Page 103 - Rock Mechanics For Underground Mining

4 Rock strength and deformability

4.1 Introduction

The engineering mechanics-based approach to the solution of mining rock mechanics
problems used in this book, requires prior definition of the stress–strain behaviour of
the rock mass. Important aspects of this behaviour are the constants relating stresses
and strains in the elastic range, the stress levels at which yield, fracturing or slip occurs
within the rock mass, and the post-peak stress–strain behaviour of the fractured or
‘failed’ rock.
In some problems, it may be the behaviour of the intact rock material that is of
concern. This will be the case when considering the excavation of rock by drilling
and blasting, or when considering the stability of excavations in good quality, brittle
rock which is subject to rockburst conditions. In other instances, the behaviour of
single discontinuities, or of a small number of discontinuities, will be of paramount
importance. Examples of this class of problem include the equilibrium of blocks of
rock formed by the intersections of three or more discontinuities and the roof or wall of
an excavation, and cases in which slip on a major throughgoing fault must be analysed.
A different class of problem is that in which the rock mass must be considered as
an assembly of discrete blocks. As noted in section 6.7 which describes the distinct
element method of numerical analysis, the normal and shear force–displacement
relations at block face-to-face and corner-to-face contacts are of central importance
in this case. Finally, it is sometimes necessary to consider the global response of a
jointed rock mass in which the discontinuity spacing is small on the scale of the
problem domain. The behaviour of caving masses of rock is an obvious example of
this class of problem.
It is important to note that the presence of major discontinuities or of a number of
joint sets does not necessarily imply that the rock mass will behave as a discontinuum.
In mining settings in which the rock surrounding the excavations is always subject
to high compressive stresses, it may be reasonable to treat a jointed rock mass as an
equivalent elastic continuum. A simple example of the way in which rock material
and discontinuity properties may be combined to obtain the elastic properties of the
equivalent continuum is given in section 4.9.2.
Figure 4.1 illustrates the transition from intact rock to a heavily jointed rock mass
with increasing sample size in a hypothetical rock mass surrounding an underground
excavation. Which model will apply in a given case will depend on the size of the
excavation relative to the discontinuity spacing, the imposed stress level, and the
orientations and strengths of the discontinuities. Those aspects of the stress–strain
behaviour of rocks and rock masses required to solve these various classes of prob-
lem, will be discussed in this chapter. Since compressive stresses predominate in
geotechnical problems, the emphasis will be on response to compressive and shear
stresses. For the reasons outlined in section 1.2.3, the response to tensile stresses will
not be considered in detail.

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