Study On Dynamic Characteristics Of Damming Earth-rock Materials By Bouc-Wen Model

A large number of high earth-rock dams existing or under construction are located in the areas of high seismic intensity in China,which makes the issue of safety of earth-rock dam under earthquake becomes of vital importance.The main damage characteristics of earth-rock dam under strong earthquake are subsidence,dam crack and so on.Subsidence and cracks of dam are all related to seismic deformation.The maximum settlement of Bikou core embankment dam was up to 24 cm,and the largest 90cm subsidence of Zipingpu dam exceeded the prediction of design stage during Wenchuan earthquake,which indicat that the earthquake deformation prediction of earth-rock dam is unsatisfactory at present.As a result,the earthquake safety evaluation of earth-rock dam is difficult to control.The prediction of seismic deformation of the dam is greatly influenced by the dynamic constitutive relationship of the dam material and its parameter selection.Although a variety of soil dynamic constitutive models have been proposed and a large number of cyclic triaxial test have been carried out to obtain the parameters.However,due to the complexity of soil dynamic properties,the stress conditions and stress history of each soil element under dynamic cyclic load are very different and its energy dissipation mechanism is not clear enough.Therefore,the stress-strain constitutive relationship of damming earth-rock materials remains to be further studied.The gravel clay not only has the advantages of seepage stability performance of clay,but also has the characteristics of high strength,small compression deformation,and coordination of deformation of the dam material.It is widely used in the core wall anti-seepage body of high earth-rock dam.As the transition layer between the core wall and the rockfill body,the inverted filler has the function of filtering soil drainage,ensuring stable penetration,and filling the cracks in the core wall to make it self-healing.The inverted filler and the core-walled clay material are the key to ensuring the safety of the dam.At present,there are few studies on the dynamic constitutive applicability of inverted filler and gravel clay.The main purpose of this paper is to test and analyze the dynamic characteristics of dam earth and stone materials,and propose an improved constitutive model and carry out simulation verification to analyze the energy variation law of dam earth and rock materials during cyclic loading and unloading.By solving the first-order nonlinear differential equations,the Bouc-Wen model could simulate the nonlinear,hysteresis,deformation accumulation,strength and stiffness degradation of materials,which has good adaptability.Bouc-Wen model can be used to describe the dynamic characteristics of damming inverted filler and core-wall gravel clay,which can enlarge the adaptability range of soil dynamic strain and overcome the problem that the dynamic model of damming earth and rock material adapts to the narrower dynamic strain level.The model can also be used to simulate the dynamic characteristics of soil under the condition of large dynamic strain,which can be used to calculate the seismic residual deformation.The Bouc-Wen model is convenient to calculate the dynamic deformation of dammed earth and stone,but fails to reflect the development characteristics of pore water pressure.The method in this paper is to take the inverted filter and core wall gravel clay material as the research objects.Bouc-Wen model is applied to research and improve the dynamic characteristics of damming materials combined with the analysis of dynamic triaxial test results,and the energy change rule of damming materials in the cyclic loading and unloading process is studied by thermodynamic method.A series of research results were obtained,and the conclusions of the dam earth and stone test and the Bouc-Wen constitutive model study were obtained.The main contents and conclusions are shown as followed:(1)The commonly used equivalent linear dynamic viscoelastic model overestimate the damping ratio of the dam material during large dynamic strain,and can not reflect the deformation accumulation process.The paper studied the characteristics of the Bouc-Wen model and the influence of model parameters on stress and deformation.It was found that the reference shear strain γr has the most significant influence on the modulus and damping characteristics.The parameters g,n and η0 mainly controlled the shape of the hysteresis loop under different dynamic strain level.The statistical mean curve based on dynamic shear modulus and damping ratio test data of dam building materials were fitted by Bouc-Wen model and the model parameter scopes in dam building material were determined.(2)The static and cyclic triaxial test were carried out on the inverted filler and the core-walled gravel clay of the Nuozhadu core rockfill dam by using the GDS high-precision cyclic triaxial apparatus.The static and dynamic characteristics of two kinds of dam building materials under different stress conditions were analyzed.It was found that under static loading,saturated inverted filler showed obvious dilatancy and strain softness characteristics,while the stress-strain relationship of gravel clay was hardened.Under dynamic loading,the consolidation ratio had a significant effect on the failure mode and dynamic strength of saturated inverted filler.The failure mode of the inverted filler under isotropic consolidation condition was cyclic liquefaction failure,while the failure mode under anisotropic consolidation conditions was cyclic plastic cumulative failure.The larger the consolidation ratio,the greater the cyclic stress ratio required for the inverted filler to failure vibration.The initial effective confining pressure had less effect on the dynamic strength of the inverted filler.Compared with inverted filler,the failure mode of gravel clay was cyclic plastic cumulative failure,the cyclic cumulative deformation was more obvious,and the dynamic strength was sensitive to confining pressure.According to the cyclic mobility of the inverted filler after initial liquefaction,the expression of the pinching parameter λ was improved,which reflected the influence of the cumulative energy consumption on the pinching effect of the hysteresis loop.Considering the dilatancy of the inverted filler,the dilatancy equation was established by introducing the dilatancy parameter.Combined with the pore water pressure model,the transient fluctuation and growth law of the dynamic pore water pressure of the inverted filler were simulated and calculated.Based on the static and cyclic triaxial test of the inverted filler,the model parameters were determined.The numerical simulation results showed that the improved Bouc-Wen constitutive model could simulate the pinching effect and cyclic mobility of the hysteresis loop under isotropic consolidation condition,and the ratcheting effect and strain accumulation of the hysteresis loop under anisotropic consolidation condition.(3)Based on the relationship between the Bouc-Wen model and the endochronic theory,the relationship between the plastic volume densification of gravel clay and the Bouc-Wen type endochronic function was established,and the ability of the model to calculate the permanent deformation was extended.Considering the influence of dynamic shear stress ratio,the logarithmic relationship between pore pressure and endochronic function was established,and the pore pressure model of gravel clay was proposed.The model was used to simulate the cyclic triaxial tests of gravel clay under different stress conditions and the cumulative deformation and dynamic pore water pressure of gravel clay under multiple cyclic loads were well simulated.Compared with the hysteretic loop calculated by the boundary surface model,the damping energy consumption characteristics of the gravel clay simulated by the Bouc-Wen model were more accurate.(4)The Bouc-Wen constitutive model was analyzed by thermodynamic theory.The relationship between the physical components model of the soil representative volume element and the Bouc-Wen constitutive model was established.The dissipation incremental and Helmholtz free energy function were derived.The physical significance of the main model parameters was explicated.The energy dissipation mechanism of dam building gravel clay in the process of cyclic loading and unloading was studied.It was found that during the cyclic loading and unloading process,most of the plastic work was dissipated irreversibly by the dissipative stress on the plastic strain increment,and the stored plastic free energy accounts for a small proportion in the plastic work.