Environmental Engineering Reference
In-Depth Information
that this evidence would have been found, and probably understood, if the site investiga-
tion had included bulldozer trenches.
Following that example the sites for most dams built in Australia have been explored
extensively by bulldozer trenches which have been designed to provide the answer to geot-
echical questions and also access for drilling rigs.
These trenches have totalled more than 3 km in length at each of the following large
embankment dams - Talbingo, Dartmouth, Sugarloaf and Thompson.
The effectiveness of well prepared trenches in providing an understanding of dipslope
stability at the Sugarloaf site is described in Chapter 2, Section 2.10.3.4.
A major benefit of trenching is the experience gained from the use of plant of similar
size and type which may be used during construction. It is good practice to record
machine and ripper types, material types and excavation rates.
At sites for large structures in geologically complex areas with little natural exposure,
trenching can be the most practical method of providing the answer to vital feasibility and
design questions. A disadvantage of trenching is that it causes much disturbance of the site
but, as with test pits, trenches can be backfilled and revegetated if construction work does
not proceed.
5.7
SLUICING
In some situations during site investigation the bulk removal of soil by slicing using high-
pressure water jets can expose the rock surface in the dam foundation and thus extend the
information obtained from natural exposures on rock condition and structure.
Sluicing is an effective option on steep, rocky slopes where the soil is relatively thin and
sandy. Stiff to hard clay is difficult to sluice. Sluicing requires a powerful pump and careful
control of the operation to limit environmental effects. The movement of the mud slurry
produced by the sluicing has to be controlled and contained using sedimentation basins.
Sluicing has been used with success at the Little Para Dam (South Australia), Gordon
Dam (Tasmania) and Upper Ramu diversion dam (Papua New Guinea).
5.8
ADITS AND SHAFTS
In investigations for a major dam it may prove necessary to investigate a part or parts of
the site, e.g. an area underlain by cavernous limestone or disturbed by a landslide, in more
detail than can be achieved by the combination of surface excavations and drilling.
In such situations the excavation of adits or shafts into the area of concern provides the
opportunity for:
- direct observation of the ground conditions;
- measurement of orientation of defects and comparison with surface measurements;
-
in situ
testing;
- underground investigation drilling.
Exposures in adits and shafts should be logged and photographed in detail using the
same general approach as for test pits and trenches. All exposed faces, including the floor
if practicable, should be logged.
Adits and shafts are expensive and slow to excavate. They are likely to require support,
ventilation and drainage.
The requirements for, and location of, adits or shafts should be carefully considered in
relation to all the available design and geotechnical information. It is necessary to justify the
expense by the clarification of site conditions which they may provide. Wherever possible
