The physical significance of the shrinkage coefficient of clays and soils

Abstract

1. The investigation described is a continuation of the researches of certain workers in the British West Indies on the significance of the shrinkage coefficient of clays and soils. A brief outline of these researches and their results is given.2. Tempany's hypothesis, which explains shrinkage as due to contraction consequent on loss of water by evaporation from the saturated gel-skeleton that ramifies throughout a mass of soil at its point of maximum plasticity, is criticised on the grounds that it does not completely fit the facts. In particular, it fails to account for the abnormally low shrinkage coefficients exhibited by lateritic soils, notably the red upland soil of Barbados. Mason explained this abnormality by assuming that kneading does not entirely destroy aggregation of soil particles, which is especially well marked in the Barbados red soil. The writer demonstrated, however, that soils of similar colloid content, and belonging to one and the same geological type, but exhibiting different degrees of aggregation, possess similar shrinkage coefficients. A different hypothesis to explain soil shrinkage was therefore sought.3. The hypothesis finally accepted is based on the belief that colloidal gels possess a reticulate structure. At the point of saturation, a hydrogel probably contains water in two phases. The first of these is adsorbed in the walls of the gel; it has been shown by Wilsdon, in the case of soil colloids, to represent the moisture content at the hygroscopic coefficient stage. The second phase fills the vesicles of the gel, and is a crystalline phase.