Deformation-failure Mechanism And Prediction Of The Landslides In Jurassic Red Beds In The Three Gorges Reservoir

The Three Gorges Reservoir (TGR) is the area with a large number of geological disasters in China, which develope in all strata, especially in Jurassic stratum. The Jurassic lithology of the Three Gorges Reservoir is continental lake facies sedimentary rocks, the colors of which are mainly fuchsia, maroon, or red brown, so the Jurassic stratum is often called "red bed" by domestic academe. According to the incomplete statistics, among the landslides in the TGR of the second and the third phase of mitigating and monitoring projects, the landslides in Jurassic red bed are more than70%. With the formal operation of water storage and the further expansion of human engineering activities in scope and scale in the TGR, the deformation and stability of reservoir bank in Jurassic red bed will directly restrict the sustainable development of economy to reservoir area, and also affect people's life and property security. In result, it has been one of the most important environmental and geological problems.The thesis selected the landslides in the TGR Jurassic red bed as the research objects, based on elaborating its development characteristics, reservoir bank structural characteristics, and deformation characteristics, and then summarized its failure mode and set up a kinematic conceptual model of landslide activity. Using the large ring shear apparatus to research the stress-strain mode and residual strength characteristics of landslide zone soil in typical Jurassic red bed under the condition of large shear displacement, and based on the kinematic conceptual model, the deformation and failure mechanism of the landslide in red bed was revealed from two aspects, which were the recovery and rate effects of residual strength, and the influence of groundwater level fluctuation of the landslide. Taking the landslide in typical red bed for example, and according to the data of groundwater level and surface displacement monitoring, a landslide deforming trend forecast model of the TGR Jurassic red bed which could response the fluctuation of groundwater level was established. This study directly related to the measures and effects of reservoir area landslide treatment, people's lives and property security, and the normal operation of the reservoir, which is significant to the disaster prevention and mitigation of the TGR Jurassic red bed reservoir bank. By synthesizing the research contents of this paper, the following staggered results are obtained: (1) The development characteristics and failure mode of the landslides in Jurassic red bed in the TGR are obtained by statistical analysis.Landslides in TGR Jurassic red bed are mainly large thick-bedded, which usually developed in J2s and J3P, and located within the influence of reservoir level fluctuation and human engineering activities from the view of the distribution of height. This kind of landslides mainly distributes in the linear-shaped bank slope, and secondly, in convex and Ladder-shaped bank slope. The concave shape at the present stage is relatively stable, so the landslide hazards are infrequent. When the linear-shaped bank slope between10°and20°(slow dump slope), large-scale landslide would probably develop, and usually a thick-bedded one; when between20°and30°(middle slight grade), giant landslide with huge thick layer could relatively develop; whereas in nearly horizontal slope (gradient<10°), large thick-bedded landslide casually appears. While the non-linear bank slope between10°and20°(slow dump slope), the giant and large landslides would be most likely to develop, both of which could form thick-bedded or giant thick layer landslide; when the bank slope between20°and30°(middle slight grade), or between30°and40°(middle abrupt slope), large thick-bedded landslide grows better, nevertheless, giant landslide with huge thick layer and medium-sized landslide occasionally appear,landslides in the TGR Jurassic red bed are usually developed from the consequent slope and diagonal slope in slow dump or middle slight rock strata, and widely distributed in the gentle (nearly horizontal) rock strata. In contrast, for consequent slopes, the degree of development of landslides in red bed is higher than that of diagonal slopes, but for retrorse slopes, landslides in red bed are less. According to the combined features of bank slope formation lithology, landslides in the TGR Jurassic red bed can be divided into three categories, each class corresponding to a failure mode, respectively they are:the layered bank slope controlled by soft layer (class Ⅰ), the layered bank slope controlled by weak substrate (class Ⅱ), and the layered bank slope controlled by bedding and structural surface (class Ⅲ). Among them, the first class is divided into five subclasses based on the dip angle of sliding bed rock strata, and the combined relationship of the angles between dip shift of the rock strata and the bank slope inclination.(2) Though the statistical analysis, the characteristics of landslide activity in the TGR Jurassic red bed were obtained, and the conceptual kinematic model describing its deformation stage before failure was established.So far, most of the landslides in Jurassic red bed stay in a stable condition (either in a creep stage, or in a slow deformation stage), and only a few come into deformation stage. For those landslides at a stable state, their cumulative displacement curve characteristics manifest the "random oscillation" and "trend oscillation", while landslides in the deformation stage will present a periodic "ladder" type, their displacement rates display a process of periodic acceleration and deceleration. This feature is closely related to the external environment i.e. the fluctuation of reservoir level and rainfalls, but sometimes it shows a sharp increase of the displacement, or an acceleration of the deformation which lags in rainfall or fluctuation of reservoir level.The deformation process of the landslides in the TGR Jurassic red bed before failure can be divided into two stages, namely motional stage and stationary stage. In the motional stage, the slide zone soil is kept in the shearing process at different rates; whereas in the stationary stage, it is in the consolidation process with different time span. The periodic activation of landslide from stationary stage to the motional stage is caused by the effective normal stress changes which are aroused by periodical changes of the pore water pressure generated in slide zone soil, when the groundwater level changes.