| Hydraulic flushing pressure relief gas extraction is the key measure to eliminate the risk of coal and gas outburst.Hydraulic flushing unloads stress near soft coal cavern walls,activates coal seam gas desorption,and substantially increases coal seam permeability.Coal mass under load will undergo non-linear volume expansion,this phenomenon of increased volume deformation of the material due to shear damage microcrack expansion is called shear dilatancy,shear dilatancy effect often occurs with the yield fracture of the coal mass.The change of coal mass permeability is closely related to the fracture development,and the shear dilatancy effect is the dominant factor in the volume deformation and fracture development of coal mass after yielding,but the shear dilatancy angle is often assumed to be 0 in the numerical simulation analysis,resulting in the lack of research on the shear dilatancy effect in the numerical study of the physical field of cavern making and the unclear mechanical mechanism of the surrounding rock of the cavern wall.In this thesis,firstly,the conventional triaxial experiments and cyclic loading and unloading experiments of coal mass are carried out to obtain the complete stress-strain curves under different confining pressures.Based on the principle that the total strain of coal mass is composed of elastic and plastic parts,the plastic strain of coal mass is separated and used to calculate the shear dilatancy angle and accumulated plastic strain.By studying the relationship between the shear dilatancy angle and the changes of confining pressures and accumulated plastic strains,the evolution model of shear dilatancy angle considering both confining pressures and accumulated plastic strains is constructed.Secondly,the secondary development of Flac3 D strain-softening constitutive model was completed,and the reliability of the developed model was verified by comparing the experimental data with the simulated data,and the results were in good agreement.Finally,the evolution laws of the plastic zone,radial stress and tangential stress in the surrounding rock near the cavern wall under different working conditions are compared by using the penetration hydraulic flushing technology as an engineering calculation case.The main conclusions are as follows:(1)The triaxial cyclic loading and unloading experiments are carried out based on the conventional triaxial experiments of coal mass,and a variety of basic mechanical parameters of coal mass can be obtained.By analyzing the experimental data in detail,we can obtain the change laws of coal mass elastic parameters and strength parameters under different confining pressures.The results are: the modulus of coal elasticity and cohesion gradually increase and the internal friction angle gradually decreases with the increase of confining pressures,and the degradation damage degree of cohesion and internal friction angle of coal mass from peak strength to residual strength under the conventional triaxial condition is compared,which shows that the degradation damage degree of cohesion increases from 26.6% to 67.9% and the degradation damage degree of internal friction angle increases from 41.6% to 43.25% with the increase of confining pressures.The effect of cohesion deterioration damage is more significant.(2)Based on the experimental data of cyclic loading and unloading under different confining pressures,the plastic strain was calculated by subtracting the total strain from the elastic strain,and the shear dilatancy angle and the accumulated plastic strain were calculated separately using the plastic strain values,and the relationship between the shear dilatancy angle and the accumulated plastic strain under different confining pressures was studied,and it was concluded that the shear dilatancy angle of coal mass has the characteristics of gradually decreasing with the increase of accumulated plastic strain and continuously decreasing with the increase of confining pressures.Based on the above analysis,shear dilatancy angle evolution model considering both confining pressures and accumulated plastic strains was constructed,and shear dilatancy angle evolution model was used to fit the experimentally obtained shear dilatancy angle values under different confining pressures and different accumulated plastic strains,and the results were fitted with high accuracy.(3)Due to the built-in strain-softening constitutive model of Flac3 D uses the form of interpolation function to describe the evolution of strength and shear dilatancy angle parameters,there are computational errors.Therefore,based on the strain softening model,the secondary development of constitutive model is carried out by modifying the plastic parameter,defining the strength function and shear dilatancy angle evolution function,and comparing the triaxial compression experimental data with the simulated data under different confining pressures to verify,and it is found that the stress-strain evolution behaviors of both are in good agreement,which illustrates the reliability of the development constitutive model.(4)In this thesis,the hydraulic flushing technique of penetrating borehole is used as an engineering calculation case.The physical field variation laws such as the range of plastic zones,radial stress and tangential stress distribution of cavern wall surrounding rocks produced by mohr-coulomb,strain softening and strain softeningshear dilatancy models under different lateral pressure coefficients were studied respectively,and the effect of shear dilatancy effect on the displacement of cavern wall surrounding rocks under the strain softening-shear dilatancy constitutive model and the physical field evolution characteristics of cavern wall surrounding rocks under the condition that the lateral pressure coefficient is 1 and the vertical pressure is different were analyzed.The plastic zones produced by the mohr-coulomb model are 0.1m,0.1m,0.2m and 0.2m under the horizontal stress of 10 MPa,12MPa,14 MPa and 16 MPa and the vertical stress of 10 MPa,respectively;the plastic zones produced by the strainsoftening model and the strain-softening-shear dilatancy model are 0.2m,0.3m,0.4m and 0.4m,respectively.The maximum extent of the plastic zone for the strain-softeningshear dilatancy model is similar to that of the strain-softening-shear dilatancy model,but the strain-softening-shear dilatancy model has more damage areas in the plastic zone;meanwhile,the tangential stress concentrations produced by the mohr-coulomb model are 12.6MPa,15.28 MPa,17.62 MPa and 19.32 MPa,respectively;the tangential stress concentrations produced by the strain-softening model are 12.08 MPa,14.26 MPa and 19.32 MPa,respectively.12.08 MPa,14.26 MPa,15.52 MPa and 18.29 MPa,respectively,and the tangential stress concentrations from the strain-softening-shear dilatancy model are 14.06 MPa,15.79 MPa,17.95 MPa and 20.61 MPa,respectively;further indicating that the shear dilatancy effect has a greater influence on the displacement of the surrounding rock of the cavern wall.The thesis has 65 figures,14 tables,and 104 references... |
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