Comparisons of different suction control techniques by water retention curves: theoretical and experimental studies

The axis-translation (ATT), osmotic (OMT) and vapour equilibrium (VET) techniques are widely used suction control techniques for investigating hydraulic and mechanical behavior of unsaturated soils in the vadose zone. Yet there is still debate in the literature regarding their validity and consistency. In this study, four purposely designed experiments have been carried out to measure the water retention curves (WRCs) of a compacted silty sand at different densities using the ATT and OMT. The four WRCs obtained from this study together with extensive existing data published in the literature are then analysed using newly derived thermodynamic equations. The aim and novelty of this study are to apply the thermodynamic theory to verify and explain the validity and consistency of WRCs obtained by the three commonly used experimental techniques. By allowing for some possible experimental errors, analysed results reveal that the WRCs obtained from the three techniques are remarkably consistent. These consistent results can be explained by the derived thermodynamic equations, which illustrate that the final soil water content at the thermodynamic equilibrium state must be the same regardless of the suction control technique used, even though the techniques involve different processes of moisture exchange.