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Geotechnical News • June 2016
THE GROUT LINE
Remedial works, 2014 season
Although after the 2003 season
remedial works the total relief well
seepages had been reduced by 44%,
there had been no further reduction
over the intervening 11 years. The
client wanted a permanent solution to
reduce the seepages through the grout
curtain to a level which would effec-
tively remove the risk of degradation
of the rock formation by the erosion
and washout of clay material within
fissures. The client’s designer decided
to construct a new, single row grout
curtain, downstream of the original,
directly below the base plinth of the
dam, and extending over a length of
150 m up to the left abutment.
In order to achieve the core objectives,
the designer prepared an extremely
demanding specification which
required
• a residual permeability of just 0.1
Lugeon units, equivalent to 1 x
10
-8
m/s.
• a grout mix with reduced cement
content to resist the degradation
caused by the super soft reservoir
water, which had a Langelier index
of -2.5.
• a very high level of process control,
including the use of computer-
piloted pumps.
• the use of GIN grouting to control
the works and avoid excessive
uplift pressures on the relatively
light dam structure
• a full-scale verification trial on site
to verify the effectiveness of the
selected grout mixes, injection
parameters, and curtain geometry
Given the range of permeabilities -
from the 0.1 Lu specified residual
permeability, up to the 3’000 Lu
identified in the extensive pre-grouting
investigations, and the difficulty of
addressing this range with the single
row grout curtain, it was clear that
the mix design would be a very key
element of the works, to allow for per-
meation of both very fine fissures, and
heavily fractured zones with signifi-
cant voids and open fissures.
An extensive programme of mix
design was carried out within the
laboratories of Bachy Soletanche to
develop the grout mix(es) necessary
to deal with such a very wide range
of rock conditions. It was decided to
use a slag based cement to address the
issue of grout degradation arising from
the Langelier index, and over a period
Figure 4. Grout absorptions at the dam foundation.
Figure 5. Wimbleball Dam and its foundation stratigraphy.
