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Geotechnical News
June 2013
25
THE GROUT LINE
sive reduction in the Apparent Lugeon
Value is a most wondrous and desir-
able thing to see when monitoring
the grouting of a particular stage, an
absolute target Apparent Lugeon Value
criterion is neither an appropriate
target for stage refusal, nor the quanti-
fication of the residual permeability of
the curtain.
Let’s assume that a misguided fellow
has established an Apparent Lugeon
Value of 5 as a criterion for every stage
refusal regardless of depth. So, for a
higher stage wherein the refusal pres-
sure is say 5 bars and the stage length
is 6 m, the rate of grout flow, at refusal,
must be of the order of 15 l/min. But,
for a deeper 6 m stage, wherefore the
refusal pressure is 10 bars, the rate
of grout flow at the same Apparent
Lugeon Value of 5 is of the order of 30
l/min. Thus, the lower stage’s injection
is terminated at a higher rate of flow
than the upper, meaning that it has
been less effectively and comprehen-
sively treated.
The point is that whereas ALT is a
very useful and expressive method for
helping to correctly bring a stage to
refusal, an Apparent Lugeon Value is
not a logical criterion for determining
stage refusal. The Stage refusal crite-
rion must be constant, regardless of
depth, and should be an absolute value,
reflecting constancy of treatment. Inci-
dentally, a refusal criterion of 1 l/min.
(over 5 minutes at maximum pressure)
is regarded as very “tight,” whereas a
more liberal criterion of 4 l/min. (over
5 minutes at maximum pressure) may
be regarded as acceptable on a project-
specific basis.
The second point is that, while ALT
aids in bringing a stage to a “good”
refusal, it is not a measure of the resid-
ual permeability of the treated rock
mass. This residual permeability must
be measured in dedicated Verification
Holes, preferably by multistep Lugeon
Tests, using water as the test medium.
Casual grouters may regard this article
as little more than splitting hairs or,
worse, as sophistry. No, my friends,
this issue is fundamental, and would
not have been raised unless there were
a crying need to do so.
References
Houlsby, A.C. (1976) “Routine
Interpretation of the Lugeon
Water-Test.” Quarterly Journal of
Engineering Geology. 9, 303-313.
De Paoli, B., Bosco, B., Granata, R. &
Bruce, D.A. (1992) “Fundamental
Observations on Cement Based
Grouts (1) Traditional Materials (2)
Microfine Grouts and the Cemill
Process.” Proc. ASCE Conf. on
Grouting, Soil Improvement and
Geosynthetics. (eds. Borden, Holtz
& Juran). Vol. 1, 474-499.
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,
.
For grouting stories, case histories or
only to comment, you can write to me
@ Paolo Gazzarrini,
,
or
.
Ciao! Cheers!