Investigation Of Stress State-dependent Soil Water Characteristic Curve And Permibility Behavoir For Unsaturated Soil

Soil water characteristic curve is one of the significant aspect of constitutive relationship, while the coefficient of permeability is one of the index properties for soils, which are the key points for unsaturated soils. In this paper, the influence of void and stress on soil water characteristic curve, as well as the approach to direct measurement of permeability for unsaturated soil are analyzed. The details are as follows:1. A series of tests of soil water characteristic curve for remoulded loess specimens at different bulk densities are carried out by means of GCTS-type SWCC device. Based on test results, the influence of bulk density on soil water characteristic curve is qualitatively analyzed. In addition, soil water characteristic curve is described by measured data so that the best-fit equation is acquired on the basis of hyperbolic model. As a result, Gardner model is verified to be a well-adapted selection to describe soil water characteristic curve for loess under different bulk densities.2. The affect of vertical stress on soil water characteristic curve for unsaturated loess is then analyzed under Ko stress state. Comparing with the condition under no stress state, the degree of saturation become higher under Ko stress state when matric suction is same. Furthermore, the soil water characteristic curve, with matric suction lower than air entrance value, is more saliently affected by stress state. In one word, stress state plays a key role on influencing soil water characteristic curve.3. According to the above-mentioned test results, the affect of wetting-drying cycle on soil water characteristic curve for loess is analyzed. With the increase in the number of wetting-drying cycle, the hysteresis effect of soil water characteristic curve become less salient, simultaneously, the number of larger voids disabled for absorption is rising. In sum up, bottle-neck effect results in the difference between wetting and drying curve mainly,4. According to the above-mentioned test results, the suction-strain behavior for unsaturated loess is also analyzed. During the drying process for unsaturated soil, the matric suction-dependent areas become smaller with the increase in matric suction, while the matric suction-strain effect can reach a peak value with the water content varying from 18%to 20.5%. The matric suction-dependent strain become more salient with the increase in the number of wetting-drying cycle.5. For Saskatchewan silt, Indian head till, and Xi'an loess, the result by introducing 'water-solid volumetric ratio'Rw shows that evolutions of Rw with matric suction under different consolidation stress can be normalized by single curve for the certain scope of consolidation stress and matric suction. On the basis of the acquaintanceship, a new density-dependent soil water characteristic curve model, with a few parameters easily determined, is proposed based on van Genuchten model for the drying curves of these three types of soils.6. The new auto-controlling apparatus for measuring air-water movement is successfully debugged so that the direct technique to measure the water permeability for unsaturated soil is achieved. Under a single hermetic state, the volume and pressure for one phase of unsaturated keep constant during the infiltration process, while the other phase become connective so that the infiltration takes place under the pressure gradient of this phase. Finally, the coefficient of permeability for loess specimen at a certain bulk density is measured under different degrees of saturation based on this principle, which shows that the exponential behavior occurs over the testing range for the evolution of permeability with degree of saturation, furthermore, the test results are more credible, comparing with former results.