Study On Methods For The Static And Dynamic Analysis Of Earth Dam Constructed On Deep Overburden

It is of great political and economic significance to develop the abundant hydropower resources in West China safely and reasonably for promoting economic development and national unity,improving the industrial structure and ecological environment.However,the geological structure of the western region is complex,the riverbed overburden is deep and uneven,the number of active fault is large,and the earthquake is great in intensity and high frequency,which bring many major technical problems to hydropower development.With the social-economical demand of high earth dam,the analysis method for earth dam has made great progress in recent decades,but it is still in the initial stage that constructing earth dam on deep overburden.The complicated deformation characteristics,dynamic nonlinear behavior and liquefiable of soils in the overburden are big obstructions,so that the deformation and stress state of impervious structures,dynamic response behavior and reasonable seismic safety evaluation for earth dam constructed on deep overburden are difficult to be captured.The connection part between the earth core in the dam body and the concrete cut-off wall in the overburden is a weak link of the seepage prevention system for earth core dam constructed on deep overburden.Existing research mainly based on small deformation method which is good at studying the deformation rule of the whole dam,but the large deformation issue at the connection part caused by overburden deformation cannot be modeled reasonably.Thus,the safety of the connection part is hard to be evaluated.The concrete cut-off wall is a crucial important structure to control the leakage through the overburden.However,there is quite few knowledge about the deformation and stress behavior of the wall in complex Three Dimensional(3D)stress condition.And,there are great limitations of traditional method for safety evaluation based on linear elastic analysis and strength criteria for the wall.In addition,the uniform excitation method for seismic input which cannot precisely reflect the interaction of Infinite-Foundation-Dam is mostly adopt to study the dynamic response behavior of the dam.Furthermore,the studies of dynamic response,liquefaction and slope stability of the dam are conducted separately.Thus,the evaluation on seismic safety of the earth dam constructed on deep overburden is unreasonable without consider the coupled effects of multi-factors.In view of the above question,a system researches on numerical methods for the static and dynamic analysis are carried out to solve the critic technical problem in evaluating the safety of earth dam constructed on deep overburden.The present research is supported by the Mega-project of Natural Science Foundation of China "Disaster simulation and safety control of high earth and rock dams during earthquake"(No.90815024),"Simulation and integration research on the disaster process of high earth and rock dams during earthquake"(No.91215301)and the Program for New Century Excellent Talents in University "Simulation system for seismic response of high earth and rock dams"(No.NCET-12-0083).The main contents of this study are as follows:(1)The numerical method for large deformation analysis of earth dam constructed on deep overburden is established on the basis of coupling the Updated Lagrangian method(UL)with Remeshing and Interpolation Technique with Small Strain model method(RITSS).The technology of mesh remeshed and information mapped locally is presented and realized,and the soil constitutive models commonly adopted in earth dam engineering is integrated.In addition,the corresponding control software is developed based on VC++ platform.The validity and reliability of the developed numerical method and the applicability of UED information mapping method based on shaped function are verified by examples.(2)The interaction behavior of soil and structure in the connection part of earth core dam constructed on deep overburden are studied with different numerical methods.The rule of the whole dam deformation is obtained with small deformation analysis.But,the deformation behavior of the connection part is not described precisely,and the inhibition effect of the soil to the penetration of the concrete cut-off wall is underestimated.The local element distortion is eliminated with the large deformation method presented in this research.And,the process of the wall penetrating into the highly plastic clay is simulated properly.Therefore,the stress level of the highly plastic clay and the stress in the wall is larger than those of small deformation analysis,and the deformation ability in the horizontal direction is stronger.In order to ensure the impermeable of the connection part,the range of the highly plastic clay should be optimized according to the stress state and the effective thickness with large deformation analysis.