Draw Points / Draw Bells:
driven horizontally from the production drifts. Usually they are driven
at an angle to the production drift to facilitate the entry of LHDs to the
draw point. The distance from the brow of the draw point to the opposite
rib of the production drift must be sufficient for LHD to have entry to the
muck pile with near zero articulation.
of the brow should be enough to allow the bucket of the LHD to raise its
load but not so high that the ore will flood on the production drift.
are lined with concrete to maintain the size and avoid major deformations.
Steel arches (3-6) as well as rock bolts (5-10) are used in the brow to
ensure the life of the draw points.
adjacent draw points are connected by a crosscut called a draw bell
draw bell is used to create the draw zones feeding broken material to two
For example, the
production level at the Premier mine had been developed as an offset
This layout was chosen after several options had been considered and
modelled for ease of operation, structural strength, and the use of electric
LHDs. The production tunnels are 4 by 4.2 m in size, and are spaced at
30m centres, the draw bells are at 15m centres. The draw bells were
planned to be 13 m long to allow for drawpoint wear of 1m at either end.
point for the grizzly and slusher systems are driven at right angles to the
grizzly or slusher drift. They are also inclined to the horizontal, the
angles varies nearly 90° to 45°. The cross section should be large enough
to allow the fragments to pass through the opening with
driving the inclined section of the draw point, a vertical section may be
driven upwards to the elevation of the undercut level. This allows the
undercut level to be raised further above the production level to
an adequate pillar between the cave area and the production level.
A concrete liner will allow a better flow of the broken rock and will retain
the size of the draw point better than unlined rock. If repairs are
required because of the excessive erosion, then it
be easier accomplished if the draw point was originally lined. It is
very important to maintain the size of the draw points to prevent flooding
of the production drift with broken rock.
In an LHD
system, the draw point entry must be nearly horizontal to allow the entry of
the production unit. The connection between the production level and the
undercut level is usually a large draw bell created by drilling and
blasting. The draw bell allows for larger pieces of rock to be moved by the
The brow of
the draw point should be high enough to allow the LHD bucket to lift a load,
but not so high as to allow broken rock to flood out into the production
drift where the movement of the LHD could be restricted.
spacing is important to insure a good recovery of the ore. If the draw
point spacing is too far apart, then good ore will be lost or dilution may
become so great that the ore becomes uneconomic. If the draw point spacing
is too close together, then the mining costs are greater than they should
be. Each draw point has a certain area of influence on broken rock above.
The draw point spacing should be spaced that these areas of influence will
overlap slightly to insure that the total column is moving downward as the
(1979) has constructed a regression curve based on data that can be used to
approximate the draw area based on median fragment size. For example if the
median fragment size is 0.8 m, then the draw area would be equal to 131 m2
Median Fragment Size (m)
point spacing (m2)
Fragment size & Drawpoint spacing in various
mines (White, 1979)
spacing for grizzly and slusher layouts reflect the spacing of the draw
zones because of the close spacing of the draw points (Laubscher, 2001).
However, in the case of LHD layouts with a nominal draw point spacing of
15m, the draw zone spacing can vary across the major apex (pillar) from 18
to 24 m, depending on the length of the draw bell. There is a trade off
between optimizing the ore recovery through the use of a short draw bell and
the desired usage of the LHD.
tests have shown that there is a relationship between the spacing of drawpoints and the interaction of draw zones. Widely spaced draw points develop
isolated draw zones, with diameters defined by the fragmentation. When draw
points are spaced at 1.5 times the diameter of the isolated draw zone (IDZ)
then interaction among rock fragments occurs. The interaction improves as
the draw point spacing is decreased, the question is whether the interactive
draw theory can be wholly applied in coarse material, where arches of more
than 20m have been observed. Low-friction material could flow greater
distances when under high overburden load, and this could mean a wider draw
model results (Kvapil, 1965) have been confirmed by observation of the fine
material extracted during cave mining and by the behaviour of materials in
stress increases just over the brow of the drawpoint. This effect
demonstrates that particles tend to go to the centre of the draw zone,
exhibiting a horizontal component of the total particle’s movement.
according to the design criteria of the Premier Mine, the drawpoint
spacing, largely a function of fragmentation, must be addressed to reduce
hang ups, maintain production levels, and avoid damaging stress levels.