20
Geotechnical News • June 2012
GEOTECHNICAL INSTRUMENTATION NEWS
follow the same general format but
with significant updating, including
remote methods for measuring defor-
mation. Information will be posted on
the same website in late summer this
year.
Closure
Please send contributions to this
column, or an abstract of an article for
GIN, to me as an e-mail attachment in
MSWord, to
co.uk, or by mail: Little Leat, Whis-
selwell, Bovey Tracey, Devon TQ13
9LA, England. Tel. +44-1626-832919.
Sveiketa (Lithuania).
Update of the fully-grouted method for piezometer installation
Iván A. Contreras, Aaron T. Grosser, Richard H. Ver Strate
[The same authors wrote a two-part
article about the fully-grouted method
for piezometer installation for June
2008 GIN (Contreras et al., 2008),
including a description of the method,
grout permeability requirements, a
laboratory testing program and field
examples, followed by my discussion.
Readers are encouraged to read this
article and discussion for background
to the current update. They are on
-
mentation_news.php. JD, Ed.]
Introduction
The fully-grouted method for piezom-
eter installation consists of installing
vibrating wire piezometers in bore-
holes directly surrounded by cement-
bentonite grout. The method is gaining
popularity within the geotechnical
community because it is a simple,
economical, and accurate procedure
to monitor pore water pressure in
the field. The method allows for
easy installation of single or nested
piezometer configurations and can
also be used in combination with other
instrumentation. However, appropriate
permeability of the cement-bentonite
grout is crucial for the success of the
fully-grouted method.
As the method becomes more popular
and is used more extensively in prac-
tice, several questions and concerns
have arisen on its application in the
field. These questions and concerns
relate to the response time, the behav-
ior of the fully-grouted installation in
soft ground, field verification of the
relative permeability of the cement-
bentonite grout with respect to that of
the soil, and the impact of barometric
pressure on measured pore water pres-
sures. These concerns are addressed in
this article. The article is based Con-
treras et al. (2011) and is published
in GIN with permission from the 8
th
FMGM Organizing Committee.
Response time
One of the main advantages of vibrat-
ing wire piezometers is the short
hydrodynamic time lag, i.e. changes
of pore water pressures in the soil are
measured fairly quickly. To evaluate
the response time of vibrating wire
piezometers in fully-grouted instal-
lations and for further validation of
the method, we evaluated the time
response in the laboratory and in the
field.
Laboratory
To evaluate the response time a
response test was performed in the
laboratory. The test consisted of plac-
ing a vibrating wire piezometer within
a grout specimen and letting it cure for
28 days. The cement-bentonite grout
mix consisted of a water-cement-
bentonite ratio of 1:2.5:0.3 by weight.
The specimen was formed by using
a cylindrical mold with a diameter
of 100 mm and height of 200 mm.
In addition to the specimen with the
piezometer tip inserted, four identical
cylindrical specimens were prepared
for permeability and strength testing.
After the grout specimen containing
the piezometer tip was cured, it was
set up in a triaxial cell. An opening
provided with an O-ring seal was
built at the top of the cell to pull the
piezometer cable out while maintain-
ing a watertight cell. The cell was then
filled with water and the cell pressure
was applied. The applied cell pres-
sure and the pore water pressure in the
piezometer tip were measured inde-
pendently and simultaneously during
application of cell pressure.
Figure 1 shows the results of the
response test. For the plot at the
left, the cell pressure was increased
incrementally in three steps. For the
plot at the right, the cell pressure was
increased in a single increment. As
can be seen in Figure 1, in both tests
the elapsed time for the piezometer to
read the correct value is generally 2
minutes or less. This elapsed time for
actual field applications can be con-
sidered instantaneous. Mikkelsen and
Green (2003) presented similar results
of response tests.
Field
The time response of the fully-grouted
method was also evaluated in the field.
The following field example consists
of a comparison of the time response
