Environmental Engineering Reference
In-Depth Information
Figure 3.16.
Probable way in which foliation shears and cross-cutting faults are formed in some schistose
rocks.
After 20 years of operation the dam height was increased to 64 m by placing a rock-
filled reinforced concrete trough across its crest.
In 2003, thirty four years after its initial completion, the maximum creep settlement at
the original crest level was 184 mm, which is 0.3% of the initial height and 0.29% of the
revised height.
3.4.5
Structural defects of particular significance in schistose rocks
Three defect types are discussed below.
3.4.5.1
Minor faults developed parallel and at acute angles to the foliation
Schists commonly contain minor faults (narrow, sheared zones or crushed seams, or both)
parallel to the foliation. Deere (1973) refers to these features as foliation shears. In folded
schists, the foliation shears have probably been formed by inter-layer slip, as shown on
Figure 3.16.
Also present in many folded schists are similar “shears” cutting across the foliation at
acute angles, generally less than 20° (Figure 3.16). In some cases these are thrust faults.
Residual shear strengths of both foliation shears and cross-cutting faults have been found
in laboratory tests (i.e. excluding the effects of large scale roughness), to lie in the range
7° to 15°. Such defects commonly form the initiating failure surfaces of landslides in schis-
tose rocks (
Figure 3.17
)
and may provide potential sliding surfaces into spillway or foun-
dation excavations, or within the foundations of an embankment dam.
As these features are often only 50 mm or so in thickness, they can escape detection dur-
ing site investigations unless the investigator sets out to look for them, using appropriate
techniques, e.g. well cleaned up trenches and high quality core drilling. The sheared zones
can be particularly difficult to detect. In these zones the rock is more intensely foliated
than elsewhere and is usually rich in chlorite and/or sericite. The sheared material is there-
fore appreciably weaker than the normal schist and is readily recognizable when the rock
is fresh. However, in distinctly or extremely weathered exposures in which both sheared
and unsheared materials are greatly weakened by weathering, it can be quite difficult to
recognize the sheared zones, because the strength contrast is much reduced, and the shear-
induced cleavage or foliation is similar in appearance to, and may be parallel to, the foli-
ation in the normal schist. Stapledon (1967) describes how initial, poor quality core
drilling at Kangaroo Creek (South Australia) failed to indicate the presence of foliation
shears and associated infill clay seams. Discovery of these in a later exploration pro-
gramme led to abandonment of a thin concrete arch design in favour of a decked rockfill
dam. Paterson et al. (1983) describe how surface mapping and diamond drilling were not
adequate to define the full extent and frequency of foliation shears at the site for Clyde
