Environmental Engineering Reference
In-Depth Information
Table 20.16.
95% and 99% equalization time lag for Casagrande Piezometers (Standards Association
Australia 1979).
Sand
Silt
Clay
10
3
10
4
10
5
10
6
10
7
10
8
10
9
Material Permeability cm/s
Average time lag for
12 s
2 min
20 min
3.5 h
36 h
14 days
150 days
95% equalization
Average time lag for
18 s
3 min
30 min
5.2 h
54 h
21 days
225 days
99% equalization
20.4.4.3
Fluctuations of pore pressure with time and the lag in response of
instruments
In most practical situations the pore pressure being measured varies with time, e.g.:
- Pore pressures in a dam vary with reservoir level;
- Pore pressures in a landslide will vary with rainfall;
- Pore pressures around a pump well will vary with rate of pumping and the time lag
between spells of pumping;
- Pore pressures under an embankment on soft clay will vary with the height of the
embankment and degree of dissipation by consolidation of pore pressures induced by
the embankment;
- Dynamic loading, e.g. earthquakes, may induce an increase in pore pressure.
In all cases, the piezometer which is selected should be constructed so that the time
taken for the piezometer to respond to the change in pore pressure is sufficiently short to
give a meaningful measure of the actual pore pressure.
In a Casagrande piezometer (
Figure 20.17
)
the piezometer responds to a change of pore
pressure by flow of water into or out of the piezometer. This is a function of the geome-
try of the piezometer, the permeability of the soil and the change in pore pressure.
The response is discussed in Hanna (1973, 1985) and Brand and Premchitt (1980a, b).
Table 20.16 is reproduced from Standards Association Australia (1979) and gives times
for 95% and 99% equalization of Casagrande type piezometers with a 73 mm “piezome-
ter tip” and 9.5 mm inside diameter standpipe.
Because of the small volume of water flow required to activate hydraulic, pneumatic,
vibrating wire and strain gauge piezometers, their time lags are very small compared to
that of Casagrande piezometers.
For hydraulic piezometers (
Figure 20.18
)
the “response” time depends on whether the
piezometers have to be flushed, in which case time has to be allowed for pressures to re-estab-
lish equilibrium due to the flushing affecting pore pressures around the piezometer tip. It
also depends whether individual pressure gauges are provided for each piezometer, or
whether a master gauge is used. In the latter case, time must be allowed for equilibrium to
be achieved.
Pneumatic piezometers (
Figure 20.19
) have a small but finite response time, as a small
displacement of the diaphragm is needed. The response time varies depending on whether
high or low air entry tips are used.
Vibrating wire (
Figure 20.20
) and strain gauge piezometers have virtually zero response
time, as there is no transfer of water involved.
20.4.4.4
Types of instruments and their characteristics
Observation well
. As shown in
Figure 20.16
an observation well, or “open standpipe”, con-
sists of a slotted plastic, reinforced fiberglass or steel pipe attached to a standpipe, installed
