24
Geotechnical News • June 2012
GEOTECHNICAL INSTRUMENTATION NEWS
method because the piezometer tips
are “sealed.”
We have found that barometric pres-
sure corrections are needed when the
piezometers are installed using the
fully-grouted method. The piezometer
tips are not “sealed” from atmospheric
pressure. The following discussion
illustrates the need for barometric
pressure corrections. We installed two
adjacent piezometers, one an open
standpipe and the other a vibrating
wire piezometer installed using the
fully-grouted method. The vibrating
wire tip and the porous stone of the
standpipe installation were within
the same soil stratum and at approxi-
mately the same elevation. A barome-
ter was installed separately to monitor
the atmospheric pressure. All instru-
ments were connected to a datalogger
programmed to take readings every
half hour.
Figure 5 illustrates the total head mea-
sured in the vibrating wire piezometer
without a barometric pressure correc-
tion, the barometric pressure, the total
head from the standpipe piezometer,
and the corrected total head after
barometric pressure correction over
time. The influence of the barometric
pressure in the uncorrected data is
apparent. It can be seen from Figure
5 that the change in total head in the
uncorrected data mimics the changes
in the barometric pressure recorded by
the barometer. The changes in baro-
metric pressure on the order of 2 kPa
are reflected in a total head change of
about 20 cm. After the uncorrected
data are corrected by the barometric
pressure correction, the total head is
smoothed out and the changes in total
head are only on the order of a few
centimeters.
Figure 5 also illustrates the compari-
son of the corrected total head from
the vibrating wire piezometer and the
total head from the standpipe piezom-
eter over the same time period. The
comparison of both values (standpipe
and vibrating wire) is remarkable. This
example illustrates the need for cor-
recting the vibrating wire readings for
barometric pressure.
Summary and conclusions
The fully-grouted 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. However, adequate installation
procedures (including grout mixing)
and appropriate permeability of the
cement-bentonite grout are crucial for
the success of the method.
This article discusses laboratory and
field experiences for response time of
vibrating wire piezometers installed
using the fully-grouted method. It is
shown that the hydrodynamic time lag
of piezometers is very short.
Additionally, the article presents a
discussion of the permeability required
for the fully-grouted method to func-
tion properly. It is found that the
permeability of the grout can be up to
three orders of magnitude higher than
the permeability of the surrounding
soil without inducing significant error
in the measured pore water pressure.
This fact is further verified by pre-
senting field evidence of installations
where the measured error is not sig-
nificant for a permeability ratio k
grout
/
k
soil
of up to three orders of magnitude.
Data presented in this article show
that the behavior of the fully-grouted
installation (using a sacrificial grout
pipe) in soft ground is adequate when
the amount of vertical compression
is less than 15 percent. In projects
involving installation in soft ground
where the vertical compression is
expected to be greater than 15 percent,
the sacrificial grout pipe should be
removed. We recommend attaching
the piezometers to plastic-covered
stranded wire (“aircraft cable”) which
would accommodate the compression,
to use a more compressible grout mix
and to extract the grout pipe.
Finally, the impact of barometric
pressure on the measured pore water
pressure is discussed. It is found that
a barometric pressure correction is
required for vibrating wire piezom-
eters installed using the fully-grouted
method.
Figure 5. Comparison between measured total head in open standpipe and
vibrating wire piezometer using the fully-grouted method, with and without
barometric pressure correction.
