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Geotechnical News • September 2013
www.geotechnicalnews.com
THE GROUT LINE
Grouting Emancipation
Mohamed El Tani
Diversity is the new grouting order.
Actually, there is a general trend in
grouting practice for rock fractures
to move towards tailored and per-
sonalised methodologies. This fact is
shared by many professionals. López-
Molina [1] asserts from experiences
made in areas of varying geology that
“any methodology can equally be use-
ful as long as assessment and adjust-
ment” are made to attain the target.
The increasing experience in rock
fractures grouting, as well as the tech-
nological developments and advances
in grouting science favour such an
orientation. All the known grouting
schemes are questioned and recon-
sidered. Strangely enough, the North
American Refusal Criterion (NARC)
remains the ultimate phase of all the
known new or old, or personalised
grouting procedures and methodolo-
gies. Recent developments in grouting
science changed the status of NARC
from an empirical criterion to a reli-
able one, making it an essential tool
among the numerous means for grout-
ing enhancement. Two complementary
tools are the zero flow path (ZFP) for
hierarchical grouting and the waiting
time (WT). An introductory descrip-
tion of NARC, ZFP and WT will open
the way to discuss some grouting
models, which use them or which may
be improved, by incorporating them in
their schemes. No details are given on
the success or failure of these models.
When it is deemed necessary their fun-
daments are examined in the light of
the recent developments.
ACG practice
The “Aperture Control Grouting” is
a methodology that was presented by
Carter et al. [2] and Bonin et al. [3] at
the New Orleans 2012 conference. Its
slogan is: the best of GIN and the best
of Aussie! In practice, the designer
specifies the grout’s volume to be
injected depending on the Lugeon
value at that stage and the mixes to be
used on a feed-back monitoring basis.
Apparently this is not sufficient. At the
ultimate phase of the ACG decision
chart NARC comes into play to decide
over closure, i.e. with a flow rate limit
(per unit stage length and pressure)
that is arbitrarily fixed at 0.1 l/min/m/
bar for a 10 minute time period. The
arbitrariness in deciding on the flow
rate limit is pointed out in many Swed-
ish publications that qualify NARC as
an empirical criterion with no founda-
tion in physics ([4], [5] and [6]).
NARC
The “North American Refusal Crite-
rion” is known in Western Europe as
the “Minimal Flow Criterion”. It states
that grout injection is stopped when
the flow rate is smaller than a pre-defi-
nite flow rate limit at a given grouting
pressure for a definite time interval.
The purpose of the time interval is to
ascertain that the current flow rate is
stable and follows a real decreasing
trend, neither being accidental nor
volatile. It is a criterion that was used
by many generations of practitioners.
It still is and will probably continue to
be so for a long time.
Currently, the hefty criticism concern-
ing NARC does not apply anymore.
The flow rate limit was parameterized
on a physics basis by the author [7]
in 2012. Before elaborating on how
this limit is parameterised, some basic
properties of grouting and cement
based mixes are recalled.
Figure 1
gives a schematic representa-
tion of grout advancement in a planar
fracture. The mix rheology is defined
in terms of viscosity η and yield stress
c. The grout filled space is defined by
the diameter of the injection hole 2r
and the fracture thickness 2H. The cur-
rent grout advance is denoted d. The
specified advance D is the target to
where the practitioner should drive the
grout. The span S is the maximal dis-
tance that a grout may travel at a given
excess pressure P. The span is calcu-
lated using the following equation
Figure 1. Schematic representation of a cement mix advance in a planar
fracture.