| Deep geological is one of the most effective and feasible methods for high-level nuclear waste(HLW)disposal.Granite with high stability,high strength and low permeability is the ideal engineering surrounding rock in geological disposal.However,no matter in the early engineering construction or in the later service period,the mechanical properties and permeability properties of granite will change,which will affect the stability and safety of nuclear waste repository.Therefore,it is of great significance to study the mechanical and seepage characteristics of granite under early dynamic and long-term static loading conditions,especially for the engineering design and construction of geological disposal.In this thesis,the granite in the preselected area of Beishan nuclear waste repository in Gansu Province is taken as the research object.Laboratory experiments,theoretical modeling and numerical simulation are carried out to study the deformation and permeability evolution of surrounding rock.According to the loading conditions,the main research content is divided into two parts.Under dynamic loading,considering the viscoelastic plasticity of geotechnical materials(i.e.,loading rate dependency and loading path dependency),a viscoelastic-plastic constitutive equation is established by introducing fractional-order theory and continuum damage theory.Its good applicability to various loading conditions is verified by experimental data.In addition,the effect of dynamic load on rock seepage properties is also studied.The effects of axial pressure and confining pressure on permeability are studied through experiments,and the corresponding permeability models are proposed.For long-term static load,the deformation characteristics of granite under long-term constant load are also studied through the multi-level creep tests,and the corresponding creep models are proposed.The main conclusions are as follows:(1)Firstly,the deformation characteristics of rock under dynamic loading are studied.On the one hand,it is concluded that constant-rate loading,loading-dead loading and loading-unloading are the three most basic loading modes to characterize the viscoelastic-plastic properties of geotechnical materials(i.e.,loading-rate dependency and loading-path dependency).Meanwhile,various complex loading conditions in the engineering and test are simplified into the combination of these three linear loading methods.On the other hand,loading-rate dependency of geotechnical materials is discussed and a three-parameter fractional order model is proposed,based on the fractional viscoelastic model(i.e.,Scott-Blair element)and the damage equivalent theory of continuum.The solutions of the new model under three basic linear loading conditions are given,and then it is extended to any linear loading conditions,which provides a theoretical basis for solving the problem of complex loading.The fitting of polymer and geotechnical material shows that it can reproduce the mechanical behavior of viscoelastic plastic body under different linear loading conditions.(2)Secondly,the relationship between axial stress and rock permeability is studied.To characterize the stress-induced permeability evolution of granite,the authors performed triaxial compression-seepage tests with measurements of permeability and acoustic emission(AE)characteristics.According to the variation of several essential properties(i.e.,permeability,volume,and AE)in the tests,the whole deformation process is divided into five stages,and the permeability evolution is divided into four stages.Considering the influence of microstructures on the volume evolution,the authors regard the fractured rock as a dual media,a composite of fractures and matrix.The volume-axial stress relationships of two parts are modified according to the experimental observation,and a dual-medium permeability model is proposed.The model calibration shows that the proposed model can characterize the permeability evolution and reflect the mutual transformation between micro-cracks/pores and fractures of fractured granite throughout the deformation process.Besides,the permeability model is implemented in FLAC3 D,where the permeability evolution of fractured granite throughout deformation is reproduced,and the stress and permeability distributions around Beishan HLW disposal are simulated.(3)Then,the relationships between confining pressure and seepage properties(i.e.,permeability and porosity)are studied.In view of the influence of effective confining pressure on rock permeability characteristics in geological storage,especially the longtailed characteristics in the permeability and porosity decays,the author found the "memory effect" of permeability characteristics dependent on effective stress by the experimental observation and theoretical analysis.Empirical relationships based on laboratory-measured data,typically exponential or power laws,have been compared.Based on the physical interpretation of the exponential models,this work presents fractional relaxation equations that,in full respect of the memory effect of permeability and porosity,lead to an accurate description of effective stress-dependent porosity and permeability decays by the Mittag-Leffler function.The experimental data from the literature are employed to validate the derived relationships.The fitting on lowpermeability shales and relatively high-permeability sandstones shows that the MittagLeffler law agrees better with the experimental data,especially those with "long tail" characteristics,than two classical laws.Moreover,the numerical solutions for the proposed Mittag-Leffler law models are presented by employing the predictor-corrector method.The relationship of Mittag-Leffler law with power and exponential law is also discussed.(4)Moreover,the creep characteristics of rock under long-term constant loading are also studied.The time dependency of rock strength is observed in triaxial creep tests.At the same time,the creep properties of granite are obtained.According to the test,the creep of granite can be divided into two types: when the stress level is less than the long-term strength,the rock experiences stable creep,where the strain rate decreases with time and eventually approaches zero.When the stress level is greater than the longterm strength,the rock will undergo unstable creep,where the strain rate will not decrease when it drops to a certain value.On the contrary,with the accumulation of damage and the deterioration of the rock,the accelerated creep stage will appear,and the strain rate will continue to increase until the rock failures.(5)Finally,according to the creep characteristics of rock under long-term loading,fractional order creep constitutive models are proposed.Based on the classical Nishihara model,this thesis introduces the fractional-order theory and damage theory,and establishes the fractional-order Nishihara model.Through fitting the creep data of granite,it can be found that the fractional-order Nishihara model can effectively simulate the three stages of rock creep.In addition,the physical meaning of fractional order in rheological mechanics is discussed.The analysis shows that the viscoelasticity of the material changes in the creep process,which means that the fractional order changes.Based on this idea,a variable-order fractional order creep model is proposed,and the validity of the model is proved by fitting the experimental data.It is also proved that the fractional order does change in different creep stages,which reflects the process of rock changing from elasticity to viscoplasticity. |
