Research On The Mechanism Of Reservoir Landslide Based On A Coupled Thermo-hydro-mechanical System With Multiphase Flow

With the development of hydropower engineering in China,a number of high dams with large reservoirs have been constructed and put into use.Reservoir region is one of the highest-risk areas of landslides for its complex geological condition and influence of the factors,such as change of reservoir water level,rainfall and river erosion.Reservoir landslides are mainly triggered by the changing water level and rainfall.The development and occurrence of reservoir landslides are outcomes of the interaction between external factors,such as rainfall,rising and drawdown of reservoir water level and change of atmosphere temperature,and internal conditions of topography,geology and hydrology.The interaction is a coupled multi-phase and multi-physical system involving solid deformation,water flow,air migration,temperature transfer and other processes.Therefore,based on the theory and method for coupled multiphase and multi-physical processes,the reservoir landslides triggered by rainfall and rising and drawdown of reservoir water level are investigated in this research.Modeling and experimental investigation are performed first to understand the coupled multi-phase and multi-physical behavior for unsaturated soils.Numerical approaches to solve the coupled problem are then proposed.At last,the mechanism of reservoir landslides triggered by rainfall and change of reservoir water level are investigated based on numerical simulations,respectively.The main researches are as follows:(1)A soil-water retention characteristics(SWRC)model is proposed to consider the effect of temperature and porosity.Experimental tests on red soil in three gorges area are designed and conducted to measure the SWRCs at different temperatures and porosities.The applicability and feasibility of the SWRC model are validated by the test results.The final form of the SWRC model is a function of suction,degree of saturation,temperature and porosity with only five parameters.Every parameter has clear physical meaning.The good agreements between model predictions and experimental data indicate that the proposed model is capable of describing the water retention behavior of unsaturated soils,and can be readily incorporated into numerical study of reservoir landslides.(2)Conbined air-water two-phase flow simulations with grid search parameter estimation algorithm,a method for measuring hydraulic conductivity for water and air in unsaturated soils is developed from experimental data of water outflow and applied air pressure.The proposed method can save the considerable time needed for establishment of equilibrium conditions at each suction.The grid search algorithm is an effective way to obtain precise results by identifying the presence of multiple minor optima in the predefined parameter space.An application to red soil shows the feasibility of the method to rapidly measure the hydraulic conductivity of water and air in unsaturated soils.The results can be used in numerical simulation of multiphase flow.(3)To avoid the layering phenomenon by compacted sampling method and reproduce the suction distribution,a new sampling method is proposed based on drying consolidation techniques.Based on the new sampling method,triaxial shear tests are conducted to study the strength characteristics of unsaturated soils at different degrees of saturation.The relationship between friction angle,cohension and degree of saturation are obtained,respectively.The results can provide shear strength parameters in numerical simulations of multi-physical processes.(4)Based on least square finite element method,a method for measuring thermal conductivity of unsaturated soil is developed using experimental measurement of temperature as input data.The appeal of the method lies in its high efficiency and accuracy because no forward simulations and iterations optimizations are needed.Experimental tests on red soils in Three Gorges area are performed to measure the thermal conductivity at different degrees of saturation and porosities.A law expressing the change in thermal conductivity with porosity and degree of saturation are established based on parameter calibration of Tong's model.The results can be incorporated into thermal simulation of reservoir landslides.(5)We proposed a numerical approach based on finite element method to solve the coupled multiphase and multi-physical problem with high nonlinearity and intrinsic ill-posedness.The numerical techniques such as adaptive time step,subsequently coupling iteration and storage optimization of non-zero data are used to improve the efficiency of numerical calculation.Degree of saturation and air pressure are set as the basic variables to solve air and water flow problem,instead of water pressure and air pressure.The proposed approach enlarges the principle element in the matrix system to avoid the instability problem from the ?pressure form? scheme,and thus has good performance on convergence,stability issues.The numerical results for the code written by FORTRAN language are validated against the Liakopoulos drainage test and a self-designed rainfall infiltration test,and good agreements are observed.Based on the numerical study on multi-phase and multi-physical processes,a method was developed to calculate the factor of stability for reservoir landslides.(6)Taking Tan Jiawan landslide in three gorges reservoir area as an example,the numerical results of temperature distribution,air and water flow,deformation and stability under real conditions of rainfall and fluctuations of reservoir water level is analyzed.Base on the understanding of multiphysical processes under rainfall and fluctuations of reservoir water level,the evolutions of rainfall infiltration and stability under conditions of different rainfalls and atmosphere temperatures are then addressed.The mechanism of rainfall-induced landslides is analyzed at last.The results show that the influence of rainfall infiltration mainly due to the effects of generalized loading and weakening of material strength.A shallow layer of saturated zone is formed in the subsurface of slope during the process of rainfall infiltration.At early time of an intense rainfall,the downward pore water pressure gradient in the saturated zone leads to a slight increase of slope stability.With the infiltration of rainwater,pore air pressure increases to slow down the infiltration process and transfer the water pressure at the top to the slope toe.The coupled air-water force transmission becomes a leading factor to decrease the stability of landslide.In addition,the degree of saturation in sliding mass will be very high as a result of rainfall infiltration and significant difference of permeability between sliding bed and sliding mass.The high degree of saturation reduces the strength of unsaturated soils,accelerates the slope deformation and decreases the slope stability.(7)Taking Shuping landslide in three gorges reservoir region as an example,the numerical results of temperature distribution,air and water flow,deformation and stability under real conditions of changes in water level and rainfall are analyzed to understand the multiphysical processes.The influences of rise and drawdown rate of water level and permeability of sliding mass on slope stability are addressed.The mechanism of landslides induced by changes in reservoir water level is analyzed.The results show that the effect of the rising and drawdown of water level on landslides is quite different for landslides with different permeability coefficients.When permeability coefficient of the sliding mass is far smaller than change rate of the reservoir water level,the changing of the reservoir water level affects the slope stability mainly through the changing of water weight on slope toe.Rising of reservoir water level increases the water weight on slope toe,and the stability increases.When the difference between permeability coefficient of the sliding mass and change rate of the reservoir water level is not significant,the influence of the changing of reservoir water level on landslide stability mainly comes from the effect of ?hydrodynamic pressure?.The response of pore water pressure to the rising and drawdown of reservoir water level are obviously different in temporal and spatial scale,which will produce additional pore water pressure gradient and affect the stability of slope.The stability of a landslide has a significant positive correlation with the rising and drawdown of reservoir water level.When permeability coefficient of the sliding mass is much larger than rising rate of the reservoir water level,the decrease of stability during the rising of reservoir water level is a result of "floating weight reduction" effect.(8)The coupling effect of temperature change and water and air flow is studied in reservoir landslides under the conditions of rainfall and rising and drawdown of reservoir water level.Atmosphere temperature change affects rainfall-induced landslide in an indirect way,while the influence on landslide induced by change of reservoir water level is relatively small.Changing of atmosphere temperature has an effect on temperature distribution in limit depth.However,the temperature change in the shallow layer of slope changes air and water permeability and suction.The inflow and outflow through slope surface and process of rainfall infiltration will be much different.Eventually,the stability is affected by the changing of atmosphere temperature.