Study On Shear Strength Of Unsaturated Compacted Loess

Unsaturated loess is widely distributed,building backfill foundations,loess roadbeds and loess dam foundations,etc.The soil used in these projects is unsaturated compacted loess.The soil-water characteristic curve is an important part of studying the strength characteristics of unsaturated soils.For compacted loess,the compaction procedures,including compaction effort and compaction water content,have a significant influence on the mechanical and hydraulic properties of soil.In comparison to the compaction effort,the influence of compaction water content is much more pronounced.Soils compacted at different water content are regarded as totally different soils in spite of the same compositions.In order to study the shear strength characteristics of unsaturated compacted loess,this paper takes Heifangtai Loess as the research object,and uses the standard compaction method to obtain the compaction curve to determine the optimal moisture content(17.0%).Remolded samples were compacted in 3 kinds of water content,which were the optimal moisture content(17%),lower than it(8.0%)and higher than it(19.0%).The densities of compacted samples were 1.57 g/cm~3(D157),1.71 g/cm~3(O171),1.67 g/cm~3(W167),respectively.And the sample(D135)was compacted below the optimal moisture content(8.0%),whose density is the same as the density of the undisturbed soil,D135.The soil-water characteristic curves(SWCCs)of four compacted loess in wetting processes were determined by filter paper method,and the measured data points were fitted by Fredlund&Xing equation.The results show that the soil samples with the same compacted water content,namely D135 and D157,have little difference in SWCCs.While the samples with different water content,namely D157,O171 and W167,varies greatly in the transition zone(matrix suction 10-3000 kPa)of SWCCs.Specifically,SWCC of D157 sample is steepest,W167 is the second,and O171 is the slowest.The cause of this phenomenon is closely related to the pore distribution of the soil sample.In the residual zone(matrix suction greater than 3000kPa),the matrix suction is mainly related to the mineral composition.Soil samples compacted in 8%water content,namely D135 and D157,are both aggregated structures with similar microstructures.The initial void ratio of the soil significantly affects the stress-strain characteristics.The soil sample D135 with larger void ratio of is easy to contraction,which is characterized by ideal plasticity or strain hardening.The microstructures of soil samples with different compacted water content,D157,O171,W167 are largely different.But the normal stress and soil water content have similar effects on the stress and strain characteristics of the soil.Under the same normal stress and water state,the soil sample is more susceptible to strain softening.The normal stress is larger,while the minimum water content of soil under strain hardening is smaller.The shear strength of four compacted loess at different moisture contents was determined by conventional direct shear test,and the shear strength characteristics were discussed.The results show that there is a clear functional relationship between cohesion and water content.And the friction angle is related to the moisture content of the sample preparation.The friction angle hardly changes with the change of water content when below the optimal and optimal moisture content.The friction angle showed a linear decrease with the increase of water content when higher than the optimum moisture content.Based on the results of the filter paper method and the conventional direct shear test,the relationship between the matric suction and the shear strength was established.The results show that the shear strength of the four unsaturated compacted loess is consistent with the change of matric suction,showing obvious stage,and the turning point of each stage completely corresponds to the turning point of soil-water characteristic curve under different normal stresses.Among them,in the boundary effect zone,the matrix suction is lower and the shear strength is slowly increased;entering the transition zone,the shear strength increases remarkably with the increase of the matrix suction.The unsaturated shear strength of the D157 sample was measured by an unsaturated direct shear test and compared with the results of conventional direct shear test.The results show that the absolute error of the shear strength measured by the two test methods is below 8 kPa,and the relative error is about 10%.This shows that it is feasible to estimate the shear strength of unsaturated compacted loess in the Heifangtai area by the combination of conventional direct shear and filter paper test.The theoretical models of three shear strengths,Vanapalli(1996),Oberg&Sallfors(1997)and Khalili&Khabbaz(1998),were selected to compare the predicted values of the model with the measured values of the direct shear test.The analysis shows that when the matrix suction is less than 100 kPa,Vanapalli(1996)The predicted values of the model are basically consistent with the measured values of the conventional direct shear test;in the range of full matric suction,the predicted values of the Oberg&Sallfors(1997)model are far from the measured values,and the predicted and measured values of the Khalili&Khabbaz(1998)model.The values are the closest,and the latter can reasonably estimate the shear strength of unsaturated compacted loess in the Heifangtai area.