Page 68 - Rock Mechanics For Underground Mining

ROCK MASS STRUCTURE AND CHARACTERISATION

Figure 3.5 Diagrammatic longi- East Rand Proprietary Mines. Another major mining problem caused by dykes in
tudinal section illustrating inrush
of water from Bank compartment, South African gold mines is the compartmentalisation of water-bearing dolomites
causing severe differences in head between adjacent compartments after the water
West Dreifontein Mine, 26 Octo-
ber 1968. Total inflow was approx- level in one has been drawn down during mining operations. At the West Driefontein
imately 100 000 gal/day ( 45.5 × Mine in 1968, a stope hangingwall failure adjacent to a fault in the compartment on
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10 1/day) (after Cartwright, 1969).
the non-dewatered side of a major vertical dyke triggered the flooding of a portion of
the mine (Figure 3.5).
Joints are the most common and generally the most geotechnically significant
structural features in rocks. Joints are breaks of geological origin along which there
has been no visible displacement. A group of parallel joints is called a joint set,
and joint sets intersect to form a joint system. Joints may be open, filled or healed.
They frequently form parallel to bedding planes, foliations or slaty cleavage, when
they may be termed bedding joints, foliation joints or cleavage joints. Sedimentary
rocks often contain two sets of joints approximately orthogonal to each other and to
the bedding planes (Figure 3.2). These joints sometimes end at bedding planes, but
others, called master joints, may cross several bedding planes.
Veins, or cemented joints, are mineral infillings of joints or fissures. They may be
sheet-like or tabular or irregular. They are generally of igneous origin but may also
result from sedimentary processes. They are commonly associated with metalliferous
orebodies and have been found to have major influences on orebody cavability and
fragmentation as at the El Teniente mine, Chile. They may be weaker or stronger than
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