Physical modeling of shrink–swell cycles and cracking in a clayey vadose zone

• Crack origin far from the soil surface in a clayey vadose zone is addressed.
• Soil structure, layer size, loading, water content range, and cycling are caught.
• Clay, intra-aggregate matrix, soil without and with cracks are studied.
• Primary and scanning shrinkage and swelling curves are derived.
• Crack volume hysteresis for steady shrink–swell cycles is predicted.

Physical understanding of the crack origin and quantitative physical prediction of the crack volume variation far from the clay soil surface are necessary to protect the underlying aquifers from pollutants. The basis of this work is an available physical model for predicting the shrink–swell curves in the maximum water content range (the primary curves) and crack volume variation in the range. The objective of the work is to generalize this model to the conditions of the deep layer of a clayey vadose zone with the overburden pressure, multiple shrinkage–swelling, and variation of water content in a small range. We aim to show that the scanning shrinkage and swelling curves, and steady shrink–swell cycles existing in such conditions, lead to the occurrence of cracks and a hysteretic crack volume. The generalization is based on the transition to the increasingly complex soil medium from the contributive clay, through the intra-aggregate matrix and aggregated soil with no cracking, to the soil with cracks. The results indicate the single-valued physical links between the scanning shrink–swell cycles and crack volume variation of the four soil media on the one hand, and primary shrinkage and swelling curves of the media on the other hand. The predicted cycles and crack volume hysteresis can be expressed through the physical properties and conditions of the soil at a given depth. The available observations of the cracks and crack volume variation in the clayey vadose zone give strong qualitative experimental evidence in favor of the feasibility of the model.