Geotechnical News • December 2015
41
THE GROUT LINE
formed more cautiously, by applying
a lower grouting intensity. Table 1, as
a rough indication, shows the relation-
ship between some common GIN val-
ues, the grouting intensity scale, and
in accordance with the above, gives
a direct correlation with the geome-
chanical rock mass quality.
Thus,
Grouting intensity number, GIN
~ Rock mass quality
It is worthwhile noting that, in contrast
to many other fields of engineering,
the design of a grouting job strongly
depends on the rock mass - a natural
medium which is not designed by
ourselves. As consequence, there is
always an unavoidable uncertainty in
the definition of the generic mechani-
cal or hydraulic parameters, and the
engineer must be aware of this vari-
ability when using those parameters as
basis for the grouting design.
It frequently occurs that the actual
rock mass conditions do not cor-
respond to the ones anticipated and
assumed in the initial design phase. If
this discrepancy becomes significant,
it might indicate the need to change
the grouting intensity according to
the new findings. Optimally, the GIN
value for any given rock formation
should be chosen at the beginning
of the design procedure, and kept
constant for each phase, or for the
whole, grouting programme. For some
sites the GIN value might require to
be adjusted after the initial results are
analysed, and possibly even reviewed
further as the grouting works progress.
However, any abrupt and frequent
changes are to be avoided in order to
keep the control and analysis of the
grouting as simple as possible. Occa-
sional modifications might be neces-
sary, but should be always based on
a rational basis to avoid the grouting
becoming confusing and obscure. It
is noted that test grouting sections on
the site into the actual rock mass allow
to significantly reduce any possible
changes of the grouting design to a
minimum.
Apart from geological aspects, the
general project requirements and
grouting objectives should be care-
fully considered when establishing the
GIN value. For many applications, it is
possible to assign priorities to certain
zones, which are then treated using
higher grouting intensities.
Thus,
Grouting intensity number, GIN
~ Project requirements
Considering a grout curtain, for
example, after impounding of the
reservoir, a lower water pressure is
to be expected in the higher abut-
ments than in the central part of the
dam. Consequently, a lower grouting
intensity might be acceptable at higher
locations. A similar allocation can be
made for the constraints related to the
hydraulic gradient imposed by the
project. The hydraulic gradient in the
rock zone to be treated will highest
at a shallow depth and diminishes
quite fast while depth reaching its
minimum in the lowest point of the
curtain. Accounting for the fact that
in this lowest part the real efficiency
of the curtain is by definition zero, the
requirements for the grouting intensity
might actually also be defined less
stringent in this lower zone.
In this way unnecessary grouting in
zones of minor importance can be
avoided, while the main effort can be
focused on the most relevant zones.
This helps to significantly optimize the
whole grouting process.
Accordingly, the GIN number itself
incorporates both geological and
project design aspects. The intensity
is therefore directly related to the rock
mass quality as well as the relevance
of the grouting result for the project.
Once selected, the GIN value controls
the injection parameters within a safe
working envelope. However, the GIN
value needs to also reflect the con-
straints of the practicable values for
the minimum flow rate and minimum
controllable pressure of the grout
pump ( typically 200-300 l/ hr, and
approximately 2 bars ).
For any given grout type, and injec-
tion rate, the evolution of the GIN
value over the duration of the injection
will depend upon the rock conditions,
the grout rheology, and the injection
rate. Once the plot of P x V reaches
the boundary curve, the injection flow
rate, controlled by computer piloted
grout pumps, is progressively reduced
or increased automatically to maintain
the product P x V at or just below the
GIN curve until either the maximum
target volume is injected, or until
the flow rate reduces to a minimum
practicable level, at which point the
injection is complete.
When establishing a GIN value it is
therefore also necessary to consider
particularly the likely flow rate dur-
ing the latter stages of the injection,
(approaching the target volume) to
ensure that this is compatible with the
minimum practicable flow rate for the
grout pump, and grout gelling proper-
ties, to avoid line blockage.
Table 1 GIN values with typically correlated geomechanical rock mass
quality ranges. Note: the indicated GIN values should be consistent with
the project requirements, and borehole location.
Intensity
GIN [bar.
litre/m]
RMR
RQD
Very high
> 2’500 81-100 very good 91-100 excellent
High
1’500 - 2’500 71-80 good
76-90 good
Moderate
1’000 - 1’500 41-70 fair - good 51-75 fair
Very low -
low
< 500 - 1’000 <40 very poor -
poor
<50 very poor -
poor
