Effects of chemical solution on the deformation and strength characteristics of expansive soil

Pore fluid chemistry can obviously influence the deformation and strength of expansive soil. To investigate the change rules and the underlying mechanisms, free swelling rate and direct shear experiments are conducted on expansive soils immersed with different concentrations of NaCl and CaCl 2 solutions. The free swelling rate of the expansive soil, with increasing solution concentration, first rapidly decreases and then continues to gently decrease. Moreover, the NaCl solution has a greater effect on the free swelling rate of the expansive soil than the CaCl 2 solution. The soil shear strength and cohesion decrease with increasing solution concentration; nevertheless, the effect of solution concentration on the internal friction angle is less significant. The test result analysis indicate that the expansive soil surface carries a fixed negative charge, forming an electric double layer on the particle surfaces. With increasing salt solution concentration, the double layer thickness decreases, resulting in reduced particle repulsion and increased intergranular stress. Consequently, soil particle settlement occurs, which reduces the free swelling rate. Specifically, twice as much Cl − is dissociated from CaCl 2 as NaCl for salt solutions with the same molar concentration, resulting in greater intergranular stress and smaller free swelling rate. Theoretically, the shear strength should increase with the salt solution concentration, due to the increase of intergranular stress. However, the scanning electron microscope experiments and the mercury intrusion porosimetry experiments show that the expansive soil microstructure changes based on the salt solution concentration, contributing to the reduction of the shear strength. The above two factors have opposite effects on the shear strength. For the Ningming expansive soil considered herein, the shear strength decreases with increasing concentration, as the impact of structure surpasses that of intergranular stress.