Geotechnical News • June 2012
23
GEOTECHNICAL INSTRUMENTATION NEWS
the range of 1.3 to 1.7 m took place
and the initial 3 m lift was placed
above the frozen ground. During and
after construction the vibrating wire
piezometers functioned without any
problems with total settlements of
up to 2.87 m, which corresponds to a
vertical compression of about 11 %.
Table 2 summarizes other sites where
we have used the same approach with
satisfactory performance. It can be
seen from Table 2 that the sacrificial
grout pipe can be used in soft deposits
in which the expected vertical com-
pression is up to about 11 percent.
However, based on our experience at
other locations not included in Table 2,
the sacrificial grout pipe can be used
when the vertical compression of the
soft deposit is up to about 15 percent.
We recognize that, because of the
two concerns identified above, this
conclusion cannot yet be extrapolated
to projects where the predicted verti-
cal compression is greater than 15
percent
.
An option for this application
is to attach the piezometers to plastic-
covered stranded wire (“aircraft
cable”) which would accommodate the
compression, to use a more compress-
ible grout mix and to extract the grout
pipe. The lack of a sacrificial grout
pipe would mean that an alternative
method for tracking the changing ele-
vations of the piezometers is needed,
so that piezometric elevations can be
determined. This can be achieved by
installing a magnet reed switch probe
extensometer nearby.
Barometric pressure correction
For fully-grouted vibrating wire
piezometers, changes in atmospheric
pressure can affect the measured
pressures. Manufacturers generally
provide the correction as a function of
elevation above sea level to facilitate
the correction. Sometimes users ignore
these corrections because they are
considered insignificant or not relevant
to the project being monitored. While
this may be acceptable for some
projects, it is not appropriate for most
projects where accurate pore water
pressure readings are required. Addi-
tionally, in some cases, it is assumed
that the barometric pressure correction
is not needed when the piezometers
are installed using the fully-grouted
Table 2: Locations of fully-grouted installation in soft ground
Site Thickness of
Soft Layer (m)
Settlement Since
Piezometer Installation (m)
Vertical
Compression (%)
1
11.28
1.11
9.8
2
19.96
1.11
5.6
3
18.14
1.15
6.3
4
13.5
1.20
8.9
5
13.81
1.52
11.0
6
18.04
1.35
7.5
7
14.63
1.35
9.2
8
10.97
1.16
10.6
9
10.91
0.78
7.1
Figure 4. Pore water pressures and settlements response in soft ground.
