Unsaturated soil mechanics & near-ground-surface phenomena

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There were two primary subjects that have been historically been by-passed in textbooks on classical soil mechanics; namely, the behavior of unsaturated soils and the quantification of moisture flux conditions at ground surface. The last 40 years have witnessed rapid strides in our understanding of unsaturated soil mechanics as it applies to relatively homogeneous continua. There are; however, two aspects of unsaturated soil mechanics that stand in great need of further research. The first aspect involves the structural characterization of the soil conditions near ground surface. Often these soils contain extensive fissures, fractures and cracks that significantly affect the unsaturated soil properties. It is proposed that the secondary macrostructure of these soils be characterized through use of bimodal mathematical functions. It is proposed that there be one air entry value associated with the macrostructures features and another air entry value associated with the intact matrix of the soil. The unsaturated soils near to the ground surface thus become bimodal, nonlinear and hysteretic. Many of these soils also exhibit significant volume change during wetting and drying. The second aspect of unsaturated soil mechanics in need of research is the quantification of moisture flow conditions at the ground surface. Both the infiltration of water and the exfiltration during evaporation (as well as evapotranspiration) are subject areas of great interest to practicing geotechnical engineers. These conditions are closely related to our understanding of the impact of climate change. It is through the ability to mathematically quantify water balance conditions at the ground surface that the effects of climate change can be taken into consideration through numerical modeling studies. In my estimation, the characterization of fissured, fractured and cracked soil masses near the ground surface, and the quantification of ground surface moisture flux conditions are two of the main topics requiring further research in unsaturated soil mechanics.