“Ask a geologist and you get an esoteric, impractical and academic answer: he hides his lack of technical understanding by using descriptive terms of value only to fellow geologists”.
This sort of reasoning is very common among engineers and it is explained by the fact that many geologists have an education that does not make them able to fully understand technico/geological issues. The engineer should therefore have an own insight in geology that is sufficient to take geological features into consideration and to interpret the rock decription given by geologists.
From a strict technical viewpoint the composition of the rock material is not of major importance, but it is essential that all the components that affect the physical and chemical properties of the rock mass are considered. Hence, why terms like mica shale or amphibolite do not give valuable information per se on the mechanical properties of a rock mass, the conventional petrographic description gives indirect information on the mode of formation and thereby on the minerals and fabric and consequently on the technically important properties, and it should therefore be specified as a basis of any rock engineering project.
A matter that is of great importance for rock mechanical analysis, and that has become increasingly importance in recent time, is that of structural discontinuities, of which faults and fractures are wellknown examples.
The definition of a complete hierarchy of discontinuities of rock mechanical importance is a major issue in the present book. Since experience shows that the most difficult conditions for excavation work, particularly in underground projects, are related to the presence of clay seams and weathered zones in crystalline rock, we will examine their origin, composition and performance in some detail in the book.
Clay minerals, especially smectites, will therefore be in focus of the present chapter, and the behavior of clay in rock examined rather deeply in forthcoming chapters on strength and stability.