Environmental Engineering Reference
In-Depth Information
Vented cap
Bentonite/cement
plug
Groundwater level
Sand or gravel
backfill
Standpipe
Filter tip
FIGURE 4.25
Observation well. (Courtesy of Slope Indicator Co.)
or lower than the free water table, and in moderately impervious to impervious soils is
subject to time lag.
Casagrande Open-Tube Piezometer
Casagrande-type piezometers are similar to the open-tube system, except that the tip is
surrounded by a bulb of clean sand and a clay seal is placed above the tip as shown in
Figure 4.26
to confine response to a particular depth interval. Before the clay seal is placed,
the annular space around the tip is filled with gravel or sand to prevent soil migration and
system clogging. The hole is backfilled above the clay seal. The casing is withdrawn
slowly as the clean sand, clay seal, and backfill are placed.
In granular soils, large-diameter (1 in.) tubes adequately reflect changes in pore pres-
sure, but small tubes are required for soils of low permeabilities. Water levels are read in
the same manner as for simple systems, except that in small tubes readings are made with
electric probes with a 1/8 in. O.D.
Double-tube systems
are a variation, which permits flushing to remove entrapped air or
gas that clogs lines, a common occurrence in single-tube systems of small diameter. In cold
weather, tubes are filled with alcohol or antifreeze.
Pneumatic-Type Piezometers
In pneumatic piezometers the sensor is a sealed porous tip that contains a gas- or fluid-
operated diaphragm and valve, which are connected by two lines to a pressure supply and
outlet system on the surface as shown in
Figure 4.27.
When air pressure applied to the con-
necting line is equal to the pore-water pressure acting on the diaphragm, the valve closes.
This pressure is assumed equal to the pore-water pressure in soil, or the cleft-water pres-
sure in rock masses.
