Page 496 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

Figure 15.41 Idealised vertical sec-
tion showing (a) excessive draw-
point spacing with non-overlapping
draw zones, and (b) closer spacing
with overlapping draw zones (after
Richardson, 1981).

In the past, heavy steel set and concrete supports have been used with limited suc-
cess (e.g. Van Sint Jan et al., 1987). It is now widely recognized that the best results
are obtained with flexible systems consisting of reinforced shotcrete, rock bolts and
cable bolts designed using the principles discussed in Chapter 11 (Brown, 2003, Van
Sint Jan et al., 1987). Concrete, steel sets and steel liner plates may be used to form
drawpoints and provide wear resistance.

15.5.5 Fragmentation
The overall success and profitability of a block caving operation will depend to a
significant extent on the fragmentation produced in the orebody during the caving
process. The prediction of rock fragmentation during caving requires understandings
of the natural fragmentation of the rock mass and of the fragmentation processes that
take place in the draw column. It is generally accepted that there are three levels of
fragmentation. In situ fragmentation is represented by the blocks that are naturally
present within the rock mass before mining takes place. They are defined by the pre-
existing discontinuities. As the undercut is mined and caving is initiated, the blocks
thatseparatefromthecavebackdefinethe primaryfragmentation.Thefragmentation
that occurs subsequently as the blocks move down through the draw column to the
drawpoints in known as secondary fragmentation.
Once caving has been initiated, primary fragmentation will result from the loading
conditions imposed on the rock mass in the vicinity of the cave back. Most failures
at this stage can be expected to occur on pre-existing planes of weakness, but under
high stress or stress caving conditions, fracture of intact rock may also occur. The
extent of these failures will depend on the strengths of both the discontinuities and
the incipient rock blocks relative to the magnitudes and orientations of the imposed
stresses. The primary fragmentation size distribution produced in this case is likely
to be finer than in the case in which gravity rather than induced stresses causes the
detachment of blocks from the cave back. To a large extent, the network of pre-existing
discontinuities will govern the formation of blocks during primary fragmentation.
Secondary fragmentation will occur as the caved ore resides in, and passes through,
the draw column. The nature and degree of secondary fragmentation can be expected
to vary with the stress regime within the caved mass, the composition and mechanical
properties of the orebody, the rate of draw, the height through which the material
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