Study On Deformation Failure Behaviors And Nonlinear Models For Rocks Under The Influence Of Mining Disturbance

In order to solve the problem of engineering disasters,in addition to studying the inherent mechanical properties of engineering rock mass,the influence of human and natural factors,such as geological environment and stress field caused by mining disturbances,should be fully considered.Before the excavation of roadway or coal seam,the surrounding rock of coal seam,roof and floor is in a stable and balanced triaxial stress state.Afterwards,with the excavation of roadways and the exploitation of coal seams,the surrounding rock undergoes complicated loading and unloading processes,and different excavation methods and mining layouts produce different stress paths and mining influence,which in turn causes the different deformation,strength and failure modes of surrounding rocks.Under the influence of mining,stress concentration occurs in the crack tip in the surrounding rock,which then the crack initiates,propagates and coalesces and gradually evolves into a large discontinuity,causing deformation and loosening of the surrounding rock,resulting in the failure of coal seam,inbreak of roof,wall caving which cause mine disasters.Therefore,recognizing these mining induced stress paths and conducting the experiments in the laboratory according the stress path is of great of significance for studying the deformation and failure mechanism of rock mass in mining and the preventing and controlling engineering disasters.The typical sandstone selected from coal mine was regarded as the research object and a combination of laboratory testing and theoretical analysis were adopted in this dissertation.The mineral composition and microstructure of the rock were obtained by means of X ray diffractometry and high resolution scanning electron microscopy.High-rigidity electro-hydraulic servo testing machines were used to perform conventional uniaxial and triaxial compression,cyclic loading-unloading compression,different unloading confining pressures rates and different mining layouts on rocks,the progressive failure and crack evolution characteristics of rocks under conventional compression and mining disturbance were analyzed.In combination with rock mechanics,elastic mechanics and damage mechanics,an axial crack closure model,axial crack propagation model and axial crack recovery model were established theoretically.On this basis,the pre-peak stress-strain relationship model for mining rocks was proposed.Through this study,some progress has been made in the following areas:(1)The post-peak stress-strain curves of sandstone are greatly affected by the confining pressure.With the increase of confining pressures,the post-peak strain softening stage becomes obvious and the ductility increases.While,decreasing the confining pressures,the strain softening modulus of sandstone decreases and the brittleness increases.The elastic modulus of sandstone is about 8.318 GPa and the uniaxial compressive strength is about 23.35 MPa.The cohesion and the internal friction angles are about 6.810 MPa and 33.86 °,respectively.After the failure of sandstone,its cohesion was reduced to 0.736 MPa.With the increasing in confining pressure,sandstone is gradually transformed into shear failure by tensile failure and the failure angle increases with the confining pressures.(2)The deformation and failure characteristics of sandstone under the influence of mining are analyzed.Under the cyclic loading,the sandstone shows strong brittleness.The axial residual strain first decreases and then increases with the increasing in cyclic number.While,circumferential residual deformation and volume residual deformation increase.The axial elastic deformation increases with the increase of cyclic load,the volume elastic deformation increases first and then decreases,and the circumferential elastic deformation decreases.In the process of cyclic loading and unloading,the elastic modulus of loading gradually increases.Before the sandstone specimens fail,the absolute damage parameters of the sandstone in the axial,circumferential and volume directions had an increasing trend,and the rate of increase of the absolute and accumulate parameters of the circumferential and volumetric damage increased suddenly,which was the premonition of the instability of the specimen.With the increase of the unloading confining pressure rate,the strain-softening modulus of sandstone decreases,indicating that the sandstone is more brittle.The larger the unloading confining pressure,the higher the secant modulus,peak strength,and peak volumetric strain of sandstone,and the lower the peak axial strain and residual strength.That’s to say,when the unloading confining pressure rate is large,a small deformation will cause the rock to be failure.Under different mining methods,the strain-softening modulus of sandstone decreases with the increase of the axial stress loading rate,indicating that the higher the axial stress loading rate,the stronger the sandstone brittleness.The strain-softening modulus of the sandstone in non-pillar mining is the least,and the brittleness is relatively strong.The strain-softening modulus of the top-coal caving and protected coal seam mining method increases in turn,and the brittleness decreases.Under different mining layouts,the peak axial strain and peak radial strain of sandstone decrease with decreasing axial stress loading rate.Namely,axial and circumferential deformations of sandstone in non-pillar mining,top-coal caving and protected coal seam mining are reduced in sequence.Under uniaxial cyclic loading and unloading,the failure of sandstone is mainly caused by tensile failure.Under the different unloading pressure rates and under different mining methods,the sandstone mainly exhibits shear failure,and the unloading confining pressure rate and the axial stress loading rate are greater,the failure angle is smaller.The rock burst tendency of sandstone under the influence of mining was analyzed,and The higher the unloading confining pressure rate is,the greater the impact energy index of sandstone is.When the unloading confining pressure rate is 0.8 MPa/min,the sandstone has a weak rock burst tendency,and when the unloading confining pressure rate reaches 1.6 MPa/min,the sandstone has a strong rock burst tendency.Under the protected coal seam mining mode,the impact energy index of sandstone is the smallest,and there is no rock burst tendency.Under the top-coal caving and non-pillar mining,sandstone has a strong rock burst tendency.