Hydraulic soil barriers play a very important role in geoenvironmental engineering by isolating a variety of wastes from the biosphere, including sources of drinking water. For that reason, cracking of such barriers poses a major challenge for engineers. Indeed, the resulting permeability increase may easily undermine the very purpose of constructing a barrier. The use of low permeability hydraulic barriers, such as landfill clay liners or top covers is mandated by the EPA recommendations regarding landfills in the US. Cracks in compacted clay barriers may form during construction due to desiccation when clay is left unprotected against evaporation. Drying may also occur due to the elevated temperatures in the landfill. Evaporation may also cause cracking of liners at the contact with the unsaturated soil beneath. Eventually, cracks may form networks for preferential water flow, and may increase dramatically contaminant transport into the aquifer. Despite intense research in this area, cracking remains difficult to prevent, and cracked soil permeability is difficult to control and predict. To engineer better barriers using compacted soils, a better understanding is needed of fundamental mechanisms of desiccation cracking and related changes in permeability. The goal of the research is to investigate such mechanisms as coupled physico-chemical and hydro-mechanical phenomena. Experiments with clay shrinkage due to water mass loss during drying, on the one hand, and with hardening of the unsaturated clay soil due to capillary force growth, on the other, will be conducted. Advantage will be taken of substantial advances made in recent years in the mechanics of unsaturated soils. Critical physical variables controlling the behavior of unsaturated soils include suction, saturation degree, humidity, clay fraction and the effect of its water chemistry. The mechanisms of possible suppression of cracking will also be addressed. Experiments involving changes in permeability to water and air concomitant to the formation of cracks and crack networks will be performed in a suitably adapted oedometer. Micro-geophysical techniques of measuring acoustic emission during cracking and other non-invasive methods will be applied to monitor cracking progress. The acquired data from the experiments will be used to establish models allowing engineers to predict advective transport of contaminants through a liner subjected to a certain degree of cracking. The intellectual merit of this work is in addressing fundamental properties of drying soils that remain unexplained and poorly controlled. This research requires an interdisciplinary approach, with the use of mechanics of geomaterials, soil physics, micro-geophysics and hydrology. Broader impact of the work will be reached via several avenues. First, undergraduate students will be engaged in the lab activities, which proves to be to great advantage to both sides. Second, student members of underrepresented minorities and African students have found a niche at Duke University in the Soil Mechanics area. They will be part of this project and will attract other students from this group. Third, the obtained results will attract attention of engineers in tropical countries, for which the cracking soils are inherent. They will also be of interest to eco-hydrologists and agricultural soil scientists. The Duke University team will form a partnership with researchers at Swiss Federal Polytechnic Institute at Lausanne, who have very similar interests.
