Study On Diffused Failure Mechanism Of Loess Based On Discrete Element Simulation

Loess,a specific type of strong structural soil,is characterized as highly compressible,strongly hygroscopic and water sensitivity,and is highly susceptible to diffuse failure under the action of water,which in turn triggers flow type loess landslides.The high speed and remote movement of static liquefaction flow-type loess landslides has caused a lot of economic losses to the Loess Plateau region,affecting the property and life safety of people in the area.The current research on the strength characteristics of loess and the destabilization and deformation damage characteristic is mainly conducted through indoor tests to obtain its macroscopic mechanical properties,and there is not enough research to analyse the initiation mechanism of loess liquefaction from the perspective of interaction of fine particles.As a granular material,loess formed by the accumulation of a large number of powder particles with a small amount of clay and sand particles,the discrete element method of simulation can be used to regulate different macro and mesoscopic parameters to investigate the influence of different factors on the strength properties of loess,and also can analyse the destabilisation and deformation mechanisms of loess from the perspective of fine-scale particle interactions.In this paper,the loess in Lanzhou New Area is taken as the research object.Firstly,the basic physical parameters of the loess are investigated,and the stress-strain curves of the loess under different confining pressures are measured through laboratory triaxial experiments.On this basis,a biaxial numerical model of granular flow applicable to loess is established and calibrated with relevant parameters.he strength characteristics of the loess and the influence on the destabilisation damage of the soil under different macro and mesoscopic parameters are investigated,and the main research contents are as follows:(1)A constant strain rate loading method was used to simulate the consolidation and drainage test of loess,the strength and deformation characteristics under the influence of different macro and mesoscopic parameters were analysed.The increase of inter-particle friction coefficient will increase the initial modulus of elasticity and peak strength of the loess sample.The increase of particle stiffness ratio will increase the peak strength and brittleness index of the loess sample.The increase in initial porosity ratio will reduce the peak strength of the loess samples and their macroscopic volume expansion will be reduced.The increase of water content decreases the peak strength of the loess sample and the bulk strain of the sample decreases accordingly.The relationship between water content and internal friction angle and cohesion was obtained according to the Mohr-Coulomb criterion,and it was found that the internal friction angle,cohesion and water content were basically negatively linearly correlated.(2)The effect of confining pressure,relative density and water content on soil destabilisation damage was analysed by simulating the consolidation undrained test of loess using the constant volume loading method.The result of the test shows that the increase of the confining pressure and relative density decreases the liquefaction capacity of the loess,while the increase of the water content increases the liquefaction capacity of the loess.The instability of the soil during the loading process is analysed based on the second-order work criterion,where the second-order work changes from positive to negative during the loading process,and the point where the second-order work value is zero corresponds to the static liquefaction instability of the soil.The water content has a greater influence on the peak strength and effective mean stress of the soil,when the sample is close to saturation and the initial relative density is lower,and the increase in pore water pressure at high water content leads to liquefaction type instability,the sample appears to be liquefied after loading and the diffused failure occurs.(3)Based on the above two studies,the following summary of the mechanism of diffused failure has been made.The interaction and connection between the particles of the loess and the binding of water formed the basic structure of the loess,which is capable of taking and transmitting effective stresses.When the soil collapse and destabilisation damage occurs,the particle contacts and connections are correspondingly disrupted,leading to a significant reduction in cohesion,the contact occlusion between the particles begins to be lost and the angle of internal friction is reduced.The pore water pressure surge,internal contact deformation of the soil particles and changes in the microstructure occur during the loading process.Based on the changes in second-order work during the loading process in the numerical test,and the determination of the destabilisation damage mode of the soil via the stress,pore pressure,displacement and the structure changes during the loading process.This paper reveals the internal reasons for the change of soil strength and different deformation and damage modes in terms of microscopic mechanisms,,providing theoretical bases for the treatment and early warning of diffused loess landslides,and to guarantee the safety of human and property in loess regions effectively.