Unsaturated Soil Mechanics Topics for All Geotechnical Engineers

Important unsaturated soil mechanics topics for all geotechnical engineers and geotechnical engineering students are reviewed. These key topics include: (1) Soil is an elastoplastic material for which the macro-level response, in general, is controlled by two separate stress variables: total stress (net stress) and negative pore water pressure (suction). (2) Pore water pressures are always negative above the groundwater table-and should not be "conservatively" assumed zero; (3) shear strength and volume change of unsaturated soils are dependent on soil suction, as well as confining stress, and therefore geotechnical site investigations and testing must account for both stress variables; (4) water flow follows Darcy's law, but hydraulic conductivity is a strong function of water content such that fine-grained soil can have a higher conductivity than course-grained soil, leading to unexpected results when using saturated flow thinking processes; (5) unsaturated soil response is complex and difficult to intuit in the absence of laboratory testing and simulation. Features of unsaturated soil behavior most frequently encountered in geotechnical practice are highlighted, with discussion and demonstration from existing literature. Suggestions are given for relatively simple approaches for first steps in taking unsaturated soil mechanics principles into consideration in site investigation, laboratory testing, and design-related decisions.