A microstructure-based conceptual model was proposed for the determination of soil swelling pressure.
Higher swelling pressure with SC method was due to compressing “extra” eM created during initial swelling under low pressure.
Similar swelling pressures by other methods were attributed to similar eM (or em) generated during hydro-mechanical loadings.
Wetting generated similar eM (or em) under very high pressure in SC and other methods, leading to similar swelling pressures.
In this study, swelling pressures of MX80 bentonite were determined by different common methods, and the microstructures were investigated by performing mercury intrusion porosimetry (MIP) tests. The obtained results and in-depth analysis allowed the microstructure-based conceptual model to be proposed for the determination of expansive soil swelling pressure. This conceptual model implied that the comparable swelling pressures determined by the swell-under-load (SUL) method, the constant-volume (CV) method, the zero-swell (ZS) method, the pre-swell (PS) method and the Cui's method were attributed to the similar macro (or micro) void ratios (eM or em) generated during hydro-mechanical loadings. The higher swelling pressure with the swell-consolidation (SC) method corresponded to the “extra” pressure that was needed to compress the “extra” macro void ratio eM created during initial swelling under low pressure. However, the swelling pressures determined by the SC method and other methods were expected to be comparable in the case of swelling under high pressure because swelling generated similar eM (or em) in that case. This conceptual model also provided useful information for better understanding of the swelling mechanism involved at microstructure level for expansive soils.