Field and Laboratory Suction- Soil Moisture Relationship of Unsaturated Residual Soils

Soils located above the groundwater table such as residual soils are generally unsaturated and possess negative pore-water pressures. A soil-water (moisture) characteristic curve (SWCC) that relates the water content of a soil to matric suction is an important relationship for the unsaturated soil mechanics. The SWCC essentially shows the ability of an unsaturated soil to retain water under various matric suctions. It has a similar role as the consolidation curve of a saturated soil that relates void ratio or water content to effective stress. This study describes a study that has been carried in the field and in the laboratory to examine the suction-soil moisture relationship of unsaturated residual soils of granite and sedimentary rocks origin. The field measurement shows a decreasing trend of suction with depth for both soils. The suction-soil moisture relationship shows two distinct curves, a wetting (sorption) curve and a drying (desorption) curve. While from the laboratory study, it is observed that there is a significant decrease in the soil moisture with increasing suction in the lower suction ranges, until a de-saturation or air entry point for both soils. Beyond this point, the magnitude of the decrease in soil moisture for the equal increment of applied suction is less. The de-saturation point of a particular soil appears to be dependent on the amount of clay content. Higher amount of fines in the soil constitute a more compact particle arrangement and a smaller pore size. Soils with smaller pore sizes de-saturate at higher matric suction.