Deformation Mechanism Of Bedding Landslides Based On Rate Effect And Interface Shear Of Shear-Zone Soil

As the most frequent geohazards worldwide,landslides account for more than half of natural disasters taking place in China over the last ten years.Especially in some mountainous areas,natural slopes with inclined structures are more prone to localize deformation within clay-rich layers of sedimentary rocks,leading to the development of bedding landslides.Characterized by the bedding-plane shear zones,such bedding landslides are particularly sensitive to some external perturbations,such as intensive rainfall,melting snow,earthquake,reservoir water level fluctuations,and anthropogenic activities.As the weakest part within the landslide body,the basal shear zone plays a critical role in dominating the landslide kinematics.Once the balance between shear resistance and the driven force of the shear zone is broken,these landslides could accelerate unexpectedly with the release of huge energy,resulting in the collapse of a large amount of sliding mass.On the other hand,they may also exhibit creep movement with very slow velocity and show unstable activities intermittently.These landslides are moving slowly along the basal shear zone,leaving uncertainties in transition to catastrophic sliding.Therefore,the deformation mechanism of bedding landslide is tightly related to the properties of the bedding-plane shear zones.The studies on mechanisms of bedding landslides have been developed over decades due to their special structures.However,there is still a lack of integrated analysis on both kinematic features and deformation mechanisms of bedding landslides.Most researches only focus on the shear strength of landslide mass or the numerical prediction without considering the unique mechanical behaviours of the shear-zone soil.Hence,a comprehensive and specific study covering both the mechanical behaviour of shear-zone soil and its role in dominating landslide activity should be taken into account to further explain the deformation mechanism of bedding landslides.In this dissertation,both experimental and numerical investigations are carried out to explore the mechanism regulating the complex behaviour of bedding landslides.First,two well-document bedding landslides located in the Three Gorges Reservoir area and the coastal area of China are selected for case studies.Their structural and kinematic features are analyzed based on geological data,field investigations and monitoring results.According to the detailed geological setting and material composition,their sliding bodies are characterized by bedding-plane layers with interbedded clay layers,which overly on the intact bedrocks,are more prone to formation basal shear zones.Both bedding landslides are driven by hydrological conditions,such as rainfall and reservoir water level fluctuations.Based on the monitoring result and patrol records,the kinematic features of these two bedding landslides exhibit slow-moving and rapid acceleration behaviour,respectively.The movement patterns of these two landslides are analyzed.Both landslides involve unstable processes with velocity variation.This finding can be linked to the landslide activity in different development stages.Second,the shear-zone soil is collected for experimental study.By means of macroand micro-analysis,the shear-zone soil is classified as clay with low plasticity.Physical properties of the shear-zone soil show that it is characterized by high clay content and coarse grains.For obtaining the compression behaviour,oedometer tests are carried out with various water contents from natural to saturated state.In addition,the shear band formation is observed in the plane strain test employed by a biaxial system in a triaxial apparatus.Due to the overconsolidation of the soil sample,the strain-softening behaviour and dilatancy are observed during the test.Third,the mechanical behaviour of the shear-zone soil at residual state is investigated by a series of ring-shear tests.The mechanism dominating the landslide deformation from slow-moving to rapid acceleration is tightly linked to the residual-state rate-effect behaviour of the soil within the basal shear zone.The test conditions include different overconsolidation ratios,normal stresses,displacement rates,and applied shear stresses.By adopting strain-and stress-controlled ring-shear test,the residual strength and rate-effect behaviour,which play important roles in regulating landslide movements,of the shear-zone soil are investigated.The test inspection indicates there exists a critical rate distinguishing the opposite mechanism of rate effect,which also affect the viscous flow behaviour of the shear-zone soil.Fourth,the interface shear between the shear-zone soil and bedrock within bedding landslides is mimicked by interface ring-shear tests.By means of utilizing manufactured concrete model,strain-controlled ring-shear test is employed for investigating the residual-state rate-effect behaviour of the shear-zone soil with different interfaces.The test results show that the existence of interfaces greatly changes the residual strength of the shear-zone soil.A monotonous rate-weakening behaviour is observed in both smooth and rough interface tests.Moreover,the normal stress also has an impact on the residual strength of the interface behaviour.These complex interface behaviours can be explained by the surface morphology after the tests,which evidences that the shear zone may migrate from the interface to the inner soil part.Finally,the landslide deformation at the pre-failure stage is numerically investigated through finite element analysis.An advanced hypoplastic constitutive model for overconsolidated clays is implemented into finite element code for this simulation.Two cases with different boundary conditions are designed for studying the formation progress of shear surfaces under the effects of various rainfall scenarios.The numerical results suggest that the bedding-plane failure develops quickly when there is a saturated zone at the uppermost part of the landslide,implying water infiltration at the main crack.However,the localized failure will take place without this crack.The FEM results reveal that the shear zone evolution within the bedding landslide can be properly predicted by using the hypoplastic constitutive model.