Compression index of clays and silts

Abstract

The consolidation settlement of a structure founded on a normally consolidated soil can be calculated from the knowledge of the compression index Cc. The value of Cc is usually obtained from the laboratory oedometer test on the assumption that the void ratio (e)-logarithm of pressure (log σ′v) plot is linear. While the literature reveals that in the case of highly plastic clays the plots are concave upwards with their Cc (computed between successive load increments) decreasing with an increase in σ′v, little is known regarding soils that are less plastic, in particular, silty soils. This paper reports the e-log σ′v plots of clays and silts. It was found that the e-log σ′v curves of the silts are not linear but are convex upwards. This results in an increase in Cc with an increase in σ′v. Convex curves are to be regarded as a normal feature in such soils and can be explained from consideration of the mechanical and physicochemical factors that govern the compressibility of clays. These factors establish that Cc increases with σ′v for those soils whose compressibility behavior is governed by mechanical factors, in contrast to soils like very plastic clays whose compressibility behavior is governed by physicochemical factors. In the latter, Cc decreases with an increase in the consolidation pressure.