Effect of montmorillonite content and sodium chloride solution on the residual swelling pressure of an expansive clay

Expansive clays may continue to swell even after obtaining fully saturated condition due to the continuing montmorillonite hydration process. Expansive clays in this case, if confined, will generate swelling pressure, which is termed as residual swelling pressure in this study. The residual swelling pressure poses significant impact on the long-term safety and stability of adjacent geotechnical infrastructure. Despite its significance, currently, there are very limited studies on the residual swelling pressure. In this paper, the influence of (1) the montmorillonite content and (2) sodium chloride (NaCl) concentration in the pore water on the residual swelling pressure of a compacted expansive clay was evaluated using constant-volume swelling tests. Scanning electron microscopy analyses were applied to investigate the influence of NaCl solution on the microstructure of the compacted expansive clay. Tests results showed that the residual swelling pressure increases with montmorillonite content and dry density of specimen and decreases with an increase in the NaCl concentration. The NaCl in the pore water used for compacting the clay influences the particle orientation and arrangement. High NaCl concentrated solution contributes to soil structures with higher integrity that exhibit lower residual swelling pressure. An equation is developed to interpret and predict the residual swelling pressure of expansive soils considering their montmorillonite content and dry density. The proposed equation is verified using experimental data determined in this study as well as data obtained from the literature.