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24
Geotechnical News
June 2013
www.geotechnicalnews.com
THE GROUT LINE
Paolo Gazzarrini
Overture
31
st
episode of the Grout Line and
summer is here at last! For this issue
a clarification from Dr. Donald Bruce
President of Geosystems, L.P., P.O.
Box 237, Venetia, PA 15367, U.S.A.,
Phone: 724-942-0570, Fax: 724-942-
1911,
.
The clarification is related to the
“refusal and closure” of grouting in a
stage, during grouting in rock, using
ALT grouting as the guiding philoso-
phy.
I hope you will find this brief article
interesting, as I did.
Refusal and closure in rock grouting:
Let’s get it right!
Dr. Donald A. Bruce
As recently documented in this
column, and on the wider stage of
international conferences, rock grout-
ing theory and practice in the U.S.
has largely evolved towards “Appar-
ent Lugeon Theory” (“ALT”) as its
guiding philosophy. This statement
of fact does not ignore the reality that
other approaches — most obviously
GIN Theory, as formerly discussed
herein— are being applied, and to
very good effect in appropriate condi-
tions. Focusing on ALT, however, it is
clear that not all its practitioners have
fully appreciated some of its subtle but
very important nuances.
The rock grouting industry has for
long been comfortable with Lugeon
Testing as a method for demonstrat-
ing the in-situ permeability of rock
masses. This comfort level, despite
the somewhat involuted, but entirely
logical units of the measure (liters
per minute per meter at an excess
pressure of 10 bars), was elevated by
the publication of Clive Houlsby’s
seminal 1976 paper entitled, “Routine
Interpretation of the Lugeon Water-
Test.” This paper was a Godsend to
junior engineers who previously had
to navigate through pages of manuals
to determine corrections to perme-
ability test results, reflecting flow
rates, pipe diameters, bends, estimated
elevations of groundwater, and so on.
Houlsby gave the grouting world an
approach that was admittedly simpli-
fied (“routine”), but arguably robust
and acceptably accurate.
With the advent of the use of stable
grouts (stable not only in bleed but,
more critically, in pressure filtration
resistance), ALT evolved in the mid-
1990’s. Credit is due to the late Alex
Naudts, building on blocks laid down
by DePaoli, et al. in 1992. In essence,
such stable grouts do not experience
significant changes in rheology as a
result of loss of water during injection.
Although, of course, continuation of
the hydration reactions will effect sig-
nificant change after periods of several
hours, or so. Therefore, the injection
of grout itself constitutes a perme-
ability test, of sorts, since the basics
of calculation (i.e., flow rate divided
by effective pressure divided by stage
length) remain in play. These basics
are then corrected by a factor, namely
the ratio of the Marsh Cone Value of
grout to that for water.
When bringing a stage to proper
refusal, one wishes to see a progres-
sive reduction in grout flow rate at
constant pressure, i.e., a gradual
reduction in the Apparent Lugeon
Value. This is monitored on the
computer screen in contemporary
grouting practice, and has been well
described in many technical papers. A
zero Apparent Lugeon is synonymous
with zero grout take, i.e., complete
grout refusal. However, zero take is
rarely achievable in typical fissured
rock conditions, and its relentless
unthinking pursuit most often involves
lengthy periods of excruciatingly slow
injection rates with all the associated
mechanical, human and economic
frustrations.
This is the hub of the argument and
the cause of misunderstanding in
certain circles. Whereas the progres-