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Geotechnical News •March 2015
GEOTECHNICAL INSTRUMENTATION NEWS
All information provided here consti-
tutes a robust geological model of the
area and the ongoing subsidence. To
control the evolution of subsidence
where vulnerable buildings or lifelines
are in the vicinity of susceptible soils,
monitoring the subsoil pore water
pressure is sufficient (green bars in
Figure 1).
Francesca Bozzano
CERI Research Center and
Department of Earth Sciences -
Sapienza University of Rome,
Piazzale Aldo Moro 5, 00185,
Rome, Italy.
Tel. +39-6-49914924
email:
The use of fully-grouted piezometers in a streambed
Raymond D’Hollander, Paul Roth, Shane Blauvelt, James O’Loughlin
The site is a stream located in the
northeastern United States with
contaminated sediments in the chan-
nel bed. Data regarding both verti-
cal hydraulic gradients and absolute
piezometric pressures were required
during remedial design to evaluate
stability of the bed and banks for an
excavation scenario and for use in
modeling a potential chemical isola-
tion cap.
Selection of fully-grouted
method of piezometer
installation
Available data during the pre-design
planning indicated that the stream
water surface and adjacent ground-
water elevations are variable with a
typical annual range of about 1 m. The
groundwater data indicated the poten-
tial for significant upward gradients
and for some of the groundwater to
be saline. The water depth above the
proposed piezometer locations was
typically about 1 to 3 m. Shearing by
ice, debris, and high flows as well as
the potential for artesian groundwater
made an open standpipe piezometer
impracticable for measurements per-
formed over an extended period.
Vibrating wire piezometers with on-
shore data acquisition systems were
selected for measuring the groundwa-
ter pressures in the streambed. It was
desirable to position the top piezom-
eter in the creek at about the expected
post-remediation sediment surface
to evaluate the piezometric pressure
and gradient likely at that point. This
position ranged from 0.6 m to 1.8 m
below the sediment surface. The shal-
low depth of these piezometer raised
concerns with the effectiveness of
conventional bentonite seals, particu-
larly given the potential for erosion
in the stream bed. Also, access to the
locations was difficult and the ability
to install the two piezometers quickly
in the same borehole was desirable.
Based on these considerations, the
fully-grouted method was selected for
installing the piezometers in the creek,
as described in McKenna (1995) and
Contreras et al. (2008).
Stream cross-section
instrumentation
Instrumentation cross-sections were
installed at six locations along the
stream. Each instrumentation cross-
section included two vibrating wire
piezometers in the channel, a stilling
well, and two open standpipe piezom-
eters installed at the top of the bank,
as shown on Figure 1. The fully-
grouted piezometers in the channel
were installed in vertical pairs with the
bottom piezometer approximately 2.1
m to 3.3 m below the top piezometer.
The on-shore standpipe piezometers
were installed so that the top piezom-
eter was located near the groundwater
surface and the deeper piezometer
at about the elevation of the bottom
piezometer in the channel pair. Due
to the potential for saline groundwa-
ter, bentonite seals for the standpipe
piezometers were installed using ben-
tonite pre-hydrated with fresh water
and then tremied into the borehole.
Fully-grouted piezometer
installation
Drilling
The fully-grouted piezometers were
installed in the center of the channel
using a CME 45C drill rig on a seg-
mented barge, as shown in Figure 2.
The barge was disassembled and reas-
sembled between some of the cross-
sections due to the presence of low
bridges. The borings were advanced
using mud rotary and casing.
Piezometer and tremie pipe assembly
Unvented vibrating wire piezometers
with a range of 0.2 MPa were used.
They were taped to the Schedule 40,
19-mm diameter PVC threaded pipe
used to tremie the grout, as shown
in Figure 3. Depending on the water
depth, the top pipe length was 1.5 m or
3 m to allow for a convenient stick up
out of the water for grouting; this top
length was unscrewed after grouting
so that the finished top of the pipe was
below the sediment surface. The total
pipe length was measured to fit the
finished depth of the borehole, so that
the pipe would rest on the borehole
Figure 1. Typical instrumentation
cross-section.
