Study On Moistening Triaxial Tests Of Undisturbed And Unsaturated Loess

The properties of unsaturated soil are more complicated than saturated soil because of the shrink film. Due to the deep groundwater level, loess is a typical unsaturated soil. Using loess mechanics as a branch of unsaturated soil mechanics has become a logical trend. So the study of strength characteristics and changing rules of unsaturated soil can provide reliable basis for engineering design. On the other hand, the special nature of mineral composition and microstructure of loess determines the effect of water on loess. Changes in the moisture content of the soil can cause deformation by changing the soil matric suction, cementing force, expansion and contraction force and gravity. The changes of soil moisture content and stress characteristics are often coupled to reflect the real soil deformation on the specific conditions. So it is important to study strength and deformation characteristics of undisturbed loess under different water content, confining pressure and flooding on different partial stress levels. In order to research the strength and deformation characteristics of undisturbed loess slope under different water content(humidify with no stress conditions) and confining pressure in Shaanxi Jingyang, using FSY30 type strain controlled triaxial apparatus for unsaturated soils and consolidated undrained test to obtain the value of υb in the shear strength formula of Fredlund’s double stress state variables, then established a practical shear strength formula used of the water content. After that, continue cut the saturated specimen after flooding under different partial stress levels, analyze its mechanical properties. Conclusions are as follows:(1) Relationship between water content and initial matric suction is hyperbolic. The confining pressure for consolidation showed no effect on the initial matrix suction. The matric suction and pore water pressure showed no significant changes throughout the process of shearing. The unsaturated and undisturbed loess samples under different initial water content were shaping destructed. The stress-strain curves of loess in this region showed a hardening type. At the same confining pressure, the higher of water content, the lower of the breaking stress.(2) The cohesion of shear strength index decreased significantly with the increasing of water content but the impact on internal friction angle is smaller. The cohesion is proportional to the value of the matric suction. The tested strength parameters of soil are: c ’ =8.89 kPa, υ’=20.45°, υb=21°. The higher of water content, the lower of shear strength. The shear strength parameters ctotal and υtotal are proportional to water content. Then use water content(w) as the only variable to calculate a shear strength(τ) formula:71.262 1.9083 tan(16.718 0.1632)f ft = -w +s +w(3) The strength of specimen reduced significantly after flooding but it has little effect on the stress-strain curves, the lower of the partial stress when humidified, the lower of the stabilized partial stress values. The higher of confining pressure, the higher of the breaking stress.(4) At the same confining pressure, the strength of humidified samples is more than half of the samples which had not humidified. With the increasing of partial stress level when humidified, shear strength parameters cohesion c values showed a significantly decreasing trend, while the impact on internal friction angle υ is smaller.