Environmental Engineering Reference
In-Depth Information
the tip pressure and pressurize the gauge to this pressure before connecting the piezome-
ter. Any imbalance will involve flow of water into or out of the piezometer tip, with result-
ant lag time, and possibly influence the pressure, particularly if the soil is partially
saturated and water flows out of the piezometer tip. Use of a gauge for each piezometer
with a check master gauge overcomes this.
The terminal structure and tubes must be located so that no more than about 5 m neg-
ative head is generated, or cavitation will occur in the tubes, giving incorrect readings.
In embankment construction, the tubes have to be laid loosely in trenches to avoid
damage by construction equipment and differential settlement effects. Trenches are usu-
ally filled with sand, with bentonite clay or clay cutoffs to prevent water seepage along the
trenches. (See examples in US Department of Interior, 1981) and USBR (1987). However
in many dams the trenches were backfilled with loose soil similar to the dam core, rather
than sand, and piezometers were located close to the upstream face of the earth core. This
leaves a potential weakness in the dam from an internal erosion and piping point of view,
since high gradients may occur from the upstream face of the core to the first piezometer
and, if erosion were to initiate, it would quickly extend through the loose soil. This would
potentially put high gradients across the bentonite/soil cutoffs.
The authors are not aware of any cases where this has happened, but strongly recom-
mend against this practice for new dams.
Provided that high-air-entry tips are used, and the tips installed with intimate contact
with the soil, hydraulic piezometers will read pore water pressure. Subatmospheric pres-
sures up to about 50 kPa to 70 kPa can be read.
Twin tube hydraulic piezometers are relatively simple and medium cost to construct,
although when the cost of the terminal house (which may have to be dewatered to posi-
tion it satisfactorily, to avoid excessive negative pressures) is included, the overall cost
may be comparable with vibrating wire and pneumatic systems.
Auto-data logging is possible by using electrical pressure transducers. Hydraulic
piezometers have a medium long term reliability (15 years to 20 years in many cases), and
have been the favoured instruments in British and Australian dams. However most
hydraulic piezometers more than 30 or 40 years old are now not functioning. They were
used in USBR dams until 1978 when pneumatic piezometers were introduced, because
they are easier to read and maintain in good working condition. Disadvantages include
“growths” in the tubes; damage of tubes by differential settlement; the need for an elabo-
rate terminal house, often requiring dewatering; skill and care to keep them operating and
the need to avoid high subatmospheric pressures in the lines. Particular advantages are the
ability to flush air from the piezometer tip if necessary, hence ensuring measurement of
u w , ability to measure moderate subatmospheric pressures,and the possible use of the
piezometer to carry out in-situ permeability and hydraulic fracture tests of the soil. As
pointed out by Sherard (1981), hydraulic (and pneumatic and vibrating wire) piezometers
should not be installed in a surround of sand. Doing so only slows the response time if the
sand surround becomes partially saturated and since the piezometers are essentially no
flow, a large sand surround has no benefit in collecting water.
The authors know of no organization installing hydraulic piezometers in new dams and
do not recommend them.
Pneumatic piezometer . Figure 20.19 demonstrates a pneumatic piezometer consists of
a piezometer tip, from which two tubes are led to a convenient terminal measuring point.
Pressure is measured by applying pressurised gas (usually dry nitrogen) on the inlet tube.
The diaphragm is moved, and gas escapes to the outlet tube where it can be observed. The
pressure on the inlet tube is then reduced until the diaphragm closes (no flow from the
outlet tube). The pressure at which the diaphragm closes is the pore pressure (after cor-
rection for any closure spring effect). Some manufacturers measure the pressure on open-
ing of the diaphragm but most use the principles outlined above.
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