E10160-200722
FOP
Page 15
In non
– or partially – saturated soils, one should not expect to read exactly 0 kPa after
installation because of the above mentioned factors, and because of the potential suction
in the soil and potential air infiltration in the piezometers in the case where these are fitted
with low-air entry filters. But from our experience, as soon as water gets back in the soil,
pressure increases to normal levels.
It is recommended after installation to take as many readings as possible and if possible
to use a datalogger to record the readings every hour for a period that can be up to one
week. This will greatly help assessing if the piezometers are responding normally or if
there is any problem and to define a stable initial reading.
If the readings remain stable and within a certain range, from about -25 to +25 kPa
(depending on the
piezometers’ range), we should assume that the piezometers are still
working properly, unless other events lead us to believe otherwise. In such case, the best
method to check consists in retrieving and calibrating the piezometers.
It should be kept in mind that these piezometers are designed for giving reliable relative
readings, provided that temperature are stable and barometric pressures at the level of
the piezometers are properly measured. Accurate absolute readings are slightly more
difficult to obtain.
3.3 INSTALLATION IN FILL
Compacted clay
Excavate a trench or recess about 30 cm deep by about 0.8 m
2
. Form a cylindrical hole in
the side wall of the trench. The hole diameter should be slightly smaller than the piezometer
body.
Push the piezometer into the side of the hole. Make sure that the piezometer filter is in direct
contact with the host material. If necessary to ensure continuity with the saturated high air
entry filter and the pore water, smear the filter with a thin paste of the saturated material.
Before backfilling, the cable must be laid with the utmost care. Loop the cable around the
recess, making sure it rests on a bed of hand placed and compacted screened clay.
Make sure that the cable does not cross over itself or other cables in the same area.
Backfill the recess with screened clay containing no particles larger than 2.5 mm in diameter.
The backfill should have a water content and density equal to that of the surrounding
material.
Make sure that the cable is protected from potential damage caused by angular material,
compacting equipment or stretching due to subsequent deformations during construction or
fill placement.
Granular materials
Install the piezometer as described above in a recess excavated for this purpose. Place the
piezometer within the trench, loop cable and backfill with screened material containing the
same moisture content and compacted to the same density as the surrounding fill. In rock
fill, it is necessary to place a graded filter around the piezometer. Use fine grain clean sand