TABLE OF CONTENTS The subject of permeability is concerned with fluid flow through a material, or rocks and rock masses in our current context, and is one of the most difficult topics facing the practising rock engineer. There are formal definitions of permeability and the associated characteristics for continuous materials but, as is emphasized throughout this book, a rock mass contains discontinuities, and discontinuities are preferential flow paths. In this chapter, therefore, we will initially present the definitions of permeability and hydraulic conductivity and discuss the tensorial nature of permeability. We will then consider fluid flow in discontinuities and the associated ideas of primary and secondary permeability. The subject of the permeability of the rock mass can be studied in terms of the effective permeability of discontinuity networks: there is then the corollary of a natural scale effect and the representative elemental volume. Finally, we will discuss the difficulty of dealing with effective stresses in rock mechanics and the permeability-related applications of grouting and blasting.
We should note in passing that the phenomenon of water flowing through rock has been observed since antiquity. In Creech's (1683) English translation of Lucretius' six books of Epicurean philosophy on the nature of things, in Book I are the lines,
Tho free from Pores, and Solid Things appear,
Yet many Reasons prove them to be Rare:
For drops distill, and subtle moisture creeps
Thro hardest Rocks, and every Marble weeps .
Permeability, k ij, is a mathematical quantity directly analagous...
