Equivalent granular void ratio for characterization of Singapore's Old Alluvium

Abstract

Characterization of the Old Alluvium (OA) Formation in Singapore is difficult due to its natural heterogeneity. The OA formation was deposited by a braided river system, and it exhibits a wide range of engineering properties. Its strength and stiffness at a given depth, within a few metres distance horizontally, may differ by an order of magnitude, and this cannot be explained by differences in void ratio alone. The formation consists of sand, silt, and clay particles in varying proportions with no consistent relation with depth. Since the majority of OA is sand mixed with fines (silt and clay), the concept of granular void ratio, which treats silt and clay particles as void, was first introduced to try to characterize the formation. Though the concept of the granular void ratio was more useful than that of the void ratio, it could not take into consideration the different contributions that nonplastic silt fines and plastic clay fines make to the shear strength and thus was found to be inappropriate for a natural soil such as OA. To deal with this problem, the relative contribution of different kinds of fines is incorporated through the concept of equivalent granular void ratio. The merit of equivalent granular void ratio for the characterization of mixed soil formations is demonstrated in this paper through the analysis of results from triaxial tests on in situ OA samples.Key words: natural soil, clayey sand, Singapore Old Alluvium, heterogeneity, granular void ratio.