Page 490 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

Figure 15.36 Comparison of post-
and pre-undercutting in panel caving
(after Rojas et al., 2000).

the starting point and direction of undercut advance;

the rate of undercut advance;

the height of the undercut; and

the shape of the undercut in both plan and vertical section.

Undercutting strategy. The undercutting strategy adopted can have a significant
influence on the stresses induced in, and the performance of, the extraction level
installations and on cave propagation. Basically, three main undercutting strategies
and their variants may be used – post-, pre- and advance undercutting.
The post-undercut strategy is also referred to as conventional undercutting. As
illustratedinFigure15.36,undercutdrillingandblastingtakesplaceafterdevelopment
of the underlying extraction level has been completed. Cones, drawbells or troughs
are prepared ahead of the undercut and are ready to receive the ore blasted from the
undercut level. The advantages of this system are that blocks can be brought into
production more quickly than with some other methods, no separate ore handling
facility is required on the undercut level, drifts on the undercut level are required
only for drilling and blasting and so can be widely spaced, and the probability of ore
compacting on the undercut level is low. The main disadvantages are that, other than
in low stress environments, the rock mass between the undercut and extraction levels
is subjected to high and variable stress levels and support and reinforcement must be
installed well ahead of the undercut stress abutment zone. This can constrain the rate
of undercut advance.
In the pre-undercut strategy, the undercut is mined ahead of extraction level devel-
opment (Figure 15.36). This method may be considered to be a variant of the advance
undercutting method to be discussed below. The minimum horizontal distance that the
extraction level development lags behind the advancing undercut is often the separa-
tion distance between the two levels. This is sometimes referred to as “the 45 degree
rule”. However, in higher stress environments, it may prove necessary to use larger
lag distances. Even with the 45 degree rule, it may be possible that the extraction
level excavations will not be in a full stress shadow zone or may be subjected to some
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