Volume 32, Issue 6 (November 2009)
Development of a Modified Axis Translation Technique for Measuring SWCCs for Gravel Soils at Very Low Suctions
For gravel soils, water content changes at very low suctions (e.g., smaller than 1 kPa) are significant, and such changes affect the permeability and shear strength of the soils significantly. Difficulties in studying the soil-water characteristic curves (SWCCs) at very low suctions include the lack of a proper experimental device and the suction difference induced by the gravitational hydraulic gradient along the sample height. In this study, a large size modified axis translation device was developed. The device can control the soil suction accurately (i.e., a precision of 0.005 kPa) using a water-head control method. A new interpretation procedure was also proposed to consider the suction difference along the sample height and to extend the minimum measurable suction from 0.1 to 0.01 kPa. The device was used to measure the SWCCs for two gravel soils. The experimental results demonstrate that the SWCCs for gravel soils in the low-suction range show bimodal features, where the water content decreases sharply at suctions smaller than 1 kPa. Such bimodal behavior may not be revealed using conventional SWCC devices. SWCCs at low suctions are sensitive to soil density. Significant hydraulic hysteresis is also present in the SWCCs at low suctions.