Discontinuities and rock deformability

Abstract

Discontinuities can have an influence on rock mass deformability by a combination of one or more of the following processes:

• (i) Displacement of the adjacent blocks can create an air space, or open aperture, that has negligible stiffness compared with the surrounding rock material.

• (ii) Shear displacement along a discontinuity with an irregular surface can create a mismatch between the adjacent surfaces. Normal compression across such a discontinuity will lead to local crushing at the contact points and a relatively complex normal force-normal displacement characteristic that depends on such factors as the initial surface geometry, the elastic properties of the rock material and the post-peak characteristics of the rock.

• (iii) The rock material adjacent to the discontinuity can weather, can become fractured by shear displacement, or the discontinuity can become filled with imported material, to create a zone that has different mechanical properties from the surrounding rock material.

In most cases the above processes produce a zone of material that is more deformable than the surrounding rock, with an areal extent that reflects the geometry of the original discontinuity and a thickness ranging from a fraction of a millimetre to several metres. In this chapter we are concerned with the influence that such discontinuities can have on the deformability of the rock mass. Rock mass deformability can be a crucial parameter in the design of foundations for large structures such as dams, bridges and high-rise buildings, and in the design of pressure tunnels. Sections 10.2 to 10.4 present some of the basic principles of rock material deformability, discontinuity stiffness and strain energy. Section 10.5 draws on experimental results and analytical models in a discussion of the factors that influence the normal and shear stiffness of discontinuities. This section is followed by a brief survey of the analytical and numerical methods that have been adopted to predict the influence of discontinuities on rock mass deformability.