Effects of confining pressure on soil-water characteristic curves of sands

Abstract

Conventional geotechnical research and structural designs are generally based on saturated soil mechanics. However, soil in the unsaturated condition is encountered most of time. In the past few decades, researchers have found an increasing demand on the study of the unsaturated soil behaviour. The soil-water characteristic curve (SWCC) is a significant property and can help in the understanding of the principles and mechanics behind shear strength and seepage of unsaturated soils.SWCC is dependent on soil types. It is usually plotted as volumetric water content of the soil as the ordinate and matric suction as the abscissa. Typically, the water content of the soil under the drying process is higher than that in the wetting process. The difference in these two processes is the hysteresis. Generally, studies of SWCC were mainly focused on the drying process and soil samples are usually under zero net confining pressure. Hence, the study on the SWCC of soils under different net confining pressures can contribute to the on-going unsaturated soil researches. In this project, both the effects of confining pressure and the percentage of fines in the soil on the SWCC were investigated. Repeated drying and wetting SWCCs were obtained for sands with 10%, 30%, and 50% fines content subjected to 0 kPa, 20 kPa, and 200 kPa net confining pressures. The Tempe cell was used to determine the initial drying SWCCs of sands with 10% and 50% fines content under zero confining pressure. A newly fabricated SWCC apparatus was used to obtain the multiple drying and wetting SWCCs for sands containing 10% and 30% fines content at 20 kPa and 200 kPa net confining pressures. The SWCCs were fitted with Fredlund and Xing’s equation (1994). Effects of net confining pressures and fines content on Fredlund and Xing (1994) equation parameters – a, n, and m – were discussed. Moreover, a simplified method – Chin’s method (2010) – to estimate the SWCC was also discussed on its accuracy and applicability. Furthermore, the hysteresis in the SWCC was calculated and studied. Test results revealed that sand under a higher net confining pressure generally gave higher air-entry value, higher residual matric suction, and larger hysteresis. Sand with a higher percentage of fines tends to have higher air-entry value, higher residual matric suction, less water retention ability, and higher hysteresis.