Geotechnical News • December 2015
45
THE GROUT LINE
Further, providing care is taken with
the mix design to control the evolu-
tion of the mix viscosity, the gel
time, and the setting time, so that the
mix remains rheologically consistent
throughout the injection, the injection
can be used as a surrogate hydraulic
or packer test. Real-time plotting of
the Equivalent Lugeon can indicate
visually the increasing ‘tightness‘ and
reducing permeability of the forma-
tion as the injection proceeds. Field
experience has shown this value - the
misnamed Equivalent Lugeon - to be a
remarkably good and consistent indi-
cator of the true residual permeability,
expressed in Equivalent Lugeons.
In summary, a carefully designed
single mix greatly facilitates the work
of the grouting engineer and the opera-
tives in the field, has real technical
advantages, and provides an accurate
and reliable basis for comparison of
grout absorptions between different
injections stages and different bore-
holes, and between successive phases
of grouting.
Use of multiple mixes, including
accelerator and/ or gelling agent
When employing the GIN grouting,
the flow rate is automatically con-
trolled to ensure that the function P x
V remains within the boundary curve.
It follows that towards the end of a
given injection, the injection rate may
be approaching the limit of the pump,
i.e. approximately 180 L per hour.
Considering for example a grout
curtain. Due to its geometry and the
need to keep a constant length for the
grout injection line to ensure constant
head loss at a given flow rate, the
total volume of grout in the injection
system might be as high as 450 L (150
L in the grout line, 250 L in the grout
agitation tank, and 50 L in the grout
Packer and stage). Clearly, if the new
mix is introduced into the system,
whether with or without an accelera-
tor, it could take up to 2 hours for this
mix to arrive at the point of injection,
particularly as flow rates are progres-
sively reduced.
This suggests that the use of an
accelerated mix, where the accelerator
is added at the mixing station, is not
compatible with the GIN idea when
following the standard GIN procedure,
as this could lead to premature sealing
of the borehole before the required
volume is injected. Therefore, acceler-
ated mixes might only be applicable
when either:
• a pre-injection stage water test
indicates an exceptionally high
Lugeon value
• there is a high hydraulic gradient
across the injection zone, with risk
of grout dissipation
• grout is being freely absorbed with
minimal pressure increase at the
point where the target volume has
been injected
at which point a decision could be
made to introduce an accelerated mix
for a single one-off, non-GIN injection
to deal with a significant local feature
such as a major fissure or preferred
seepage path. Whether an accelerator
is added for a single on-off injec-
tion, or used systematically in poor or
voided ground, the accelerator should
be added at the point of injection, via
the packer, using a separate supply
line for the additive, an in-line mixer,
and with a variable flow or proportion-
ing pump to adjust the flow according
to the rate of injection to maintain the
correct additive proportion in the mix.
The same considerations should be
made to changing the grout mix at any
point within a GIN injection, since as
the injection progresses, and the flow
rate gradually reduces, it is highly
likely that the new grout mix could
still be advancing within the injec-
tion lines at the time that the injec-
tion is nearing completion. We would
strongly recommend therefore the use
of a single grout mix throughout any
GIN injection, and wherever possible,
the use of a single grout mix through-
out the whole injection program for a
given phase of the works.
Grouting procedure
Tracking the GIN boundary curve
Injection of an individual stage pro-
ceeds on the basis of pre-set injection
rates, until the value of P x V reaches
the limit of the boundary envelope
defined by the GIN value. Once the
product of P x V reaches the boundary
envelope, it is necessary to progres-
sively reduce the flow rate as the
cumulative volume increases, in such
a manner that the product of P x V
remains constant at or just below the
limiting GIN value. This operation
could be, and has been in the past,
carried out manually - but this might
be extremely difficult. Current best
practice is to employ piloted grout
pumps which have the facility to be
controlled by computer at all stages
of an injection, utilising continuous
real-time feedback of data on the pres-
sure, cumulative volume, and flow rate
to the grouting computer, in such a
manner that in real time the computer
can respond to the incoming data and
can automatically slow down the rate
of pumping to allow P x V to track the
GIN curve until one of several criteria
are reached.
These are
• maximum pressure - no further in-
jection is possible without exceed-
ing the allowable pressure
• maximum volume - the cumula-
tive volume of grout injected has
reached the target limit for the
borehole / injection
• minimum flow rate - this is a condi-
tion where in order to maintain
the plot of P x V coincident with
the boundary curve of the GIN
envelope, the injection rate falls
to a level which is impractical,
un-desirable on economic grounds,
or poses considerable risk of
blockage of the grout pump and/or
injection lines.
Consistent
injection rate
There is no inherent advantage, techni-
cally or commercially, to either client
or contractor in injecting grout slowly.