(3)The deformation and stress behavior of concrete cut-off wall constructed in deep overburden are studied,and the mechanism and development rule of tensile stress in the wall are discussed.In order to describe the stress state of the wall reasonably in a bending deformation mode,a 3D nonconforming element is realized and validated.The results show that the wall is in a serious bending deformation state under the action of water pressure on the upstream and the constraint of the bedrock,the tensile stress is aroused at the downstream face of the wall near the bank slope.The angle between the tensile stress and the axial direction of the wall is increased with the range in tension extend to the deep gradually.In addition,the valley topography has a significant effect on the direction,peak value and extend depth of the tensile stress in the wall,and vertical tensile stress will be aroused at the bottom of the wall when the valley is particularly wide.(4)The peak tensile stress in the concrete cut-off wall far exceed the yield strength the concrete material and the range of the wall in tension is very large with liner elastic analysis,that the safety of the wall is underestimated.Based on a self-developed finite element software,the plastic damage model for concrete is integrated and its validity is verified.On this basis,the 3D damage analysis of the concrete cut-off wall in deep overburden of an earth dam is carried out.The results show that the tensile damage caused by bending mainly appears on the downstream side of the wall and extends to the deep gradually with the water level raising.The damage localization and stress redistribution behaviors in the wall are captured reasonably with plastic damage analysis,that the peak tensile stress and the range in tensile of the wall are reduced obviously compared with linear elastic analysis.The dangerous locations of the wall can be clarified according to the damage degree and the stress state at the end.(5)The mechanism of slurry cake in the construction joints is studied with the fine model for the concrete cut-off wall constructed in deep overburden.The compatible deformation capability between wall panels and the risk of compression damage at the bottom of the wall are underestimated when the wall is simulated as continuous structure.The interaction between adjacent wall panels and the support and constrain of the bank slope on the wall are affected by the panel width and analysis parameters for the construction joints,and the damage range and damage degree in the wall has change significantly.When impounding completed,construction joints at the downstream side of the wall will be open,that the degree of tensile damage in the wall is reduced and the effective thickness of the wall to control leakage is also decreased.(6)A series of parametric studies are performed to study the error of seismic uniform excitation method with analyzing the acceleration response of the earth dam constructed on overburden.Meanwhile,the simple and effective nonlinear seismic input method is investigated.The error of seismic uniform excitation method is closely relative to spectrum behavior of the earthquake wave and direction of the input motion.The vertical acceleration with seismic uniform excitation are about twice the result with seismic wave input,and the error is significantly greater than the horizontal direction.A simplified model for nonlinear dynamic response analysis of free field is established with consist of consistent deformation mode of the lateral boundary and artificial boundary at the bottom.This model with high precision improves the computational efficiency.The nonlinear seismic wave input method for earth dam constructed on deep overburden was developed by combining the simplified model and nonlinear artificial boundary.The presented method reduced the computation effort remarkably,meanwhile,reasonable dynamic response of dams is obtained.(7)The dynamic response characteristics under strong earthquakes is studied and the numerical analysis method for safety evaluation of liquefaction and slope stability is discussed based on an earth dam constructed on 500m-deep overburden.The acceleration is amplified by comparing results of the top and bottom of the overburden.The spatial distribution of the acceleration response in the dam varies considerably due to the difference of the modulus betwen the upper and downstream dams.In addtion,the acceleration response at the top of the overburden is decreased on account of dam-foundation dynamic interaction.The dynamic interaction is represented in two aspects mainly.One is that the spatial variety of overburden modulus is enhanced with the gravity of dam.The other is that the earthquakes energy is absorbed by the damping of the dam body.The liquefaction analysis method based on the effective stress dynamic response analysis can properly describe the decrease of the dynamic shear stress ratio and soil modulus caused by the accumulate of pore pressure.Therefore,the liquefaction possibility of sand layer is lower than the conventional method based on total stress analysis.The slope stability analysis method based on seismic wave input method and effective stress dynamic response analysis not only reflects the actual acceleration distribution behavior of the dam,but also solves the key problem that the strength parameters of liquefaction sands is difficult to be determined,thus the position and depth of sliding surface is more reasonable than the conventional method based on standard.