Basic HTML Version
40
Geotechnical News • December 2013
www.geotechnicalnews.com
THE GROUT LINE
5 meters). Tertiary holes, would be as
required, depending on the take of the
Secondary holes.
The main part of the Grouting Speci-
fication (
please remember, copied and
pasted from 50 year old Specs
) called
for:
à
à
Use of very unstable grout
mixes W/C= 4 and gradual
thickening to W/C=3, W/C=2
and finally W/C=1 (“
colored
water”
to “
dirty water
”).
à
à
Use of grouting pressures
based on the principle (rule of
thumb) of 1 psi/ft or 0.23 bar/
meter.
à
à
“Refusal” (
poor rock mass
that is obliged to refuse grout,
despite maybe being quite
thirsty!
) criteria 1 liter/minute/
meter.
à
à
Acceptance criteria
< 2 Lugeon.
The D&G contractor started the job on
time, following the Specs, and after
completing a few additional Tertiary
holes, in only one month (earlier
than expected, WOW!), completed
the grouting job. To verify the grout
curtain, the Engineer instructed some
WPT that (oops!) provided terrible
results, with very high values of
Lugeon.
The poor Rock Mass felt very sad,
about having “refused” some grout
mix, and the Grout Curtain was very
upset because it was not able to satisfy
everyone’s expectations!
Panic on site was evident! The Owner
fighting with the Contractor, the Con-
tractor fighting with the Engineer, etc.
The risk was huge, considering that if
the dam was not completed on time,
before the window of the weather, the
production of energy would need to
be postponed by one year with great
losses for the Owner.
So the Contractor hired a Grouting
Consultant, the Engineer another
Grouting Consultant (both consultants
very happy!) and a new strategy was
defined.
It was decided, using the same criteria
of P, S and T, to:
Firstly, use STABLE mixes.
Secondly, increase the grouting pres-
sures.
And thirdly, use computers to monitor
in real time, and record the behavior of
the grouting.
A brand new row was constructed and
the result..... was astonishing!
The Lugeon value dropped to the
required level, the job was done in
three weeks -permitting the comple-
tion of the dam on time in the
remaining four months- with great sat-
isfaction to the Owner, Engineer and
Contractor. And more importantly, to
the Rock Mass and the Grout Curtain,
the two of whom felt very “tight” and
happy!
and they all (Rock Mass, Grout Cur-
tain and Dam)
...... lived happily ever after!
Merry Christmas with a happy end!
Morals of the Fairy Tale
• Never copy and paste yesterday’s
Specs to today’s job. Instead, let’s
try to stay with the state-of-the-art
of the Grouting industry.
• Always use stable mixes! It is
time to stop after the unsuccess-
ful results of the past decades to
continue in this dangerous direc-
tion. It is not admissible anymore
to say, “..but I did it that way all
my life!”.
• Engineers, please, specify comput-
ers, computers and computers for
monitoring and recording of the
grouting behavior. It is no longer
acceptable that in a grouting job on
a Dam site, computers for grouting
are not used.
• Why not increase the grouting
pressures? Also here I think it is
time to stop asking the poor rock
mass to “refuse” grout at 15 psi (1
bar!). How can the mix penetrate
small fissures if the pressure is not
enough? Engineers, please, don’t
specify the maximum pressure;
let’s let the rock mass, with its ex-
isting network of fissures, decide
the right pressure to be used! (Un-
derstand that the poor rock mass
also has a soul and desires!).
The pressure shall, of course, be
controlled with real time monitor-
ing to avoid hydro- jacking or
hydro-fracturing.
Rhetorical questions: if you have
granite to grout with no fissures,
what is the maximum grouting
pressure you can use at 20 meters
(60 ft)? 4 bars = 60 psi?
And with fissures?
The logic (see Dr. Lombardi and
Dr. Deer articles @Lombardi.
ch) behind this higher pressure
theory is that it is very improbable
to have hydro-jacking or hydro-
fracturing of the rock mass if there
is not a combination of high grout
take and high pressures.
I hope you enjoyed my brief fairy tale
and if you have some comments or
grouting stories or case histories, send
me your material to:
Paolo Gazzarrini,
E:
,
or
Ciao! Cheers!.. and a FABULOUS
2014 to everybody!