TABLE OF CONTENTS In this chapter, we will concentrate on extending the ideas discussed in the previous chapter on discontinuities to provide a predictive model for the deformability and strength of rock masses. In Chapter 12, we will be discussing rock mass classification (which is a method of combining selected geometrical and mechanical parameters) to semi-quantitatively provide an overall characterization, mainly for assessing excavation support requirements.
Consider first, as an initial step in the overall development of a deformability model, the deformation of a set of parallel discontinuities under the action of a normal stress, assuming linear elastic discontinuity stiffnesses. This circumstance is illustrated in Fig. 8.1. To calculate the overall modulus of deformation, the applied stress is divided by the total deformation. We will assume that the thickness of the discontinuities is negligible in comparison to the overall length under consideration, L. Additionally, we will assume that the deformation is made up of two components: one due to
deformation of the intact rock; the other due to the deformability of the discontinuities.
The contribution made by the intact rock to the deformation, ? l is aL/E (i.e. strain multiplied by length). The contribution made by a single discontinuity to the deformation, ? D, is ?/ ED (remembering that E D relates stress to displacement directly). Assuming a discontinuity frequency of ?, there will be ?L discontinuities...
