Study On Strength And Parameters Of Unsaturated Loess

Loess is a typical unsaturated soil.Unsaturated strength is the basic theory of engineering stability analysis and evaluation in Loess area.In this paper,the Quaternary L5-layer loess along the South Bank of Jingyang County,Shaanxi Province,is used to test the soil-water characteristic curve of undisturbed and remolded unsaturated loess by pressure plate instrument,the conventional triaxial saturated strength parameters and the unsaturated strength parameters under unsaturated triaxial conditions.The test data are analyzed by two different strength theories,and the corresponding strength parameters are obtained.This paper mainly carries out the following research:（1）The natural water content of unsaturated loess in L5 layer on the South Bank of Jinghe River is 17.3%,density is 1.83,specific gravity is 2.71 by gravity bottle,liquid limit by butterfly liquid limiter is 28.8%,plastic limit by rubbing strip method is 19.3%.Saturated volume water content is 39.8%and void ratio is 0.727.The dry density was1.59 and the plastic index was 9.5.（2）The stress-strain curves and stress paths of undisturbed and remolded loess under different water content and confining pressures are obtained through conventional triaxial tests.The stress-strain curves of undisturbed and remolded loess are compared and the effective strength parameters of undisturbed and remolded loess are calculated.（3）Soil-water characteristic curves of wetting and dehumidifying of undisturbed loess with matrix suction in the range of 10-1000kPa were drawn by pressure plate instrument.The relationship between matrix suction and moisture content was obtained.It was found that when matrix suction increased to 100 kPa,the curve of wetting and dehumidifying basically coincided.（4）Through unsaturated triaxial tests,the stress-strain curves of undisturbed unsaturated loess under different net confining pressures and matrices suction are obtained,and the effective strength parameters are calculated.（5）Based on Lu Ning’s suction stress theory and conventional triaxial test,the parameters of suction stress strength theory are obtained.Based on Vanapalli’s strength theory and unsaturated triaxial test analysis,the strength formula suitable for L5formation in the South Bank of Jinghe River is obtained.The two strength theories are compared and the relevant conclusions are drawn.Through analysis,it can be found that the slope of the strength curve obtained by Lu Ning’s theory of absorptive stress strength is unchanged,while the concept of tanφ^b is introduced into Vanapalli’s theory of strength.The slope of the strength curve will decrease with the increase of matric suction,and the trend of the curve obtained by changing different fitting parameters kappa is different.Through the analysis of the experimental results,the strength curve is simulated.It can be found that for the net confining pressures of 40 kPa and 100 kPa,when kappa equals 1.8,the best fitting curve can be obtained,and then the shear strength formula for L₅ layer unsaturated loess on the South Bank of Jinghe River can be obtained.Finally,by comparing two different strength theories,the advantages and disadvantages of the two methods and their application in engineering practice are discussed.