EJECE – 13/2009. Failure in multiphase geomaterials, pages 869 to 888 Desiccation cracking of soils Hervé Peron* — Lyesse Laloui* — Tomasz Hueckel** Liang Bo Hu** * Ecole Polytechnique Fédérale de Lausanne, EPFL Laboratory of Soil Mechanics, Station 18 CH-1015 Lausanne, Switzerland [email protected] ** Duke University, Department of Civil and Environmental Engineering Durham, NC 27708, U.S.A. ABSTRACT. The scope of this paper is to present the global mechanisms of soil desiccation, including drying shrinkage and cracking. The paper first reviews the basic processes that are beneath the word “desiccation”. Then the results of an experimental study of desiccation are presented, in which strains, suction, water content, degree of saturation and crack geometry are investigated. The results show that cracking initiates close to the onset of de-saturation. Insights into the micro-scale are proposed to explain this observation. A scenario for the processes leading to crack initiation is further established in terms of the macroscopic variables: an assessment of the stress building up is proposed, until a critical point at which the tensile strength is met. Desiccation crack pattern formation is finally discussed. RÉSUMÉ. Le but de cet article est de présenter l’ensemble des mécanismes de la dessiccation des sols, comprenant le retrait de séchage et la fissuration. La nature de ces derniers processus dans les sols est tout d’abord définie. Les résultats d’une étude expérimentale de la dessiccation (qui consiste en une investigation de l’évolution des déformations, de la succion, de la teneur en eau, du degré de saturation et de la géométrie des fissures) sont ensuite exposés. Les résultats montrent que la fissuration commence à un moment proche du début de la désaturation et de la valeur d’entrée d’air. Cette observation est interprétée à l’échelle microscopique. Un scénario pour les processus menant à l’initiation de la fissuration est établi en termes de variables macroscopiques : une évaluation du développement des contraintes jusqu’à un critère macroscopique d’initiation des fissures est proposée. La formation des formes de fissures est finalement discutée. KEYWORDS: drying shrinkage, cracking, degree of saturation, air entry value, stresses. MOTS-CLÉS : retrait de séchage, fissuration, degré de saturation, valeur d’entrée d’air, contraintes. DOI:10.3166/EJECE.13.869-888 © 2009 Lavoisier, Paris870 EJECE – 13/2009. Failure in multiphase geomaterials 1. Introduction Drying of soils and the ensuing cracking are a crucial issue in geo-environmental engineering. Drying fractures strongly affect permeability and may compromise the integrity of structures, such as clay buffers for nuclear waste isolation. In addition, cracking is the cause of substantial damage in foundation-supported structures. Compressibility increases substantially while the rate of consolidation decreases with the appearance of desiccation cracks (Morris et al., 1992). Cracks are also a possible precursor for inception of a failure surface at the top of dams and embankments. Assessment of the potential for such damage is difficult, as the mechanisms and principal variables in the process are not fully understood, despite decades of research and substantial progress, e.g., Corte and Higashi (1960), Abu-Hejleh (1993), Konrad and Ayad (1997). The primary scope of this paper is to present the global mechanisms of desiccation, including shrinkage and cracking. For this purpose, experimental evidence as well as the substantial advances made in recent years in understanding the hydro-mechanics of unsaturated soils are used. The entire set of processes that lead to cracking and that stem from drying shrinkage is addressed. The paper first reviews the basic processes that are beneath the word “desiccation”. Then the results of an experimental study of desiccation are presented, in which strains, suction, water content, degree of saturation and crack geometry are investigated. The conditions of cracking initiation are discussed, especially with respect to the degree of saturation and suction. Insights into the micro-scale are proposed, bearing in mind that the useful variables are those at the macro-scale. A scenario for the processes leading to crack initiation is established in terms of the macroscopic variables. Desiccation crack pattern formation is finally discussed. 2. Physical processes involved by desiccation cracking of soils 2.1. Drying Drying results from an initial thermodynamic imbalance between the soil moisture and its surroundings, which causes evaporation and a transfer of fluids within the soil. In the general case, the fluid movement is accomplished through both liquid and gaseous phases. According to equilibrium thermodynamics laws, the phase change between pore liquid and vapour occurs instantaneously at the interface between the phases, so that the specific vapour and liquid Gibbs potentials remain equal (Coussy et al., 1998; Mainguy et al., 2001). This generates liquid pressure decay (i.e. suction increase) in this zone according to Kelvin’s law, and at the same time, a gradient of suction within the body. The liquid movement is due to the spatial differences in liquid head that then arise. External pressures, high temperatures andDesiccation cracking of soils 871 shrinkage deformations are also responsible for additional pore fluid pressure generation and subsequent fluid movements. We define the velocities of the gas, liquid and solid, respectively, by vg, vl and vs. The Darcy’s law then relates the liquid/solid relative velocity to the gradient of the water head. The Darcean flux of liquid ql [M.L-2.T-1] is given by: ! "! "()l r l lnS k g p##$ % $ % %& 'l l sq v v g [1] where n is the porosity, Sr is the degree of saturation, k is the hydraulic conductivity (permeability to liquid) of the medium and g is the gravity vector, #l and pl are the intrinsic density and pressure of the liquid, respectively. When a continuous gaseous phase is present within the body, the gaseous phase movement can also be