Rock masses:
deformability, strength
and failure
8
8.1 The nature of rock masses
In Chapters 6 and 7, we have posed
and answered questions concerning
the properties of the intact rock and
the fractures. In this chapter, we con-
sider the deformation, strength and
failure of a fractured rock mass, i.e. a
discontinuum comprising intact rock
dissected by fractures.
A rock mass is a complex geometrical and mechanical assemblage
resulting from a long history of tectonic forces and other natural envir-
onmental effects. Additionally, the rock mass properties can be signific-
antly affected by engineering activities, especially blasting. The follow-
ing series of photographs in Fig. 8.1 illustrates different rock masses in
different conditions.
The deformation, strength and failure properties of a rock mass are
determined by the mechanical properties of the intact rock (Chapter 6),
the geometrical properties of the fractures (Chapter 7), and the mechan-
ical properties of the fractures (this chapter).
We can consider a fracture being stressed by a normal stress and two
shear stresses, as shown in Fig. 8.2.
The compression of the fracture due to the normal stress component
a,, produces a normal displacement 6,. It is awkward to convert this
directly into strain because there is no clear gauge length over which the
displacement has taken place. Thus, assuming initially a linear relation
between a,, and S,, we can define the fracture normal deformation mod-
ulus as a,,/~, = &, where the units of Ed are stress/metre, or L-~MT-*
(cf. the units of Young’s modulus which are stress units, or L-1MT-2).
From the shear displacements, we can define shear deformation moduli
in a similar way.
Within the local co-ordinate system illustrated in Fig. 8.2, the two
shear fracture stiffnesses and the normal stiffness can be denoted as
Intact rock Fractures% Rock Mass