Research On The Interaction Of Coupling Seepage And Stress And Deformation Of Landslide Under The Condition Of Water Level Fluctuation

The Three Gorges area is a region prone to geological hazards such as landslides. Thestability is controlled under the interior and exterior factors. Over the period of 2003 and 2006,the reservoir water level has risen from initial 135m to 156m, and the majority of landslides haveexperienced the challenge without failures and reactivation. However, it is still urgent to researchon the failure and reactivation mechanism of landslide induced by the reservoir water fluctuation.This paper takes the front collapsed debris of Huangtupo landslide, which is one of the greatestlandslides in There Gorges area, and the deep slip bands as the major study objects. On one hand,based on the creep test of the slip band soil, which mostly control the stability of the landslide,the research focus on the attributes of rheology and long-term strength. The proper creepconstitutive creep equations are established which set up mechanics base for further geologicalmodel and numerical simulations. On the other hand, taking the front debris of Huangtupolandslides as the numerical simulation engineering background, the research on the coupledhydraulic and mechanics actions are conducted in theory and in numerical simulations under 5different typical engineering cases.Moreover, the research on the rheology attributes of slip band and the coupled hydraulicand mechanical action are the popular and challenging problems currently in the field ofgeotechnical engineering and engineering geology. The following major problems are researchedwith respect to the interior and exterior factors:1. The characteristics of the engineering geological conditions of Huangtupo landslide areanalyzed, and the emphasis is put on the research of the front debris slipped body. The spatialdistribution, composition and the physical characteristic of the weak layer (slip band) are studiedin details, which are the preconditions for establishment of simulation model.2. As matter of results, large-scale landslide's deformations and revival controlling factorsin Three Gorges reservoir lie in the mechanics characteristic of slip band's strength damageaccumulation and its time-depended effects. This interior geological mechanics conforms withthe exterior deformation evolvement. So the major factor influencing the landslide stability liesin slip band's long-term strength effects. It is very necessary to research on the mechanicsattributes of slip band soil and urgent to establish the proper stress-strain-time constitutive of it. In this chapter, the long-term rheology behavior of slip band soil, which is sampled from theexplorative cave TP3 in Huangtupo landslide, is studied. The Burger's creep model andparameter's non-restrain regressions are conducted, which can properly represent thestress-strain-time creep attributes. Based on the creep test, the envelope equations of transientstrength and residual strength, and the envelope equations of peak strength and long-termstrength are established.3. The differential equations and FEM equations for saturated-unsaturated, steady-unsteadyseepage in porous medium are deduced. Aimed to the type of slope containing marl interlayer,applied the FEM program to saturated-unsaturated, steady-unsteady seepage numerical analysis,the dynamic changes of free surface, the distributions of pore water pressure including thenegative suctions, volumetric water content and the variation law of FOS of slope are discussedand researched systematically. Finally, the results and conclusions are applied to the fronts ofHuangtupo landslide's unsaturated seepage research. The main conclusions are following:â‘ whenever impounding or sluicing of the reservoir, in the seepage numerical simulationcalculations, the interlayer (containing different layer of hydraulic conductivity) type of slopetakes on the wedge-shaped unsaturated zone and a double flee surface during the front time steps,which conforms the facts that the same conclusions were obtained using such experimentsotherwise under the conditions of rainfall infiltration conducted by Rulon and Freeze in 1985.â‘¡The saturated hydraulic conductivity, the characteristic curve of soil-water, and the characteristicof slop structural combination together determine the free surface and pore water pressuredistributions. Hence, the modeling results can provide evidences for the evaluating of slopestability and the design of water chaining and enforcement measurement for such type slope.4. According to the mechanical actions under the condition of reservoir water fluctuations,three typical pore water pressures are concluded. And in light of different hydrologicalgeological conditions, three mechanics patterns influencing the FOS for the ground water inslope are summed up as: preatic aquifer model, confined aquifer model, preatic and confinedcombined aquifer model. Introduced unsaturated soil mechanics theory into slope stabilityanalysis, the traditional limit equilibrium analysis is extended to consider thesaturated-unsaturated soil mechanics. The Morgenstem-Price stability method, which fullyallows for the contribution to anti-strength from suction, is applied to ideal reservoir slopestability evaluation. The results shows that during the water drawdown, there exists a minimumFOS and its general variation trend is firstly decrease rapidly then increase to a constantgradually. The FOS reflects the most dangerous water level, generally equal to a less 10-15mcontrasted with initial level. By contrast the FOS calculated via considering the slice bottom'ssuctions contributions and not involved it, it shows that the former is normally greater 0.5ï¼…thanthe latter.5. The Terzaghi qusi-3D consolidation theory and Biot true 3D consolidation differentialequations are rigidly deduced and demonstrated. The difference between them are compared andanalyzed. Based on the virtual work principles, the discretizations of FEM equations in timedomain and spatial domain of seepage and stress field are conducted. And considering the double field interactions effects, the saturated-unsaturated FEM formations and the coupled hydraulicand mechanics field FEM formations are deduced in the form of displacement incrementalvariable and pore water pressure incremental variable.6. The last not the least, based on the fully insight into the engineering geological andhydro-geological conditions present in the second chapter in details, considering the differentstrength characteristic of the lithology and variations of hydraulic conductivity of landslides, thetypical geology-seepage-stress combined coupled simulation model are established. Via of 5typical engineering cases, the FEM coupled simulation analysis is carried on the fronts ofHuangtupo landslide in Three Gorges region. The conclusions are reached as following:â‘ Before reservoir impounding, the yield zone just exists in front shallow section in very smallscale, which shows the shallow and deep landslides are stable as a whole. However, with thewater level rising, the maximum shear stress in front are increasing with time, and its principaldirections are almost parallel with the slip bands, and the stress centralized zones extendsbackwards to the landslide. The displacements and strains concentrates on the highstrain-gradients slip band zones, which show the deformation belongs to the front traction type.When water level rise to 175m, the simulation results show that yield zone extends backwardsand run-through the deep slip band, which can account for the influence on the deep wholelandslide's stability.â‘¡Due to the coupled simulation calculations allowing for the variation ofhead boundary and stress boundary under the level drawdown, during the process of impoundingand sluicing, the modeling pore water pressure results different from the single field calculationresults, and it takes on the Cryer's effects, that is excess pore water pressure occurs on thebottom near the slip band.â‘¢Though the simulation and the calculations in engineering case 5, itshows that the trends of extend, the deformation field and the FOS calculated based on the limitequilibrium are well conformed. And we can reach the following conclusion: During theprophase in case 5, the positive contributions coming from the increment of effective stress andunsaturated seep suctions are less then the negative contributions coming from the hydraulicpressure and excess pore water pressure, and subsequently, vice versa, which is the realmechanism of the FOS of landslide influenced by the water level.