Geotechnical News • June 2016
33
THE GROUT LINE
size and area of the base slab, com-
pared to the relatively light weight the
concrete face and buttresses, has led
to the dam being extremely sensitive
to drainage through the curtain and
resulting hydraulic uplift. In addition,
many fissures have been identified as
having a clay infill, and there has been
evidence over the years of progressive
washout of this clay infill, evidenced
by clay and sand sedimentation within
the under drainage system, leading to
fears that the foundation seepages may
increase in the medium to long-term
and undermine dam stability.
The dam abutment and foundations
were also significantly affected by
the development of significant stress
release fractures during construction,
and these, combined with the multiple
natural joint sets have led to a structur-
ally complex foundation rock which is
highly fissured with permeabilities of
up to 3,000 Lugeons.
Remedial works, 2003 season
It was fortunate that during the sum-
mer of 2003 a highly targeted injection
program was carried out to address
a preferred seepage path identified
via drainage and sedimentation in the
under drainage system. This limited
program included just 9 no. boreholes,
and injection used the C3S stabilised
bentonite cement grout for permeation
of fine fissures, placed using the GIN
technique.
As a precursor to this work a very
detailed 3-D model of the dam founda-
tions and the excavation profile was
prepared using historic records from
the date of the dam construction (see
Figure 2). This 3-D model proved to
be invaluable both for the 2003 season
works and also for the final remedia-
tion program which was executed dur-
ing 2014. This model was used both
for the 3-D design of all subsequent
borehole geometry, and for the pre-
sentation and analysis of the graphical
data from the injection works.
The execution of this limited amount
of works provided extremely valu-
able information which was able to
provide a basis for the design of the
2014 remedial works. The injection
program:
a. identified the presence of voids and
major fractures within the imme-
diate dam foundations below the
base slab
b. highlighted hydraulic connections
over distances of up to 32 m be-
tween individual boreholes
c. provided an average grout absorp-
tion per linear metre of borehole,
and
d. verified the geometry of the rock
profile and dam foundation.
Despite a very limited program, the
2003 works achieved three major
objectives
a. it reduced total seepages through
the relief wells by 44% in the tar-
get area, dropping the total flows
sufficiently below the critical level
to allow the client several years to
develop a comprehensive and final
solution
b. it arrested completely the progres-
sive increase in erosion within
the fissure network, so that the
reduced flows stabilised, and re-
mained constant, over the succeed-
ing 11 years
c. it verified that a permanent solution
was achievable with stable residual
flows, using fluidified mixes and
GIN technology
Figure 3 shows the trend of increas-
ing seepage up to the 2003 grouting
works, and stable seepage thereafter
through the South abutment, with
flows diverting around the curtain
into the backfill. The graphical plots
in Figure 4 indicate high absorptions
close to the dam foundations.
Figure 2. 3D model of Wimbleball Dam foundation based on historic
records.
Figure 3. Seepage development before and after the grouting works in 2003.
