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Propping |
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Propping is a process whereby timber is used to stabilise underground
mine workings. By reducing the risk of cave ins, propping improves safety for opal miners
while
also maximising the amount of opal bearing claystone that can be excavated.
In many opal mines the only workings developed are narrow exploratory
drives (tunnels) and very little opal, if any, is found. However, in
the best mines opal may be found over relatively large areas and it then
becomes necessary to install some support to to reduce the risk of cave
ins. Timber props are the most common type of support used, usually
cypress pine
(Callitris glaucophylla). Steel rock bolts are also used rarely.
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Cypress pine stockpiled ready for use in a
mine on the Grawin opal fields, Lightning Ridge. |
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Cypress pine props installed in a mine on the Coocoran opal fields, Lightning Ridge, 2001. |
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Even extensively propped opal mines may cave in, as shown below.
Such collapses may result from a mixture of water entering mine workings,
timber decay, unstable ground conditions or poor installation
techniques. Typically the props provide opal miners with some
warning that the mine workings are collapsing when they begin to fracture
due to stress.
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Props fracturing due to pressure from overlying rock layers, Coocoran opal fields, Lightning Ridge, 2001. |
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Mine workings which have begun to cave in despite extensive
propping, Coocoran opal fields, Lightning Ridge, 2001. |
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