Experimental Study On The Characteristics Of Loess Disintegration And Interface Effects

The structural characteristics of loess itself have caused its extremely strong disintegration.With the proposal of the overall layout of the Megaproject for the protection and restoration of the national important ecosystem centered on the "three regions and four belts",the ecological fragility and serious soil erosion in the loess plateau area are gradually recognized.The strong disintegration of loess can not only aggravate slope erosion,collapse and subsurface erosion,but also increase the permeability of soil mass due to the disintegration of slope,it has an adverse effect on the mechanical parameters of slope soil,and the products of slope collapse can also become an important material source for mud flow disasters.Therefore,studying the disintegration of loess has important theoretical and practical significance for soil conservation and disaster prevention in the Loess Plateau region.At present,the research on the mechanism of loess collapse is still in the qualitative analysis stage,lacking quantitative analysis.This article takes the Yan’an Malan loess,which is located in a typical loess zone,as the research object.Based on this,the differences in the collapse properties of loess in the sand loess zone and the clay loess zone are compared.The sampling and investigation points from north to south are Yulin,Yan’an,and Jingyang,followed by on-site disaster investigation and sampling,and indoor undisturbed loess disintegration tests,remolded loess with interface disintegration tests,nuclear magnetic resonance experiment and numerical simulation,in connection with unsaturated soil mechanics,the disintegration characteristics of loess are compared.The disintegration mechanism and disintegration disaster process under the influence of interface effect is analyzed.The main results and conclusions are as follows:(1)The development characteristics,interface effects,and catastrophic phenomena of the mesoscopic interfaces in the study area were summarized.The mesoscopic interfaces in the study area are mainly produced in different occurrences and combinations,and the disintegration catastrophic phenomena under the influence of interface effects are mainly crumble,falling blocks,and slope erosion.(2)Under the influence of interface effects,in the early stage of disaster development,the meso interface,as a weak mechanical property surface,also provides an preferential channel for water to quickly enter the soil.The infiltration of water causes the soil on both sides of the meso interface to disintegration and become loose deposits.Over time,the meso interface further expands,causing further damage to the soil and entering the later stage of disaster development,the soil on both sides of the interface has basically lost its strength,and phenomena such as slope erosion and detachment from the parent body have occurred.(3)In order to study the collapse characteristics of undisturbed loess,a collapse test was designed.Based on the experimental phenomena,the collapse process of loess was divided into unsaturated infiltration stage,rapid collapse stage,and slow collapse stage;From the perspective of microscopic interfaces,the disintegration characteristics of undisturbed loess under different regions,moisture content,and temperature conditions were analyzed.The results showed that all three regions have strong disintegration properties of loess.Except for Jingyang loess,which has the highest degree of cementation and does not collapse at high moisture content,the rest can completely collapse,and the disintegration rate curve shows an "S" shape.(4)Based on the geometric characteristics of the interface,a self designed interface remolded soil sample pressing mold was designed.Different types of interfaces were prepared for the remolded soil samples to undergo disintegration tests.The test results showed that the existence of interfaces would have a strong interface effect during the loess disintegration process,controlling the cracking and failure of the soil sample,infiltration mode,and thus affecting the disintegration rate.(5)In order to study the preferential flow interface effect generated by the interface during the disintegration process,nuclear magnetic resonance tests were conducted on remolded soil samples containing interfaces,using Yan’an loess as an example.The T2 spectra and water distribution images of the samples after soaking in water were obtained.The results showed that the rapid cracking of the interface would become an advantageous channel for water to quickly penetrate into the interior of the soil sample,provide boundary conditions for the soil sample to come into contact with water during its disintegration process.(6)Based on the MATLAB platform,an LBM two-phase flow model was developed,which combined the microscopic interface and Bishop’s unsaturated effective stress formula to calculate the unsaturated effective stress during the infiltration process.The results showed that when the saturation was low,the gas inside the unsaturated infiltration stage of the soil sample could not be discharged in time,and the effective stress instantly became negative,that is,the repulsive force was greater than the attractive force,corresponding to the cracking phenomenon in the early stage of disintegration.After entering the rapid disintegration stage,the unsaturated forces disappear,the contact mode and cementation characteristics of soil particles control the disintegration rate.The gas is compressed to generate repulsive force,and the cementation between particles softens,reducing effective stress is the reason for the collapse of loess.