Page 216 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN MASSIVE ELASTIC ROCK

Hoek-Brown criterion. The tensile strength of the rock mass, T 0 , is usually taken to be
zero.
The need to consider two compressive strength criteria arises because different
failure modes apply near the excavation boundary and in the interior of the rock. Under
the complex stress path and in the low confinement conditions near the boundary of
the excavation, crack initiation leads to unstable crack growth and the formation of
spalls in the excavation boundary. Under confined conditions in the interior of the
rock mass, rock failure depends on the formation of a population of interacting cracks,
i.e. the accumulation of damage in the rock fabric from crack initiation and growth.
The experimental observations and analysis supporting this formulation of criteria for
rock mass failure are presented by Martin (1997), Martin et al. (1999) and Diederichs
(2002).
While the evolution of and final state of stress around single excavations involve
three-dimensional geometry and complex stress paths, Brady (1977) and Martin et al.
(1999), among others, have shown that plane strain elastic analysis and an appropriate
failure criterion can be used to make sound engineering predictions of the extent of
zones of failure close to the excavation boundary. In a well-controlled and detailed
study, Martin et al. (1999) showed that the constant deviatoric stress criterion pre-
dicted both the extent of the zone of near-boundary microseismic activity, indicating
initiation of cracks, and the geometry of the spalled zone. The criterion could be
represented by the Hoek-Brown criterion, i.e.

1 = 3 + m 3 c + s 2 1/2

c
with m = 0 and s = 0.11. The failure criterion for the particular rock mass was
therefore expressed by

1 − 3 = 0.33 c (7.1)

In the design of any mine opening, two points need to be borne in mind. First, in
successful mining practice the existence of an extensive zone of damaged or failed
rock near the boundary of an excavation is common. Second, a basic mining objective
is to ensure that large, uncontrolled displacements of rock in the excavation boundary
cannot occur. This may be achieved by due attention to excavation shape, mining
practice, and possibly by the application of one or more support and reinforcement
devices or systems. By extension, it also involves questions of excavation location and
shape, and frequently, development of an excavation sequence, the specification of the
detail for rock support and reinforcement, and definition of the timing of support and
reinforcement installation. In this chapter, some issues related to excavation shape,
location, orientation and the effect of sparse discontinuities are considered.
The general principles and some examples of engineering design in rock have been
considered extensively in a series of papers in Comprehensive Rock Engineering
(Hudson et al., 1993). In particular, the broad concepts of engineering design and
their translation into rock engineering practice have been discussed by Bieniawski
(1993). The components and logical sequence of the design process are presented
schematically in Figure 7.1. It shows the evolution of a design from specification of
functional requirements through to final realisation of the design in the engineering
construction phase. The excavation design practice considered here can be regarded
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