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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.


Cypress pine stockpiled ready for use in a mine on the Grawin opal fields, Lightning Ridge.
Props on surface
Cypress pine props installed in a mine on the Coocoran opal fields, Lightning Ridge, 2001.

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.


Props fracturing due to pressure from overlying rock layers, Coocoran opal fields, Lightning Ridge, 2001.
Props under pressure
Mine workings which have begun to cave in despite extensive propping, Coocoran opal fields, Lightning Ridge, 2001.
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Last Update Pre 2006