Geology Reference
In-Depth Information
low-temperature slate, through phyllite, to high-temperature schist. In
all cases, the rocks are recrystallised and the new fabric and structure
dominates their mechanical properties, although original bedding may
be evident. Phyllite is intermediate between slate and schist and gen-
erally has shiny, low-friction foliation because of the presence of
minerals such as mica and chlorite. In schist, the original bedding
may still be broadly recognised by chemical layering throughout the
rock mass
some zones could be richer in silica (originally sandstone),
others might be graphitic (the original rock perhaps having been
organic mudstone with coal). Sometimes there have been several
phases of metamorphism with several different cleavage or schistosity
foliations imposed on the same rock mass, leading to blocky rock,
which may cause dif
-
culties for underground excavations, as occurred
for the power house at Kariba Dam, Zambia (Blyth & de Freitas,
1984).
Mineralogical, grain size and shear strength variability along schis-
tose foliation can result in joint styles changing very rapidly from
layer to layer, as illustrated in
Figure 3.19,
which causes obvious
dif
culties for characterisation of the fracture network. Schist is
sometimes associated with thin (say 100mm) shear zones of low-
frictional strength (15
25 degrees) often running roughly parallel to
foliation and sometimes extending laterally for more than one kilo-
metre (Deere, 1971). Not surprisingly, these often cause problems for
engineering structures, including tunnels and slopes. Deere gives
several examples, particularly of tunnels running parallel to the
strike of steeply dipping schistosity. There are various possible ori-
gins for these shear zones but many are probably the result of
slippage along foliation during folding
-
similar in origin to intra-
formational shear zones in folded sedimentary rock sequences, as
discussed below.
-
Figure 3.19 Joint
system geometry
varying with each
way along Sector
9A of the Via
Algarviana
São
Bartolomeu de
Messines to
Barragem do
Funcho, Algarve,
Portugal.
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