Experimental Study On Characteristics Of Strength Changes Of Unsaturated Soil During Drying

The rainfall-induced instability of soil slopes is one of the major geological hazards in areas with abundant rainfall in China.This is because the saturated state of the soil changes when the water content is different,resulting in a large change in shear strength.At present,scholars at home and abroad study the shear strength of unsaturated soils is less involved in the high suction stage,so it is necessary to study the shear strength characteristics of unsaturated soils in the range of full suction and find a matching shear strength model.In this paper,SWCC measurements,conventional direct shear tests,capillary cohesion analysis and strength models for unsaturated soils under high suction are studied.The test soils include:Xining silty clay,kaolin,weathered soil with different fines content and sand with different fines content.For the latter two types of soils,the influence of fines content on the capillary cohesion and matrix suction of the samples was studied.During the test,unsaturated soil samples with different suction/moisture contents were obtained by means of air-dry curing;The SWCC of eleven soils was determined by filter paper method;the strength parameters of soils under different non-saturated conditions were determined by conventional direct shear tests.Based on the experimental data,the correlation factors of the influencing factors of capillary cohesion were analyzed,the existing unsaturated soil strength model was verified,and the full-range improved Shen model for all types of soil was proposed.The main conclusions of the paper are as follows:(1)Most of the traditional models consider that the friction angle in unsaturated soils does not change with the suction force.The experimental data show that the internal friction angle of unsaturated soils increases with the increase of matrix suction under the mode of air-dry curing.(2)For the eleven soils used in the experiment,there is a "peak" effect on the non-saturated strength.That is,the non-saturation intensity has a peak in the drying process,and the corresponding saturation is about 0.02 to 0.1.The corresponding matrix suction is due to the type of soil.different.(3)Previous studies have suggested that the unsaturated strength of sand is limited.This article shows that the test data after air curing show that with the increase of fines content,the capillary cohesion of unsaturated sand increases,even up to 86kPa;for sand with a certain fine content It is said that the total dry strength is about 40%-60%lower than the peak strength,while the other types of soil are only about 3%-7%lower.(4)A correlation analysis was made on the factors affecting the capillary cohesion and combined with the experimental data on the existing Khalili model,Lamborn model,and Shen model.It was found that these three models have poor prediction effect on the capillary cohesion of the high suction section.(5)In this paper,the improved Shen model is proposed based on the Shen and Vanapalli models.The prediction of the capillary cohesion of 11 soils proves that the improved Shen model has good applicability.