Experimental Study on the SWCC Transition Zone Features of Unsaturated Soil

In order to study the soils SWCC transition zone features under different steady-state and transient conditions, a series of pressure plate and model tests were carried out; the internal structure adjustment of soil caused by different test conditions is the underlying reason for the change in SWCC transition zone curve. Results show that the slope of the SWCC transition zone decreases with the reduction in the applied vertical stress and the number of drying–wetting cycles during the pressure plate test; for the model test, the change in SWCC transition zone of high compaction degrees and steep slope soil under the same condition is obviously smaller than that of low compaction degrees and gentle slope soil, that is, the slope of the former SWCC transition zone is obviously slower than the latter. Also, the soil parameters a and b which represented the intercept and slope of the fitting line in the plane log(θ) − log(φ) have a strong linear relationship, implying all the models and natural soils are of similar grading, and these transition zone features do not alter with test conditions changing. The above research will be useful and meaningful as reference for the engineering characteristics and governance of the residual slope.