Page 498 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

It is essential to recognise that there is a relation between the natural rate of caving
Figure 15.42 Vertical slice through
a draw column in the early stages of and the permissible rate of draw of the caved material. Consider a vertical slice of
draw showing (a) caved ore to the cave width, w, in a cave developing in an area under draw as shown in Figure 15.42a. If the
back, (b) the formation of a small air
slice is drawn down by a vertical distance, d, as in Figure 15.42b, caving can occur
gap following draw, and (c) the filling
above the slice until the space created by drawing is filled again as shown in Figure
of the void following the next episode
15.42c. During the process of caving, the ore will increase in volume, or bulk, with
of caving (after Brown, 2003).
an in situ volume, V , becoming a caved volume of V (1 + B) where B is the bulking
factor, likely to be in the order of 0.2 or 20% for initial caving. If the in situ unit
weight of the ore is , the overall unit weight of the caved ore will be /(1 + B).
It will be noted that, in order for the cave to propagate, it is necessary that an air
gap be created above the ore pile by drawing caved ore. However, in order to ensure
that the cave is filled with each successive episode of caving and that an excessive air
gap does not develop, the rate of draw must be related to the rate of caving and to the
bulking factor, B. The volume of the air gap BCEF in Figure 15.42b and the volume

of in situ ore BCC B in Figure 15.42c must together equal the bulked volume of the

newly caved ore EFC B in Figure 15.42c. Thus
(c + d)w = c(1 + B)w
or d = cB (15.16)
This means that for caving to proceed as illustrated in Figure 15.42 so that a
permanent air gap is not created, the volume drawn from the slice after each episode
of caving should be only the difference between the in situ and bulked volumes of the
newly caved ore, sometimes referred to as the swell. In other words, the time rate of
volume draw, d, should be B times the time rate of caving by volume, c. If the time
rate form of equation 15.16 is written in terms of tonnages using the relation volume
= tonnes / unit weight, we obtain the result

d t = c t B/(1 + B) (15.17)

where c t and d t are the time rates of caving and draw in terms of tonnages.
There is sometimes a temptation to draw excessively from drawpoints that come
into early production in a new operation or block in order to begin generating cash
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