Unsaturated soils - Applications to structural design

The mechanics of unsaturated soils (cf. articles "Water in unsaturated soils" and "Mechanical behavior of unsaturated soils") is applied to many geotechnical structures. In addition to earth structures built on compacted soil (road embankments, embankment dams, dykes), we know that the stability of slopes is often conditioned by the unsaturated state of the mass. Superficial foundations can also be affected, as demonstrated by the extreme damage caused to buildings by the effects of drought. Excavations in urban areas and retaining structures can also be affected. Finally, environmental geotechnics involves the use of compacted materials to isolate waste of all kinds. The interface between the earth and the atmosphere is most often made up of a layer of unsaturated soil, often subject to pollutant infiltration in industrial zones, which poses problems when industrial wastelands are redeveloped.

Compared with saturated soils, unsaturated soils have deformability and fracture properties that change with water content. This results in complex couplings between water movement, internal stress redistribution and the deformation response of the massifs. At a constant state of stress, there may be time-delayed responses that may sometimes appear unexpected. These aspects are described in the first part of this article, devoted to compacted soil structures. The second part deals with the effects of unsaturation on slope stability, while the third places the problem of the effects of drought on constructions in the context of soil-atmosphere exchanges. The article concludes with the description of two case studies concerning the collapse of a bridge abutment and the collapse of loess in Picardy.