Soil Water Characteristic Curve of an Unsaturated Soil under Low Matric Suction Ranges and Different Stress Conditions

Soils with negative pore pressures are usually referred to as unsaturated soils. The voids of these soils are normally filled with both water and air. The Soil Water Characteristic Curve (SWCC), which represents the capacity of the soil to store and release water when matric suction is applied, is defined as the relationship between the matric suction and the degree of saturation or gravimetric water content for unsaturated soils. In this regard, the SWCC has widely been identified as the key soil information required for the analysis of seepage, stability and volume change problems involving unsaturated soils.

In fact, many experimental works have reported on the unsaturated soil properties, and the soil-water characteristic curve (SWCC) test has contributed significantly to the interpretation of matric suction. Since traditional instruments cannot apply stress in SWCC tests, some researchers have developed suction controlled triaxial apparatus, by which SWCC tests are performed under different stress states. Determination of SWCCs under stress conditions similar to those in the field is key for interpretation of the hydro-mechanical behavior of unsaturated soils.

This study conducted SWCC tests of unsaturated silt soil in low matric suction ranges under both drying and wetting conditions. The SWCCs were measured under one-dimensional and isotropic confining stresses ranging from 50 to 450 kPa. The micro porous membrane method was used instead of high air entry ceramic disk for controlling relatively low matric suction. The range of matric suction controlled was from 0 to 20 kPa.

The results revealed that different stress conditions have an effect on the shape of SWCCs in low matric suction ranges. And the SWCCs revealed that an increase in the net confining pressure decreased the void ratio and consequently increased the degree of saturation of the soil specimens. Besides, there were considerably minimal deviations in the correspondence between the measured SWCCs under low suction ranges and different stress conditions and the fitted SWCCs.

In short, the current methods adopted for estimating unsaturated soil properties require further development to take into account the effect of different stress conditions.

Article by Paul Habasimbi and Tomoyoshi Nishimura, from Ashikaga University, Omae-cho, Ashikaga Tochigi, Japan.

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Image by USDA NRCS South Dakota, from Flickr-cc.