Operator's guide
3.2 Installing a sensor using sand backfill
Dry sand backfill is a convenient and effective way of installing a
borehole or posthole sensor in a time-stable environment. The
presence of sand not only fixes the sensor in place at the bottom of the
hole, but also reduces noise due to air convection.
The ideal type of sand to use is the fine, kiln-dried sand used for
children's play sandpits. This is readily available in airtight bags, is
thoroughly washed and clean, and will contain little sediment. (When
dried out after wetting, sand containing foreign matter may solidify
and “concrete” the sensor in position.) This sand is suitable for use in
both dry and damp boreholes.
In the procedure outlined below, the sensor rests on a pad of sand
around 300mm thick. This pad will absorb any residual moisture at
the bottom of the borehole, and ensure that the surroundings of the
instrument are kept dry.
After positioning the sensor, more sand is added to fill the space
between it and the borehole casing, holding it firmly in place. The
sand should reach within 30mm of the top of the instrument, but
should not cover it. This way, the instrument can be more easily
recovered when it requires maintenance or replacement. This is
particularly important if the borehole is not completely dry, since
moist sand does not flow well.
The following photographs show the steps involved in backfilling with
sand:
November 2006
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