Environmental Engineering Reference
In-Depth Information
Figure 17.17.
Tumut 2 concrete gravity dam foundation cleanup in progress.
- Mechanically loosened
in situ
rock mass with clay-infilled joints. This may be adequately
strong in substance but the mass is too compressible/weak as it contains too many
seams which cannot be treated effectively by jetting and grouting.
The excavation is aimed at reaching the required foundation surface, while causing
minimal damage to the rock below it. Some drilling and blasting may be required, but
usually the rock next to the surface is removed by rock breaker, backhoe and hand meth-
ods. The surface reached in this way is cleaned up by compressed air/water jetting, with
further use of hand tools. The cleaned-up surface should contain no detached rock frag-
ments or loosened or drummy blocks and details of the exposed rock substances and
defects should be clearly visible. Figure 17.17 shows cleanup in progress at Tumut 2 con-
crete gravity dam in New South Wales.
The cleaned-up surface should be mapped geotechnically in detail.
Figure 17.18
is a sample
of such mapping at Sturt River Dam in South Australia. The purpose of this mapping is to:
-Confirm that the foundation surface rock is of the required and predicted quality, and
- Confirm that the rock structure is essentially as predicted in the geotechnical model and
allowed for in the design and, in particular, that there is no unexpected geological situation,
e.g. a kinematically feasible failure surface, which may require a change to the design.
The mapping is preferably done while the cleanup is in progress, as this can allow spe-
cial attention to cleanup of important defects and timely decisions to be made, e.g. on the
need for dental treatment, further deepening or design changes.
17.8.3
Treatment of particular features
17.8.3.1
Dental treatment of localised weak/compressible features extending into
acceptable foundation rock
Features for which localized dental treatment is suitable may range from less than 10 mm
to several metres in thickness and include faults (sheared and/or crushed zones) and zones
