Soil-water characteristic curve of expansive soils considering cumulative damage effects of wetting and drying cycles

A novel experimental program was undertaken to explore crack development inside an expansive soil column under cyclic Wetting and Drying (W-D) and its influences on the Soil-Water Characteristic Curve (SWCC). Experimental investigations revealed that in the first drying cycle, fine and evenly distributed cracks developed from initial defects in the upper surface. In the subsequent drying cycles, the unhealed cracks during the wetting cycle further grow in width, length and depth while the initiation zone of new cracks was limited. The crack development characteristics within the soil column were described using fractal dimensions. The cracking degree of soil cross-sections gradually reduced with increasing depth. Also, each W-D cycle process contributed to a similar increment in the crack fractal dimension of soil at the same depth. In addition, the saturated water content, air-entry value and residual suction of cracked expansive soils gradually reduced with an increase in the number of W-D cycles. An obvious bimodal phenomenon was only observed in the SWCC of soil samples after the third W-D cycle. In order to quantify the damage induced by crack development to the soils, the damage variable was proposed based on the crack fractal dimensions. Furthermore, the Damage Impact Factor (DIF), as a function of the volume damage variable and soil plasticity index, was introduced into the traditional Fredlund and Xing (F-X) model to estimate the SWCC of cracked expansive soil. The form of modified F-X model is relatively simple and can be conveniently extended into geotechnical engineering practice applications.