Constitutive Model And Permeability Evolution Of Pre-Damaged Granites

In order to improve the safety and the stability of underground water-sealed caverns in long term,it is necessary to conduct a research on the quality of water sealing and the long-term stability of surrounding rocks in underground water-sealed caverns systematically and deeply.As for the quality of water sealing in underground caverns,the seepage field of groundwater plays a decisive role,and the change of rock permeability is one of the main reasons for the change of seepage field of ground water.As for the long-term stability of the surrounding rock,although the surrounding rock of the cavern is mainly hard rock with great long-term stability,the long-term strength and deformation characteristics of the surrounding rock in the excavation disturbed zone will bring potential threats to the underground caverns,especially once the underground water-sealed cavern starts to operate,it is almost impossible to carry out any repair and remedy.Returning the problem of the quality of water sealing and long-term stability of surrounding rock in underground water-sealed caverns to the theoretical study of rock mechanics,it is necessary to study the damage deterioration mechanism,permeability evolution mechanism and damage creep mechanism of rock more systematically and deeply under complex conditions.In the world,the lithology of underground watersealed reservoirs is mainly granite;in China,the majority of underground water-sealed reservoirs are granites.Therefore,this paper takes granites as the research object,and carries out the Uniaxial Impact test,uniaxial compression test,triaxial compression test,triaxial stress-seepage test and triaxial creep test in real water environment.The main results and conclusions are as follows:(1)Based on Weibull statistical damage theory,a uniaxial statistical damage constitutive model considering the interaction of initial damage variable and damage variable varying with loads is established.Then,on the basis of the model,a piecewise function is established to simulate the concave trend of rock stress-strain curve in the stage of pore compaction.Finally,the coincidence degree between the model curve and the test curve is high,which basically conforms to the variation trend of uniaxial stress-strain curve of granites.(2)Assuming that the granite is composed of two types of micropores and microcracks,and on the basis of the triaxial permeability evolution model of granites considering the average effective stress and the damage variable,the initial damage impact factor is introduced to revise the model,and the triaxial permeability evolution model of granites considering the initial damage impact factor is established.The model can be used to simulate the evolution trend of triaxial permeability of granite under different damage degrees.(3)By introducing the theory of fractal derivative and initial damage impact factor,the creep constitutive model considering the initial damage impact factor and variable parameter fractal derivative is established.By fitting the test data,the model can describe the creep characteristics of granites with different initial damage degree,and the fitting accuracy is high.(4)The uniaxial dynamic test of granite was carried out by using the Hopkinson bar(SHPB)test system.The threshold of the impact rate of failure and damage in granites is determined.Secondly,the change of porosity in granites depends on the impact rate and impact times.Finally,the porosity has a linear relationship with damage variables,which is helpful to quantify and control the porosity change of granites in the field engineering.(5)Granite will have a significant softening effect on strength under the action of long-term water environment.Based on SEM scanning,failure form of granites is not the traditional intergranular failure between the mineral particles,but the transgranular failure along the feldspar scaly joint plane,which is the main reason for the low strength of the granite.The uniaxial strength of granites is negatively related to the initial damage degree,while the triaxial strength of granites is not directly related to the initial damage degree.This is mainly due to the fact that the micro-fractures developed after granite damage are closed under the action of confining pressure,which leads to the enhancement of the ability of the fracture interface in the rock to resist the relative dislocation of the rock.In the test,the upper and lower end faces of the sample are constrained by the test device,and the relative displacement along the development direction of the microcracks cannot be generated.In uniaxial and triaxial compression tests,the greater the initial damage degree of granites,the greater the lateral deformation of granites and the smaller the volume expansion point.(6)Under different initial damage degrees,there are certain differences in the permeability evolution and creep characteristics of granites.In terms of permeability evolution,the initial permeability is positively correlated with the initial damage degree.And the higher the degree of initial damage,the higher the rate of drop in permeability.This is mainly due to the higher the degree of damage,the higher the degree of development of cracks inside the rock,resulting in higher initial permeability of granites,and the faster rate of crack closure under stress loading.In the stage of unsteady development and expansion of the cracks,the average effective stress required for the undamaged granite sample to reach the maximum permeability is the smallest,while the average effective stress for the damaged granite sample to reach the maximum permeability is much larger.In terms of creep characteristics,granites have the similar creep characteristics of hard and brittle rocks.The failure modes are all vertical splitting and tension failure parallel to the loading stress.The fragments are mainly strips and strips of cones.The damaged granites will suddenly increase in deformation during the creep process.This may be due to the higher degree of cracks in the damaged granite.During the creep process,the granites are prone to local damage,which will eventually lead to a sudden increase in deformation.Finally,with the loading of each level of stress,the increments of instantaneous deformation and creep deformation of granites under different damage degrees show a trend of first decreasing and then increasing,and the instantaneous deformation increment and creep deformation of all granites increase.The amount reaches the maximum under the last stage of loading.