Environmental Engineering Reference
In-Depth Information
Figure 2.12.
Convergence indicated by traces of presplit holes, near base of railway cutting in fresh,
strong siltstone.
to 460 m 3 /s caused erosion of a channel about 20 m deep in fresh, massive granite. The
granite apparently failed progressively by buckling upwards. As each slab of rock was car-
ried away a new sheet joint formed, by buckling. Isolated rockbursts continued to occur
in the channel floor for more than 6 months after the floods. The channel was eventually
stabilized by rock anchors and dental concrete.
Inward movement or “convergence” of the walls of relatively shallow excavations in rock
is another common destressing effect. Figure 2.12 shows part of a railway cutting 40 m deep
and about 500 m long near Paraburdoo, Western Australia. After excavation of the lowest
15 m of the cutting, by presplitting and trench-blasting methods, a maximum possible con-
vergence of 410 mm was indicated by displacement of traces of presplit holes. Although part
of this convergence was undoubtedly caused by the development of blast-initiated cracks
behind the rock faces, it is believed that much of the 410 mm was caused by destressing of
the gently dipping siltstone. This belief is based on many other similar situations where the
authors have observed inward movements to continue for several days after completion of
excavations. Similar observations have been reported by Wilson (1970).
2.6
WEATHERING OF ROCKS
Broadly speaking, weathering of a rock is its response to the change from the pressure,
temperature, moisture and chemical environments in which it was formed, to its new
environment at and near the ground surface. Weathering processes are of two fairly dis-
tinct types, namely mechanical, and chemical.
2.6.1
Mechanical weathering
Mechanical weathering includes all of the near-surface physical processes which break rock
masses down to progressively smaller rigid blocks or fragments, and cause those blocks
to separate. Mechanical weathering generally precedes chemical weathering. It renders the
 
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