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access to seam from surface |
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Special Issues |
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Special Issues
There are occasions where seam accesses have to be excavated in particularly difficult conditions, typically where there are very high water makes (too high to handle with pumps practically) or very poor strength strata.
In cases of high water make a method used in the past has involved drilling a series of boreholes around a shaft location through which a refrigerant liquid is pumped to freeze the ground around the shaft. This frozen ring is maintained while the excavation is carried out and the shaft sealed and/or permanent water handling facilities installed. This is a very costly process and the writer is not aware of any recent application of this method in Australia (it was used at John darling Colliery around 1970).
In more recent times, grouts have been developed which can be pumped into the strata around an excavation through boreholes drilled in advance, to effectively seal off any potential water flow paths. Provided there is no future ground movement, this seal should be permanent and prevent any water make for the life of the excavation. For water control, the grouts need to seal all (or at least most) of the flow paths so the grout needs to flow well through the strata. They are typically high expansion grouts and do not need to be high strength.
The same process can be used to inject grout into poor strata to consolidate it prior to excavation. Different types of grout may be suitable as the need for consolidation is essentially to glue most of the strata together rather than to seal every opening. These grouts do not have to fill all flow paths but do require sufficient strength to enable the ground to stand for the period between excavation and installation of supports and/or lining, often under conditions of high stress and significant strata movement
Example of using grout to control water – not a coal mine, but grouting principles are the same