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Version 1.4 May 09
1.
Before drilling the hole it is important to ascertain the depth at which the
piezometer is to be installed. An indication of the ground conditions may also
be helpful.
2.
The borehole should be formed to a depth of approximately 500mm below the
intended elevation of the piezometer tip. If a Bentonite plug is required at the
base of the borehole, it will have to be drilled further to accommodate the plug.
3.
Before commencement of installation the depth of the hole should be re-
checked and the procedure you intend to follow should be confirmed with the
engineer and discussed with the driller.
4.
Confirm that all materials are available ( filter sand, Bentonite pellets or balls
and backfilling grout ). The piezometer to be installed should be transported to
the borehole locations in a container of clean water ( see section 5.1.13 ).
5.
Check the drill hole to ensure that the full depth is clear and free of obstructions.
6.
CHECK BASE READING OF PIEZOMETER. Where necessary, record a
reading from the piezometer to confirm that it has not been damaged since
establishing the base readings ( see Section 5.1 )
7.
Where necessary and possible, fill the borehole with clean water.
8.
If a Bentonite plug is required at the base of the borehole, slowly drop Bentonite
pellets / balls down the borehole. Be sure not to let the pellets / balls plug or
stick to the inside of the borehole. Feeding pellets / balls in to the borehole too
quickly will result in ‘bridging’ of the hole and make completion of the installation
very difficult ). The top of any base-plug must be below the intended
piezometer installation elevation.
9.
Once the plug has been formed ( if required ), filter sand should be slowly
added to fill the borehole up to the depth at which the piezometer is to be
installed. Use a weighted fibre tape to control the level of the filling materials.
10. With the filter securely fitted to the piezometer, lower it slowly down onto the
sand at the base of the borehole and record another reading.
11. Add more clean filter sand on top of the piezometer to provide the required filter
above the piezometer level. Continually check the level of the sand using the
weighted tape.
(Continued on page 23)
5.3.2 Open ( Un-cased ) Boreholes.
When forming a borehole for instrument installation, it may not be necessary to use a
temporary steel sleeve or ‘casing’ to hold the hole open during drilling and installation
operations. These holes may be described as ’open holes’. The following describes
a series of steps that could be adopted to carry out a piezometer installation in an
open or uncased borehole. It is similar to, but simpler than, the procedure for the
cased borehole ( 5.3.1 )
