E10160-200722
FOP
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around the instrument and increase the particle size as the backfill proceeds outwards to
the rock fill. The sand placed in the recess around the instrument and cable should range in
size from 0.5 mm to 2.5 mm in diameter.
3.4 INSTALLATION IN BOREHOLES
The method used to install a piezometer in a borehole depends on the particular conditions
in which the installation must be carried out. Artesian conditions, borehole stability,
available drilling equipment and sealing material are among the factors that will influence
the method chosen. The method described below will cover most applications.
1. Drill the borehole below the required depth at which the piezometer is to be
installed. Drive the casing thirty centimeters below the required piezometer
elevation. Wash until the water (or biodegradable drilling mud) emerging from the
borehole runs clear. This will prevent the backfill materials from sticking to and
plugging the casing.
2. Raise the casing fifteen centimeters and pour sand below its bottom. Repeat the
operation once and lower the piezometer to the top of the sand. Check the
borehole depth after each operation.
3. While holding the instrument in place, repeat step 2 until thirty centimeters of sand
are placed above the piezometer.
4. Raise the casing fifteen centimeters and pour compressed dry bentonite below its
bottom. Repeat the operation until a seal of at least 1.2 m is in place. When
pouring bentonite, keep the cable taut to prevent the bentonite from hooking up in
the casing. Pour the bentonite slowly in the hole to avoid bridging. A brush or a
thirty centimetre layer road salt can be used to unplug a blocked hole. Check the
borehole depth after each operation.
5. Wait for the bentonite to set up. Two hours are generally enough. Refer to
supplier’s instructions for exact time. Keep the borehole filled with water. This will
prevent the bentonite of drawing water from surrounding soil during its setting.
6. If only one piezometer is to be installed in the hole, backfill the casing with a
bentonite/cement grout.
If more than one piezometer is to be installed in the borehole, backfill with a
bentonite/cement grout to an elevation of a meter and a half below the second
piezometer. Then use 1.2 m of bentonite. Repeat operations 1 to 5 for the second
piezometer. When all instruments are installed, backfill with a bentonite/cement
grout.
7. Pull the casing without rotating it during removal. Top off the borehole with grout.
If the deepest piezometer has for purpose to measure the pore water pressure in a specific
horizon, it is necessary to drive the casing below the instrument and set a 1.2 m bentonite
seal at the bottom of the borehole. Pull the casing as the bentonite is set in place. Proceed
by stages of fifteen centimeters. Be very careful not to plug or allow bentonite to stick to
the inside walls of the casing. This is accomplished by making sure the bentonite level is
at all times below the casing and by slowly dropping bentonite in single file down the hole.
Trying to feed bentonite too rapidly will result in bridging in the casing or borehole.