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40
Geotechnical News • September 2013
www.geotechnicalnews.com
THE GROUT LINE
new flow rate equation and presented
in the next section. In practice, the
graphical representation is used to
monitor and control the grout advance.
This may explain why this practice is
often called the “Real Time Grouting
Control Method”.
Closure in the Swedish practice is
asserted when the grout advance
in the smallest fracture is above a
certain minimal value or in the largest
fracture before a maximal value. The
latter condition is always satisfied
and needs to be amended otherwise
grouting is stopped right from the
beginning. Anyhow, the real problem
with this practice is that maintaining a
constant pressure all along cannot be
guaranteed. In case of uncertainties,
NARC should be considered. Indeed,
the Swedish practice is predisposed to
incorporate NARC since the closure
criterion is based on the achievement
of a specified grout advance, at least,
in the smallest fractures.
Waiting time
The time versus advancement curves
when grouting at a constant pressure
are shown on Figure 2 using non-
dimensional variables. Every curve
is labelled with the ratio of the radius
of the injection hole to the span.
The non-dimensional time τ, current
advance ξ and radius ξ
r
are defined as:
P2
tc
2
η
=τ
S
d
=ξ
S
r
r
=ξ
A precise approximation that cap-
tures the main features of the grout
advancement is compared to the exact
advancement using a dot representa-
tion in Figure 2. The approximation is
The calculated time will be affected
by delays due uncertainties such as
unexpected cavities, hydro-jacking or
hydro-fracturing. Also, the pumping
material cannot always react instanta-
neously to maintain a constant pres-
sure. These delays will break up the
correspondence between the time and
grout advance. Therefore, the calcu-
lated time will be considered a waiting
time before expecting the grout front
to be in proximity to the target. At this
point NARC inequality comes into
play.
A practical example concerning
grouting at constant pressure is given
here. Two professionals are asked to
grout a fracture with a given mix. The
specified advance, fracture thickness,
injection hole radius and grout mix
properties are:
• D=12 m
• H=0.4 mm (thickness is 0.8 mm)
• r=30 mm
• c=5 Pa
• η=0.02 Pas
The excess pressure needs to be
greater than 0.15 MPa to be able to
grout further than the specified grout
advance. The first professional selects
a 0.18 MPa excess pressure and
obtains the following quantities:
• S=14.4 m
• δ=0.167
• ξ=0.833 for d=D
• ξ
r
=0.0021
• L=0.7 l/min
Figure 2. Time ratio versus advance ratio and radius ratio.
τ
ξ
ξ
ξ
ξ
ξ
ξ
ξ
ξ
= +
+
−
+
−
+
(
) (
)[
(
) ]
1 1 1
1
2
3 1 2 1
2
r
r
n
n