Page 440 - Rock Mechanics For Underground Mining

ARTIFICIALLY SUPPORTED MINING METHODS

ratio, the stope crown may require little or no support. When the development of
induced fractures is expressed as incidents of local instability in the stope crown,
rockbolting may be used for securing loose or potentially unstable surface rock. The
generation of penetrative fractures across and at depth in the stope crown, and their
interaction with the rock structure, may create conditions under which rockbolting
cannot assure crown stability. A number of mining options exist, which allow further
exploitation of the orebody. For example, the overhand stope may be abandoned,
and an underhand stope commenced at a higher elevation, as indicated in Figure
14.8a. Alternatively, a pillar may be left above the stope crown and overhand stoping
resumedat thehigherelevation, inthe mannerillustratedin Figure14.8b.The resulting
floor pillar might be recovered by some other method subsequently. Finally, a more
practical alternative may be to reinforce the stope crown in such a way as to allow
mining to proceed even though an extensive fractured zone exists above the active
mining domain. This is illustrated in Figure 14.8c.
Large-scale reinforcement is used routinely in many overhand cut-and-fill opera-
tions. Reinforcement technology and field practices are discussed in Chapter 11. The
function of the cable or tendon reinforcement system is to maintain the integrity of the
fractured mass in the crown of the stope. The system acts in a passive mode, so it is
necessary to consider the loads mobilised in the reinforcement by the displacements
of the host rock. The method of analysis of reinforcement mechanics described in
Chapter 11 may be used to design a stope crown reinforcement system. Further, a
reasonable check on the design may be based on the ultimate requirement to suspend
any potentially unstable rock in the crown of the excavation. This simple procedure
may be illustrated by the example shown in Figure 14.8d. Suppose intrascope inspec-
tion of holes shows transverse cracking to a depth, h, of 1.5 m into the crown, that
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the unit weight of the rock mass is 30 kN m , and that tendons each with a yield
load capacity of 260 kN are to be used. The design of the tendon assembly is such
that grout failure produces a shear surface of diameter, d, equal to 50 mm.
For a yield load of 260 kN, and a factor of safety of 1.5, the allowable load per
tendon is 173.3 kN. If the tendons are emplaced at a × a m centres, the weight of
rock W to be supported per tendon is

2
2
W = a h = 30 × 1.5 × a kN
= 173.3kN
Hence

a = 1.96 m
This ensures that the load capacity of the tendon can support potentially unstable
rock. It is also necessary to demonstrate that the grout annulus passing through the
potentially unstable block can support its dead-weight load. If the shear strength of
the grout is 1.4 MPa, the maximum shear resistance S of the grout column is given by

S = dh × 1.4MN = × 0.05 × 1.5 × 1.4MN
= 329.9kN

The factor of safety against grout column shear failure is therefore 1.90.
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