Geotechnical News September 2011
27
THE GROUTLINE
should be reduced and adapted to
the actual conditions of the rock
mass.
• By the way, it is strange to have to
take notice that the GIN is criti-
cized by Dr. D. Bruce because of
volume limits considered to be too
low and at same time because the
method allows one to overpass the
same limits due to the fact that they
are considered to be a point of deci-
sion not an absolute limit. The GIN
method has also been blamed for
having adapted, at some date, the
interpretation of the limit for the
maximum take. Maybe, this was
just an improvement of the method
in order to remain “new”! In any
case only the “last version” of the
method needs to be discussed.
• All this is independent of the fact
that the three limits are to be de-
fined by the designer on the basis
of preliminary grouting tests and
not “specified” a priori.
• It has to be noted that in many cases
a wrong use of the method was car-
ried out, leading to poor results.
This fact apparently authorized a
number of authors to accuse the
GIN method of not working prop-
erly and obviously at same time to
excuse the engineer who did not
understand it. A typical, many times
repeated case, is the one of karstic
rock. It should be finally clear to
everyone that the method is de-
signed to be used in “solid, fissured
rock masses”, not in karstic nor in
too weak rock or loose ground.
Old Concepts Still in Use in the
“New” Methods
A number of very old concepts are still
in use in the so-called “new” methods
and, in the opinion of the writer, should
finally be changed:
• The first concept is the expression
“refusal”. In the way it is gener-
ally used, it suggests that the rock
mass would “refuse” any additional
grouting. Indeed it is the designer
who refuses to use higher pres-
sures, or possibly the pump that
does so! Never the fissured rock.
Therefore, a different word should
be used to describe the fact that
the specified, designed, or arbi-
trarily selected pressure value was
reached while the flow rate is nil.
• A second old habit should also be
updated. To stop the grouting pro-
cedure it is usual to maintain the
prescribed pressure for a certain
duration until the flow rate falls be-
low an “arbitrarily” defined value.
By experience it is much more effi-
cient to overpass slightly the given
pressure by - let’s say, 5% or 10%
- to stop the pump and to observe
the falling of the pressure at flow
rate nil. According to the pressure
arrived at after a short duration, the
operation can be stopped or contin-
ued. Obviously, a certain tolerance
of a few percentage points on the
final pressure should be allowed. In
many cases the procedure appears
to be quite simple and effectively
time saving. At least the two ways
of defining the ending of the grout-
ing should be accepted.
• It is felt also that in a number of cas-
es some attention should be paid to
the grain size of the cement.
Also the concept, about one centu-
ry old, of having to limit the grouting
pressure to the weight of the overbur-
den should finally be abandoned be-
cause, while a limited heave can gener-
ally not be avoided, depending on the
conditions, pressures many times high-
er (up to 3 times) can be used at no risk
of substantially heaving the ground.
All of this is just to recall that the
newly used concepts are not always as
“new” as claimed or as they should be.
General Comments on Grout
Curtains
It has to be taken into account that
with the depth below the ground the
conditions are changing due to the
increased stresses in the rock mass; so
the permeability generally decreases.
Also due to hydraulic considerations,
the requirements to the grout curtain
are decreasing with the depth because
of the longer paths for the water to
flow from upstream to downstream.
Therefore one should not prescribe as
a general rule a number of rows, but
instead, adapt its number to the depth.
Additionally, the constant length of
the grouting stages generally chosen
for all boreholes of the curtain (e.g. 5
m), is the result of an “old concept”
which can be improved by increasing
the length of the stages with the depth.
(No indication or examples are given
here in this respect in order to avoid
that they would be understood as firm
numerical “recommendation”!)
This is a “new” concept not yet consid-
ered in the “new” methods, except in
the GIN one.
The opinion that the depth of the
curtain should not be fixed a priori but
adapted to the geotechnical conditions
is entirely shared by the writer.
The actual permeability across a
grout curtain is always quite difficult
to be measured and defined with preci-
sion. The relatively high pressure used
by the Lugeon tests (not to be compared
directly with the grouting pressures but
with the reduced grout pressures at
some distance from the borehole) may
cause some damages to the curtain.
The question thus arises whether the
decreasing of the takes from borehole
to borehole and to the control holes
should not be used as a criteria to bet-
ter define the results of the grouting
curtain instead of using water pressure
tests with possible damages.
Additionally, it has to be questioned
whether in many cases a distinction
and a difference must still be made be-
tween “consolidation” and “grout cur-
tain grouting” or whether the definition
at a unique, comprehensive treatment
zone should not be preferred.
Conclusions
From the above comments it results
that a number of improvements in
grouting rock masses are still possible
in matters of “new concepts” not yet
considered in the “new” methods.
In any case it appears quite clear-
ly that, for the time being, the GIN
method is by far not outdated and that
it can in no way be presented as retro-
gressive. This could obviously happen
in the future, should radical improve-
ments of any kind take place in the field
of grouting.
