Geotechnical News • June 2016
37
THE GROUT LINE
In addition to the original grout cur-
tain, remedial grouting of the founda-
tions using classical stage grouting
methods with multiple grout mixes of
varying water-cement ratios, had been
attempted on three or four occasions
of the 20 year period, without success.
In 1999 Soletanche was engaged for
a further attempt to seal the reservoir,
and to carry out real-time monitor-
ing because of the sensitivity of the
upstream concrete face to differential
movement. Grout pressure regulation
during injection, and uplift control of
the dam facade and foundations were
therefore key issues during execution
of the works.
The solution called for a stable grout
capable of penetrating fine fissures,
and also filling larger seepage path and
voids without bleed. The mix selected,
C3S, was a stable cement-bentonite
slurry based upon locally sourced
ordinary Portland cement, de-floccu-
lated bentonite slurry, and fluidifier.
It was decided to use ascending stage
grouting with water flush drilling, and
to execute pre-saturation of the rock
mass, which had dried out rapidly
after the drawdown of the reservoir. It
was noted during pre-saturation that
pressures of 10 bar or more resulted
in dilation of fissures and increased
flows. Because of the sensitivity of
the structure, and its concrete facade,
it was decided to execute the new
grout curtain at the upstream toe of
the dam, and to restrict pressures with
grout to below 10 bars.
Because of the regular geometry of
the solution features, and evidence
of numerous sub-vertical fissures, a
three-row grout curtain was designed
with inclined boreholes to ensure that
all fissures and solution channels were
intersected within the thickness of the
grout curtain.
The GIN technique was used through-
out for the grouting, but with a
stripped down very simplified control
system with capacity for piloting 8
pumps.
The works were executed with fully
piloted pumps and the results dis-
played graphically in real-time and
updated continuously on the 2-D
Model as the works progressed. Final
equivalent Lugeon values we used
to control the progress of the works,
and will reduce progressively from in
excess of 30 on the primary holes to
below 2 Lugeon on completion of the
tertiary holes of the final row.
The outcome of the works was
extremely successful and efficient,
requiring an average of just 175 litre
per linear metre to complete the works
(Figure 11). A total of 647 m³ of grout
was injected over the course of the
injection program.
Grouted volumes were progressively
reduced, from in some cases over
1’000 litre per linear metre to below
40 litre per linear metre in the later
stages of the works. The higher value
absorptions were sometimes isolated,
but there was a concentration below
the spillway on the right abutment,
where numerous voids were encoun-
tered during drilling.
On completion of the works the reser-
voir was successfully impounded and
brought into service.
Clif Kettle
Bachy Soletanche Ltd.
Tom Dando Close
Normanton Industrial Estate
Normanton, UK WF6 1TP
E:
Maren Katterbach
Lombardi Consulting Engineers Ltd.,
Via R. Simen 19
C.P. 1535
Minusio, CH 1648
T: +41 91 735 3163
E:
Figure 11. Sketch of grout volumes.
