Isotropic volumetric behaviour of compacted unsaturated soils within specific volume, specific water volume, mean net stress (v, vw, p) space

A detailed description of the volumetric behaviour of compacted unsaturated soils is essential for modelling compacted soil behaviour. It is more complex than when the soil is saturated, as unsaturated soils exhibit a range of responses, such as yielding under loading, swelling and collapse under wetting, and shrinkage and cracking during drying. In unsaturated modelling, (v, s, p) or (v, s, p') is commonly used as the state space to describe volumetric behaviour, where v (= 1 + void ratio, e) is the specific volume; s is the soil suction; and p and p' are the mean net and mean effective or skeleton stress, respectively. An alternative approach is to use (v, v(w), p) space to describe volumetric behaviour, where vw is specific water volume. In either case, coupled water retention behaviour is needed to describe the overall macroscopic process more completely by including the fourth state variable (v(w) or degree of saturation, S-r, for the former and s for the latter). Following from work undertaken under one-dimensional conditions, the current paper presents the volumetric behaviour of compacted kaolin in (v, v(w), p) space. A series of state path tests comprising various loading, unloading, and (or) wetting paths with nondecreasing degrees of saturation was carried out. The results show that a state boundary surface that is also the virgin compaction surface depicting the loosest state of soil takes control of the volumetric behaviour in (v, v(w), p) space, which can be used as a more practical approach to modelling compacted soil behaviour, especially for analysing major wetting events.