Environmental Engineering Reference
In-Depth Information
from the parallel arrangement of platy minerals, commonly clays, muscovite, biotite, chlo-
rite and sericite. Also often present and in parallel arrangement, are tabular or elongate clus-
ters of other minerals, usually quartz and felspars and occasionally amphiboles.
Although the foliation is referred to as “planar”, the foliae or layers are commonly
folded. The folds can range in amplitude and wave-length from microscopic up to hun-
dreds of metres. Small-scale folds, which cause the surfaces of hand-specimens of schist to
appear rough or corrugated, are called crenulations.
In some schists the foliation has been so tightly and irregularly folded as to give a con-
torted appearance. Such rock is called “knotted schist”.
Slate, phyllite and most mica schists have been formed by the regional metamorphism of
fine grained sedimentary rocks (mudstones or siltstones). This has involved relatively high
temperatures and directed pressure over long periods of geological time. Under these con-
ditions the clay minerals present in the original rocks have changed partly or wholly to
mica minerals, usually muscovite, chlorite and biotite. These minerals have become aligned
normal to the direction of the maximum compressive stress. The proportion of these new
minerals is least in slate and greatest in schist.
Greenschists, which are well foliated rocks containing a large proportion of chlorite
and other green minerals, have been formed by the regional metamorphism of basic
igneous rocks (e.g. basalt and gabbro).
Schists can be formed also as a result of shear stresses applied over long periods of geo-
logical time to igneous rocks or to “high-grade” metamorphic rocks, e.g. gneisses. This
process is known as retrograde metamorphism and usually produces schist containing abun-
dant sericite and/or chlorite. These are weak minerals and so the rocks produced by this
process tend to be weaker than other schists.
3.4.1
Properties of fresh schistose rock substances
Most schistose rocks have dry strengths in the very weak to medium strong range (see
Table 2.4). Schists containing abundant quartz are generally medium strong or strong,
while greenschists which are rich in chlorite are generally very weak.
The most significant engineering characteristic of schistose rocks is their pronounced
anisotropy, caused by the cleavage or foliation.
Figure 3.12
taken from Trudinger (1973)
shows the results of unconfined compressive tests on fresh schist samples from Kangaroo
Creek Dam, South Australia. This schist is stronger than most; it contains generally about
40% quartz and felspar and about 60% sericite and chlorite. It is foliated but not excep-
tionally fissile. It can be seen that the strengths recorded for samples loaded at about 45°
to the foliation were about one third of those for samples loaded at right angles, i.e. the
anisotropy index of this schist in unconfined compression is about 3.
When tested by the point load method, i.e. in induced tension, the schist at Kangaroo
Creek shows anisotropy indices ranging typically from 5 to 10. Failure along the foliation
surfaces in this test is by tensile splitting, rather than in shear.
Most other schists, slates and phyllites show similar anisotropic properties (Donath,
1961). It is clear that foliation angles in relation to loading directions should always be
carefully recorded during tests and reported with the results.
In weak or very weak schists (often those rich in chlorite) the effective angle of friction
along foliation surfaces can be low. Landsliding is prevalent in areas underlain by these
rocks.
In knotted schists the foliation surfaces are often so contorted that shearing or splitting
along near-planar surfaces is not possible. As a result, knotted schists are usually appre-
ciably stronger than those in which the foliation surfaces are near-planar.
Figure 3.12 also indicates that the schist at Kangaroo Creek Dam showed a 25-65%
reduction in strength, after soaking in water for 1-2 weeks. The greatest strength reduction
