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Version 1.4 May 09
4.
As the rods are lowered into the borehole, the cable must be restrained to
prevent the piezometer dropping out of the shoe and to maintain control of the
cable.
Never rotate the lower rods when adding further rods. This
may cause the piezometer cable to become wrapped around
the rods and will result in a failed installation due to either
cable damage or pulling the piezometer back out of its
intended location when the rods are extracted.
5.
Always count the rods into the holes so as to be sure of the piezometer
elevation at any time. ( In soft ground, heavy rods could push the piezometer
past its intended location without applying any driving force. )
6.
When the piezometer is just above the base of the borehole connect a portable
readout to the piezometer cable to monitor any changes in the pressure
registered by the piezometer, particularly when it is being pushed into the base.
These pressures MUST NOT exceed the maximum pressure
shown on the calibration sheet by more than 50%. If it does
exceed the calibrated values by more than 50%, irreparable
damage to the piezometers may occur. The calibration and
Zero values would then be invalidated. The piezometer would
not then provide any useful data.
7.
If necessary, use hydraulic equipment ( rig head or jacking ram ) to apply
gradual loading to the rods to push the piezometer to its intended elevation.
8.
No filter zone is required. Bentonite pellets can be used to provide a plug
behind the piezometer, but generally the borehole would be backfilled with
Bentonite / Cement grout once the rods have been extracted. When water is
present in the borehole, grouting must be carried out using a tremie pipe.
Where the borehole is dry, liquid grout can be placed from the top.
Where a Cone Penetrometer Testing Rig ( CPT rig ) is to be used to push the
piezometers, it will be necessary to use a special adaptor shoe and to run the
piezometer cable up inside the CPT rods. Each rod must be threaded over the cable
as pushing is carried out. A standard electronic CPT pushing head can be used to
allow the cable to pass though under the pushing head. A pre-driven CPT hole with
a fake head can also be utilised to prevent pressure build up on the piezometer.
This system can be a cost effective solution where piezometers are to be installed in
soft ground but care must be taken not to damage the cable when the CPT rods are
withdrawn.
(Continued from page 24)
