Research On The Hypoplastic Constitutive Model And Stability Criterion Of Tangjiao Landslide In Three Gorges Reservoir

China is a very large country,which the water resources are very rich.Therefore,water conservancy and hydropower projects have been developed rapidly.To some extent,the reservoir alleviated the flood problem,on the other hand,it also brings greater economic benefits to the society.But it aggravates the risk of revival instability of bank landslide.The three gorges reservoir is the largest reservoir project in China.Since the formal impoundment in June 2003,many landslides in the reservoir area have caused instability and damage.Landslide is in a relatively stable state under natural conditions,but under certain external disturbances,the stability of landslide will be declined.The landslide disaster seriously restricts the development of national economy and worsens the environment in some areas,which has a significant impact on the country's maintenance of social stability and realization of sustainable development.Therefore,To study the deformation and failure mechanism of bank landslides is more practical and significant in the Three Gorges Reservoir area.It can also provide help and guidance for landside prevention and control.Reservoir bank landslide not only has the basic characteristics of ordinary landslides,but also its particularity is affected by the reservoir impoundment and reservoir operation,which causes the geological environment of landslides to change constantly.Under the condition of periodic change of reservoir water level,especially sudden drop or sudden rise,the dynamic water pressure in the slope may induce the deformation and failure of the landslide.In addition,the reservoir bank landslide will also be affected by rainfall.According to the occurrence mechanism and deformation process of landslide,landslide can be divided into abrupt diffuse type and progressive failure type.Progressive failure is the most important failure mode of landslides in the Three Gorges Reservoir area,which basically develops from local failure to overall failure,called shear band.Moreover,landslides with abrupt diffuse type are also occurred in the Three Gorges Reservoir area,and the most important type is the rainfall induced shallow landslide.In this case,the failure of the soil may occur without reaching the plastic limit criterion.For the two failure modes which mentioned above,the traditional limit equilibrium analysis method can not analyze such complex problems because it doesn't take into account the change of strain,while the numerical analysis method combined with the appropriate criterion has great advantages in dealing with the above problems.The accuracy of numerical calculation is not only related to the calculation method,but also closely related to the basic mechanical properties of soil.In terms of the current research progress,the understanding of the soil is not deep enough,and the constitutive model which can describe the behaviour of soil comprehensively has not been formed.In view of this,the constitutive model of soil has become a very important research topic in geotechnical engineering.Soil medium is a kind of complex engineering material,and the stress-strain relationship under external load is very complex.Therefore,it is very important to establish the constitutive model which is simple,practical and including soil mechanics behaviour as many as possible.Through the field investigation,monitoring analysis and theoretical research,this paper focus on the failure mode of Tangjiao landslide in the Three Gorges Reservoir area.Based on the analysis of the physical and mechanical properties of landslide soil,the hypoplastic constitutive model is used to describe the mechanical behavior of soil.The potential unstable region and progressive failure process of the landslide under rainfall condition are calculated by introducing the second-order work instability criterion through the numerical calculation method of the model.The thesis consists of seven chapters.The first chapter describe the significance of the study on the deformation and failure mode of the reservoir bank in the three gorges reservoir area.It is considered that there is a theoretical deficiency in the process of analyzing the deformation and failure mechanism of this type of landslide.In view of these shortcomings,on the basis of previous studies,the theoretical system and idea of combining numerical simulation method with field investigation and laboratory test as the main means are proposed.Furthermore,the accuracy of numerical calculation is considered to be closely related to the choice of constitutive model.Therefore,the hypoplastic constitutive model is used to describe the mechanical behavior of landslide soil.The second chapter according to field investigation and long-term monitoring of landslide,The deformation area,main influencing factors and present state of Tangjiao landslide are analyzed in detail.In chapter3 th,the physical and mechanical behavior of Tangjiao landslide is analyzed in detail by using laboratory direct shear test and triaxial test.By means of numerical simulation,the deficiency of mohr-coulomb model in describing the stress-strain relationship of soil was revealed.In chapter 4th,the basic framework and mechanical characteristics of the hypoplastic constitutive model has been introduced in detail.According to its main characteristic,the failure surface,flow rule and boundary surface of the constitutive model are deduced.In addition,by introducing the concept of critical state and the structure tensor,the constitutive model can describe more physical and mechanical properties of soil.In addition,the numerical