Page 261 - Rock Mechanics For Underground Mining

IDENTIFICATION OF POTENTIAL BLOCK FAILURE MODES – BLOCK THEORY

the medium. Stable, continuous behaviour of a jointed or granular medium exploits
frictional resistance to shear stress, and this resistance is mobilised by compressive
normal stress. Thus, generation and maintenance of a state of mechanically sustain-
able compressive stress in the excavation boundary rock, which may involve the
installation of support and reinforcement, is a basic objective of design in this type of
medium.
In addition to considering the quasi-continuous behaviour of a jointed medium
in compression, it is also necessary to take account of its explicitly discontinuous
properties. Because the rock mass prior to mining consists of an assembly of topo-
graphically matched blocks created by the joint surfaces, the problem is to predict the
behaviour of the individual blocks after a transecting surface is formed by an excava-
tion. For blocks defined in the crown and side walls of an excavation, the requirement
is to examine the potential for displacement of each block under the influence of the
surface tractions arising from the local stress field, the fissure water pressure and the
gravitational load.

9.2 Identification of potential block failure modes – Block Theory

The critical issue in design in blocky rock is identification of the rock units or prisms
that may be created and left in an unstable state after the mining of an excavation.
‘Block Theory’, the topological theory used to perform the identification, is primarily
due to Goodman and Shi (1985) on whose work the following discussion is based.
The specific objective of Block Theory is to identify so-called ‘key blocks’ or critical
blocks which present particular risk to the stability of an excavation boundary. The
theory provides for the design of appropriate support and reinforcement and for
orientation of excavations and their boundaries to mitigate the effects of difficult
block geometries.

9.2.1 Removable blocks
While the shapes and locations of key blocks are a fully three-dimensional problem,
the basic principles of block analysis can be understood from consideration of the
two-dimensional problem. In Figure 9.2 are shown three different types of blocks

Figure 9.2 Two-dimensional views
of types of blocks that can be formed
in an excavation boundary.

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