8 Rock masses
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.
8.1 Deformability
Consider first, as an initial step in the overall development of a deforma-
bility 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 com-
parison to the overall length under consideration, L. Additionally, we will
assume that the deformation is made up of two components: one due to
~t -- U i
/
Intact rock
(units of
stress MPa)&Discontinuities
(units of stress/
length)
MPdm
6~
N discontinuities,
frequency = A
Figure 8.1 The modulus of deformation of a rock mass containing a discontinuity set.