Errors in Conventional Calculations of Soil Phase Relationships

Soil physical properties derived from phase relationships such as void ratio, degree of saturation, water content, and unit weight conventionally are formulated and calculated under the implicit assumption that the volumes of the three soil phases (i.e., solid, water, and air) are independent and unaffected by interactions among phases. However, significant variations do occur, including variation in the unit weight of soil water resulting from adsorptive soil–water interactions and variation in void volume resulting from bulk swelling or shrinking with changes in moisture. The importance of accounting for soil–water interactions in calculating soil physical properties was examined by introducing a new variable-volume soil phase diagram, and was illustrated using experimental data from consolidation and shrinkage tests for silts, nonswelling clays, and swelling clays. Errors in calculating void ratio and soil unit weight in saturated consolidation tests using conventional phase relationships can be significant—as high as 48% relative error in void ratio and as high as 16% relative error in soil unit weight. Errors in calculating moisture ratio and saturation in shrinkage tests can be as high as 53% relative error in moisture ratio and 42% in saturation. The occurrence of maximum error is highly correlated with soil water content corresponding to the maximum amount of adsorbed water. Liquid limit (LL) and specific surface area (SSA) were identified as index properties that can be used to account for volume variations in phase calculations. Examples are provided to illustrate the implementation of the proposed variable-volume soil phase formulation using LL or SSA in practical problems. Analysis showed that for fine-grained soils, it is practically necessary to use general variable-volume phase relationships to define basic soil properties in lieu of the conventional phase relationships.