Environmental Engineering Reference
In-Depth Information
Figure 2.31.
Tooma Dam, cross section and sketches through downstream shoulder.
At the same time, 400-700 mm wide zones of extremely to highly weathered granite
which were wet and showed local seepages became exposed close to river bed level on
both banks. Both zones were undulating but dipping generally between 10 degrees and 20
degrees towards the river. Each zone contained one or more remoulded seams which
included surfaces with slickensides (Figure 2.31A). In the right bank seams these were
trending downslope and obliquely downstream. Displacements of 50 mm were observed
across one slickenside seam during a period of two days. Monitoring across this seam
ceased when several cubic metres of rock toppled from immediately above it.
When all of these features appeared the contractor withdrew from the upstream part of
the site until it could be demonstrated that large-scale landsliding was not imminent.
Close examination of the largest cracks on the right bank showed that these had formed
by the collapse of surface soils into pre-existing gaping joints which appeared to extend
almost down to the extremely weathered zone (Figure 2.31B). Thus, although the cumu-
lative past downslope extension appeared to be between 1 m and 2 m, the amount of cur-
rent movement appeared to be much smaller, probably just enough to cause the bridging
soils to crack and fall into the already gaping joints. This was confirmed by monitoring
which was started after the appearance of the cracks, and showed only about 150 mm of
further movement over a period of about 2 weeks.
It was concluded that the gently dipping joints were probably sheet joints and that the
basal weathered zones had been formed by weathering along the lowest of these joints.
The pre-construction block movements towards the river were probably initiated by stress
relief, and continued due to gravity.
On each bank the largest of the old movements appeared to have occurred between the
upstream soil-filled gullies and the cutoff trench (
Figure 2.30
).
Movements of the right bank cliff area were stopped by early placement of rockfill in
the river bed. Some of this is visible at the right hand edge of
Figure 2.32
.
