Environmental Engineering Reference
In-Depth Information
sand because the pore pressure regimes are different. Several piezometers would be needed
in each soil unit, because of the rapidly varying pressures away from the well points.
20.4.4.2
Pore air and pore water pressure
The effective stress in a soil
is given by:
(20.1)
u
where
pore pressure.
In partially saturated soils the pore pressure (u) is a function of the pore air pressure (u
a
)
and pore water pressure (u
w
):
total stress and u
uu
(u u)
(20.2)
a
a
w
where
is a function of the degree of saturation. For fully saturated soils
1.0, and in
a dry soil
0.
In partially saturated soil, pore air pressure is commonly greater than pore water pres-
sure due to surface tension effects on the air-water meniscus. Hence, when measuring pore
pressures in partially saturated soils, the piezometers must be constructed so that either
pore air or pore water pressure is measured. This is achieved, in most cases, by using high-
air-entry filters for the piezometer element which measures pore water pressure, and low-
air-entry filters where pore air pressure is to be measured. These are constructed of porous
stone, ceramic or bronze. The pores are very small and uniform in size and, when satu-
rated with air free water, air cannot pass into the filter until the applied air pressure
exceeds the pressure developed on air-water meniscus at the entrance of each pore in the
filter. Hence, if fine “high-air-entry” filters are used, the piezometer will measure pore
water pressure and, if coarse filters are used, pore air pressure will be measured.
Air and other gases may also be dissolved in the pore water so even in saturated soils,
air may enter the piezometer tip if it is too coarse.
In such cases, the air can lead to incorrect readings by lengthening the response time of
the piezometer and forming air-water menisci in the tubes and standpipes which affect the
measured pressure.
Sherard (1981) includes a discussion of the differences between pore air and pore water
pressure in dam construction. He concludes that:
- In granular cohesionless soils u
a
u
w
;
- In clay soils u
a
u
w
, but not by more than 2-3 m of water head where u
w
5m water
head. At higher pressures the difference rapidly decreases;
- The difference is always highest during construction when the soil is partially saturated.
Sherard quotes case histories where low-air-entry and high-air-entry tips have been used
adjacent each other, with only small differences in readings. However, this experience is in
large dams where pore pressures are large in magnitude. In smaller embankments the dif-
ferences may be significant.
Sherard (1981) recommends use of high-air-entry tips for both hydraulic and vibrating
wire piezometers. He suggests use of tips with an entry pressure of 500-600 kPa where
these are available. He recommends use of tips with low-air-entry pressures for pneumatic
piezometers, the entry pressure being selected to be just greater than the anticipated soil
suction (in partially saturated soils). This prevents transfer of water to the surrounding
soil from the piezometer tip, while still allowing the transfer of the small volume of water
that is required to operate the piezometer.
