Study On Permeability And Strength Characteristics Of Compacted Loess Under Wetting-drying Cycles

In recent years,due to the acceleration of urbanization,Yan’an municipal government decided to expand the city by cutting mountains,filling ditches,building land and building cities in order to break through the shackles of the narrow and unfavorable terrain of the original valleys.In the process of urban expansion,engineering such as mountain cutting,ditch filling and land making will destroy the original topography and hydrogeological conditions of the area,change the original stress state of the soil and the runoff or seepage path of surface water and groundwater.Moreover,the climate characteristics of dry and rainy in spring and hot and rainy in summer in Yan’an City will make the original soil and compaction site in the adverse conditions such as the increasing humidity and reducing humidity cycle,which will lead to great changes in the engineering mechanical properties of the soil itself.If we don’t pay attention to it in the actual project construction,it is easy to cause related engineering accidents,and result in economic losses and adverse effects.Therefore,it is of great practical value and engineering significance to study the permeability and strength characteristics of compacted loess under the action of dry and wet cycles.In this paper,Yan’an compacted loess is taken as the research object.Based on the self-developed test device of soil column moisture increasing and decreasing cycle,the micro structure characteristics,macro crack characteristics,permeability characteristics,soil-water characteristic curve and shear characteristics of compacted loess under the dry-wet cycle are studied by means of SEM(),NMR()and direct shear tests.According to the microstructure and crack development,the macroscopic test phenomena are explained.The main research contents and achievements are as follows:(1)Through the SEM and NMR tests of compacted loess with different dry density under the action of dry-wet cycles,it is found that: With the increase of dry density,the soil particles of the sample contact more closely,the connection mode of surface to surface contact increases,and the overhead structure is gradually replaced by mosaic structure;with the increase of the number of dry-wet cycles,the number of large-sized particles decreases,the number of medium-sized particles basically remains unchanged,the number of small-sized particles increases,while the micropores basically disappear,the small poresdecreases,the medium-sized pores obviously increases,the large pores increases,and the overall pore volume increases after the dry and wet cycles.(2)Based on the self-developed increasing and decreasing humidity cycle device,the macro crack characteristics and permeability of compacted loess under the action of dry-wet cycle were studied.The results show that: The initial crack position on the surface is within one third of the center point of the sample surface under different densities;with the increase of dry density,the matrix suction became larger,the moisture content became smaller,but the total number of cracks decreases;at the same density,the number of dry-wet cycles has little effect on the moisture content and matrix suction while the initial crack occurs,but the amount of soil shrinkage decreases with the increase of the increasing-decreasing wet cycles;the saturated permeability rate decreases with the increase of dry density,while with the increase of the number of dry-wet cycles,the surface cracks of the same density soil increase,the pore volume of the soil increases,and the saturated permeability increases rapidly.(3)In the process of dry wet cycle,under different dry density,the test and analysis of soil water characteristic curve show that: The first dry wet cycle has the strongest influence on the shape of soil water characteristic curve,and the higher the dry density is,the greater the slope of soil water characteristic curve is;The curve in the process of soil dehumidification is always lower than that in the process of soil dehumidification,showing obvious hysteretic characteristics;Also the curve under the soil dehumidification is always lower than that in the process of soil dehumidification,it showing obvious hysteretic characteristics;The fitting effect between the FX model and measured soil water characteristic curve is the best.The residual water content increases with the increase of dry density,decreases with the increase of the dry-wet cycles.(4)The direct shear tests of compacted loess under dry-wet cycles are carried out,which compacted methods are static pressure,layered static pressure and indoor standard compaction.The following conclusions are obtained: The shear strength of the samples compacted in layers and indoor is larger than that of the samples with the same density,which is closer to the parameters of the shear strength of the compacted loess;the effect of dry and wet cycles on the shear strength of compacted loess with different dry density is significant.The cohesion decreases with the increase of cycles,but the increase of dry density caneffectively reduce the effect of dry and wet cycles on the shear strength;The matrix suction and shear strength of compacted loess in the process of humidification and dehumidification are approximately linear.The shear strength in the process of humidification is higher than dehumidification which causes soil cracking and particle connection weakening...