Geomechanics is concerned with the deformation and flow of geomaterials. A specific aspect of geomaterials, i.e. the materials that make up the planet earth, is their complex combination of solid and fluid phases.

In engineering geomechanics it is customary to talk of rock masses in a way that refers to both the rocks and the fracture systems that affect the volumes of concern. This concept, however, is often too vague for efficient mechanical modeling, and specific attention must be given to both the geomaterials and their discontinuities.

In the two first sections of this chapter we define more precisely the notion of a geomaterial and the related concept of a representative elementary volume (REV), with a brief reference to the various methods available for identifying geomaterials. In the third section we discuss the concepts of fracture sets and faults, with special attention to scaling laws. Finally, in the fourth section our attention turns to the various loading processes that may be encountered in geomechanics, whether of human or natural origin.

Rocks, soils and other geomaterials

Three kinds of rock can be identified: igneous, metamorphic and sedimentary. This characterization refers to the origin as well as to the past thermal and loading history of the rock. It implies strong consequences for the rock fabric, namely, the structure of its constitutive (solid) grains and of the complementary pore space, which is generally filled with fluid. And this introduces immediately the fact that materials of geological origin are most often multiphasic in their natural environment, i.e. they include solid, liquid and gas phases.

First, we introduce definitions that are used to describe the relative volumes occupied by the various phases. Then we introduce Goodman's classification of rocks according to their texture (Goodman, 1989), since in this book attention is given to the behavioral rather than to the genetic attributes of rocks.