integration algorithm of the hypoplastic constitutive model has been discussed.The adaptive implicit integral algorithm and stress correction algorithm are introduced into the model,and the corresponding algorithm is integrated in ABAQUS software by UMAT.Taking triaxial compression test and slope stability calculation as examples,the stress correction algorithm is compared.In chapter 5th,the second order work instability criterion is introduced.This thesis deduces the stability surface of the hypoplastic constitutive model in the principal stress space,and compared with the yield surface and the boundary surface.The criterion is embedded into the numerical calculation.In the chapter 6th,Based on the detailed finite element method,the potential unstable area of Tangjiao landslide under rainfall condition is analyzed by using the finite element software of ABAQUS.In chapter 7th,the thesis has been summarized,and looked forward to the problems that have not been solved well in this thesis.Specifically,the thesis has made periodic achievements in the following aspects:(1)Failure model of Tangjiao landslide in Three Gorges Reservoir areaThe stability of the front of landslide is mainly controlled by the rise and fall of reservoir water level.The main performance is that the decline of reservoir water level leads to the formation of large head difference inside and outside the slope,and when the groundwater seeps outward,it forms a large seepage force,resulting in the slope with an unstable state.The middle and rear parts of the landslide are mainly affected by rainfall.It is mainly manifested as the erosion of slope surface by rainfall water,the weakening and deterioration of the shear strength of rock and soil mass inside slope by rainfall water infiltration,and the lubrication of sliding surface.The deformation and failure of Tangjiao landslide can be roughly divided into three parts.The front of the landslide body is a strong deformation area,and the left side of the front of the landslide body is the most severe deformation in the strong deformation area.The middle platform is divided into slow speed sliding zone.The rear of landslide platform is divided into basic stable zone.(2)Physical and mechanical characteristics of the soil mass of Tangjiao landslideAccording to the direct shear test and triaxial test results,the soil mass of Tangjiao landslide shows obvious mechanical behavior and nonlinear mechanical characteristics of strain hardening and softening.The deformation and failure of soil show obvious progressive failure mode.However,the mohr-coulomb model can well describe the hardening and yielding process of soil,but it cannot well reflect the process of strain softening.In addition,the reflected stress-strain relationship is linear.Therefore,in describing the physical and mechanical behavior of soil,we need a more accurate model to describe the deformation and failure of soil under external disturbance.In this paper,the hypoplastic constitutive model is used to describe the mechanical behavior of soil.The results show that the model can well describe the process of strain hardening and softening,and the relationship between stress and strain is nonlinear.(3)Numerical integration algorithm for hypoplastic constitutive modelsThe accuracy of integral calculation can be improved by reducing the time increment,that is,the small time increment can ensure the accuracy of integral calculation.The most straightforward way to do this is to divide the time increment into several substeps.However,constant substeps usually result in low computational efficiency or uncontrollable errors.In view of the above problems,the paper introduces an adaptive display integral algorithm,which can automatically adjust the user-defined sub-step size according to the local truncation error,thus improving the integral calculation efficiency and robustness of the constitutive model.In this paper the modified Euler algorithm is compared with the Runge kutta algorithm.The accuracy of the modified Euler algorithm is within the acceptable range.In addition,this method has the characteristics of high calculation efficiency and stability.(4)Stability and second-order work instability criterion of geotechnical materialsLimit equilibrium method is widely used to evaluate the stability of rock and soil mass,however,the results obtained by this method are often not accurate.The failure criterion of plastic limit is satisfied at each point of failure.In fact,the development of stress-strain in the process of failure is often inhomogeneous.Moreover,the rock and soil materials show obvious non-association,and the failure may occur before the plastic limit failure criterion is reached.Bifurcation failure region appears in stress space.The second order work instability criterion can describe the failure mode which is mentioned above.The second order work failure criterion is introduced into the hypoplastic constitutive model and the stability surface of the hypoplastic constitutive model is drawn in the principal stress space.In addition,the method is used to calculate two boundary value problems,namely slope excavation and rainfall slope stability.(4)Numerical calculation of Tangjiao landslide under rainfall conditions in Three Gorges Reservoir areaBased on the hypoplastic constitutive model,the deformation and failure process of tangjiao no.1 landslide under two different rainfall conditions was calculated by numerical method.Combined with the second-order criterion of work instability,the potential unstable region of landslide under rainfall conditions is identified.The calculated results can provide some helps for landslide prevention and control under rainfall infiltration.