necessary condition for self-closing of fractures.When the normal stress of the fracture is greater than 2.0~2.5 MPa,the fracture closes quickly and completely.The erosion of mudstone by water,the softening of mudstone by water and the creep of mudstone all promote the closure of fractures.(2)The closure mechanism of rough fractures is the relative displacement of the fracture walls.We design a MATLAB program to extract the CT tomography information of the experimental rock sample and reconstruct the three-dimensional fracture.We use the reconstructed model to calculate and simulate the law of fracture closure.The calculation results are the same as the experimental results,but are slightly larger than the latter.The final result of viscoelastic deformation fracture can be calculated by changing the elastic modulus of mudstone.The calculation shows that the softening of mudstone plays a major role in fracture closure,and creeping further promotes fracture closure.After denudation,the fracture opening decreases slightly,but does not change in magnitude.However,the fracture opening decreases by more than one order after hydration and creep.(3)The seepage-stress coupling model of fractured mudstone takes mudstone softening,expansion and fracture opening changes into account.The influence of fracture opening on permeability of mudstone block can't be ignored.The stress field and fracture seepage field of mudstone matrix are calculated by COMSOL Multiphysics and MATLAB respectively,and the error of coupling calculation is less than 5%.There is a competitive relationship between the expansion deformation of saturated mudstone matrix and its elastic deformation after softening.Non-penetrating fractures promote mudstone matrix swell and soften,and the influence on permeability coefficient of mudstone blocks reaches an order of magnitude.(4)The leakage tolerance of surface water and shallow groundwater is affected by the type,thickness,mutual position,saturation of surface soil and precipitation.When the thickness of silty loam or silty heavy clay is large and they are in the upper part of the overlying stratum,the soil moisture content always satisfies the growth of grassland vegetation.In this case,it does not depend on the water resistance of the overlying stratum.In certain conditions,the self-healing of cracks in barrier layer can reduce the permeability coefficient of overlying strata and hinder the shallow water leakage.Without excessive water loss and water supply,the self-closing of fractures promotes the restoration of surface vegetation.The necessary remediation measures(including irrigation,grouting,etc.)can effectively reduce or prevent the degradation of surface vegetation.There 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