Formation Mechanism And Evolution Process Of Jurassic Bedding Landslides

Landslides are significant natural occurrences with frequent catastrophic results.The concept of progressive strength weakening along the rupture surface of Jurassic bedding landslide remains to be fully understood.This study aims to investigate the mechanisms and progressive instability evolution of Jurassic bedding landslides.During this research,field investigation,image acquisition,and soil sample collection were carried out.The thesis uses this information to study the fundamental aspects of landslide documentation,soil characteristics,and instability evolution.The target area selected for the Ph D.research is the Shizibao landslide,located on the left bank of the Tongzhuanghe River,a sub-branch of the Yangtze River in Zigui County,Hubei Province.This area is characterized by severe landslide activity induced by rainfall,reservoir impacts and slope excavation.Structurally,the thesis is mainly composed of three parts: Part A,Part B,and Part C.Regarding the research methods and the scientific objectives,and these components differ from one another.The common objective of these parts is to map and study the failure mechanisms of Jurassic bedding landslides.Because of the lack of background information and geotechnical data required for assessing landslide evolution,the UAV technique and ring shear test help to document the landslide and determine the shear zone soil behaviour.Part A of the thesis focuses on applying UAV imagery for landslide mapping.This includes image processing,point cloud generation,and DSM and DOM generation.These data are then used to map the Shizibao landslide in detail and offer relevant quantitative information,such as structural information and topographic surface changes(depletion and accumulation of material).From the 3D point clouds,the landslide’s sliding plane and left boundary were extracted.Results from this extraction state that the sliding direction is 354o N with a sliding angle of 32o to 35o.The landslide is small and shallow,covering a horizontal surface area of 11.9 x 103 m2.A significant tension crack in the crown area and a 50 m sliding surface occurred in the upper part of the landslide body.Field investigation and survey data showed that the landslide slid along the discontinuity plane.Furthermore,pre-and post-landslide point clouds generated from a topographic map and UAV-acquired photos were compared.The analysis provided information on mobilized material from the source area,including both depletion anduplift zones.Overall,the documentation of the landslide in this case,based on the UAV technique coupled with field investigations,strongly improves the information on the geological and geomorphological features of the study area and determines the topographic changes that have triggered the Shizibao landslide over the last 15 years.Part B is the laboratory tests.The drained shear strength of soils is important in evaluating geotechnical structures’ stability.A series of drained ring shear tests have been conducted at relatively normal stress levels(200 k Pa,300 k Pa,400 k Pa,and 500 k Pa)and water contents(7%,12%,17%,and 20%).A total of 16 tests were conducted to measure the drained shear strength of the silty mudstone slip zone.The following conclusions can be inferred based on the test findings:1.The water content affects the drained shear strength of the slip zone soils,and the shear stress-displacement curves showed strain-softening curves.2.The shearing displacement corresponding to peak shear stress is positively related to normal stress.However,under the water content,the peak shear displacement is negatively associated with the water content.3.The vertical-horizontal displacement curves show a similar trend in dilation characteristics for all water contents.4.The residual friction coefficient and friction angle had power function relationships with water content and normal stress.As the water content increases,the residual friction coefficient decreases.Additionally,the residual friction coefficient decreases with increasing normal stress.5.For the peak and residual shear strength of silty mudstone,the peak cohesion of the silty mudstone slip zone increased with water content to a certain limit,above which the cohesion decreased.In contrast,the residual cohesion showed the opposite trend,indicating cohesion recovery above a certain limit of water content.However,the silty mudstone slip zone’s peak and residual friction angle decrease steadily with increasing water content.Furthermore,the macroscopic morphological features of the shear surface show that the sliding failure was developed under high normal stress at low water content.In contrast,discontinuous sliding surfaces and soil extrusion occurred when the water content increased to a saturated degree.The localized liquefaction developed by excess pore water pressure reduced the frictional force within the shear zone.The results providea new understanding of the slip surface mechanisms operating within the Jurassic bedding landslides.Part C is the evaluation of slope instability evolution.An S-curve model is proposed to explain the progressive strength weakening and progressive instability evolution along the rupture zone.The model demonstrates that the strength weakening initiates at the rear part of the slope,and then progressive weakening extends downward to the slope.To calculate the instability evolution of the slope,two methods,i.e.,the Culmann method and the Mogernstern-Price method,were used.The Culmann method shows that the rockslide is unstable at the initial stage(0)under a water content of 20%.However,Morgenstern-Price method results reveal that the rock is unstable under 17% and 20% at the initial evolution stage(0).In addition,based on the Morgenstern-Price slice method,the sliding zone can be divided into four subzones(I,II,III and IV)depending on the vertical depth from the surface.The calculation results show that zones I and II(the lower portion of the landslide)are the most stable,and zones III and IV are less stable.Therefore,the rear portion of the landslide is regarded as an active zone,and the toe area is a passive or locking zone before the road construction.The results confirm that the onset of the Shizibao landslide is associated with strength degradation induced by rainfall activity and slope excavation in the toe area.Therefore,by developing high-quality datasets that combine laboratory and field data,the findings add new knowledge about the mechanisms of progressive strength weakening and instability evolution in the Jurassic silty mudstone slip zone.The research also offers a new approach to predicting the future behavior of Jurassic bedding landslides.