Environmental Engineering Reference
In-Depth Information
As at Sugarloaf, the site was trenched extensively during the feasibility investigations
but the trenches were not so deep, and the same standard of cleanup was not achieved.
However the Ski-jump slide (
Figure 2.40
) on the upstream left abutment was identi-
fied from highly disturbed rock in a trench cut into an area showing typical landslide
topography.
Diamond drilling in the upper right bank core foundation area indicated closely jointed,
seamy and partly weathered rock down to 70 m. High losses occurred during water testing of
this zone and it was suspected that it may have been disrupted by past landsliding. This
was allowed for in the designed depth (up to 30 m) of the cutoff trench on the upper right
bank. Also, the owner, Melbourne and Metropolitan Board of Works, decided to excavate
the whole of the cutoff trench on this bank, prior to awarding a contract for construction
of the dam. During this excavation, and excavation of associated haul roads, it was found
that past landsliding in the site area had been more widespread than expected and had
caused disruption of the rock mass in the three areas shown on Figure 2.40. In each area
the sliding had occurred where the combined effects of valley erosion and the fold shapes
had caused downslope dipping beds to daylight. Details of each slide and the nature of the
disturbed materials are given in Marshall (1985).
The Ski-jump Slide proved to be a relatively small feature. The folded rock here day-
lighted into a gully immediately upstream (Figure 2.40). The slide mass was buttressed
effectively by placing rockfill in the gully and the rockfill shoulder of the dam was built
over it.
The Core Trench Slide proved deeper than expected, downslope movements having
occurred along folded beds daylighting at river level. The trench excavation ranged from
10 m to more than 40 m deep and involved more than a million cubic metres of excava-
tion, all by Caterpillar D9 bulldozer without ripping. In the upper half of the trench deeper
excavation appeared likely to give rise to instability of the sides and so it was decided to leave
the deeper, moderately disturbed rock in place and treat it with a 5-row grout curtain. This
rock contained many joints which were open or clay-filled, from 1 mm to 20 mm. It was
appreciated that most of the clay would remain after the grouting but could be washed out in
the long term, when the reservoir was filled. To allow for this possibility a reinforced concrete
gallery was built in a slot cut into the upslope half of the cutoff trench. The gallery provides
access for monitoring and for drilling and grouting equipment if regrouting should ever be
needed.
The Synclinal Slide (Figure 2.40, Section AA) was discovered during construction when
cracks and displacements appeared in haul roads near the toe of the ridge at the right bank
upstream shoulder of the dam in mid-1977. The estimated slide mass was about
350,000 m
3
, but, once the slide had been defined geologically, the potential slide mass was
estimated at well over 3 million m
3
, with a basal surface that would have sliced through the
right abutment of the main dam and extended downstream to affect the top of the spillway.
Movements in the toe area ranged from 1 mm/day in dry weather to 3 mm/day after rain.
Movements stopped after 9 months (a total of 300 mm cumulative movement) on comple-
tion of a 50 m high stabilising rockfill built against the ridge toe. This required diversion
of the river into a channel cut into the left bank, as the diversion tunnel was still under
construction.
Detailed investigations showed that past movements in the ridge had extended right up to
its crest. The sliding had occurred directly down the 12° plunging axis of the Thomson
Syncline. Upslope from the toe area the base of the disturbed rock zone was stepped as
shown diagrammatically on Figure 2.40, Section AA. In the ridge crest area rock dis-
turbed by this slide occurred in the top few metres of the gate shaft (alongside and just to
the left of the spillway) and also at the top of the cutoff trench. The inferred basal surface
of the potential slide was estimated to be nearly 75 m below the top of the gateshaft and
generally 60-70 m deep below the crest of the dam on the right abutment.
