Geotechnical News • March 2016
41
THE GROUT LINE
GIN Method (II) Case Histories
Clif Kettle & Maren Katterbach
Case History 1 - Carno Dam, South Wales, 2009
Carno dam project involved the
stabilisation of an extremely sensitive
embankment dam which had exhib-
ited significant seepage through the
main core. The dam was potentially
unstable, posing a high risk to public
safety and the dam reservoir had to
be completely drawn down allow
excess water to drain from within the
core (see photos below). The works
involved the construction of a 44 m
deep slurry wall cut-off within the
original clay core extending over the
full length of the dam and into the clay
core contact trench.
Once the slurry wall had been com-
pleted, a twin row grout curtain was
constructed to heavily fractured rock
formation which was suspected of
having been mined for coal early in
the 20th century. The grout curtain
geometry was critical with respect to
its relational position to the core, the
new slurry wall, and the existing draw
off culvert. Drilling accuracy was
important and rock drilling was exe-
cuted using the Wassara water-pow-
ered DTH hammer. Drilling through
the body of the dam was executed by
means of open hole drilling with grout
flush to fill any localised voids as the
drilling progressed.
The Engineer specified a residual
permeability for the grouted cut-off of
3 Lugeons, to be verified by multiple
stage Lugeon testing on completion of
the works.
Design considerations for
grouting Ooerations
• weakened embankment - history of
seepages and sinkholes
• embankment cavitation leading to
settlement of dam crest
• suspected presence of old coal
workings in foundation bedrock
• steep and irregular rock head /
plinth profile on left abutment
• impossibility of achieving full
closure of slurry wall with plinth
& rock-head
• complex dam geometry requir-
ing 3-D design to avoid seepage
‘windows’
• unexpected rock profile required
20% lateral curtain extension up to
spillway
• the draw-down of reservoir limited
injection pressures due to risk of
hydro-fracture and heave.
• particular care was required to
close the contact with slurry wall
complexity & sensitivity of the
works led to the employment
of computer piloted injection
equipment with computer design,
control, analysis, and reporting of
the injection operations
• use of Wassara water powered DTH
hammers to avoid clogging of
fine fissures whilst ensuring high
production and accuracy of drilling
• computer controlled GIN grout-
ing selected for fissure treatment
in bedrock to maximise injection
control and minimise risk of high
pressures
• fine fissures and tight specifica-
tion led to the selection of C3S
- a highly penetrating stabilised
cement-bentonite slurry used to
ensure high grout mobility &
penetration
• difficult confined space work
conditions in the culvert led to a
re-design of the works around the
culvert to maximise the drilling
from the dam crest
The C3S mix is a fluidified bentonite-
cement slurry based on ordinary
Portland cement and a de-flocculated
bentonite slurry.
Figure 1 indicates the degree of seep-
age through the core and embankment,
Figure 1. Seepage evidence at the Carno dam.