(3) Based on the features of residual shear strength generated in slide zone soil, the mechanism of landslides in the TGR Jurassic red bed from deformation to stationary condition was revealed.At the different shear rates, the residual shear strength of the slide zone soil shows the "positive-rate effect" and the "negative-rate effect". For the slide zone soil with high content of clay particle, the residual shear strength of it will gradually increase along with the increase of the shear rate, namely " positive-rate effect"; Conversely, if the clay particle content in the slide zone soil is very low, its residual shear strength will gradually decline, namely "negative-rate effect". For the controlled slip zone of landslide in the TGR Jurassic red bed, that is the argillation intermediate layer, the process in which the "positive rate" of its residual shear strength takes effect is as follows:after the landslide reliving, the deformation rate of activated landslide increases gradually under the action of initial acceleration, owing to the "positive-rate effect", the residual strength increases, thus leading to the decline of acceleration until a negative acceleration appearing, with the deformation rate gradually reducing to0, the landslide will come into the stationary state afterwards. This process reveals the kinematic mechanism of displacement rate fluctuation in the process of landslide deformation in the TGR Jurassic red bed.In a relatively short consolidation period, the residual strength of slide zone soil with high content of clay particle will recover by a small margin, but there is no obviously recovery features for the slide zone soil with low content of clay particle. According to the cumulative displacement curve characteristics of landslides in the TGR Jurassic red bed, when the landslide deformation rate gradually reduced to0, the residual shear strength of slide zone soil in a short time could recover by a small margin, which guarantees the landslide living in a static condition before the next activity recovery.The shear zone in slide zone soil has been formed before the failure of landslides in the TGR Jurassic red bed, and the slide zone soil only depends on the residual shear strength of shear zone to resist the effects of slipping shear force. Usually, the shear zone formed in slide zone soil is very thin (about1cm), and the sliding trace is parallel to the sliding direction.(4) From the view of groundwater level fluctuation, the mechanism of landslides from stationary condition to deformation revival in the TGR Jurassic red bed was presented.The mechanical analysis of the infinite slope model shows that there is a negative correlation between the shear strength in shear zone and the height of groundwater level, a negative correlativity between the stability coefficient and the height of groundwater level, and a positive correlation between the movement velocity and the height of groundwater level. In addition, under the condition of the same groundwater level in height, the higher the water content (or saturation) of unsaturated zone is, the lower the shear strength and stability coefficient of shear zone become, and the greater the movement velocity is.The analysis of Numerical Simulation shows that in Wanzhou District,2009, the groundwater level fluctuation of Huayuanyangjichang landslide and the rainfall have significant corresponding relations; the influence of reservoir level fluctuation is limited, which has little effects on groundwater level and seepage field in the middle or the rear edge of the landslide. The changes of pore water pressure in landslide rock and mass correspond with the groundwater level fluctuation; moreover, the pore water pressure in landside mass is more sensitive to the groundwater level fluctuation than that in landslide soil. The whole stability of landslide basically synchronizes with the rainfall process, the heavier the rain falls, the smaller the stability coefficient becomes.The results of ring shear test show that periodic fluctuation of groundwater level of the landslide is the trigger causing the periodic resurrection of landsides in the TGR Jurassic red bed. When the deformation rate of landslide declines to0, the residual shear strength of slide soil in the short-term consolidation process will recover to a certain extent. Since then, under the circumstances the groundwater level of landslide changes, both of the pore water pressures in landslide mass and slide zone soil have changed; when effective normal stress reduces to the point where the shear strength is less than the shear sliding force, the landslide deforms again, and so on, over and over again until instability and failure. Under the effect of head difference, the pore water in landslide mass forms the infiltration pressure increasing the shear sliding force at the same time reducing the effective normal stress of the slide soil, which is an important reason why large landslide deformation occurred during the period that reservoir level rapidly declined.(5) The deformation tendency prediction model of the landslides in the TGR Jurassic red bed, responding with the underground water level, was established; moreover it had solved the problem of the delayed effect of inducing factors, which was not fully considered in the previous prediction model.The accumulated displacements with time of all the monitoring points in the landslides can be considered as a set of non-stationary time series. The method of time series analysis is scientific to deal with the monitoring data dynamically. The time series of accumulated displacement is divided into three parts, namely trend item, periodic item and random item, which reflects the response of deformation of landslide to external trigge. In the extraction of periodic item the the cycle was determinated by the of groundwater level based on the calibrated hydrogeological model, which considered fully the responselag between the deformation of landslide and rainfall and reservoir water level fluctuation. The results of Baishuihe landslide with the predicted model shows that it is reasonable to use the time series method based on the fluctuation of groundwater level.