Effect of pore-size distribution on permeability function of residual soils from Bukit Timah formation

Abstract

Classical soil mechanics deals with soil that is saturated. However, in tropical region like Singapore, soils are often unsaturated. Unsaturated soil has more than two phases and has negative pore-water pressure relative to the pore-air pressure. This negative pore-pressure poses geotechnical problems such as slope failures which are evident in residual soils.

Residual soils that cover more than two-thirds of Singapore’s land area are mainly from the Bukit Timah granitic formation and Jurong sedimentary formation. Properties of residual soils are a function of degree of weathering. Large amount of rainfall coupled with hot and humid climatic conditions encourages weathering of the bedrock in Singapore. As a result, the permeability of the residual soil also varies along the soil profile and increases the pore volume and produces a larger range of pore-size distribution.

The objective of this report is to examine the effect of pore-size distribution (PSD) on permeability function of residual soils from Bukit Timah formation. Water retention curve, saturated permeability and basic index properties tests were used to quantify the PSD.

Results determined that variation in the permeability and microstructural characteristics of the residual soils with depth is associated to the degree of weathering. The PSD has a profound effect on the unsaturated permeability function; the smaller the mean pore diameter of a soil specimen, the lower will be the permeability of the soil.