Residual soil and weathered rock slopes are typically found in mountainous and hilly terrains in most tropical regions like Thailand and ASEAN countries. These earth slopes are often in the unsaturated condition with negative pore-water pressure or matric suction. The presence of matric suction is a favorable condition because it has a beneficial effect on the shear strength of unsaturated soils and thus increases the stability of slopes. However, when the rainfall infiltrates into slope, pore-water pressure increases and matric suction decreases, thus reducing the stability of the slope. This phenomenon commonly known as rainfall-induced slope failure, is one of the most destructive natural disasters, often random and hard to predict.