Fundamental relationships of crack formation in expansive clay soils

Abstract

The cracks which form in expansive clay soils during desiccation, due to their unique swelling and shrinking property, greatly influences agricultural and geotechnical processes, such as water infiltration, solute movement, evaporation, and aeration. The objective of this research was to investigate how soil mechanics might be used to explain the formation and spatial distribution of desiccation cracks. Previous studies which have attempted to model the mechanics of desiccation crack formation have assumed that the internal soil stress, which causes crack formation, results from constraining shrinkage perpendicular to the crack plane. The approach taken in this study was to model the soil with unconstrained boundary conditions. At these boundaries there is no stress, but for a secondary crack to form, the stress must increase with distance from the boundary until it exceeds the soil strength. The hypothesis proposed to explain this phenomenon is that the internal soil stress which builds up with distance from the unconstrained faces, results from the soil's internal resistance to distortion. This distortion is the consequence of a non-uniform change in the soil moisture profile. Laboratory experiments using remoulded clay soil samples with unconstrained sides were used together with a two-dimensional finite element analysis, to evaluate the stresses developing within the soil. The results obtained support the proposed hypothesis that it is the soil's internal resistance to distortion which can generate the stress necessary for crack propagation. The results also provide an explanation for the soil compaction and horizontal cracking which have both been shown to occur in the field. Overall the work provides a model for understanding the fundamental relationships of crack formation in expansive clay soil.