SOIL-WATER CHARACTERISTIC AND UNSATURATED PERMEABILITY OF ENGINEERED SOIL WITH MULTI-MODAL POROSITY

Abstract

Slopes formed by residual tropical soil are prone to slope instability triggered by rainfall.  Rainwater seeps into the soil, increasing the water content and reducing soil suction. Changes in water content and soil suction of unsaturated soil are best understood through unsaturated soil parameters, such as the soil-water characteristic curve (SWCC) and unsaturated permeability curve. While a great deal of study has been dedicated to unimodal and bimodal soils, little has been done on multi- or tri-modal soils. Multi-modal porosity soil refers to soil that contains a range of pore sizes—such as several micro- and macropores—that influence its hydraulic behavior and water retention characteristics. This research is conducted to investigate the correlation between the soil's multi-modal porosity, derived from particle size distribution (PSD), and its soil water characteristic curve (SWCC). Compacted soil samples of sand-kaolin (SK), sand-kaolin-mica (SKM), and sand-kaolin-activated carbon (SKAC) were studied. It was found that the drying SWCC of SK and SKM can be categorized as bimodal, while the drying SWCC of SKAC is trimodal. On the other hand, the wetting SWCCs of all three soil samples are of the unimodal type. In addition, SKAC has the lowest permeability throughout the suction range as compared to SKM and SKAC due to a higher percentage of fine particles.