Civil Engineering Reference
In-Depth Information
which pressure fluctuations are not likely
to be significant, a standpipe piezometer is
likely to be suitable. However, if the pur-
pose of the piezometer is to measure the
response of the ground water pressures to
a drainage system such as a series of hori-
zontal drains, or to detect transient water
pressures in response to precipitation, then
a diaphragm piezometer with a much shorter
time lag would be more appropriate.
end, which is encased in clean gravel or sand
to provide a good hydraulic connection with
the rock (Figure 5.12). This perforated sec-
tion of the piezometer, which is the point
Steel (schedule 40)
protective casing with
hinged cap
0.9 m
Vented cap
Master lock
(g)
The filter material in the completion zone
should be suited to the rock type. Installa-
tions in clay shales or weathered micaceous
rocks should use fine grained filter material
that will not be clogged by rock weather-
ing products washed in from the walls of
the hole.
Drain
0.8 m
0.6 m
0.3 m
Concrete pad (min. 0.10 m
thick on undisturbed or
compacted soil)
Sand
Frost sleeve
Concrete seal
1.1 m
1.2 m
Granular bentonite seal
(h)
Cost and reliability are other factors to con-
sider in selecting piezometer types. Stand-
pipe piezometers are simple to install and
can be read with inexpensive well sounders,
while pneumatic and vibrating wire piezo-
meters are expensive and require more costly
readout units. In situations where the slope
is moving and piezometers may be lost,
it would be preferable to install standpipe
piezometers for reasons of economy.
1.4 m
2% Bentonite-cement seal
50 mm PVC casing
(schedule 40, flush joint,
threaded)
The following is a brief description of piezometer
types and the conditions in which they may be
used (Dunnicliff, 1993).
Granular bentonite seal
1.5 m
•
Observation wells
—Ground water pressures
may be monitored in open holes if the per-
meability of the rock mass is greater than
about 10
−
4
cm/s, such as coarse-grained sand-
stones and highly fractured rock. The major
limitation of observation wells is that they
create a vertical connection between strata
so their only application is in consistently
permeable rock in which the ground water
pressure increases continuously with depth.
Observation wells are rarely utilized in mon-
itoring ground water pressures in rock.
Silica fine-grained sand
(mortar sand)
0.6 m
0.3 m
50 mm PVC well screen
with 0.25 mm slot open
1.5 m
Filter pack (clean medium
to coarse silica sand)
50 mm PVC casing
0.6 m
•
Standpipe piezometers
—A standpipe piezo-
meter consists of a length of plastic pipe, with
a perforated or porous section at the lower
Figure 5.12
Typical standpipe piezometer
installation.
