Damage Evolution Study Of Beishan Granite Under Complex Stress State And Environmental Conditions

Granite is internationally recognized as the surrounding rock for geological disposal of high-level radioactive waste.In order to ensure the safety of the repository,it is necessary to systematically study the mechanical properties of granite under complex stress and environmental conditions.Firstly,the mechanical experiments of Beishan granite under uniaxial and triaxial loads are carried out.Based on acoustic emission(AE)monitoring,the damage evolution law of Beishan granite under complex stress and environmental conditions is revealed.Based on Weibull distribution,a heterogeneous model of Beishan granite is built.By numerical simulation,the variation laws of stress and strain of Beishan granite under complex stress state are revealed.In order to study the damage characteristics of Beishan granite under hydraulic coupling,the hydro-mechanical(HM)coupling experiments under different fluid pressures were carried out.The mechanical response and damage state of Beishan granite under hydraulic coupling were obtained.Based on the transient pulse method,the permeability of Beishan granite at different stress and strain levels is calculated.The evolution law of pre-peak permeability with volume strain is revealed,and the relationship model between permeability and volume strain before and after expansion point is established.The conventional triaxial compression(CTC)tests of Beishan granite after heating treatment are carried out,the mechanical responses are analyzed.The variation laws of mechanical parameters versus heating treatment temperature are revealed.The damage evolution law of Beishan granite under thermal stress is revealed by numerical simulation.The main conclusions are as follows:(1)The results of uniaxial and triaxial experiments show that the average tensile strength of Beishan granite is 7.66 MPa,the average compressive strength of uniaxial compression is 161.4 MPa,the average modulus of elasticity is 59.51 GPa,the average Poisson's ratio is 0.260,the internal friction angle is 52.4 degrees,and the cohesion is 35.4 MPa.Deformation analysis shows that the stress at dilatancy point is about 81.1%of peak stress(?c).With the increase of confining pressure,the rock compression at the dilatancy point increases.Observing the failure mode of rock,it is found that with the increase of confining pressure,the failure mode of rock transforms from splitting failure to shear failure,which is supported by AE monitoring.According to AE monitoring,the crack closure stress(?cc),crack initiation stress(?ci)and crack damage stress(?cd)are 12.3%,48.7%and 81.1%of the peak stress,respectively.The AE amplitude and frequency centroid(FC)fluctuate with the increase of stress,and the vibration density increases.After the peak,the AE amplitude increases evidently,while the FC decreases.(2)The cyclic loading-unloading experiments of Beishan granite show that the weak damage generated during pre-peak cycles has little effect on the peak strength.Based on AE monitoring study,it is found that the AE counting rate increases gradually with the increase of stress level.The pre-peak AE events mainly generate in the loading stage when stress is higher than ?cd.The study of energy evolution shows that the energy behavior during the pre-peak cycles is mainly manifested by elastic energy accumulation and release.During the peak stress cycle,the dissipated energy increases and the internal structure of rock changes evidently.During the post-peak cycles,the proportion of dissipated energy increases gradually,and the development of fracture surface leads to rock failure.By analyzing the confining pressure effect,it is shown that the rock energy storage limit and the dissipated energy required for rock failure increase linearly with the increase of confining pressure.(3)The results of single cube element simulation show that FLAC3D and its strain softening model can well reflect the mechanical response of Beishan granite at different stages.The analysis of heterogeneity shows that Weibull distribution can effectively characterize the heterogeneity of rock materials,and the heterogeneity model of Beishan granite is successfully established.By analyzing the simulated failure process,it is shown that during the peak stress and post-peak failure stage,there are stress concentration and tensile stress areas.And the failure zone develops from the boundary to the interior of the model.By analyzing the strain distribution,it is confirmed that the rock failure mode transforms from splitting failure to ductile shear failure with the increase of confining pressures.(4)Based on HM coupling tests,it is found that the bearing capacity of Beishan granite decreases with the increase of fluid pressure under the same confining pressure.Based on AE monitoring,it is found that ?cc disappears under the condition of hydraulic coupling.And ?ci accounts for 65.8%of ?c,which indicates that micro-cracks are relatively stable under relatively low stress level due to the existence of fluid pressure.?cd accounts for 82.4%of ?c.(5)It is shown that under confining pressure of 20 MPa,the initial permeability of Beishan granite is 2.58 × 10-18 m2 tested by transient pulse method.In the initial compression stage,the permeability decreases with the volumetric strain,and the can be reduced to 50%of the initial permeability.In the elastic stage,the permeability almost remains constant.When the stress exceeds ?cd,the volume of rock begins to expand and the permeability begins to increase.At the peak stress,the permeability can increase to 10 times of the initial permeability.Based on the analysis,the evolution of Beishan granite permeability with volumetric strain before peak stress is divided into two linear stages with ?cd as the demarcation point,and its relationship model is given.Through the HM simulation,it is shown that during the failure stage of rock,the tensile stress region develops faster and has a wider area compared with the CTC,which indicates that the rock deterioration process is accelerated due to the existence of osmotic pressure.(6)The results of heating treatment show that the size of rock increases exponentially versus temperature in the range from 300?500?,and the reduction of rock mass increases logarithmically with the increase of temperature.Based on the CTC tests after heating treatment,it is found that the strength of Beishan granite does not change significantly,but the stress at dilatacy point decreases,about 65%of ?c.Research on failure pattern shows that not only the main shear crack generates,but also the obvious transverse secondary crack is produced,when the rock is destroyed.Based on AE monitoring,it is found that ?ci and ?cd almost coincide.Although the AE counting rate increases gradually,it is smaller than that of CTC(without heating treatment),and the rock has undergone a slow dilatation process.(7)By analyzing the physical and mechanical parameters of Beishan granite,it is shown that the thermal conductivity decreases gradually with the increase of temperature;the tensile strength,Poisson's ratio and internal friction angle first increase and then decrease,reaching the maximum at about 200?;and the elastic modulus decreases gradually,while the cohesion increases gradually.The results of thermo-mechanical coupled numerical simulation show that the stress and strain distribution caused by heating is not uniform due to the heterogeneity,and there will be a local area changed significantly.The results of CTC simulation after heating treatment show that the volume strain increases with the increase of temperature,which indicates that the higher the heat treatment temperature is,the easier the later loading will lead to rock expansion,while the shear strain will decrease.