Operator's guide
14.Whilst monitoring the depth of the sensor, carefully approach
the sand layer at the bottom of the borehole. The lifting cable
will go slack when the sensor makes contact with the sand.
If the lifting cable goes slack before the sensor has reached the
sand layer, it may have become caught on a bad joint or lip in
the borehole; carefully raise and lower the instrument to free it.
15.When you have reached the bottom, use the winch to lift the
package slightly, taking the slack off the cable. This ensures that
the sensor is hanging vertically within the borehole, and is no
longer in contact with the sand bed.
At this point, you may wish to use an inclinometer monitor unit
to check that the instrument is sufficiently close to vertical to be
properly centred. See section 3.5, page 46
16.Calculate the volume of dry sand required to fill the gap
between the sensor and the borehole liner to the level of the top
of the sensor (
v = πr
2
h using the internal radius of the borehole,
less the volume of the instrument determined in step 1.)
17.Pour this sand into the borehole. If you can, check how much of
the sensor is covered with sand. Do not overfill the hole.
18.Carefully slacken the load bearing cable. This will engage the
locking arm of the strain relief mechanism and secure the
installation within the borehole.
19.Without pulling or lifting the sensor, lightly shake the cables to
remove any sand that may have fallen onto them or onto the
strain relief mechanism.
20.Clamp the load bearing cable to the top of the borehole, and
remove the winch.
21.The sensor can now be centred and unlocked ready for use.
November 2006
35
