Environmental Engineering Reference
In-Depth Information
Australia. They present evidence which suggests that these have developed due to solution
along localized calcarenite zones rendered more permeable by small movements along
faults in the bedrock at a depth of 100 m or more.
(1983b). It has extremely low substance permeability, but can be permeable where joints
are present. Bell (1983b) and Goodman (1993) point out that large cavities are rare in
chalk, but solution pipes and sinkholes can occur, near its contact with colluvium or other
rocks.
3.7.2
Watertightness of dam foundations
Many dams have been built successfully on sites underlain by
Category O
carbonate rocks,
despite the fact that solution cavities have been present at most sites and beneath at least part
of their storage areas.
Selection of dam foundation levels and treatments can be difficult, due to the highly
irregular nature of the uppermost surface of the fresh rock, as shown by
Figures 3.25
to
3.28. This applies particularly to embankment dams, because of their large foundation
areas and the need to provide stable, non-erodible surfaces for placement of embankment
materials.
Treatments to fill cavities and prevent excessive leakage have included cement grouting,
concrete curtain walls and selective mining and backfilling with concrete. The presence of
clay infilling in cavities presents a problem when cement grouting is proposed. Clay usu-
ally prevents grout penetration and is not readily removed by flushing between boreholes.
However, if left in place, it may be flushed out later when the dam is filled. High pressure
cement grouting designed to cause hydraulic fracturing of clayey infills has been shown by
Zhang and Huo (1982) to give significant improvement in their resistance to leakage and
piping.
At the 35 m high Bjelke-Peterson earth and rockfill dam in Queensland about 250 m of
the left bank was formed by a landslide deposit underlain by limestone. In this section the
cutoff trench was excavated down to the water table, 15-20 m below the original surface
and 5-15 m below the irregular upper surface of the limestone (See Chapter 8, Figure 8.8).
At the trench base the limestone was mainly fresh, dense and very strong, but contained
open and infilled solution cavities within and next to sheared and crushed zones (Eadie,
Figure 3.28.
Weathering profile on very weak, porous calcarenite near Perth, Western Australia. Note
the extremely irregular nature of the rock surface.
