Prediction Study Of Landslides With Step-like Deformation In The Three Gorges Reservior

With the fastest growing population and global industry development in the world since the mid-age of the 20th century, the expanding urbanization and changing land-use practices lead to many environmental problems, such as landslides.In addition, the climate change and frequent earthquakes are the important factors in increasing the frequency of their occurrence. Landslides, as one of the major natural hazards, account for enormous economical losses and human casualties each year in terms of both direct and indirect costs in the world A large number of landslides exsit in the Three Gorges Reservoir in China,which exert a heavy social, economic, and environmental toll. Since the first impoundment of the Three Gorges Reservoir in June 2003, a number of new landslides have occurred and existing landslides made worse.Due to the big rise and periodic fluctuation of water level, the shape of cumulative displacement-time curve of landlides in the special geology environment of the Three Gorges Reservoir is step-like. The landslides with step-like deformation behave multi-acceleration beforce failure.The velocity of displacement fluctuation with the combined effects of rainfall and water level in the reservoir. The displacement curve consists 'power law' acceleration phases during the external loading separated by phases of decreasing velocity during the non-loading. The landslides with step-like deformation also result in the economic loss and human casualties.In order to reduce the risk and loss of the landslides, it is necessary to study the prevention methods. Remedial countermeasures are not often useful when coping with large landslides in the Three Gorges Reservoir because of the extremely high kinetic energy potentially and deep depth involved in the phenomena. In such conditions, landslide prediction is the only effective tool to reduce the vulnerability of the potentially involved areas by means of evacuation or river closure. Prediction of landslide failure is an inherently difficult task because of the difficulty in quantifying the temporal evolution of the driving forces and the shear resistances where both of which are complicated by the effects of rainfall and reservoir water level.Predticon of landslides with step-like in the Three Gorges Reservoir is researched based on site investigation and the analysis of monitoring data. There are seven chapters in the thesis, the first chapter introduces the practical significance of the research and discovers the existing questions by the means of literature review. The second one describes site condition, landslide deformation, influencing factors and failure mode of Baishuihe landslide, Xintan landslide, Bazimen landslide, Kaziwan landslide and Woshaxi landslide which belong to the landslides with step-like deformation. The third one discusses the temparal and spatial evolvement characteristics of landslide deformation, involving progressive failure, velocity fluctuation and hysteresis, movement patterns during the phase of acceleration, double sliding surface, local failure and deformation zoning of landslide surface. The fourth chapter firstly reviews prediction models of landslide. Then aiming at the exsiting problems of the traditional methods the couping GM(1,1)-BP model is established based on the influencing factors of the water level and rainfall. Finally, the validity of this model was confirmed by investigating two landslide cases through compairing the GM(1,1) model and BP model. The fifth chapter discusses the applicability of Voight model and proposes the prediction method of alert velocity on the basis of displacement curve shape. The alert velocity of Baishuihe landslide and Xintan landslide are computed. The sixth one analyzes the applicability of the single prediction criteria of landslide including velocity, acceleration, ratio of displacement, LURR. In order to improve the precision of landslide prediction,the three-dimensional criteria of landslide prediction is proposed on the basis of the alert velocity, earth crack of landslide surface and macroscopical evidences.The seventh part is the conclusions of the thesis and prospect of the next step research. The achievement are set forth in details as follows:(1) Typical landslides with step-like deformation in the Three Gorges Reservoir are investigated.Typical landslides incuding Baishuihe landslide,Xintan landslide, Bazimen landslide, Woshaxi landslide and Kaziwan landslide are introduced in the chapter 2, involving the site investigation, deformation character, influencing factors and tpyes of deformation and failure. The shape of displacement curve is step-like.There are multi-acceleration and the large displacement through long period's deformation before landslide failure. The velocity fluctuates with the external factors of water level and rainfall. The failure modes are various, inculding retrogressive failure and advancing failure. Local failure often occurred before general failure. The landslides result in large loss of property and human life due to the scale and the secondary hazard arosed by the slope sliding.