Environmental Engineering Reference
In-Depth Information
Figure 20.20.
Vibrating wire piezometer - Maihak type (from ANCOLD, 1983).
this is not a problem. Australian experience is that it can be. Other advantages are that,
by using high-air-entry tips, the saturation of the tips can be checked by observing the
development of negative pressure as the tip is allowed to dry slightly and, in partially sat-
urated soils, water will not be sucked from the tip into the soil (unlike hydraulic piezome-
ters which may wet the soil during the flushing operation).
Disadvantages have included damage by lightning strike, particularly but not only dur-
ing construction when the cables are exposed. This has been overcome by shielding
cables, earthing, and provision of overvoltage protection. Stray currents from nearby
power stations have also caused problems.
Strain gauge piezometer.
Strain gauge piezometers are similar in construction to the
vibrating wire piezometer, but the deflection of the diaphragm is measured by either
bonded electrical strain gauges or Carlson type unbonded strain meters. Pore pressures
are recorded as changes in resistance. The instruments are not as widely used as vibrating
wire piezometers, mainly because of the possibility of drift in the long term and with
