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Geotechnical News • September 2013
39
THE GROUT LINE
Attaining the span means attaining
a zero flow rate at that excess pres-
sure. The time the grout advance will
take before attaining the span is very
long and exceeds every imaginable
waiting time. Since the span cannot be
attained, it has to be greater than the
specified advance. If not, the target
will never be achieved. If there is a
pressure limitation, the correspond-
ing span should be greater than the
specified advance. A correct selection
of the grouting sequence and distances
between the injection holes will take
this fact into consideration.
The difference between the specified
advance and the span is the grouting
margin. Denoting δ the margin ratio,
then
S
DS
− =δ
The margin ratio is an important quan-
tity that enters the parameterised form
of the flow rate limit. This latter is
denoted L and is given by the follow-
ing equation
)
r
S 1ln(
P H
3
4 L
23
+
δ
η
π=
Based on energy considerations and
the properties of Bingham’s mate-
rial, it is proven that if the flow rate,
denoted Q, is smaller than the flow
rate limit then the relative difference
of the current grout advance to the
span is smaller than the margin ratio.
Abbreviating, for an advancing grout
with a span larger than the specified
advance, if Q<L then d>D.
The recurring question formulated
by Shuttle et al. [8] on “how small an
injection rate should be?” now has a
parametric response. There is no small
or large flow rate to stop grouting. The
closure requirement is that the flow
rate be smaller than the limit, which
depends on the selected parameters.
The refusal terminology is no more
satisfactory since no zero flow rate is
ever required. Therefore, no zero flow
rate should be expected. Furthermore,
there is no past or history limitation
to check the closure inequality. It is
independent of the anterior pressure or
flow rate.
ALT practice
The “Applied Lugeon Theory” is,
according to Bruce [9,10], a general-
ized US practice and a pillar of the
North American methodology. The
basic argument of ALT is that com-
plete refusal is synonymous to a zero
apparent Lugeon value. Since ALT
closure is a difficult objective to attain,
it is explained that when a stage is
brought to refusal, grout consump-
tion at target pressure is less than
0.1 gpm over a period of 5 minutes.
This explanation alludes to NARC
with an arbitrary grout consumption.
Practitioners are also warned against
a wrong interpretation of the grout
consumption. On one hand the grout
consumption is arbitrarily fixed and
on the other hand it can be misinter-
preted. The real problem with ALT is
that the target is a zero flow rate. This
means that the grout advance should
attain the span that corresponds to the
grouting pressure, which in ALT is the
maximal admissible pressure at that
stage. This is impossible and the target
cannot be attained whatever the stage
or the maximal pressure . Bruce con-
firms this fact based on his personal
experience, noting that “relentless
unthinking pursuit (to attain a zero
flow rate) involves lengthy periods
of(...)frustrations”. Hence, ALT needs
to be modified and completed, by
upgrading it into NARC, for instance.
GIN practice
GIN is a popular grouting model with
a simple modus operandi [11,12]
The development in grouting experi-
mentation and simulation revealed
difficulties in attaining the GIN target.
This problem is discussed in length
by Rombough et al. [13], Shuttle et
al. [8] and El Tani [7,14]. It is now
well established that: 1- There are as
many GIN targets as the number of
fractures with different thicknesses
and 2- None of the targets can be
attained without infringing the GIN
rules. The second point stems from the
fact that GIN is a constrained model:
the specified advance and the span are
equals independently of the fractures.
This clarification is useful to turn the
second point around without going
into complex details or elaborating
sophisticated grouting schemes: when
a target is identified, grout injection
is stopped before attaining the span
within a given margin. This implies
modifying the GIN rules and proce-
dure by incorporating NARC. The
span and the specified grout advance
needs to be separated to create a mar-
gin. More information are required to
proceed than was before. As well, it is
necessary to know the number of frac-
tures and their thickness in advance to
identify the different targets. With this,
GIN “lovers” can continue to use it
safely and efficiently.
Swedish practice
The time evolution is a basic element
of the “Swedish Practice” that includes
in its jargon new terms such as the
characteristic time and dimensional-
ity [4,5,6,15,16]. But its fundament,
which is grouting at a constant excess
pressure, is generally not formally
dealt with or ignored. The Swedish
practice was promoted by Gustafson
and Stille [4] after Gustafson and
Claesson [15] had deduced a graphi-
cal representation of the advancement
versus time using non-dimensional
variables. Since the applicability of
this practice is limited to a constant
excess pressure, Kobaysahi and Stille
[5] studied a possible extension to a
succession of pressure steps.
Gustafson and Claesson used the
extended flow rate equation of Dai and
Bird [17] to plot the graphical repre-
sentation of the time evolution. A new
radial flow rate equation that satisfies
the energy balance is currently avail-
able [7,14]. An update of the graphical
representation is obtained using the
S PH
c
=