"Soil suction and cracking from the onset to the end of drying: micro-scale evidence and model” or “Why drying soils crack in tension when they are compressed and still completely wet"
Tomasz Hueckel
Civil and Environmental Engineering Department
Mechanical Engineering and Materials Science Department
Duke University
Abstract
A multi-scale, multi-physics sequence of processes is discussed as developing during drying of non-clayey soils. Two variables are believed to be central in drying: suction resulting from the evaporation and effective stress associated with external constraints imposed on drying shrinkage. These two variables are tracked across the scales, both in experiments and simulations. The effective stress is critical as leading eventually to soil drying-cracking. Cracking is a most unwanted development in soil undergoing dewatering. Drying - cracking is shrouded by two paradoxes. First, it takes place when the effective stress is compressive, and second, it occurs when soil is still almost entirely saturated. Drying cracks often arise in the apparent absence of external forces. Hence, a tensile eigen-stress pattern resulting from stiff inclusions, or (relatively weak) tensile total stress produced by reaction forces at the external boundary constraints need to be contemplated to reach cracking criteria. An earlier tubular micro-scale model of porous drying medium indicates that transport of water toward evaporating surface during saturation phase induces a high suction. As a result the effective stress is compressive, which is consistent with widely observed drying shrinkage. This paradox has been solved by George Scherer (1992), who postulated that in the presence of any surface imperfection, a total stress amplification is expected, which despite high suction yields a tensile effective stress locally. A critical suction value is reached at which water body boundary is penetrated in an unstable manner by air. We postulate that at the meso- scale such air penetration indeed constitutes a surface imperfection, and a tensile total stress amplification near its tip, and a rapid increase in local tensile effective stress and crack propagation. Recent experimental results from a configuration of a cluster of grains provide geometrical data suggesting that an imperfection resulting from of air entry penetrates deep into the granular medium over 4 - 8 radii of the typical pore. Percolation studies indicate that capillary fingers can penetrate over tens to several hundreds grain radii forming a drying front prompting formation of discrete cracks at the point of maximum tensile stress. Further evolution entails separation of grain clusters by funicular bridge instabilities, and in the last stage formation of two-grain pendular capillary bridges. The final phase is associated with a gradual decrease of the micro-scale suction within these elementary bridges, which eventually evolve into a positive pressure before the bridge rupture.