Geometric and fractal analysis of dynamic cracking patterns subjected to wetting-drying cycles

Abstract

Desiccation cracks detrimentally affect the hydraulic and mechanical properties of soils. Soil cracking dynamic subjected to wetting-drying (W-D) cycles aids in the understanding of soil shrinking-swelling mechanism and the regularity of water movement in unsaturated soils with variable solid phase. In the present paper, we experimentally investigate the geometric and fractal features of dynamic cracking patterns with the soils undergoing four W-D cycles. Crack patterns were recorded by a digital camera mounted above the soil specimen. Digital image processing and morphological algorithms were employed to yield the geometric and fractal indexes that depict crack patterns (incl. crack ratio, length, mean width, similarity degree, box-counting fractal and multifractal dimensions). The analysis of dynamic crack patterns suggest an inconsistency exhibited in the crack morphology towards the drying (DR) process and backwards the wetting (WT) process, which could be possibly attributed to the different suction stress between soil particles during moisture desorption and sorption, closely analogous to the hysteresis phenomenon of soil-water characteristic curve. The gradient of crack propagating and dissolving related to water content reaches different peak values during an individual W-D cycle. High repeatability is found across the stabilized crack patterns in the four drying cycles and the cracking degree reaches equilibrium with the increasing number of W-D cycles. The crack patterns in the subsequent drying processes are significantly dominated by the first cracking. The similarity degree between the first and respectively the second, third, fourth crack patterns decreases with the increasing W-D cycles, suggesting the soil cracked in the previously cracking vicinity, and saturated wetting/submerge hardly healed the failure zones that caused the repeatable cracking. Attempt of fractal and multifractal methods applied to the crack networks corroborates the applicability of the scaling method to the crack patterns. The analysis indicates that soil cracks progress in hierarchical networks in the first drying cycle, while the gradational crack segments nearly simultaneously emerged or vanished in the subsequent drying or wetting cycles. The study reveals the discrepancy of crack morphology between the crack formation and dissolution during W-D cycles, and demonstrates the repeatability degree and hierarchy of cracks across W-D cycles, which would aid in quantifying physical processes in cracked soil shrinking/swelling and modelling of water transport in unsaturated soils of dynamic structure.