Water retention curves of a deformable lean clay under various stress ratios: Emphasizing the pore shape effects

Existing studies on water retention curves (WRCs) of unsaturated soils focus on pore size effects, while knowledge regarding pore shape effects remains limited. This study aims to investigate effects of pore shape on WRCs of a deformable lean clay under various stress ratios. Pressure plate tests were carried out on the specimens prepared by isotropic and one-dimensional compressions (i.e., ISO and 1D). The pore size and pore shape of tested specimens were quantified by microstructure analysis. Results show that stress ratio has a significant impact on WRC, particular for air entry value (AEV). The average AEV of 1D specimen is 15.8% larger than that of ISO specimen, mainly because the pore shape of the former is 9.2% more elongated than that of the latter. Moreover, the difference in AEV between 1D and ISO specimens exhibits a parabolic pattern in relation to void ratio, aligning with the changes in pore shape factor (pore width-to-length ratio). Based on test results, a new WRC model is developed for deformable soils under various stress ratios. Different from previous models, the new model incorporates the evolution of pore shape with respect to void ratio and stress ratio. Six parameters are included in the new model, of which three are the same as those in Gallipoli's model, while the other three consider pore shape effects under various stress ratios and void ratios. The measured WRCs from this study and the literature were used to validate the new model, demonstrating its good predictive capability.