Page 282 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN BLOCKY ROCK

It has already been observed that it is the normal component of traction on a joint
surface which is responsible for mobilising friction to prevent displacement of a rock
prism. Thus if at any stage in the mining life of an opening in jointed rock, the
peripheral rock is de-stressed, wedge collapses will occur from the excavation crown
and sidewalls. De-stressing may be due to such effects as mining adjacent stopes,
local fracture of rock and its subsequent non-transmission of stress, blasting practice
causing local stress relief, and local stress relaxation due to time-dependent effects.
In initially low-stress environments, of course, the internal forces available to prevent
block failures will always be low, and pervasive peripheral failures are to be expected.
In all cases, the design of the excavation should take account of the near-field state of
stress to be expected throughout its projected mining life.
The rˆole of friction in controlling peripheral rock performance was discussed briefly
in Chapter 7. In the current context, it is noted that, in their initial, topographically
matched state, joints are almost universally dilatant in shear. Thus the effective angle
of friction exceeds the value which might be determined by a shear test on a disturbed
specimen of a joint surface. Any mining activity which disturbs the initial, interlocked
stateofajointsurfaceautomaticallyreducesthecapacityoftherockmasstosupportits
constituent blocks at the excavation periphery. The chief sources of joint disturbance
are local blasting effects, transient effects due to the impulsive nature of the excavation
process, and large scale, far-field blasting.
The effect of wedge size on the possibility of boundary collapse may appear to be
obvious, on a superficial examination. However, there are some subtle considerations
which may have serious practical consequences if ignored. An example is illustrated
in Figure 9.18, in which an opening has been developed in a rock mass in such a way
that a rock prism has been generated in the crown of the opening. If it were decided to
widen the opening, a stage would inevitably be reached where the roof prism would
collapse. This is so since the wedge increases in weight with the square of the span,
while the mobilised support force, to a first approximation, increases only linearly
with span. For a three-dimensional problem, the same conclusion applies, since the
wedge weight always increases by a power of the linear dimension higher by unity
than does the surface area. The important principle demonstrated by this example
is that a marginal increase in the span of an opening in jointed rock can cause a
significant reduction in the stability of the system, through a marked increase in the
disturbing force (the block weight) relative to the mobilised resisting force.

Figure 9.18 Problem geometry
demonstrating how an increase in
excavation span increases the volume
of a roof prism, without comparable
increase in the restraining force.

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