Environmental Engineering Reference
In-Depth Information
short length (usually 1 metre) of bentonite. The hole should be backfilled with cement
bentonite grout. In dam cores grouting should be staged to avoid inducing hydraulic frac-
ture. The standpipe is kept as small a diameter as practical to keep the response time to a
minimum. Dunnicliff (1982) recommends the standpipe be not smaller than 10 mm diame-
ter so that gas bubbles will not be trapped. In any case, this is a practical minimum for use
of a dip meter to measure the water level. The sand filter acts as a large collector of water
compared to the small diameter standpipe.
The bentonite seal may be formed using dry pellets which are available commercially, or
by making 25 mm diameter moist balls of powder bentonite. These are dropped down the
hole, possibly tamped to compact and allow checking of the position and depth of the seal.
Casagrande, and all standpipe piezometers, measure pore water pressure (u
w
). They are
potentially accurate, depending on how carefully the water level is determined. The time
lag can be determined as outlined in Section 20.4.4.3. Time lag may be a problem in lower
permeability soils, and where the pore pressure is changing rapidly.
The Casagrande type piezometer is low cost, simple to construct with readily available
equipment and materials. They are self de-airing, measure pore water pressure and have a
long, satisfactory performance record. Disadvantages include susceptibility to damage by
construction equipment or animals and by consolidation of soil around the standpipe,
possible leakage from the surface runoff if the grout seal is not adequate, susceptibility to
freezing and inability to measure pressures higher than the level of the standpipe or pres-
sures lower than atmospheric. If porous filter tips are used, these may clog with chemical
deposits with flow in and out of the tip. Reading is simple but relatively time consuming.
Auto-data logging is not practicable.
Hydraulic twin tube piezometer
.
Figure 20.18(a)
shows an Imperial College (Bishop)
type hydraulic tube piezometer. Variations of the principle include the USBR type shown
in Figure 20.18(b).
These consist of a porous tip, which is usually installed during construction (e.g. in a
dam embankment), from which two tubes are led to a convenient terminal measuring
point where gauges are used to record the pressure. The total pressure equals tip pressure,
plus elevation difference from the tip to the measuring gauge.
Two tubes are provided so that air (gas), which is trapped in the tubes or which enters
through the porous tip, can be flushed from the system using de-aired water. The tips are
constructed of fine porous ceramic, stone, aluminium or sintered bronze. The tips are
required to be strong enough to withstand the total pressure, and fine enough to prevent
clogging by the soil. More importantly, the tips must be “high-air-entry” to limit entry of
air into the system. Commonly, high-air-entry tips can withstand an air pressure of 100 kPa
to 200 kPa, but may be up to 500 kPa to 600 kPa.
Coarse, low-air-entry tips are not used with hydraulic piezometers because they allow air
to enter the tip, the piezometer and tubes. This leads to pore air pressure rather than pore
water pressure being measured in partially saturated soils, a longer response time for the
piezometer, and incorrect elevation pressure adjustment if the air enters the tubes. The
high-air-entry tips are only effective if they are completely de-aired before installation.
This is usually done by boiling for 2 hours to 6 hours. Sherard (1981) suggests 12 hours
to 24 hours boiling.
The tubes leading to the measuring gauge should be constructed of Nylon 11, covered
by polyethylene. The Nylon 11 is impervious to air, the polyethylene impervious to water,
and the combination is impervious, strong, flexible and not affected by chemical attack.
Despite the use of high-air-entry tips, and the proper tubing, gas may still accumulate in
the tubes, and periodic flushing with de-aired water is necessary. This should be done
under back pressure to ensure that negative pore pressures are not induced. Some organ-
isations add bacteriacides and algacides to the water, to prevent algae growth blocking the
tubes.
