Effects of Different Degrees of Hydrophobic Treatment on Soil–Water Characteristic Curves and Infiltration Coefficients of Hygroscopic Soils

Soil treated with silicone hydrophobic material for a long time can effectively improve the saturated permeability of the ground, which has been confirmed. However, the long time will increase the cost in engineering construction. Secondly, the mechanism of alteration of soil–water properties by silicone hydrophobic materials is not understood. This lack of understanding is not conducive to the engineering application of silicone hydrophobic materials. Therefore, variable-head permeability and matrix suction tests under 15 different test conditions were conducted with different hydrophobic material dosage and action time as variables in this paper. The soil–water characteristic curve (SWCC) was plotted according to the test results, the unsaturated permeability coefficient was calculated, and curve fitting was performed. It can be seen from the test data that, with the increase in the dosage of hydrophobic materials and the action time, the permeability coefficient and the air-entry value showed a downward trend. Nevertheless, as the reaction proceeded, a mutation occurred at 4 h. When the minimum dosage and action time were used, the soil’s permeability coefficient and air-entry value decreased by 80.65% and 71.09%. The test results show that, even when the action time was 2 h, the hydrophobic material could maintain the permeability of the soil and reduce its air-entry value; thus, the hydrophobic material could effectively reduce the rise of capillary water in the soil and protect the roadbed.