Installing the 3TB in a borehole
18. Use the winch to drag the assembly up within the borehole for a distance of
15 to 30 centimetres. This will ensure that the hole lock arm and 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 lifting and signal cables together above the strain relief mechanism
using tie wraps.
22. The sensor can now be levelled and unlocked ready for use.
4.2 Installing a sensor using sand backfill
Dry sand backfill is a convenient and effective way of installing a borehole or post-
hole 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 300 mm
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 30 mm 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.
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Issue G - November 2019
