Study On Creep And Constitutive Model Of Granite Under Condition Of Seepage

Nuclear power is a mature,clean,deployable and economical technology which is playing an increasingly important role in strengthening energy supply and mitigating climate change.With the rapid development of nuclear power industry,it becomes an inevitable problem to deal with and dispose of high waste safely and effectively.Deep geological disposal of high waste is considered to be a reliable and safe disposal method.High-level radioactive waste geological disposal repository contains multiple barriers in500-1000 meters underground from the surface.As the final natural barrier to prevent radioactive substances into the environment,the physical and mechanical properties of surrounding rock is the most important factors to determine geological disposal repository can be long-term safe operation.In general,high-level radioactive waste geological disposal of the surrounding environment of the library has a large buried depth,osmotic pressure and high temperature,in the short term by excavation disturbance,groundwater seepage factors such as joint action,rock mass initial stress field changes,leading to mechanical characteristics of surrounding rock and the characteristics of the permeability change.From the long-term perspective,geological disposal repository for in-situ stress of surrounding rock,seepage,nuclide decay heat,chemical erosion caused by aging mechanics characteristics and penetration characteristics change,will be the long-term safety of disposal repository have a significant impact.Therefore,the study of seepage under the condition of mechanical behavior of surrounding rock and the in-depth study of evolution law of permeability will not only contribute to determining rational engineering design and implementation of the scheme,but also to the success of the high-level radioactive waste geological disposal repository construction and the use of China in 2050 is of great significance.This paper firstly uses the method of fractional calculus to common non-darcy percolation in low permeability media problem as the research objects,the equations of fractional Swartzendruber,analyzed the non-darcy seepage phenomenon from a new angle.Secondly,for high-level radioactive waste geological disposal repository as the research background,carried out the Gansu Beishan granite triaxial compression under the condition of seepage experiment,obtained under different stress conditions of beishan granite permeability evolution characteristics,this paper discusses the internal mechanism of permeability evolution,and analyzes the relationship between volumetric strain and permeability evolution,permeability before peak strength is established as a function of volumetric stress.Besides,the granite hierarchical load creep experiment under different confining pressure is conducted to obtain the triaxial creep damage characteristics and analyzes the steady creep,stable creep stage of deformation and deformation rate.Combining with the fractional order theory,the granite creep constitutive model considering damage evolution is established,and the validity of this model is verified with single-stage loading experiments.On this basis,combining with the acoustic emission technology to carry out the granite creep seepage experiment,the fluid-structure coupling under the condition of acoustic emission signal characteristic,the permeability evolution characteristics and creep deformation characteristic is obtained,the volume strain of rock permeability and deformation rate,along with the time evolution of relations between the deformation is analyzed,and then compared the creep compression with conventional permeability evolution characteristics.In addition,by using statistical model based on Gaussian mixture model realizes the clustering analysis of acoustic emission events,the relationship of numbers and ratio of tension crack and shear crack in stable creep and unstable creep is obtained.In the last,by expanding the fractional creep constitutive model to the three-dimensional space and in the numerical differentiation after processing to be embedded into the numerical computing platform software,the deep underground laboratory near field after excavation and its influence on permeability of rock creep deformation is simulated.The main research work and achievements are summarized as follows:1.In order to solve the problem of non-darcy seepage in low-permeability media,a new non-Darcy seepage equation is established by using fractional calculus method.First,based on the Darcy law,the Laplace transform method is used under the definition of Caputo,and the equation of the flow motion of Hansbo in the form of power function is obtained,which indicates the rationality of fractional order method in describing the non-Darcy problem.Secondly,according to the characteristics of interval uncertain Hansbo equation piecewise function,a continuous function in the form of fractional order Swartzendruber motion equation is conducted.When the fractional order equals 1,the equation is in accordance with the original Swartzendruber equation function expression.Finally,based on the fractional Swartzendruber motion equation,soil-rock-mixture of bentonite and two kinds of medium to simulate the experimental data,the fractional percolation model for two types of experimental results in good agreement.2.Based on the experiment of three axial compressive seepage of granite,the