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Geotechnical News • December 2015
THE GROUT LINE
ance of bleed within the voids and
fissures of the formation.
Stable mix
Generally a stable mix is a grout con-
sisting of a cement-based slurry, with
additives if necessary, to ensure that
no water is expelled from the suspen-
sion when injected at pressure (i.e. no
pressure-filtration). The stability of the
grout ensures that
• the grout rheological properties
remain constant throughout the
injection to maintain the fluidity
and penetration capability
• the progressively reducing ab-
sorption of grout can be clearly ob-
served, understood, and measured,
as the works progress
• no water filled zones are left
Consistent rheological properties
ensure a realistic comparison of grout
injection data between subsequent
phases of injection, and during the
course of a single injection.
This is why the mix should not be
fluidified with excess water. Water
should be mainly considered as trans-
port medium for cement grains not as
physical component of the mix.
Current practice is to employ a grout
of low water cement ratio (typically
0.6 -1.1), so that once an individual
injection is completed, the potential
for bleed in-situ is minimised. It
also ensures long-term strength and
durability reducing the requirement for
successive re-injections.
Single mix
For successful and efficient grouting,
it is highly recommended to inject a
single grout type with a consistent
water/cement ratio for all injections
and all phases of the works. Combined
with the stability of the grout, a single
mix enables the accurate verification
and control of the increasing compe-
tence and water-tightness of the strata
with the grouting works progress.
Recognizing the importance of using
a single mix is one of the main aspects
where the GIN approach differs from
classical grouting practice of 30 years
ago. Traditionally, the w/c ratio was
lowered in steps (see Figure 4) to
increase the cohesion, and in this way
lower the normalized pressure, P/c.
The introduction of the GIN concept
can be said to present a turning point
away from this traditional approach of
thickening the mixes in steps.
For GIN, (as indicated by the blue line
in Figure 4), it is recommended to
• Use 1 unique stable mix throughout
the grouting works
• Limit the grouting pressure with
increasing volume take
• Reduce the normalized pressure
(P/c) by progressively decreasing
the pressure.
The use of a single, stable, grout mix
avoids many potential errors in mix
formulation and in the interpreta-
tion of the most relevant injection
data - the volume per linear meter
injected. In the past, much effort has
been expended in trying to accurately
convert injected volumes into a dry
weight of material per linear metre
- a pointless exercise in terms of the
specified objectives and technical
management of the works, and only of
interest for assessing payment.
Multiple mixes, changed during a
single injection according to certain
volumetric or pressure criteria, have
resulted in a flawed understanding of
the grout absorption due to the fact
that insufficient consideration was
taken of the distance over which the
grout has been pumped, and/or the
volume of grout in the system. There
have been sites where mixes have
been changed in a rigid succession,
when one of the mixes in the sequence
has been still wholly or partly within
the delivery system, without ever
reaching the point of injection. Con-
sequently, the basis for changing the
grout mix was flawed, and a calcula-
tion of the total dry weight of material
injected into a grout stage at the time
of refusal was incorrect, so that deci-
sions on subsequent injections were
based on a false premise and under-
standing.
The changing of mixes, in particular
the thinning or thickening of the grout
mix already in the system, is prone to
errors of mix formulation and prepara-
tion, whether manually or automati-
cally batched, and this has led to errors
in calculating the effects of varying
viscosity and head loss, the extent
of pressure filtration and sedimenta-
tion, and hence in understanding the
effective penetration of grout into the
formation.
However, the real advantage of a
single mix is that it is designed specifi-
cally for the rock conditions on site,
and particularly for the finer fissures
required to be injected to achieve the
specified residual permeability,
Another real and valuable advantage is
to enable a simple and direct compari-
son of injections from stage to stage,
hole to hole, and between successive
phases of grouting. This is invaluable
in understanding and visualising in
real-time the improving condition of
the rock mass and reduction in mass
permeability.
Figure 4. Mix and pressure evolution
-Traditional versus GIN grouting.
