Page 523 - Rock Mechanics For Underground Mining

DISCONTINUOUS SUBSIDENCE ASSOCIATED WITH CAVING METHODS OF MINING

Figure 16.20 Idealisation of pro-
gressive hangingwall caving, Gath’s
Mine, Zimbabwe (after Brown and
Ferguson, 1979).

solutions. A simple iterative procedure is required to determine values of p2 and b
for a given stage of mining.
Example. Brown and Ferguson (1979) used the limiting equilibrium analysis to
predict the progress of hangingwall caving at Gath’s Mine, Zimbabwe. At several
sections, the progress of caving had been monitored as mining had progressed down-
dipfromthe99leveltothe158level(Figure16.20).Usingthedimensionsandproblem
−3
−3
idealisation shown on Figure 16.20 with = 28 kN m , c = 25 kN m , = 0,

◦

◦
U = V = 0, w = 35 ,andc = 200kPaand = 40 determinedfromBieniawski’s
geomechanics classification scheme (Table 3.5), values of Z 1 = 31.3m, p2 = 61.4 ◦
and b = 66 were calculated for mining in increments from the 99 level to the
◦
158 level. These values agreed remarkably well with field data, and provided some
confidence in the applicability of the method in this case.
Using the 158 level as the starting point, successive calculations were then made
of the locations of the tension crack and shear surface as mining advanced down-dip
at this section. The results obtained are shown in Figure 16.20. The results were
found to be sensitive to the value of H c at the beginning of each mining lift. The
results shown in Figure 16.20 were obtained using values of H c that were determined
from calculations of the volumes of caved materials and estimates of the volumes of
material drawn historically and likely to be drawn in the future.
In practice, the geometry of the problem is rarely as simple as that used in the
model. Care has to be taken in assigning values of geometrical parameters such as
orebody dip and width and depth of caved material, and of rock mass properties. In
the case of Gath’s Mine, the real problem also involved the use of backfill in the cave
(which served to steepen the angle of break), a three-dimensional effect at the end of
the mined section of orebody, the influence of a hill, the presence of a pillar designed
to protect the mill area, and the influence of a major set of geological discontinuities
which controlled cave development at one location. The effects of some of these
factors could be assessed by sensitivity studies using the analysis, but assessing the
influence of others had to remain a matter of judgement and experience.
The emphasis in this section has been on the use of a limiting equilibrium approach
to predict the progression of hangingwall caving during the mining of an inclined
orebody. Numerical methods can also be used for this purpose. Two-dimensional
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