The Structural Characteristics Of Loess Landslide And Its Formation Mechanism

Since 1976,irrigation has been adopted as a regular practice for agricultural activities on the southern tableland in Jingyang County.Consequently,the slopes along the tableland border have commonly failed.According to field investigations,there is a typical landslide with the landslide sliding surface deep into the terrace sediments in the southern tableland in Jingyang County,and the deformation and displacement of the terrace sediments were found in front of the landslide deposits,and the deformation features manifest as folds and faults,it is essentially Similar to the geologic structures,and it is defined as landslide structure in this paper.This study would be of great theoretical significance and application value for analyzing structural characteristics and formation mechanism of landslide structure.On the one hand,landslide structure as a part of landslides,the analysis of its formation mechanism is helpful to understand the interaction relationships of movement laws of landslide and landslide structure;on the other hand,the research results are not only of academic interest for revealing the formation mechanism of landslide structure,and also has important application value for explaining the movement and deposit behavior of landslide.In order to study the characteristics of the structural deformation caused by the dynamic action of the landslide and its formation mechanism,the multi-scale photogrammetry was used for obtaining the three-dimensional spatial geological information in this study area,trenches and outcrops,and the dynamic terrain data of the whole process of the model test.Based on these data,the geometry,kinematics and dynamics analysis of landslide structure were carried out.Using Miaodian 4#landslide as the prototype,numerical modeling method was adopted to simulate the formation process of the landslide structure by the matrix discrete element method(Mat DEM),and then a generalized model of the landslide structure was proposed based on the numerical simulation results.On the basis of the generalized model,a numerical model test and physical model test were carried out.Finally,the formation mechanism of the landslide structure was analyzed based on the research results and field survey data.The main conclusions are as follows:(1)The spatial scale of data obtained by multi-scale photogrammetry ranged from meters to kilometers,the spatial resolution ranged from millimeters to centimeters,and the minimum temporal resolution can reach milliseconds.The spatial scale and spatial resolution of the three-dimensional spatial image data of the entire tableland obtained by the ultra-low-altitude drone photogrammetry can reach kilometers and centimeters respectively;The spatial scale and spatial resolution can reach several hundred meters and millimeters respectively;dynamic photogrammetry can obtain the dynamic terrain data of landslides,and the spatial scale and spatial resolution of the data can reach meters and millimeters respectively,and the minimum temporal resolution can achieve to milliseconds.The resolution and accuracy of the spatial geological information obtained by the above approaches can meet the requirements of geological applications on the corresponding space and time scales,and researchers can select a suitable method according to their own research goals.(2)The landslide structure at the southern tableland in Jingyang County is a special structural phenomenon which formed under complex geological conditions.First,the landslide structure is the deformation and displacement phenomenon of the terrace sediments under the squeezing action of the landslide body.The primary growth environment of the landslide structure included lacustrine depositional environment and the fluvial depositional environment at the foot of the landslide;secondly,the basic structural style of the landslide structure is imbricated thrust napped structure,and the squeezing action and movement direction of the landslide are actinomorphic "finger-like distribution" at the front of the landslide,and the plane shape of the structural formation area is "crescent shape";In addition,during the relative sliding of the hanging wall and the footwall of the fault,due to the drag effect of friction,some bending may occur in the sheeted thrusting belt,for example,drag fold,inclined fold,overturned fold,box fold,etc.The degree of bending of traction folds is proportional to the friction coefficient,friction distance,horizontal compression stress and the dip angle of the fault,and inversely proportional to the thickness of the curved rock layer.(3)The geometric shape and mechanical parameters of faults change significantly with the development of landslide structure,and have many laws.The initial dip of the fault are close to a fixed value,which is determined by the strength of the terrace sediments.In the formation process of the structure,Since the differences in the strength of the fault plane,the dip of the fault has obvious zoning characteristics;The failure mechanisms of thrust fault is similar to the shear failure of compressed members.Under the horizontal pushing action of the landslide body,two groups of oppositely potential faults will be formed in the terrace sediments,and one group of potential faults along the thrust direction dominates the development of the fault plane;The compression stress of the landslide body acting on the terrace sediments,the shear stress of the fault plane and the compression stress of the thrust sheet,they are all similar to the brittle failure process of rock and soil mass.(4)The generalized geological model of landslide structure is mainly composed of cap rock,detachment layer and near-horizontal squeezing force.Above the groundwater level is unsaturated soil,which plays the role of cap rock in structural deformation;below the groundwater level is saturated soil,which plays the role of the detachment layer;the landslide body pushes the terrace sediments corresponds to the near-horizontal squeezing force of the generalized model.(5)The formation of landslide structure is controlled by factors such as terrace sediments,location of shear outlets,and dynamic action mode of landslide body.The essential reason for landslides is the groundwater level is increasing because of perennial irrigation;The material basis of the landslide structure is the terrace sediments(the fluvial sediments or lacustrine sediments)with some initial strength;The shear crack of landslide deep into the terrace sediments are able to meet the dynamic action of landslide body;The “shove-type” is a basis dynamic action mode of landslide body,which result in the structural deformation of terrace sediments.(6)The formation mechanism of landslide structure has important engineering geological application value.The strength of the detachment layer effects on the morphological characteristics of landslide structure at the southern tableland in Jingyang County.The saturated loess-like silt or silt layer at the bottom has undergone a certain degree of liquefaction under the dynamic action of the landslide;the sliding resistance of the landslide has experienced a rapid growth in the initial stage,it will gradually decrease to near the strength level of terrace sediments;the "deep detachment type" landslide structure has a stronger resistance effect on the landslide,and causes the "deep detachment type" landslide structure to develop into nearly round shape or lateral spread shape.Conversely,the "shallow detachment type" landslide structure is easier to develop into longitudinal spread shape.