(2)The temparal and spatial evolvement characteristics of landslide deformation are systemically studied.The temparal and spatial deformation characteristics of landslide are investigated by means of the analysis of engineering geology condiction, deformation process, deformation mechanism and influencing factors. The landslides often deformed for long time due to progressive failure of the sliding belt when experiencing from local failure to general failure.The descending shear strength with time may arise from the strain weakening soil. In addition to expericing the strain weaking process of the sliding one, the displacement velocity of landslide movement flucutated with rainfall and water level and there is hysteresis. The possible reseasons are the occurrence of the flucutation of the external factors, earth crack of landslide surface and soil viscosity. The pattern of movement approaching the landslide failure displays ian asymptotic form in a plot of 1/v against time (v is velocity). The style occurs where movement is taking place across existing sliding belt or as a result of ductile deformation processes. The slice method and infinite slope method are adopted to elucidate inherent function relationship between external factors and displacement change.The existence of viscous force.is crucial to the change of displacement velocity.The double sling of the landslides with step-like deformation is affectd by the external factors, landslide scale and the thickness of the sliding body. The shallow and deep sliding surfaces are interactional. The different deformation property in the landslide surface including tension zone, compression zone, sliding zone and falling zone ocurred in different landslides. Local failure often occurered before landslide sliding in whole. Therefore, the distinguish of local failure and general failure of landslide is important in landslide prediction.(3) Based on the affecting factors of the water level and rainfall, the couping GM(1,1)-BP model is established for landslide displacement prediction.The chapter reviews the statistical models, nonlinear models, physically-based mechanical models, the coupling models and discusses the applicability of their methods. The traditional methods of landslide prediction are used to modeling the displacement-time curve, not consideing the influencing factors. Or the uncertaintie related to the heterogeneity of soil/rock mass, the geological boundaries, the pore pressures acting on the sliding surface makes the models based on the stress-strain relation unrealistic. Aim at the exsiting shortages of the methods, the couping GM(1,1)-BP model is established on the basis of the water level and rainfall. Firstly, the displacement is divided into trend part and departure part. The trend part is extracted using GM(1,1) model. Secondly, BP model is used in calculating departure part consideing the water level and rainfall. One month rainfall, two months rainfall, the elevation of the water level, maximum change of reservoir water in one month, water level change between two months are chosen according to the literature review. The couping model, GM(1,1) model and BP model are compared througth the analysis of Baishuihe landslide and Bazimen landslide. The results find that the proposed model is the best and the precision of the model is less 10% by comparing the prediction displacement and the monitoring one.(4) The prediction method of aler velocity is set up for the landslides prediction.The Voight's relation through the analysis of acceleration and velocity of 35 landslide appoaching the failure was foud that the logarithm of acceleration is proportional to the logarithm of the velocity of slope displacement. Voight model is widely used in landslide failure time predition. Due to the step-like displacement of the landslides, the final acceleration phase of deformation could not be identified. Therefore, Voight mothed could not be directly used in prediction of failure time. As a result, a more practical and much simpler method based on the Voight model has been suggested to assess the phases of a landslide by means of threshold values of velocity by Crosta. This method has been attempted in a large rock slide and validated by analyzing historical landslide failures. Crosta's proposal give us to a new insight in changing the prediction of failure time to forecasting the alert velocity where the similar deformation patterns occurred in the landslides with step-like deformation.According to the same procedure, alert velocity could be got from the Voight's method and the paraments of the model are obtained from the nonlinear regression. In estimating the parameter values of the Voigt model, a better fit could be obtained by adopting the displacement as the variable rather than velocity. Alert velocity thresholds (corresponding to 7,15 and 30 days before predicted failure) can be computed according to Voight model, allowing the definition of different thresholds of velocity according to the needs of the emergency management.The feasibility of the method are validated through the cases of Baishuihe landslide and Xintan landslide. The calculated alert velocities are very cosed to the monitoring displacement of Xintan landslide.(5) The three-dimensional criteria of landslide prediction is proposed on the basis of the alert velocity, earth crack and macroscopical evidences.The applicability of the single prediction criteria of landslide including velocity, acceleration, ratio of displacement, LURR is researched through the landslide cases. In order to improve the precision of landslide prediction and avoid the shortage of information, the synthetical prediction criteria are also discussed. It is crucial for synthetical prediction criteria that how to express the multi-factor. The multi-factor is exhibited through the multi-dimension body. The three-dimensional criteria of landslide prediction is proposed on the basis of the alert velocity, earth crack of landslide surface and macroscopical evidences. The alert velocity could be obtained from the Voight's method.The three-dimensional criteria was proved by the analysis of Baishuihe landslide and Xintan landslide.