Mechanics of Unsaturated Geomaterials

An understanding of the mechanics of unsaturated geomaterials has become an important component of the background of a geo-engineer operating in various fields of geomechanics. Several geotechnical operations, such as compaction and excavation processes, are linked to the mechanics of unsaturated geomaterials. As more than one-third of the earth’s surface is arid or semi-arid, in addition to the less extreme cases of seasonal droughts and diurnal variations of the water table in soils, it is obvious that most soils and rocks are in a general state of partial water saturation. In other words, the pore space within geomaterials (e.g., soil, rock and concrete) is generally filled with water and air. That means that the mechanics and physics of the considered material are those of a three-phase material: solid mineral and two immiscible fluid phases. Even though most natural and engineered geomaterials are only partially saturated with water, a persistent assumption made in geomechanics and geotechnical engineering over the past decades has been the assumption of complete saturation. The study of the mechanics of unsaturated geomaterials was initiated approximately 50 years ago as a natural extension of the knowledge developed in the conventional areas of the mechanics of (saturated) soils and rocks. The mechanics of saturated geomaterials is primarily based on the concept of effective stress and on the consolidation theory. In the hydromechanical frameworks for saturated materials that have been developed, the pore fluid (water) pressure mainly contributes to the mechanical behaviour through the field equations (consolidation theory). Its contribution to the constitutive behaviour of the solid skeleton is considered “neutral” (no effect of the pore fluid pressure on the effective material compressibility or strength, for instance). When this particular assumption about materials saturated with a fluid under compression was no longer considered valid, the conventional theories needed to be revised. This was the first major development in defining the mechanics of unsaturated materials. The field equations were found to need an extension to address the effect of the degree of saturation on water permeability and on compressibility; the gas flow also had to be considered in some situations. The solid skeleton constitutive behaviour must incorporate the effect of the gas pressure, or more specifically, its difference with respect to the liquid pressure, known as suction. In addition, the extension of the effective stress concept to the unsaturated conditions revealed a need to take into account the important contribution of the water retention behaviour, linking the degree of saturation to suction. In the last decade, the advancement of knowledge regarding the mechanics of unsaturated geomaterials has been significant. Some fundamental issues were solved, and important achievements were made in certain areas, including application of the effective stres