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

Abstract

Abstract. 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 shrinkage and swelling curves in the maximum water content range (the primary curves) and crack volume variation. 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, inevitably 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. Keywords IntroductionShrinkage cracks close to the soil surface, their origin owing to the vertical water content gradient, and prediction of their volume, width, and spacing are well known [1][2][3][4][5]. Even though these cracks can extend to depths of several meters keeping a micro-width [4], their characteristics cannot explain the origin, volume, and volume variation of the cracks that are directly or indirectly observed at sufficiently large depths of a clayey vadose zone [6][7][8][9][10]. Contrary to the notion about the small permeability of clays, such cracks can essentially increase the hydraulic conductivity of the vadose zone and, correspondingly, transport different contaminant admixtures and solutes to the ground water [6]. Therefore, theoretical estimation of the crack volume, accompanying the natural multifold shrink-swell cycles far from the soil surface, as a function of soil structure and local hydrological conditions, is of theoretical as well as practical interest. In particular, the physical understanding of the origin of such cracks and quantitative physical prediction of the crack volume variation, are important to protect the underlying aquifers from different pollutants.The physical understanding and prediction should take into account the major specific limitations existing in a clay soil at large depths: (i) overburden action at a given depth; (ii) limited and frequently small range of water content variation (compa...