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3.2.3 Pull-Pin Method
In this method, the piezometer assembly is held in its closed position by means of a pull-
pin. After the filter stone and the platen are squeezed together, the pull-pin passes through
two sets of three eyebolts each, which are mounted on the filter stone and the platen.
If the grout pipe is going to be removed from the borehole:
The piezometer assembly must be pushed around the grout pipe, down to the desired
elevation. This is accomplished using a second pipe, which can be another length of the
grout tube. While holding the piezometer in position by the second pipe, the pull-pin
cable should be pulled gently until all the slack is taken out. With a sudden and strong
jerk, pull on the pull-pin cable. This will release the platens without changing the position
of the piezometer relative to the borehole.
(A bit of practice pulling the pins before the
actual installation will give some "feeling" and confidence as to how the system works.)
Piezometers should be installed sequentially, from the base of the borehole to the mouth.
If the grout pipe is going to be left in the borehole:
The piezometer assemblies should be attached to the grout pipe in a manner that does not
restrict the movement of the platens. The grout pipe is assembled, length by length, while
the piezometer assemblies, (each with its own pull-pin,) are attached to it by taping the
electrical cable to the grout pipe near the piezometer. The grout pipe and piezometer
assemblies can then be pushed down the hole as a unit. When the final position is reached
the pull-pins are pulled, activating the platens.
If installed in drill casing:
The sensor is lowered to the proposed elevation. The casing is pulled just above this
elevation before the assembly is released. The tool is then removed from the drill hole
and the next assembly prepared for installation. When lowering (or pushing) the
subsequent piezometers down the hole, feed the cables from the lower piezometers
through the middle of the assembled piezometer rather than around the outside. This will
prevent the cables from coming between the filter and the borehole wall.
When all the assemblies are installed, the hole can be grouted from the bottom up using a
bentonite cement grout. The grout pipe can either be removed from the hole or left in
place.
3.3 Splicing and Junction Boxes
Because the vibrating wire output signal is a frequency rather than a current or voltage,
variations in cable resistance have little effect on gage readings. Therefore, splicing of cables has
no effect and in some cases may in fact be beneficial.
