Effects of microstructure on desiccation cracking of a compacted soil

Desiccation cracking has a significant influence on the hydro-mechanical behaviour of soils. Most previous studies focus on the desiccation cracking of slurry soil samples, whereas little attention has been paid to compacted soils. This study aims to investigate the effects of microstructure on the desiccation cracking of a compacted lean clay. Five soil samples are mixed with different water contents including 12.5%, 14.5%, 16.5% (optimum water content), 18.5%, and 20.5%, compacted to generate different initial soil microstructures. After compaction, the soil samples are subjected to saturation and then the same drying process. The pore size distribution of each soil sample is characterized by performing mercury intrusion porosimetry (MIP) test. The change in water content and the evolution of surface crack pattern during the drying process are continuously monitored. Experimental results show that the addition of water content during soil compaction significantly influences the microstructure and desiccation cracking behaviour of soils. With increasing compaction water content from the dry side to the wet side of the optimum water content, soil microstructure transits from an aggregate structure to a dispersed structure, resulting in the change of pore size distribution from bimodal to unimodal. For soils with aggregate structures, the desiccation cracks initiate simultaneously and distribute uniformly throughout the soil body. With decreasing water content, crack geometrical parameters such as surface crack ratio and crack density increase almost linearly. Comparatively, for soils with dispersed structures, more localized growth of primary and secondary cracks are observed and the crack geometrical parameters show a two-stage linear growth during drying. This study provides microstructural interpretations to important aspects of desiccation cracking in compacted clayey soils and may guide the design of clay materials for geotechnical engineering applications.