Seismic Performance Analysis Of Deep Overburden Earth Rockfill Dams In High-cold Regions

In the numerical analysis of earth rockfill dam,the boundary conditions of the simulation model are simplified and the calculation range is truncated.By simulating the "radiation damping" effect of infinite foundation,the transmission of outlying seismic waves on the boundary and their diffusion into the far-field foundation are realized,and the influence of the model on improving the dynamic response level of the structure is reduced or eliminated.Tibet belongs to the main part of the qinghai-tibet plateau,where the climate is cold,the temperature difference is large,and the solar radiation is strong.Therefore,the freezing-thawing effect is particularly prominent in areas above 4000 m above sea level.In particular,whether the freezing-thawing effect affects the dynamic characteristics and seismic performance of earth rockfill DAMS is worth in-depth analysis and discussion.Based on the background of the actual project with complex geological conditions in the Tibetan plateau region and the multi-earthrock-fill dam near Tibet as the analysis object,a 3d simulation analysis model is established according to the actual engineering structure parameters.The equivalent linear method is used to calculate and analyze the dynamic characteristics and seismic performance of the damfoundation structure.The specific work is as follows:(1)The application of boundary conditions is accomplished by using FORTRAN programming,and the realization of fixed boundary conditions,infinite boundary conditions and viscoelastic boundary conditions in ABAQUS is verified by a 3d semi-infinite space 1m×1m×10m model.The results show that the fixed boundary,infinite boundary and viscoelastic boundary can be well realized,but the accuracy and reliability of the calculated results are somewhat different.Therefore,reasonable boundary conditions are selected to better calculate the dynamic response of the dam on the deep overburden.(2)According to the actual engineering structure parameters,the 3d simulation analysis model with different boundary conditions and different thickness of overburden was established,and the ground motion with different peak sizes was applied on the 3d simulation analysis model.The thickness of overburden was 0m,132 m and 264 m respectively,and the input peak values of ground motion were 0.1g,0.3g and 0.4g respectively.The effects of boundary conditions,overburden thickness and seismic input peak on the dynamic response of the dam are studied.The results show that: 1)the fixed boundary will significantly increase the dynamic response of the dam structure,and the damping effect of the far-field foundation is not considered although the problem of repeated energy reflection will not occur in the infinite element boundary.Viscoelasticity takes into account the elastic recovery performance and damping effect of the material,so the results are more accurate and reasonable than those of the first two artificial boundaries.2)with the increase of input ground motion peak value,the dynamic response amplification coefficient of dam structure tends to decrease.3)as the depth of foundation overburden increases,the calculation results of infinite element boundary and viscoelastic boundary tend to be consistent within 0.3g,and above 0.3g means that the calculation results of viscoelastic boundary are more accurate.(3)According to the heat balance control differential equation with the phase transition transient temperature field and the water migration formula,the distribution characteristics of the water field and temperature field are calculated and analyzed using the finite element method to determine the area where the frozen soil in the dam body-dam foundation is located.,Set the frozen soil parameters in the relevant area of the 3D simulation analysis model,and analyze the impact on the dynamic response of the dam in the presence of frozen soil.The results show that:1)Considering the conditions of frozen soil,the frequency of dam natural vibration is higher than that without considering frozen soil.2)The presence of frozen soil has a certain inhibitory effect on the acceleration peaks along the dam body in the vertical and along the river.3)When the depth of the cover layer and the input ground motion are small,the frozen soil and water storage have a greater impact on the dynamic response of the earth-rock-fill dam.When the depth of the cover layer reaches 260 m and the peak input ground motion is 0.4 g,the frozen soil and water storage It has no obvious effect on the dynamic response of earth-rock-fill dams.