The influence of soil structure and stress history on the soil–water characteristics of a compacted till

The soil–water characteristic defines the relationship between the soil suction and gravimetric water content, w, or the volumetric water content, θ, or the degree of saturation, S. Theoretical and empirical relationships are available to model the unsaturated soil properties such as the coefficient of permeability and shear strength using the soil–water characteristic and the conventional saturated soil properties. These procedures are attractive to engineering practitioners because rigorous laboratory tests on unsaturated soil are difficult and time-consuming and, therefore, costly. Various investigators generally have used the soil–water characteristic over a limited suction range (usually 0 to about 1500 kPa) to model the unsaturated soil behaviour. Soils, however, change from a saturated to a dry condition over a range of suctions from 0 to 1000 000 kPa. A rationale for extending the soil–water characteristic up to 1000 000 kPa is provided in this paper, and a method for estimating the residual state of saturation is presented. Soil–water characteristics were developed for the entire range of suctions from 0 to 1000 000 kPa on statically compacted clayey till specimens prepared at three different initial water contents. The influence of initial water content, soil structure and stress history, as it relates to the soil–water characteristic, has been studied and is reported in this paper. The initial water contents selected for this study represent the dry of optimum, optimum and wet of optimum conditions with corresponding densities determined from the standard AASHTO test. The results indicate that initial moulding water content has considerable influence on the resulting structure (and aggregation), which in turn influences the soil–water characteristics. In the low suction range (i.e. 0–1500 kPa), macrostructure governs the soil–water characteristics for specimens compacted with dry of optimum initial water contents and they exhibit a steeper slope. However, for the specimens compacted at wet of optimum, microstructure governs the soil–water characteristic behaviour. The soil–water characteristic of the specimens compacted at dry of optimum are influenced by the stress history; however, stress history appears to have no significant influence on the soil–water characteristics of the specimens compacted with wet of optimum conditions. The soil–water characteristic behaviour of specimens compacted with optimum initial water contents lies in between those of specimens compacted with water contents that are dry and wet of optimum. It appears that soil–water characteristics are not significantly influenced either by the soil structure (aggregation) or the stress history for the high suction ranges (i.e. 20 000–1 000 000 kPa).