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Geotechnical News •March 2015
GEOTECHNICAL INSTRUMENTATION NEWS
Introduction by John Dunnicliff, Editor
This is the 81st episode of GIN.
The importance of Step 2 in the
Systematic Approach to Plan-
ning Monitoring Programs
Those of you who are familiar with
my frequent sermons on planning (e.g.
red book Chapter 4) will be aware of
my Step 2: “Predict Mechanisms that
Control Behavior”.
The article by Francesca Bozzano
elaborates on and extends the impor-
tance of this by showing, with two
case history examples, that:
“A general inverse relationship
exists between the level of
understanding about the ongoing
geological/geotechnical process
and the complexity (and cost) of
an efficient monitoring system.
Said another way – the more we
understand the process, the less
is the complexity and cost of the
monitoring system.”
A very important message – we should
do intensive homework at the begin-
ning of the planning process!
More on fully-grouted
piezometers
The article by D’Hollander et al adds
to our confidence level for using the
fully-grouted method. Site specific
solutions were developed to address
the challenges of installing the
piezometers in a flowing stream with
continuous readings obtained in all
weather and stream conditions.
Having published several articles in
GIN on this subject during the past
13 years (see the next section), I’ll
now go on hold, and encourage you to
transfer your attention to interacting
with Gord McKenna, as in the next
section.
Fully-grouted piezometers. We
need your stories and insights.
Fully-grouted piezometers appeared
briefly on the stage in 1969 with Peter
Vaughan’s paper in Geotechnique, but
didn’t gain traction until much later.
Since then, the method of installing
diaphragm-type piezometer tips by
simply grouting them in (with no sand
pack) seems to have gained fairly
widespread popularity. The technique
has been supported by the following
key publications:
• McKenna, G.T., 1995, “Grouted-in
installation of piezometers in bore-
holes”. Canadian Geotechnical
Journal, Volume 32, pp.355-363.
• Mikkelsen, P.E., 2002, “Cement-
bentonite grout backfill for bore-
hole installations”. Geotechnical
News, December.
• Contreras, I.A., Grosser, A.T., Ver
Strate, R.H., 2008, “The use of the
fully-grouted method for piezom-
eter installation”. Geotechnical
News, June.
• Durham Geo Slope Indicator
(DGSI), 2009. “Grout Mixes for
Piezometers”.
-
dicator.com/support/piezometers/
technote-groutmix-piezometers.
php
• Contreras, I.A., Grosser, A.T., Ver-
Strate, R.H., 2012, “Update of the
fully-grouted method for piezom-
eter installation”. Geotechnical
News, June.
and again in this episode of GIN
(D’Hollander et al), but with a few
warnings. Some practitioners enjoy
the ease and speed of installation of
fully-grouted piezometers while others
choose conventional techniques every
time.
You now have a chance to share war
stories on how the method has been
working for YOU: your successes
and failures. There’s major evidence
of success in some parts of the world
(for example, in the West Coast of
USA, where it has become accepted
practice) but concerns remain. There is
field evidence of poor sealing, e.g.
• For those who have poor cement-
bentonite grout mixes, who add
bentonite to the water first instead
of cement, or who use a pre-deter-
mined quantity of bentonite rather
than adding enough to achieve
a consistency of thick cream or
pancake batter (details of how to
do this are in Mikkelsen (2002).
• The few who forget to add the
bentonite.
So, please send us your fully-grouted
piezometer stories:
• Your anecdotes, improvements,
failures, fears and insights.
• Vaughan made calculations to show
that the grout could be 2 orders
of magnitude
more
permeable
than the formation for a good
seal. Contreras et al (2008) did
numerical analysis to prove that
the grout could indeed be 3 orders
of magnitude
more
permeable to
seal effectively. Do you accept this
latter recommendation and use it
in your practice? Or do you favor
different permeability criteria?
• Are you using this method? If yes,
why? If not, why not?
• Do you place the filter up or down?
Proponents of “up” claim that
this prevents de-saturation during
installation.
• Do you surround the tip with a tiny
sand sock? Proponents claim that
this prevents grout from plugging
the filter.
• Are we ready to declare the fully-
grouted method as mainstream?
• And if so, subject to what provi-
sions?
Gord McKenna of BGC Engineer-
ing Inc., Vancouver has volunteered
