| Landslide is one of the most dangerous disasters in the Three Gorgers Reservoir area.Reservoir water level fluctuation and rainfall infiltration are the main extrinsic factors that could change the stress field in landslide,as well as the mechanical strength of geomaterial,hence affecting the deformation and stability of landslide.Meanwhile,the deformation of geomaterial varies with time,characterized by evident rheological characteristics,which exerts a significant impact on the overall deformation of landslide.Study on the long-term deformation and time-varying stability of reservoir landslide considering the rheological property of geomaterial is extremely important for the construction and running of hydropower engineering.As the weakest layer in landslide,slip zone controls the overall deformation and evolution process of landslide,rheological effect appears to be particularly important in landslide study.It has been acknowledged that the rheological property of soil is determined by the mineral composition,pore water,organic matter,granular component and arrangement.So,study on slip zone soil at the microcosmic level is essential and could help to understand its rheological behavior.In the long evolution process of landslide,mechanical strength of slip zone soil degrades gradually due to the intrinsic rheological effect and extrinsic wet-dry cycle caused by water level fluctuation.So,it is necessary to master the strength parameters at different deformation stage of landslide to find the deterioration law.In this thesis,Huangtupo landslide,the largest reservoir landslide in the Three Gorges Reservoir area,is select as the case study,of which the long-term deformation and the time-varying stability are thoroughly studied considering the rheological property of slip zone soil.The deformation characteristics was analyzed based on the in-situ monitoring data;then the rheological behavior of undisturbed slip zone soil was explored through laboratory triaxial creep tests,and the rheological constitutive model was proposed to describe the relation among stress,strain and time.The impact of reservoir water level fluctuation was represented by the dynamic change of underground seepage field and thereby was realized with numerical modelling.Finally,the long-term deformation considering rheological property of slip zone soil was studied,and strength deterioration was taken into account in the time-varying stability calculation.Research was carried out by means of field investigation,in-situ monitoring,laboratory experiments,and numerical simulation,to provide a true and reliable result for stability evaluation.The main research contents and results are shown as follows:?1?The deformation data obtained from in-situ monitoring indicates that deformation of ground surface and slip zone has the same development characteristics along the sliding direction.Deformation rate of the frontal part,the central part,the upper part are in descending order and increases slowly when water level is high,while grows rapidly when water level falls.The Huangtupo landslide is currently in the creep deformation stage,and the deformation characteristics is closely related to the fluctuation of reservoir water level.It can be explained by the change of underground water seepage field in landslide.In the rise process of reservoir water level,reservoir water provides an inward replenishment for the underground water seepage,and the seepage line is mainly in concave shape and inclines to slope.While the underground water creates an outward discharge to the reservoir water in the decline of reservoir water level,the seepage line is characterized by convex shape and inclines to the Yangtze River.Meanwhile,the increasing water head produces a considerable large penetration during the decline of reservoir water level,thus improving the sliding force of slip body.The stability of landslide will be reduced in the decline of reservoir water level.?2?The mineral composition of slip zone soil of Huangtupo landslide contains calcite,chlorite,illite,quartz,and feldspar,characterized by silty clay with gravels.Experimental study on the mechanical properties of slip zone soil show that the peak shear strength is c=12.0 kPa,?=26.4°and the residual shear strength is c=9.3 kPa,?=23.23°.Two creep stages were observed based on the displacement-time curves from triaxial creep tests,i.e.,the primary creep stage and the attenuation creep stage,and no accelerated failure stage was observed,which indicates that soil softening induced by structural transformation plays a dominant role in the creep process.Both the deformation and creep rate were affected by the deviatoric stress and their developments are very similar under high and low stress conditions.The calculated long-term strength parameters are c=8.13 kPa,?=19.2°,which reduces by 32.25%and 27.3%respectively compared with the peak strength,caused by the positional adjustments of particles during the creep.The strength reduction indicates that time has a greater effect on the cohesion than on the internal friction angle.And the decrease in the internal friction angle indicates the significant change in particle arrangement.A viscoelastic rheological constitutive model for the slip zone soil is established to describe the creep behaviour,and the constitutive parameters were fitted from creep test results.?3?Huangtupo landslide is characterised by a retrogressive landslide,of which the deformation mechanism is closely related to the reservoir water level fluctuation.There is an obvious deformation springback at the frontal and central part of slip mass in the rise stage of reservoir water level.The plastic zone is clearly observed in the upper slip zone and lower slip zone,and develops upwards from the upper shear outlet in the whole fluctuation process of reservoir water level.The evolution process of plastic zone can be divided into four stages,an instantaneous elasto-plasticity deformation generated at the shear outlet of landslide due to the unloading effect caused by the drop of reservoir water level.Then,stress adjustment takes place in landslide resulting to the predominant deformation in slip mass.As reservoir water level descends continuously,the overall deformation of landslide is mainly controlled by the slip zone,creep deformation develops from slip zone to slip mass gradually.Finally,deformation develops at a constant speed under the rheological effect of slip zone soil after the completion of stress adjustment.In practical,landslide deformation usually lags behind the change of reservoir water level,and the hysteresis time is about 30 days according to in-situ monitoring data.It can be explained that is because it takes a long time for the adjustment of seepage field and stress during the decline of reservoir water level.?4?Strength deterioration of slip zone soil combined with rheological property are taken into consideration in evaluating the time-varying stability of landslide.The sudden change on the displacement-time curve was selected as the failure criterion for the investigated landslide.The evolution process of the acceleration deformation stage is divided into slow acceleration,fast acceleration,and rapid acceleration deformation stage.The change of landslide stability is closely related to the reservoir water level fluctuation,the stability coefficient increases to the peak value when reservoir water level rises to 175 m and then decreases to the least value when reservoir water level declines to 145 m.When only strength deterioration of slip zone soil was taken into consideration,the stability coefficient decreases exponentially with time and it reduces to less than 1 in the 17th year,this time is shortened to 11year since rheological property of slip zone accelerates the instability of landslide.The innovative points in this thesis are summarized as follows:?1?The excavation of the large experimental tunnel on the Huangtupo landslide makes sampling of undisturbed slip zone soil become possible.A comprehensive understanding of slip zone soil was obtained through a series of laboratory experiments,which helps to explain some macro characteristics from the mirco prospect.The rheological property of undisturbed slip zone soil is studied through triaxial creep test,and a viscoelastic rheological constitutive model is established based on its rheology behaviour.Research results are more reliable because the influence of original internal structure and coarse particles were taken into consideration.?2?A viscoelastic model contains elastic and viscous indicators was proved to be more suitable for numerical modelling.Based on unsaturated-unsteady seepage theory,the change of seepage filed in landslide respect to reservoir water level fluctuation is analysed through finite element method.Then,the influence of reservoir water level combined with rheological property of slip zone soil on landslide deformation was analysed in modelling,while the long-term deformation characteristics and its failure mechanism are fully studied.?3?Strength deterioration of slip zone soil caused by wet-dry effect due to the periodic fluctuation of reservoir water level is adopted in stability calculation,and the sudden change of deformation-time curve is chosen as the failure criterion in stability analysis.Thus,the failure characteristics of the investigated reservoir landslide are obtained and discussed.On this basis,study on the time-varying stability of Haungtupo landslide is carried out. |
PDF Full Text Request