Variability in unsaturated hydraulic properties of residual soil in Singapore

• COV of saturated and unsaturated soil properties is computed and discussed.
• Variability of SWCC and permeability function for residual soil are quantified.
• Effect of variability of SWCC on permeability and shear strength is investigated.
• A new framework for simple estimation of SWCC from ks is proposed.
• Upper/lower limit in framework is advised for infiltration/slope stability analysis.

Residual soil covers most of the land in Singapore and is commonly found in unsaturated conditions due to the deep ground water table. Matric suction in residual soil plays an important role on the stability of residual soil slopes against rainfall. Matric suction in the soil will change due to infiltration during rainfall, and infiltration is mainly controlled by the unsaturated hydraulic properties (i.e., soil-water characteristic curve (SWCC) and hydraulic conductivity) of the residual soil. Variability in SWCC and hydraulic conductivity of residual soils in Singapore, such as Jurong Formation, Bukit Timah granite and Old Alluvium, were quantified and compared with the uncertainty of saturated soil properties. A new framework for the estimation of SWCC from the saturated hydraulic conductivity, ks, is proposed in this paper. The upper bound from the proposed framework is recommended for the infiltration analyses and the lower bound from the proposed framework is recommended for the slope stability analyses.