Research On The Earthquake-induced Hydrodynamic Pressures And Instability Mechanisms Of Moraine Dams

With the implementation of west development strategy in China,Sichuan-Tibet railway has been included in the national recent development plan.Sichuan-Tibet railway is expected to across the Parlung Zangbo river basin from the east of Tibet plateau,which this section has the geomorphic features of high altitude,large elevation difference,and distributes a large amount of glacial lakes.Moreover,Parlung Zangbo river basin are located in strong earthquake regions where earthquake activity are frequent.Generally,the moraine dams have a poor stability,which are formed by an unconsolidated,poorly sorted rock debris or highly heterogenerous mixture of particles.The earthquake action may directly damage the dam and affect the stability of moraine dam,leading to the instability of the slope of the reservoir bank or the whole dam.In addition,the reservoir water movement may be directly generated by earthquake,which formed the strong impact on the dam and resulted in outburst of the lake.Moreover,the surge waves generated by the glacier avalanches,landslides,snow avalanches,which may still formed the strong impact on the dam and caused the dam broken.Therefore,the moraine-dammed lake outbursts caused by earthquakes have become a special environmental geological hazard threaten to railway route scheme.The hydrodynamic pressure loads acting on the dams may lead to failure of the dams and become a critical factor causing the dam to break in the earthquake areas.Much attention has been paid to the safety characteristics of earth-rock dam under the earthquake direct action.While the studies on the stability of moraine dams,which were resulted of hydrodynamic pressures caused by earthquake or the landslide has been seldom reported.In this paper,on the basis of shaking table tests,and combined with the theoretical models and numerical method,the characteristics of hydrodynamic pressures were studied and further revealed the mechanism of the outburst of moraine-dammed lake caused by earthquake.It was expected this study has important reference value for selecting the railway's corridors and the layout of key projects.The main achievements include:?1?On basis of shaking table experiments,regularities of distribution of hydrodynamic pressure caused by 2 Hz to 6 Hz sinusoidal waves and seismic Wolong wave were obtained.The effect of the initial water depth,PGA and seismic frequency were considered to study the characteristics of the hydrodynamic pressures.The comparison of the theoretical values and experimental data shown that Westergaard's equation were in good agreement with the Wolong earthquake-induced hydrodynamic pressures in this small lake.It is reasonable that Westergaard's formula to calculate hydrodynamic pressures caused by real seismic waves in the small lake.?2?On basis of shaking table experiments,the longer time and lager amplitude of hydrodynamic pressure with water resonance was observed compared to Westergaard's method,and the distribution along the water depth changed from the original parabolic distribution to a uniform distribution.An empirical equation was generated to predict the maximum hydrodynamic pressure with water resonance combined with mass-spring model.Moreover,the frequency scope was referred to the resonant frequency of the structure system,which exhibits a ratio value of 0.75?f_e/f₁?1.25.Therefore,the presented ratio scope is applicable as the suitable condition for the equation.?3?By building physics model experiments,the effect of the volume,the sliding block velocity and initial water depth were considered to study the characteristics of the hydrodynamic pressure loads caused by ice avalanches.It was found that the hydrodynamic pressures caused by ice avalanches were increased from the bottom to the top,and an empirical equation was obtained to predict the maximum pressure load acting on the dam.Finally,the ice sliding block was taken into the water in the process of earthquake by shaking table experiment,the combined hydrodynamic pressures caused by earthquake and ice avalanches were observed,which the maximum values are in the bottom and upper,the minimum value is in the middle.The estimation method of maximum hydrodynamic pressure was proposed.?4?For using finite element software of geo-studio,analysis of the relationship among the excess pore water pressure and the different seismic parameters,dam geometry,physical and mechanical parameters,the water depth.It was indicated that the distribution of seismic excess pore water pressure is roughly extended from the left and right sides to the middle of the dam,and the most dangerous potential sliding surface always passes through the dam upstream or downstream area where the excess pore water pressures were higher.It is suggested that the principle of“foot reinforcement and mountainside strengthening”should be emphasized for moraine dam in strong earthquake regions.?5?Based on the field survey of the glacier Lakes in In the Parlung Zangbo river basin and the analysis of the historical data,three modes for the global stability analysis were established for taking into account the non-resonant hydrodynamic pressures,resonant hydrodynamic pressures,earthquake-ice avalanches-induced hydrodynamic pressures,respectively.According to three modes for the global stability analysis and the fifth generation of China seismic zoning map,which determined the safety factorsof 80 moraine dams in Parlung Zangbo river basin,and given the moraine-dammed lake risk zoning map to be quickly estimated.