Page 416 - Rock Mechanics For Underground Mining

PILLAR SUPPORTED MINING METHODS

working methods and meet production schedules. In any event, some preferred layout
of the stopes and pillars can be defined for which detailed designs are required of stope
access openings, service openings, and stope development to support the extraction
operations.
The way in which the stress distribution calculated for a mine layout or the evolving
mine structure is interpreted is the key to successful design practice, and is represented
by the following example. From Figure 13.6, it is observed that several modes of rock
fracture and interaction of fractures are involved in the mechanical degradation and
evolution of failure in a pillar. The discussion in Chapter 4 considered the different
failure criteria that describe the fundamental processes involved in rock mass failure,
including fracture initiation, fracture extension and damage accumulation, and ulti-
mately shear failure at peak strength. These ideas have been employed by Diederichs
et al. (2002) in analysing pillar behaviour at the Brunswick mine under the stress paths
imposed by adjacent stoping, and subsequently in pillar design for the mine. In their
approach, several domains are defined in normalised 1 − 3 space by stress bound-
aries which are derived from a generic form of the various failure criteria, expressed
by

(13.25)
1 = A 3 + B c
From this expression, the threshold for crack initiation and the onset of dispersed mi-
croseismic activity indicative of microscopic rock damage is defined by A = 1 − 1.5,
B = 0.4 − 0.5. This approximates the maximum deviator stress criterion of Martin
et al. (1999). The term B c in equation 13.25 is effectively the uniaxial compressive
strength of the rock mass, UCS*, which can be used to scale other strength relations. A
second threshold, described by A = 2, is identified with the onset of systematic fabric
damage accumulation and is represented by a transition from dispersed microseismic
events to localised seismic clusters. The third threshold, determined by A = 3 − 4,
represents the conventional peak strength of the rock mass and corresponds to in-
teraction of damage zones and localisation of extension fractures into shear zones.
Finally, a fourth threshold with A = 0, B = 10–20 applies close to stope boundaries,
under conditions where surface conditions tend to promote spalling.
The relation between these thresholds of damage and rock rupture is shown in
Figure 13.27. In terms of the modes of pillar degradation illustrated in Figure 13.6,
the stress path for an element of rock in a pillar typically corresponds to the trajectory
from surface spalling through interior cracking to shear failure in the body of the
pillar, which is the transition from intact pillar through a state of partial failure to
pillar failure and rock mass rupture.
Application of the method is illustrated in Figure 13.28, where the performance of a
set of 57 pillars is represented in terms of the degree of rupture at the pillar boundaries
(or overbreak) and the condition of pillars, classified here as intact, partially failed
or failed. The results suggest that the application of the different criteria provides an
appropriate discrimination between the possible modes of pillar degradation. They
also suggest that the elastic stress analysis used to determine pillar stresses coupled
with the criteria for defining domains of pillar degradation provide a sound basis for
pillar design analysis. Using this approach, a series of analyses of stress make it is
possible to trace the stress path to which the parts of a pillar will be subjected, and to
predict the overall mode of pillar response.
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