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Geotechnical News December 2011
GEOTECHNICAL INSTRUMENTATION NEWS
direction of the optical axis results in a
change of the image scale, and can also
be measured.
FMGM 2015
Looking back on a very successful
symposium in Berlin I am excited by
the prospect of the next one in four
years. The options for the organization
and city of the next FMGM symposium
were discussed, and offers were made
by Australia to organize it in Sydney
and from Brazil to organize it in their
country. Helmut Bock concluded the
discussion in stating that it will be
in Australia in four years. After that
Brazil is considered a favorable option
in eight years, but that will have to be
decided in Australia.
See you all in Sydney.
Ton Peters, Manager Urban Engineer-
ing, Deltares, PO box 177 2600 MH
Delft, The Netherlands,
email:
Evaluating Practices for Installation of
Vibrating Wire Piezometers
Garrett Bayrd
Introduction
The fully-grouted method of vibrating
wire piezometer installation has gained
wide acceptance. This method calls for
installing vibrating wire piezometers
(VWPs) directly in bentonite-cement
grout. The non-fully-grouted method
calls for installation in sand packs,
with bentonite above the sand pack,
and grout above the bentonite. In my
field experience, project managers
have instructed me to install VWPs
in canvas bags full of sand and then
grout the boring. I have also had field
experiences where clients still have
reluctance to the fully-grouted method,
and call for sand packs and bentonite.
In addition, manufacturers recommend
saturating the filter stone, and some
recommend inverting the VWP tip.
I undertook this research to evaluate
the necessity of a variety of installation
procedures, and check the accuracy
and response times of VWPs installed
in different mediums. My intent with
this study was to see if simpler instal-
lations could function as well as more
complicated ones.
Previous Research,
Publications and Practice
Diaphragm piezometers (both VWPs
and pneumatic piezometers) have
been in use for many decades. Early
installations of these piezometers
mimicked the installation procedure for
standpipe piezometers, or Casagrande
piezometers, using sand and bentonite.
Research during the late 1960s
presented and supported the hypothesis
that VWPs could be installed directly
into a bentonite-cement grout mixture.
Further research performed by
Mikkelsen (2002) and Contreras et al
(2008) have supported the hypothesis
that installations of VWPs into grout
function without error. Mikkelsen
(2002) provides grout strength and
permeability information for several
mixes of grout, and advocates for
installations of VWPs directly into a
bentonite-cement grout mix. Contreras
et al (2008) provide a theoretical model
for the ability of a VWP to function
in grout, test grout permeability, and
perform field tests of these installations.
This research was then followed by
a discussion by Dunnicliff (2008),
which supported these conclusions
with case histories of successful fully-
grouted VWP installations around
the world. Webber (2009) supports
the use of the fully-grouted method.
Additional information was presented
at the September 2011 Symposium on
Field Measurements in GeoMechanics
in Berlin, Germany by Contreras et
al (2011), and Simeoni et al (2011).
Note that I read these two papers after
completing my tests and a draft of
this article, and that there is general
agreement among us. Contreras et al
(2011) provide field and laboratory
examples of functional VWPs that are
installed directly into grout. They also
provide data from a laboratory test (of
a VWP installed in grout and tested in
a triaxial compression test chamber)
similar to the tests that I will discuss
in this article. Their laboratory test
of a VWP has results that agree with
those presented here. Simeoni et al
(2011) provide even more examples of
successful field installations of fully-
grouted VWPs, and examine pressure
responses through sections of grout. I
seek to expand on their work by testing
the accuracy and response time of
VWPs in various installation methods
(not just grout) in the laboratory.
Test Methods
I wanted to test both the accuracy
(instrument output versus the known
pressure applied to the bottom of the
test chamber), and the response times
(how long it took for the instrument to
record the change in applied pressure)
of various VWP installations.
In order to test different installation
methods, I salvaged an unused triaxial
compression test chamber. The interior
of the chamber was approximately 5.5
inches in diameter and 11 inches high.
For each test, I installed one VWP into
the chamber, varying the installation
method and surrounding material. The
VWP sensors were installed in vari-
