Operator's guide
the skids or studs on the sonde keep the sensor package vertical
within the borehole. Do not drag too far, or you will damage the
contact points.
19.Lower the load bearing cable by around 30 cm to engage the
strain relief unit inside the borehole casing, and to provide some
slack in the cables.
20.Clamp the load bearing cable to the top of the borehole.
21.Tie the lifing and signal cables together above the strain relief
mechanism using tie wraps.
22.The sensor can now be levelled and unlocked ready for use.
3.2 Installing a sensor using sand backfill
Dry sand backfll i s a c onvenient a nd e fective wa y o f i n stalling a
borehole or posthole sensor in a time-stable environment. The presence
of sand not only fxes the sensor in place at the boiom of the hole, but
also reduces noise due to air convection.
The ideal type of sand to use is the fne, k iln-dried s and u sed f or
children's play sandpits. This is readily available in airtight bags, is
thoroughly washed and clean, and will contain liile sediment. (When
dried out afer weiing, sand containing foreign maier 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
boiom o f t he b orehole, a nd e nsure t hat t he s urroundings o f t he
instrument are kept dry.
Afer p ositioning t he s ensor, m ore s and i s a dded t o f ll t h e s p ace
between it and the borehole casing, holding it frmly 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 fow well.
December 2004
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