Hydraulic Equilibrium: Is It Ever Reached?.

The soil water characteristic curve (SWCC) is determined by plotting applied pressure (0 to -1500 kPa) versus residual water content (θ_r) at hydraulic equilibrium. Equilibrium at each pressure step is commonly assumed when dewatering of the sample becomes extremely slow or when rules are imposed, i.e. equilibrium is assumed after a predetermined length of time has lapsed. When determining the SWCCs of undisturbed soil cores, identification of equilibrium proves challenging, as prolonged dewatering often occurs. The objective of this study was to identify the occurrence of hydraulic equilibrium of undisturbed soil cores at a range of pressure steps obtained via high resolution centrifugation; and hence to comment on the temporal approaches present in the literature.

Undisturbed soil samples were subjected to centrifugation until dewatering had completely ceased, with frequent measurements creating detailed descriptions of dewatering at each pressure. Time to equilibrium at each step ranged from 21 to 173 hours, and was not linearly related to the magnitude of the applied pressure. This suggests that a) the temporal rules in the literature are, in many cases, arbitrary and b) that misattribution of equilibrium at an early pressure step will result in cumulative errors at successive steps when the same temporal rules are applied. In light of this observation, it must be considered whether SWCCs, reported in the literature and measured using traditional methods, truly reflect the hydraulic properties of the soils in question?