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 suc tion and gravimetric water content, w, or the volumetric water content, the ta, or the degree of saturation, S. Theoretical and empirical relationships are available to model the unsaturated soil properties such as the coeffic ient of permeability and shear strength using the soil-water characteristic and the conventional saturated soil properties. These procedures are attra ctive to engineering practitioners because rigorous laboratory tests on uns aturated soil are difficult and time-consuming and, therefore, costly. Vari ous investigators generally have used the soil-water characteristic over a limited suction range (usually 0 to about 1500kPa) to model the unsaturated soil behaviour. Soils, however, change from a saturated to a dry condition over a range of suctions from 0 to 1000000kPa. A rationale for extending t he soil-water characteristic up to 1 000 000 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 prepa red at three different initial water contents. The influence of initial wat er content, soil structure and stress history, as it relates to the soil-wa ter characteristic, has been studied and is reported in this paper. The ini tial water contents selected for this study represent the dry of optimum, o ptimum and wet of optimum conditions with corresponding densities determine d from the standard AASHTO test. The results indicate that initial moulding water content has considerable influence on the resulting structure (and a ggregation), which in turn influences the soil-water characteristics. In th e low suction range (i.e. 0-1500 kPa), macrostructure governs the soil-wate r characteristics for specimens compacted with dry of optimum initial water contents and they exhibit a steeper slope. However, for the specimens comp acted at wet of optimum, microstructure governs the soil-water characterist ic behaviour. The soil-water characteristic of the specimens compacted at d ry 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-cater characteristic behaviour of specimens compacted with optimum initial water contents lies in between those of specimens compacted with water contents t hat are dry and met: 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. 20 000-1000 000 k Pa).