Laboratory tests to study stability mechanism of rainfall infiltrated unsaturated fine-grained soil slopes developing into shallow landslides and their hydraulic properties

Abstract

This study consists of two parts. In the first part, saturated soils wetting band infiltration theories and the most widely used in the world by Lumb, 1975 and Pradel and Raad, 1993 compares theoretical predictions were compared with observed results which gave poor correlations. Results showed that both theories grossly underestimated wetting-band thicknesses. Because above mentioned two theories result in constant values, instead of giving values changing as functions of time. These theories need corrections, which indicate need for further studies. In the second part, hydraulic properties were determined (water-retention, hydraulic-conductivity) of locally obtained 3 undisturbed soils near saturation with a new Hyprop testing technique using the evaporation method. As the Unified Soil Classification System (USCS) does not distinguish inorganic clay colloids by size (size <0,001 mm or 1000 nanometers), Lazer Diffraction Method was used. Results have shown that under zero overall stress; Matric suction does not stay constant, but increases with time up to a maximum point and then decreases, whereas time to reach maximum matric suction increases with decreasing plasticity index (PI) and colloid content (c). While maximum matric suction increases with PI and c, hydraulic conductivity and volumetric water content decreases with increasing matric suction. Also, hydraulic conductivity at maximum matric suction decreases with increasing PI and c.