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Benching
Hard rock quarries are usually worked from top to bottom in
a series of layers, known as benches. This is because the
rock deposit is normally thicker than can safely be worked
as one quarry face. In general each quarry face is no higher
than 15 metres, and special assessments are now required
if they exceed this height. The overall slope of the quarry
side walls will therefore have a stepped appearance alternating
between steeply sloping quarry faces and near horizontal,
or gently sloping, benches. The stability of the quarry faces
will have an impact on the possible width of the subsequent
bench because the bench forms one method of protecting
against rockfall down onto lower levels.
Wider benches will result in a shallower slope to the overall
quarry side wall, whereas narrower benches will give the impression
of steeper walls even though the actual angle of the quarry
faces from vertical may be the same. Face angles are generally
five to twenty five degrees from vertical, depending in part
on the strength of the particular rock type, the presence or
absence of any geological structures (which may weaken the
rock face) and on the methods of excavation used.
The edge, or 'crest', where a quarry face meets the bench above
may become loose (known as 'break-back') as a result of blast
damage, or the general ground conditions, and this area is
therefore particularly dangerous.
The width of benches is important in the design of the final
slope since it controls the use of that bench in respect of:
Under the Quarry Regulations of 1999, the removal or narrowing
of benches is now strictly controlled and safe access has to
be assured. As a consequence, bench widths of eight metres
to twelve metres are becoming more common and can provide safe
access for restoration purposes. |
Edge protection, in the form of a bank of material or large
boulders, is important along the crest because of the risk
of 'break-back' and rockfall protection measures from the quarry
face above are needed. However there are greater opportunities
for planting vegetation on wider benches to soften the appearance
of quarry faces once extraction has been completed.
 Restoration
on quarry benches. |
