Page 67 - Rock Mechanics For Underground Mining

MAJOR TYPES OF STRUCTURAL FEATURES

Deere (1979) has described the nature of shear zones and discussed the engineering
problems associated with them. Salehy et al. (1977) have described their occurrence
in coal measures rocks as intraformational shears.
Dykes are long, narrow intrusions of generally fine-grained igneous rock with
steep or vertical and approximately parallel sides. They may vary in width from a few
centimetres to several metres and may appear as dyke swarms. Dykes may also be of
considerable length. The Great Dyke of Rhodesia, for example, is some 500 km long.
It is a flat, trough-like structure which is extensively mineralised, particularly on the
margins. Some dyke rocks are more resistant to weathering than the country rock, but
the basic igneous dyke rocks such as dolerite can weather to montmorillonite clays
which are noted for their swelling characteristics. The dyke margins are frequently
fracturedandalteredduringtheintrusion.Theyformpotentialseepagepathsandzones
of low stiffness and shear strength in which movements will tend to be concentrated.
Because of their high stiffnesses, unweathered dyke rocks can develop high stresses
and so be susceptible to stress-induced failure or, as in the deep-level gold mines
of South Africa, be associated with rockburst conditions. Figure 3.4 (after Cook,
N.G.W. et al., 1966) shows the effect of dyke proximity on rockburst incidence at

Figure 3.4 The effect of dyke prox-
imity on rockburst incidence, East
Rand Proprietary Mines. (a) Large
dykes in continent abutments; (b)
large dykes in island or remnant abut-
ments (after Cook, N.G.W. et al.,
1966).

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