Page 66 - Rock Mechanics For Underground Mining

ROCK MASS STRUCTURE AND CHARACTERISATION

Figure 3.2 Jointing in a folded stra-
tum (after Blyth and de Freitas, 1984).

echelon or in groups. Fault thickness may vary from metres in the case of major,
regional structures to millimetres in the case of local faults. This fault thickness may
contain weak materials such as fault gouge (clay), fault breccia (recemented), rock
flour or angular fragments. The wall rock is frequently slickensided and may be coated
with minerals such as graphite and chlorite which have low frictional strengths. The
ground adjacent to the fault may be disturbed and weakened by associated structures
such as drag folds or secondary faulting (Figure 3.3). These factors result in faults
being zones of low shear strength on which slip may readily occur.
Shear zones are bands of material, up to several metres thick, in which local shear
failure of the rock has previously taken place. They represent zones of stress relief
in an otherwise unaltered rock mass throughout which they may occur irregularly.
Fractured surfaces in the shear zone may be slickensided or coated with low-friction
materials, produced by the stress relief process or weathering. Like faults, shear zones
have low shear strengths but they may be much more difficult to identify visually.

Figure 3.3 Secondary structures as-
sociated with faulting: (a) bedding
plane fault in brittle rock develops
associated shear and tension (gash)
fractures; (b) bedding plane fault in
closely bedded shale develops closely
spaced, intersecting shears; (c) bed-
ding plane fault in poorly stratified,
partially ductile rock produces a wide
zone of drag folds; (d) fault in compe-
tent,brittlerockdiesoutinweakshale;
(e) fault in crystalline igneous rock de-
velops subsidiary inclined shears and
parallel sheeting; (f ) a fault in an ig-
neous rock changes character in pass-
ing through a mica-rich metamorphic
rock (after Wahlstrom, 1973).
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