characteristics of granite deformation and the evolution of permeability are studied systematically.The experimental results show that the initial permeability of water in the deep granite of Beishan in Gansu is around 10^-19m².The permeability decreases with the stress evolution and then increases,and the minimum value appears before the peak intensity,which is an order of magnitude lower than the initial value,and increases by 1² orders of magnitude before the peak damage.The relationship between permeability and volume stress is established,and the results of fitting results are better.The confining pressure strengthens the peak strength,and the osmotic pressure is weakened.In the permeability decline phase,the confining pressure have inhibitory effect on permeability,and osmotic pressure have promoted effect on permeability.Because these two factors do not have regularity effect,it means that the permeability evolution is determined by combined effect of crack growth and coalescence.The volume strain of the minimum permeability is less than that of the expansion point.3.Triaxial creep experiments were carried out under different confining pressures.Firstly,the characteristics of creep deformation of granite specimens are obtained,namely,tensile failure under low confining pressure and shear failure under high confining pressure.Second,the analysis of the transient strain and creep strain results show that the former has a linear relation with the deviatoric stress,the latter has an exponential relationship with the deriatoric stress,and the creep deformation ratio increases with the increase of stress.On this basis,in view of the Nishihara model considering damage factors,describe the acceleration phase of the problem,according to the principle of element combination,introducing the theory of fractal derivative is established considering the damage creep constitutive equation.The experimental results show that the model can describe the three stages creep problem of granite well,and the fitting effect is better than that of the original west original equation.And the result of parameter values indicates that the Nishihara equation is a special case of the new equation.4.Combining with acoustic emission testing technology,granite creep experiment under different confining pressures is conducted.In the process of creep experiment,the pressure pulse attenuation method is used to measure the granite permeability at the same time interval,and the acoustic emission characteristic parameters are recorded by the acoustic emission monitoring system.The experimental results show that,with the similarity of the three axial compression seepage,with the partial stress increase,the permeability of granite increase at first and then decreases.Difference is that the granite permeability emerges a period of stability before bulk expansion point,in other words,there are not volumetric compression deformation and the increase of partial stress leads to the decrease of permeability in this period,and the permeability remain unchanged.After the expansion point,the permeability decreases with the volume strain,and enters the accelerated creep stage,and the permeability increases exponentially.The main reasons for the evolution by stages of granite permeability under creep condition are analyzed.It is concluded that the quantitative relation between permeability and volume strain is suitable for the conclusion of piecewise function.The ring count rate and energy rate of acoustic emission characteristic parameters are studied,these two factors have positive correlation with the creep rates and have negative correlation with the confining pressure.The time-accumulative curve and strain-time curve have the same evolution trend.The results show that the volume expansion phase is active compared with the volume compression stage.It can be used as the precursors characteristic of accelerating creep failure stage.On the base of the above understanding,using the Gaussian mixture model characteristic parameters of acoustic emission events RA,AF has carried on the statistical cluster analysis,the creep experiment of acoustic emission events corresponding crack failure mode.The results of statistical clustering show that tensile crack is the main failure mode in stable creep stage.The creep failure of rock samples is marked by a significant increase in the number of shear cracks,and the shear crack under high confining pressure is the main failure form.5.Based on the variable coefficient fractal derivative constitutive model established in this paper,the three-dimensional stress state is extended,and it is converted into a numerical integration form to be embedded into the OpenGeoSys finite element numerical calculation platform.Base on the creep deformation of the deep underground laboratory,the numerical modeling and analysis of creep deformation and permeability of near-field surrounding rocks is conducted.The numerical simulation results show that the creep deformation of surrounding rock under different confining pressures is mainly concentrated in the near-field surrounding rock of about 3m.With the increase of confining pressure,the creep deformation of surrounding rock in the top and bottom of underground laboratory increases correspondingly.The influence of confining pressure on the creep deformation of the side is smaller,and the influence of steady creep deformation on the permeability of near-field surrounding rock is less obvious.