Ground water constitutes a significant operational problem and potential hazard in open-pit and underground mining. The pressure of ground water in geologic discontinuities adversely affects the safety of height and slope angles of open-pit mines, justifying measures to reduce ground-water pressures in their vicinity. The principal hazard in underground mining and tunneling arises from massive inflows of ground water when the mine unexpectedly intersects large, water-bearing geologic discontinuities; normal inflows are on the order of 2.5 tons of water per ton of rock mined. In tunneling, pilot drilling ahead of the advancing face is used to detect such discontinuities, which can then be sealed by ring drilling and cover grouting. Extensive deformations resulting from large areas of unsupported roof make such grouting ineffective in many mine excavations.
Surprisingly few data are available with which to calculate the permeability of rock masses at depth. Two sets of data quoted yielded permeabilities of 0.8 × 10−9 m/s for tunnels, and 1.6 × 10−6 m/s for mined excavations, at a depth of about 1.5 km below surface and before the permeability of the rock mass is enhanced by deformations induced by mining. The difference between these values may be attributed to local reductions in the permeability of the rock mass by the grouting of major discontinuities in tunneling, which is not done ordinarily in mining.
Where precautions such as regional dewatering, installation of watertight bulkheads, and adequate spare pumping capacity are not sufficient to cope with potential inflows in mining, the extent of the unsupported roof in mines must be broken by regularly spaced pillars, so that pilot drilling and cover grouting can be done.