Page 217 - Rock Mechanics For Underground Mining

GENERAL PRINCIPLES OF EXCAVATION DESIGN

Figure 7.1 A general design metho-
dology for rock engineering (after
Bieniawski, 1993).

as a subset of the Bieniawski approach, concentrating on Stages 5–8 of the design
logic.
The design of a mine excavation proceeds from an initial configuration which
satisfies its duty requirements, such as minimum dimensions required for operating
equipment, or airway resistance to achieve some ventilation capacity. Siting and ori-
entation are also determined by the duty requirements and by the need for integration
with other elements of the mining layout. The suitability of the selected draft design is
then assessed by following the logical framework defined in Figure 7.2. It is observed
that a key step in the design process is the determination of the stress distribution
around the excavation. This can be achieved using any of the methods described in
Chapter 6. The logical path then involves comparison of boundary stresses with the
in situ crack initiation stress, ci , and the rock mass tensile strength, T 0 . If no bound-
ary failure is predicted, it remains to examine the effect of any major discontinuities
which will transgress the excavation. This requires consideration of both the general
effect of the structural features on boundary stresses and local stability problems
in the vicinity of the discontinuity/boundary intersection. Such considerations may
lead to design changes to achieve local and more general stability conditions for the
excavation perimeter.
Excavation design for the case where rock mass strength is low, or field stresses
are high, proceeds using the path defined by the right-hand branch of Figure 7.2.
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