Spatiotemporal Variability Analysis Of Geotechnical Parameters And Stability Assessment Of Reservoir Colluvial Landslides

Landslide is one of the most common geological hazards in China.Landslides are influenced not only by natural factors such as gravity,earthquakes,and rainfall,but also by engineering activities,such as water engineering and mining engineering.Several hydropower projects,represented by the Three Gorges dam,Gezhouba dam,and Jinping dam,have been built and operated in natural lakes.Hydropower projects inevitably change the hydrological condition and geological structure of the reservoir area.Numerous landslides are frequently induced and revived.Deformation analysis and stability assessment of reservoir landslides are conducive to hazard prediction and prevention in reservoir areas.This thesis focuses on the colluvial landslide in the Three Gorges reservoir(TGR)area,which has complex geological structures,variable hydrological environment,and heterogeneous geological properties.For reservoir colluvial landslides affected by rainfall and reservoir water fluctuations,more attention should be attached to the slope stability analysis and parameters spatiotemporal variability.In particular,the rational characterization of soil properties and the establishment of spatiotemporal models are essential for landslide stability assessment.How to construct a reasonable constitutive model,study the spatiotemporal variability of geotechnical properties,and analyze the landslide stability are key problems to be solved in landslide hazard research.The Tangjiao landslide in the Three Gorges reservoir area is taken as the research object,the scientific problems of reservoir colluvial landslides,i.e.,the spatiotemporal variability characterization of geotechnical parameters and the stability assessment of reservoir landslides,are investigated.Comprehensive research approaches including field investigations,laboratory tests,theoretical and numerical analysis are used to study the probabilistic assessment of hypoplastic constitutive model parameters,the time-varying soil properties affected by repeated drying-wetting(D-W)cycles,the back analysis of landslide random fields under monitoring data,the landslide stability assessment considering the spatial variability of geotechnical parameters and the timevarying constitutive model.The main achievements and conclusions are as follows:(1)Spatiotemporal deformation characteristics of the Tangjiao landslide are studied.The hydro-mechanical coupled model considering the spatial variability of geotechnical parameters is established.The back analysis of landslide random fields is realized using monitoring information.The landslide deformation and groundwater characteristics are investigated.The correlations between rainfall or reservoir water and groundwater level or landslide displacement were analyzed.Results show that the main deformation area is primarily found near the river and the platform connection area,the deformation of the landslide front area is more correlated with reservoir water than other areas.The landslide body composed of level 1 and level 2 platforms can be regarded as a rainfall landslide,and the leading part of the landslide can be regarded as a reservoir landslide.The variation characteristics and correlation laws of physical parameters and shear strength parameters are analyzed.The mean,maximum,coefficients of variation,distribution types,and spatial distribution are also obtained.Some procedures for the non-intrusive stochastic finite element simulation,probabilistic analysis,and back analysis of random field parameters are built.Landslide random fields of saturation permeability coefficient and shear strength parameter are inverted separately using monitoring information of groundwater level and landslide surface displacement.(2)Mechanical properties of soil that undergoes D-W cycles are explored.The uncertainty of hypoplastic model parameters is evaluated.Then,the hypoplastic clay model is used to characterize the time-varying soil properties.The shear strength and compression characteristics of sliding mass soil that undergoes different D-W cycles were studied.Laboratory tests show that the soil at the leading edge of the Tangjiao landslide is classified as low-plasticity clay with powder and clay particles.The shear strength decreases with the number of D-W cycles.And cohesion and friction angle decrease with the D-W cycle.The deterioration of cohesion is greater than that of friction angle.The compression index increases with the D-W cycles,which shows increasing compressibility.The vector sum method is used to calculate the slope factor of safety(FS)for the hypoplastic numerical model.A Bayesian analysis framework is proposed to assess constitutive model parameters.Ma(?)n hypoplastic clay model and the critical state hypoplastic model were used to demonstrate the framework effectiveness.The relationships between constitutive model parameters and the D-W cycles number were established according to the multiple test results.Then,the time-varying hypoplastic constitutive model is built.(3)The effects of parameters spatial variability and D-W cycles on long-term landslide deformation and stability are investigated.Then,the probabilistic analysis of long-term stability is carried out based on the spatiotemporal variable parameters.The effects of the hydro-fluctuation belt weakening on the landslide deformation and stability are investigated under real reservoir water operation conditions.The hypoplastic numerical model well predicts the landslide deformations.For the leading edge of the landslide,the deformation characteristic displays a step-like pattern,and the FS varies in response to the reservoir level.The influence of D-W cycles on landslide stability and deformation is mainly reflected in the magnitude.After 6 D-W cycles,the maximum reduction of FS is 0.038.The effects of spatial permeability coefficient and shear strength parameters on landslide deformations are investigated separately.Deterministic and random field models exhibit the same trends of landslide deformation and stability.However,major differences exist in the overall magnitude.Compared with the random field model,the deterministic model underestimates the landslide deformation and overestimates the stability of the landslide in most cases.The random field model has the advantage of reproducing multiple deformation possibilities as well as giving the ranges of landslide FS and deformation.Research results of this study can provide some references for the landslide model construction,uncertainty quantification of geotechnical parameters,deformation and stability assessment of landslides in the TGR area.