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4.2 Installation in Boreholes
Geokon piezometers can be installed in cased or uncased boreholes, in either single or multiple
piezometer configurations. If pore pressures in a particular zone are to be monitored, careful
attention must be paid to the borehole sealing technique. Two methods of isolating the zone to be
monitored are detailed below.
Boreholes should be drilled without using drilling mud, or with a material that degrades rapidly
with time, such as Revert
. The hole should extend six to twelve inches below the proposed
piezometer location, and should be washed clean of drill cuttings. The bottom of the borehole is
then backfilled with clean fine sand up to a point six inches below the desired piezometer tip
location. The piezometer can then be lowered into position. (Preferably, the piezometer will be
encapsulated in a canvas bag containing clean, saturated sand.) While holding the instrument in
position, (a mark on the cable is helpful) fill the borehole with clean fine sand to a point six
inches above the piezometer. Continue by following the instructions for Installations A, B, or C
as detailed below.
Installation A:
Immediately above the area filled with clean fine sand, known as the "collection zone", the
borehole should be sealed by an impermeable bentonite-cement grout mix, or with alternating
layers of bentonite and sand backfill, tamped in place for approximately one foot, followed by
common backfill. (See Figure 4.)
If multiple piezometers are to be used in a single hole, the bentonite-sand plugs should be
tamped in place below and above the upper piezometers, as well as at interval between the
piezometer zones. When using tamping tools special care should be taken to ensure that the
piezometer cable jackets are not cut during installation, as this could introduce a possible
pressure leak in the cable.
Installation B:
The borehole is filled from the “collection zone” upwards with an impermeable bentonite grout.
(See Figure 4.)
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