The electrical piezometer and the pneumatic piezometer offer rapid
response to change in phreatic level, but may prove to be relatively expen-
sive and less flexible in use than the hydraulic alternatives. They offer the
advantage of requiring only a small terminal measuring chamber, rather
than the costly instrument house generally required for twin-tube
hydraulic instruments. Both are therefore suited to isolated installations of
limited numbers of piezometers. The principle of a pneumatic piezometer
tip is illustrated in Fig. 7.3. It functions by application of a known and con-
trolled gas back-pressure to balance the porewater pressure operating on
the diaphragm. The pressure balance is indicated by deflection of the
diaphragm, allowing the gas to vent to a flow indicator and is confirmed at
closure of the diaphragm when gas pressure is slowly reduced. The electri-
cal piezometer, which is less common and is not illustrated, senses pres-
sure via resistance strain gauges bonded to a steel diaphragm or by use of
a vibrating wire strain gauge.
(c) Internal settlement and deformation
Vertical tube extensometer gauges with external annular plate magnet
measuring stations located at vertical intervals of about 3 m can be
installed during embankment fill construction. Most are based on the prin-
ciples illustrated in Fig. 7.4. A probe on a calibrated tape is lowered down
the tube to detect the plate magnet stations, allowing the determination of
relative levels and increments of internal settlement to 2 mm. The prin-
298 DAM SAFETY: INSTRUMENTATION AND SURVEILLANCE
Fig. 7.3 Pneumatic piezometer
