36
Geotechnical News • June 2016
THE GROUT LINE
ment for a cut-off approaching 9
m thick, and the specified borehole
geometry - which included an obliga-
tory tertiary phase to reduce borehole
spacing is down to just 1.5 m, with
provision for quaternary holes to
achieve the final closure. The value
selected, and modified after the full
scale site trial, was a GIN value of 770
per m.
All injections were carried out with
piloted pumps and real time graphical
plotting of key injection parameters,
including the Equivalent Lugeon
value, against both flow and time.
Although a high value had been
selected on the basis of the above cri-
teria, the final average grout take was
just 311 litre per linear metre for the
trial panel (in heavily fractured zone),
and 121 litre per linear metre the
curtain as a whole. This demonstrates
very clearly the self-regulating nature
of the GIN process, and its efficiency,
limiting flow rates and pressures in
response to actual ground condi-
tions. No quaternary boreholes were
required.
Following completion of the curtain
grouting, an extensive programme of
permeability testing was carried out
by an independent contractor using
sophisticated instrumented double
packers equipped with multiple pres-
sure transducers.
The table on the previous page
confirms that residual permeabili-
ties recorded within the plane of the
curtain were of the order of 10
-8
m/s to
10
-9
m/s, with one value of 1.5 x 10
-7
m/s. grout dispersal was controlled
with no evidence of wash-out towards
the river.
Table 1 and the seepage graph shown
in Figure 7 indicate the effectiveness
of the 2003 and 2014 grouting opera-
tions in rapidly reducing the flows
through the curtain, and the residual
flows reduced to a historic low for the
dam, and the distribution of grout plot-
ted graphically, and updated automati-
cally on a daily basis.
Case History 4 - Symvoulos
Dam, Cyprus, 1999
Symvoulos dam is located on the Brit-
ish Army complex at their main base
on Cyprus. Remedial works at the dam
were executed in 1999 to try to bring
the reservoir to full serviceability. It
had previously proven impossible to
impound the reservoir over a period
of 20 years due to significant seepage
through fissures and discrete solution
channels in the soft limestone bedrock.
Figure 8. Wimbleball Dam - Graphical plot of final grouting pressures.
Figure 9. Symvoulos Dam on soft limestone bedrock.
Figure 10. GIN curve & grouting principles for Symvoulos Dam grouting.
