Questions and answers: design of surface excavations 329extremes of this range where sliding on individual benches may occur.
This will be controlled by the local geometry of the fractures, and so will
need to be identified in situ. Usually these small instabilities can either
be reinforced, or the unstable blocks removed. However, the principal
problem is how to overcome the instability of the entire face.
345'.,/
plane sliding,
entire face\wedge sliding,
\ ------. I/ individual benches\
The engineer has essentially three options to reduce the effect of this
sliding instability: flatten the slopes, reinforce the slopes, or change the
shape of the quarry to reduce the amount of face that is subject to
instability.
The first two of these options will usually be unacceptable for an
asset such as a quarry. Flattening the slopes will reduce the amount of
material that can be extracted from the quarry, and the installation of
reinforcement over a large part of the quarry faces will be too expensive
to be economic. The third option - changing the shape of the quarry -
may be possible, and leads to some interesting design issues.
The design problem is to re-
duce the length of quarry face
that is subject to sliding instabil-
ity. One way of doing this would
be to use a rectangular quarry,
with its major axis aligned along,
say, 055"-235", as shown in the
sketch below. If the rectangle has
an aspect ratio of 1 : 2, then with
such an arrangement the length
of unstable face is reduced to
only 1/6, or 1770, of the entire
face length. This compares to 25%
for the case of the circular quarry.
It may now be economic to reduce the overall slope angle for the
southwest face of the quarry, and avoid all instability.
Only this length of face
subject to instability