Abstract: To investigate effects of the concentration and composition of the pore solution on the mechanical properties of the expansive soil, free swelling rate tests and direct shear tests are performed on Guangxi expansive soils containing different concentrations of NaCl and CaCl2 solutions. The experimental results show that with the increase of solution concentration, the free swelling rate decreased rapidly and then decreased gradually. When the concentration is the same, the free swelling rate of expansive soil containing NaCl solution is slightly larger than that of CaCl2 solution; the strength and the cohesive of soil decrease with the increase of concentration; however, the influence of solution concentration on the internal friction angle is smaller. Analysis of the test results shows that the expansive soil surface has a negative charge fixed, the particle surface will form electric double layer; as the concentration of salt solution increases, the thickness of the double layer decreases, the repulsion between particles decreases, the intergranular stress increased, soil particle sedimentation, reducing the free swelling rate; meanwhile, the higher cationic chemical price, the greater cationic radius, the more significant effects on the double layer; therefore CaCl2 solution has a greater effects on the free swelling rate than NaCl solution. On the other hand, the salt solution can change the structure of expansive soil from agglomeration to flocculation. The salt solution will increase the intergranular stress, increasing the strength of the expansive soil; however, structural changes can reduce the strength. The above two influencing factors have the opposite trend to the strength, for Guangxi expansive soils, the effect of the structure larger than the effect of intergranular stress.

Key words: expansive soil, salt solution, free swelling rate, shear strength, double electric layer, intergranular stress