Geotechnical News • June 2012
21
GEOTECHNICAL INSTRUMENTATION NEWS
of an open standpipe piezometer
installed using the traditional Casa-
grande sand-pack method and a
vibrating wire piezometer using the
fully-grouted method in northern Min-
nesota at a site involving a landslide.
Figure 2 shows the total head read-
ings versus time. The tips of both
piezometers were installed within the
same formation at about 23 m below
the ground surface (tip elevation
about 239 m). The installations are
approximately 25 m apart laterally.
The two piezometers were installed in
high-plasticity clay with permeability
on the order of 1x10
-8
cm/s. It can
be seen from Figure 2 that the total
head reading from the vibrating wire
piezometer at the time of installation
was about 277 m. This total head dur-
ing installation reflects the pressure
exerted on the tip by the column of
cement-bentonite grout in the liquid
state. As the cement-bentonite grout
set up, the total head decreased and
after approximately two days became
fairly constant.
On the other hand, the open standpipe
piezometer had an initial total head of
approximately 252 m after installation.
Then the total head increased with
time as the water level rose inside the
standpipe. It took more than 180 days
before the total head in the open pipe
piezometer reached a similar value
to the vibrating wire piezometer. The
sudden increase from about 100 to
180 days is the consequence of water
freezing within the upper portion of
the standpipe.
This field example illustrates the long
hydrodynamic time lag in standpipe
piezometer installations in low perme-
ability deposits. It also illustrates the
rather short time lag in vibrating wire
piezometer installations using the
fully-grouted method.
Grout permeability
requirements
As described by Mikkelsen and Green
(2003), the success of the fully-
grouted method is based on the fact
that the pressure gradients in the radial
direction from the borehole wall to
the piezometer tip are normally one
to several orders of magnitude greater
than those in the vertical direction
within the borehole. As a result, the
radial gradients control the piezometer
response. This holds true as long as
flow in the vertical direction does not
develop due to higher permeability of
the cement-bentonite grout than the
ground. Therefore, low permeabil-
ity of the cement-bentonite grout is
crucial for the success of the fully-
grouted method.
Contreras et al. (2008) developed a
computer model to obtain a better
understanding of those permeability
requirements. The computer model
simulated seepage conditions around a
piezometer installed using the fully-
grouted method. The results of the
computer simulation indicated that the
permeability of the grout can be up
to three orders of magnitude higher
than the permeability of the surround-
ing soil without inducing a significant
error. This was an interesting finding
and differed from previous assess-
ments (e.g. Vaughan, 1969) which
indicated that the permeability of the
grout could only be one or possibly
two orders of magnitude greater than
the permeability of the surrounding
soil.
The minimum permeability that is
commonly encountered in natural
soils is on the order of 10
-9
cm/s (k
soil
).
Therefore, the cement-bentonite grout
mix used in the fully-grouted method
is required to have at most a perme-
ability of 10
-6
cm/s for these low
permeability soils.
Field verification of grout
permeability requirements
Despite the computer model simula-
tion indicating that the permeability
of the grout can be up to three orders
of magnitude higher than the perme-
ability of the surrounding soil with-
out inducing a significant error, we
believed it was necessary to verify this
in the field. We have therefore col-
lected data from a series of locations
at which a fully-grouted piezometer
exists near an open standpipe piezom-
Figure 1. Results of laboratory tests of pore water
pressure response.
Figure 2. Response times of open standpipe and vibrating
wire piezometers in high plasticity clay.
