Study On Soil Water Characteristic Curve And Strength Of Unsaturated Soil In Zhangjiakou Area

Due to the influence of groundwater distribution and climate,the dry and wet state of the natural soil is always in a changing process.The variation of the dry and wet state causes the internal matric suction of the soil to change.The shear strength formulas applied at this stage are mainly applicable to saturated soil.The influence of matric suction on the shear strength of soil has not been reflected,resulting in waste in engineering applications.Therefore,the shear strength of unsaturated soil provided by matric suction is studied in this paper.Because the matric suction is hard to measure,the soil-water characteristic curve is usually plotted to obtain the relationship between the matric suction and the water content.The water content is used instead of the matric suction to facilitate calculation.Besides,the soil-water characteristic curves measured in the laboratory are mostly in the unstressed state,and the historical conditions of undisturbed soil stress are more complicated.It is necessary to carry out the mapping of the soil-water characteristic curve of remolded soil under different stress states better to analyze the soil-water characteristic curve of the undisturbed soil.Taking the typical silty clay in Zhangjiakou area as an example,this paper conducted soil-water characteristic curve mapping,micro-analysis,and triaxial shear strength research on unsaturated silty clay.(1)The unsaturated soil consolidation instrument and unsaturated soil triaxial instrument are used to conduct experiments on the soil body under no stress,axial stress,and isotropic confining pressure to obtain the soil-water characteristic curve of unsaturated soils under different stress states.Comparing the undisturbed soil and the remodeled soil-water characteristic curves under different stress conditions,the overall trend of the curve is the same,and the differences in air intake value,water content reduction rate,and residual water content are quite different.It is believed that the stress history of the soil mass has a more significant effect on the soil-water characteristic curve of unsaturated soils.After fitting,the Gardner model is used to analyze the soilwater characteristic curve,and the quadratic derivative and logarithmic formula are used to determine the model parameters.(2)The pore distribution of the soil surface was observed with a Zeiss stereomicroscope,and it was found that after the pre-consolidation of the remodeled soil sample,the soil surface was flatter and the pore distribution was more uniform.The BSD-PS2 specific surface area and pore size analyzer were used to perform lowtemperature nitrogen adsorption experiments on unsaturated soils with a matric suction of 385 kPa.The analysis of the measured soil pore diameter results found that when the matric suction reached a certain value,the micropore pore diameter index of the soil inside the remolded soil sample was larger under no pressure.The soil pore size index is very close,so compared with the residual moisture content obtained under no stress condition,the residual moisture content of the soil body under different stress conditions is closer to the original soil.(3)The stress-strain curves of the soil under different suction conditions are obtained through triaxial tests of unsaturated soils.The curve shows a hyperbolic state as a whole.The unsaturated soil eccentric stress tends to increase first and then decreases with increasing matric suction.With the increase of confining pressure,the ratio of soil size to principal stress gradually decreases.With the decrease of the water content,the ratio of the major and minor stresses increases first and then drops.Matric suction has a weak effect on the friction angle in the soil,and it has a more significant impact on the cohesion.The total cohesion of unsaturated soils was analyzed by using the axis translation technique.It was found that the total cohesion of the soil mass increased with the increase of the matric suction and showed a parabolic state.The shear strength formula of the soil body with the change of the suction of the matrix is obtained through analysis.When the soil body reaches the residual stage,the applicability of the shear strength formula to the unsaturated soil becomes low.Combining the relationship between matric suction and volumetric water content in the soil-water characteristic curve of the unsaturated soil,it is considered that the formula for calculating shear strength of unsaturated soils using volumetric water content is also applicable and easy to measure.The relationship between total cohesion and volumetric water content is parabolic,and the relational formula between volumetric water content and shear strength of unsaturated soil is obtained.The formula is also valid when the soil reaches the residual stage.