Environmental Engineering Reference
In-Depth Information
the sheeted structure, discing of drill cores and excessive propagation of fractures during
and after blasting. Bock et al. (1987) showed the maximum principal stress to be hori-
zontal, striking 10° to 30° to the dam axis. Values ranged generally from 10 to 20 MPa
across the river bed, rising locally to 30 to 35 MPa at the left abutment.
During construction, the depth to adequate foundation rock was established progressively
during the excavation for each monolith. In the river bed, excavation was mainly by rock
breakers and excavators. Explosives were used for some slabs which were too large to han-
dle. Cleanup was by bobcat, hand shovels and hosing. The adopted foundation surfaces
ranged generally from about 1 down to 4 metres below the river bed. They were chosen for
each monolith from the results of geological inspection of progressively exposed surfaces,
and by the drilling, cleaning out, and downhole inspection of at least 3 vertical holes, each
2.8 m deep. An Olympus IF6D2-30 Fiberscope was used for these inspections. A simple log
for each hole recorded the depth of each sheet joint present and its aperture or infilling. Based
on these logs, judgements were made on the persistence and character of the joints, and deci-
sions made on the need for either removal of the rock above them, or grouting. Extra holes
were sometimes needed. The final (adopted) cleaned up surfaces were mapped geologically
in plan view on 1:200 scale. The as-constructed record includes these plans and the logs of
the 2.8 m holes.
In the abutments, blasting was used for much of the excavation, but smooth blasting
techniques were specified. The as-constructed records include foundation geology
mapped on 1:100 scale, plus logs of the 2.8 m holes.
Significant stress-relief effects during the foundation preparation occurred only close to
the base of each abutment, during hot weather. They included rock bursting (spalling of
small, thin slices or rock), and slabbing (buckling up of slabs up to 100 mm thick and
many square metres in area).
Figure 17.26
shows an example of this.
