Environmental Engineering Reference
In-Depth Information
Vented cap
Piezometric
water level
Groundwater level
Bentonite / cement
grout
Standpipe
Bentonite seal
Sand
Filter tip
FIGURE 4.26
Casagrande-type open-tube piezometer. (Courtesy of Slope
Indicator Co.)
Other types of pneumatic transducers include the Warlam piezometer, which operates
with air, and the Glotzl piezometer, which employs a hydraulic fluid. In all cases, the lines
can be extended through an embankment to a readout so that they do not interfere with
construction operations. The lines must be protected against pinching.
Hydraulic Piezometers
Commonly installed in earth dams, the hydraulic piezometer used by the U.S. Bureau of
Reclamation (USBR, 1974) uses a porous ceramic disk as a sensor. The disk is connected by
two plastic tubes to a pressure gage near the downstream face of the embankment (
Figure
4.28).
The pressure gage and its housing are located at an elevation slightly higher than the
piezometer tip.
The system requires long tubing lines. Deairing by water circulation must be done care-
fully, and all fittings must be well made to avoid leaks. The lines require protection against
pinching. To operate, fluid is pumped into the system until a desired balancing pressure
is obtained as read on the gage; then an inlet valve to the piezometer tip is opened slightly
and the response pressure observed.
Electrical Piezometers
The tip of an electrical piezometer contains a diaphragm that is deflected by pore pressure
against one face. The deflection is proportional to the pressure and is measured by means
of various types of electrical transducers. The system components are illustrated in
Figure
4.29
.
Electrical piezometers are extremely sensitive and have negligible time lag and are
used with impervious materials, where measurements are critical and continuous moni-
toring and recording are required.
