A Research About The Axis-translation Testing Technology And Its Application In The SWCC Of Unsaturated Soil

Soil-water characteristic curve can reflect the relationship of matric suction and moisture content. It plays a very important role in a number of studies about unsaturated soil. Currently, Fredlund soil-water characteristic curve apparatus and tensiometer are the two most commonly used SWCC direct test means. Fredlund soil-water characteristic curve applies the principle of axis-translation technique to measure SWCC, and tensiometer applies the principle of negative water column to obtain SWCC. Some research results found that SWCCs of same soil samples measured by the axis-translation technique and tensiometer have significant differences, in the following two points:(1)The differences were different in different mineral composition soil samples;(2)The differences increased with increasing matric suction. In order to reasons of the SWCC difference, this paper applied theoretical analysis, SWCC experiment, capillary experiments and finite element numerical simulation. A correction calculation formula was build for reducing the SWCC differences. And the importance of SWCC amended was showed by finite element numerical simulation of slope stability. This paper accomplished the following works:1. Focus from testing principles of soil water characteristics curve, the working principle and instructions of axis-translation technique and tensiometer were described in detail, and SWCCs of sand and silty clay were measured by the two methods. Results of two kinds of soil samples verified the fact that SWCC obtained by two testing methods is different.2. On the basis of the differences of SWCCs measured by the above two methods, simultaneously, to combination the principle of the two testing methods, the environmental pressure is the root of SWCC differences.3. The micro pores and ultrafine pore of clay soil sample were idealized a simple model. The relationship between environmental pressure and air volume was established using the model. Pore water didn’t drain due to environmental pressure compressed pore air internal micro pores and ultrafine pore. It can be seen that micro pores and ultrafine pore are main reasons of SWCC differences. Meanwhile, the rationality of theoretical analysis was verified through SWCCs of quartz sand and clay soil samples.4. A relationship between environmental pressure and water retention properties was established by capillary experiments in axis-translation environment and the connection between surface tension coefficient and pressure. It can be seen that the impact of environmental pressure on the SWCC data is very small, it can be omitted.5. Based on the reasons of difference, the relationship between environmental pressure and volume of air internal micro pores and ultrafine pore was build through combining with a three-phase diagram of unsaturated soils. A correction formula was set up in the light of the relationship between environmental pressure and volume of air. SWCCs of clay and sand soil samples were corrected using build correction formula, and the effect of the correction method was assessed by error correction evaluation theory.6. The correction method was used in a slope stability analysis example of unsaturated soil considering the effect of solid-fluid coupling. Compare to the SWCC amend and SWCC measured, the pore water pressure, soil deformation and plastic zone of slope were obtained.