Environmental Engineering Reference
In-Depth Information
TABLE 4.8
Piezometer Types and Applications
Type
Application
Advantages
Disadvantages
Figure
Open Systems (Vertical Readout; Water Level below Ground Surface)
Single-tube open
Coarse-grained
Simple, rugged,
Indicates average head,
4.25
system
granular soils,
inexpensive
relatively insensitive,
(Standpipe)
free-draining rock
time lag in impervious
masses
soils
Casagrande type
Coarse-grained to
Measures response at
Time lag in relatively
4.26
(single tube or
silty soils and rock
a particular depth
impervious soils
double tube)
masses
Decreased time lag
Low sensitivity
Double tube allows
Single-tube system subject
flushing entrapped
to clogging from entrapped
air or gas from lines
air or gas
Closed Systems (Remote Readout: Water Level any Location)
Pneumatic
Fine-grained soils,
Negligible time lag
Relatively costly device and
4.27
(diaphragm
slow-draining rock
installation
principle)
masses
Increased sensitivity
Durable and reliable
Requires protection against
pinching of lines
Lines readily purged and
extended to avoid
construction
Hydraulic
Earth dams with
System can be flushed to Slow response time
4.28
soils of low to
remove air and gas
medium
permeability
Remote sensing of pore
Long tubing lines require
pressure to leave
careful flushing
construction area free
Fittings may be subject to
leaks
Requires protection against
pinching
Electric
Fine-grained soils
Extreme sensitivity
Relatively costly
4.29
(diaphragm
and slow-draining
principle)
rock masses
Fast response
Continuous recording
Decreased durability and
possible (only type
reliability over other
with this capability)
closed systems because
of electrical circuitry
Single-Tube Open Piezometer
The simplest piezometer consists of a tube or pipe which connects a tip or sensor (porous
stone, well point, or slotted pipe) (
Figure 4.25)
to the surface and which is installed in a
borehole as the casing is withdrawn.
Readings are made by plumbing with a chalked tape or with an electric probe contact-
ing the water in the standpipe. The static head measured is the average head existing over
the depth of the inflow part of the borehole below the water table. The head may be higher