(3)The progressive failure process and crack evolution of sandstone under normal loading and mining conditions are analyzed.According to the evolution law of crack volumetric strain,the progressive failure process of sandstone can be divided into five stages under conventional uniaxial and triaxial compression,namely,crack closure stage,elastic stage,crack stable propagation stage,crack unstable propagation stage.Affected by cyclic loading,sandstone begins to occur damage in the 8th cycle number and crack growth occurs.Under different unloading confining pressure rates and different mining layouts,the progressive failure process of sandstone can be divided into elastic stage,crack propagation stage and post-peak stage.With the increase of confining pressure,the initial crack volumetric strain,crack volume propagation strain,crack volumetric(radial)peak strain and peak crack propagation rate all show a decreasing trend,while the crack axial peak strain increases,which indicates that there are more crack generating when the confining pressure is larger.The crack closure stress,crack initiation stress and crack damage stress of sandstone increase with the increase of confining pressure,and the ratios between the characteristic stress to the peak strength is within a reasonable range.With the increase of the cyclic number,the axial,radial,volumetric peak strain of the sandstone,the initial crack volumetric strain and the crack volumetric propagation and the peak crack propagation rate gradually increase,which reflects the damage of the cyclic load to the sandstone.The crack closure stress and crack initiation stress of sandstone increase with the number of cycles,and the ratio of crack damage stress and peak stress increases first and then decreases.Under different unloading confining pressure rates,the crack radial peak strain,crack volumetric strain and crack propagation rates increases with the increasing in unloading rates of confining pressures,while the crack axial peak strain decreases gradually which indicates that there are fewer cracks in the sandstone when the unloading rates is faster.Under different mining layouts,the peak crack strain and crack initiation stresses gradually decrease with the increase of the axial stress loading rate.That is to say,the cracks are the highest when the sandstone fails in the protected coal seam mining mode,and the crack initiation stress is the lowest.Then followed by top-coal caving mode,and the crack is the least when there is no coal pillar,and the crack initiation stress is the largest.The peak crack growth rate under conventional loading decreases with the increase of confining pressure.The larger the cycle number,the greater the peak crack propagation rate of sandstone.Under different unloading pressure rates,the peak crack propagation rate of sandstone with the unloading The pressure rate increases and gradually increases;under the non-pillar mining mode,the peak crack propagation rate of sandstone is the largest,followed by caving coal,and the protection layer mining method is the smallest.The greater the crack growth rate,the greater the impact energy index,and the stronger the rock’s impact tendency,and the more likely it is to cause impact rock pressure.Comparing the crack growth rate under the influence of mining,it was found that the crack growth rates under different unloading pressure rates and different mining methods are greater than those under conventional loading.(4)Based on the law of axial crack evolution,the parameters of rock axial cracks were identified,and their relationship with confining pressure,wet-dry cycle,cyclic number and heat treatment temperature were analyzed.The larger the confining pressure is,the greater the axial crack propagation(peak)strain of the rock and the axial crack damage stress is,and the smaller the axial crack closure stress is.With the increase of the number of wet and dry cycles,the axial closure strain and stress increase.When the confining pressure is constant,the higher the heat treatment temperature,the higher the axial peak strain and the propagation strain,and the axial crack damage stress gradually decreases.The evolution law of rock’s axial crack under cyclic loading and unloading conditions is analyzed.With the increase of the number of cycles,the axial closure(recovery)strain and stress of the crack show an increasing trend.With the increase of unloading confining pressure rate,the axial crack propagation strain of rock tends to decrease gradually,while the axial crack damage stress increases gradually.The axial crack propagation strain and damage stress under protected coal seam mining,top-coal caving and non-pillar mining are gradually incrasing.(5)The rock is composed of matrix and cracks.The deformation of matrix and cracks is described by engineering strain and natural strain respectively.Based on the evolution law of the axial crack strain,the crack evolution models for rocks are established,namely axial crack closure model,axial crack propagation model,axial crack recovery model.Based on the established definitions of rock crack closure,propagation and crack revoery strains,a pre-peak stress-strain relationship model under rock loading conditions and a rock stress-strain relationship model under unloading conditions are proposed.Finally,a large number of existing experimental data and rock mechanics test data are used to verify the rock crack evolution model and the pre-peak stress-strain relationship model under loading and unloading conditions.The agreement between the experimental value and the theoretical value is high and can be compared.The nonlinear characteristics of the stress-strain relationship before the rock peak and the mechanical response under unloading conditions are well described,which verifies the applicability and rationality of the model.