Environmental Engineering Reference
In-Depth Information
Changes to the project resulting from this slide included construction of a permanent
stabilising fill of more than 2.6 million m
3
, that involved filling in the valley upstream of
the dam for a depth of 50 m with rockfill, constructing a larger supporting fill on the right
abutment upstream from the dam and elimination of an access bridge from the gate shaft
area to the upper outlet tower.
During detailed studies of the Core Trench Slide and Synclinal Slide the succession of
rock types and the shape of the Thomson Syncline became well known in those areas.
Projection of this known geological picture southwards raised serious questions about the
possibility of first-time landsliding within the ridge which formed the reservoir rim, imme-
diately south of the right abutment of the dam. The spillway and part of the saddle dam were
located on this ridge (
Figure 2.40
, Section BB). Assuming a full storage and using residual
strengths obtained from back analysis of the Synclinal Slide, this ridge appeared to have
safety factors of less than unity against sliding downstream along any of the numerous
bedding surface seams which daylighted above river level on the downstream side. A
major new site investigation was carried out over a period of 16 months, including 1.6 km
of trenches, adits 250 m and 460 m long, diamond drilling and field and laboratory testing.
This work is described in detail by Marshall (1985) and summarised by Stapledon (1995).
It showed that irregularities in the fold shape and rotational displacements on faults effec-
tively precluded failure of the ridge. The following additional works and changes were
undertaken, to further guarantee its stability:
-Weathered, potentially unstable rock was removed from the top of the ridge;
-
The axis and grout curtain of the saddle dam were moved 10 m upstream, to reduce the
embankment load on the active wedge;
-
An additional drainage gallery 70m below dam crest level was driven into the ridge and
connected by a drainage curtain of vertical boreholes to the previously constructed
exploratory gallery that was some 115m below dam crest level.
The site for Thomson Dam is geologically very complex and was difficult to explore due
to its steepness, dense vegetation and often deep soil cover. Some evidence of the slope sta-
bility problems at the site was found during the early planning and used in the design of the
dam. The owner's decision to excavate the right bank cutoff trench prior to awarding the
main construction contract (Hunter, 1982) was a good one. As Hunter (1982) points out,
this decision, although driven as much by concern over limiting potential industrial prob-
lems as to geotechnical aspects was more or less in accord with the approach of Terzaghi and
Leps (1958) at Vermillion Dam. Terzaghi and Leps concluded that the foundations at
Vermilion were so complex that further more detailed exploration would still leave a wide
margin of interpretation. They therefore advocated proceeding with construction of the dam
as designed on the best interpretations and assumptions, with careful monitoring of the con-
struction, and modification of the design to suit the conditions as found.
However, at Thomson, after construction was well advanced the expenditure of money,
technical manpower and time on geotechnical site investigations far exceeded that spent in
the feasibility and design stages. Most of this effort went into investigation of the down-
stream ridge, the stability of which was a question that should have been answered in the
feasibility stage.
If the construction stage studies had found this ridge to be unsafe, the options for reme-
dial works were very limited, extremely expensive and would themselves have required
further detailed site investigations to prove their feasibility. As discussed in Section 2.11.1
and Section 4.6, the authors consider that for all dams, large and small it is vital that suffi-
cient funds are made available at the feasibility stage to ensure that all questions affecting
feasibility are asked and answered satisfactorily. For dams of the size of Thomson the
exploration will often include adits as well as extensive and carefully cleaned trenches.
