Environmental Engineering Reference
In-Depth Information
trenches to avoid damage by construction equipment and looped, coiled etc. to allow for
differential movement. As for hydraulic piezometers, these trenches are a potential point
of weakness. The piezometers are installed into existing dams using a similar system to
that shown in
Figure 20.17
,
with piezometer in a sand surround, with a bentonite seal,
and cement/bentonite grout.
Because of their method of construction and reading, pneumatic piezometers cannot be
used to measure subatmospheric pressures.
Pneumatic piezometers are relatively simple and low cost construction, and the termi-
nal gauge house requirement is much simpler than for hydraulic piezometers. Pneumatic
piezometers have performed satisfactorily over 10 years to 15 years in the USA (Sherard,
1981) and were then the preferred instrument for dams in that country because they are
easier to read, easier to install and auto-data logging is simple. Less skilled operation is
needed than for hydraulic piezometers. There has been some reticence to adopt them for
dams in some other countries, including Australia. Where used, their performance has
been acceptable, but many cease to work after some years. They have been used in bore-
holes in rock (and in concrete dams) by installing with a packer, allowing replacement if
this was necessary. The time for taking measurements is dependent on the readout equip-
ment and the length of the tubes to the piezometer. Sherard (1981) quotes read times of 1
minute to 2 minutes for short tubes, but up to 10 minutes to 20 minutes for very long
tubes. He indicates this was reduced to 3 minutes to 5 minutes by initial rapid gas filling.
There are no problems of freezing with pneumatic piezometers.
Disadvantages include long measurement time for long tubes and high and subatmos-
pheric pressures cannot be read.
Vibrating wire piezometer
.
Figure 20.20
shows the principle of a vibrating wire
piezometer (also known as an acoustic piezometer).
These consist of a piezometer tip from which an electrical cable leads to a convenient
terminal measuring point. The piezometer consists of a porous tip, as in a pneumatic
piezometer with a stiff metallic diaphragm. The diaphragm is attached to a prestressed
wire. When the diaphragm deflects with changes in pore pressure, the tension in the wire
changes. The natural frequency of the wire is dependent on the tension and is measured
by plucking the wire, using an electromagnet. The wire then vibrates in the magnetic field
of a permanent magnet, causing an alternating voltage of the same frequency as the wire,
and this can be measured at the measuring point.
Hence, by measuring the natural frequency of vibration of the wire and calibrating this
against pressure on the diaphragm, the instrument can be used to measure pore pressure.
The instruments are essentially no flow devices and, therefore, have a very short
response times. Sherard (1981) suggests that they are best fitted with high-air-entry tips to
limit entry of air into the piezometer and ensure that pore water pressure is measured. A
digital readout is usually provided and the instruments are readily suited to auto-data log-
ging. The readout unit can be located at any elevation relative to the piezometer and up
to several kilometres away. The wires can be laid in any configuration, but provision
should be made for differential movement. In existing dams, vibrating wire piezometers
are installed in boreholes and sealed in place as shown in Figure 20.17. Provided they are
installed with intimate contact with the soil, vibrating wire piezometers can be used to
measure subatmospheric pressure.
Vibrating wire piezometers are more expensive than hydraulic or pneumatic piezometers,
but have been used widely in European, USA, Canadian and Australian dams, with reliable
performance for up to 20 years (Sherard, 1981). Australian experience is that most piezome-
ters more than 30 years old are no longer functioning. They are the easiest and quickest to
read of all piezometers and are not susceptible to freezing.
There are varying opinions on whether they are susceptible to long term drift, due to
creep of the diaphragm or tension wire. Sherard (1981) presents information suggesting
