Environmental Engineering Reference
In-Depth Information
Figure 20.16.
Observation well.
Figure 20.17.
Casagrande type piezometer.
in a borehole and surrounded with sand. Porous tips, as described below for Casagrande
piezometers, may be used. When slotted pipe is used, it is often wrapped in filter fabric to
prevent clogging by the sand. The hole is sealed at the top with mortar to prevent surface
water flowing in. The water level in the well is measured with a water level dip meter.
An observation well normally measures the maximum water pressure intersected in the
hole. It does not measure pressure at a point, and does not account for flownet effects (see
Figure 20.12
)
. In some situations, with confined aquifers of varying permeability, an
observation well may not measure the maximum pressure, with water from the high pres-
sure zone flowing into a more permeable lower pressure zone.
The time lag for observation wells may also be large, since a relatively large flow of
water is required for the well to reach equilibrium pressure change.
In most situations observation wells are not adequate, and piezometers should be installed.
Casagrande piezometer
. Figure 20.17 shows a Casagrande type piezometer. These con-
sist of a porous tip which is embedded in a sand filter, and sealed into a borehole with a
