Then he must present the information to the engineer in a form in which it can be understood and used. Often he may be called upon to forecast the influence of the geology on the engineering work. In so doing he has to be conscious that he has crossed from the field of geological science into the field of geological engineering—a crossing which he is entitled to make only if properly qualified by training and experience to do so.

The information that geologists can supply to the engineers about the physical and mechanical properties of rocks is mostly of a qualitative kind. This is not very satisfactory to the engineers. The design engineer wants to know the strength and deformability of the rock at a site so that he can fit a structure to the site and calculate the factor of safety of the composite work. In response to the demands of design engineers for quantitative information, there has emerged the rapidly developing inter-disciplinary subject of rock mechanics, drawing contributions from civil and mechanical engineers, physicists and applied mathematicians, as well as geologists.

Rock mechanics first makes the important distinction between rock material and rock mass. Rock material is intact rock free from fractures, joints, shear zones. Rock mass is rock in situ complete with all defects.

The properties of rock material are relatively easily measured but since the measurements relate to rock in its best condition they are of limited usefulness. The object of much rock mechanics work is to determine the strength and deformation properties of rock masses. This is a very difficult undertaking, and only limited advances have been achieved so far. A complete study involves the integration of the mechanical measurements with those describing the geological structure of the mass including its joints, bedding or foliation, and faults.

Another significant property of the rock mass is its inherent state of stress, which is due to a combination of factors such as overburden pressure and shape of the terrain, residual stresses from past geological events and stresses due to currently active tectonic forces. The engineering works modify this initial stress field through excavation and the imposition of loads by structures and water under pressure. The measurement of the state of stress of the rock mass is especially important for excavations deep underground where the initial stress field is high.

A variety of techniques for determining the state of stress of the rock mass have been devised and are now in common use.