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6
3. TAKING READINGS
Before taking readings, check the battery voltage by depressing the "TEST" button on the side of
the tape storage reel; if no beep is heard the nine-volt alkaline battery needs to be replaced. (See
Appendix B for battery replacement instructions.) Access to the battery holder is made by
removing the three small screws around the periphery of the tape reel on the side that holds the
test button and buzzer.
Readings are taken by lowering the reed switch probe down the access tube until an audible beep
is heard from the buzzer in the tape reel. A reading of the graduated measurement tape is taken
opposite the top of the access tube when the buzzer just starts to sound.
For greater accuracy, a second reading can be obtained at the same anchor by lowering the probe
until the buzzer stays beeping. Take a measurement on the tape, as before, and average the two
readings to get the position of the anchor.
Repeat this process for all the anchors and the datum anchor at the bottom of the hole.
4. DATA REDUCTION
The distance from the datum magnet to the top of the access tube can be designated D
meters .
t
The distance from the top of the access tube to any intermediate anchor can be designated D
,
1
D
, D
, etc.
2
3
If the datum magnet is located in solid ground (i.e., no settlement) then the true settlement of any
intermediate anchor is given by D
- D
meters.
t
n
For instance, assume that initially D
= 29.214 meters and the measured distance to the top
t
anchor D
is 3.632 meters.
1
Then (D
-D
) at time T
is 25.582 meters.
t
1
0
One day later at time T
, D
= 28.943 meters and D
=3.595 meters. (D
-D
) at time T
is 25.348
1
t
1
t
1
1
meters, hence anchor D
has settled (25.582-25.348) = 0.234 meters. Similarly, the top of the
1
access tube has settled a distance of (29.214-28.943) = 0.271 meters and the compression of the
ground between the top of the access tube and anchor D
is (0.271-0.234) = 0.037 meters.
1
If, on the other hand, it is required that all measurements be referred to the top of the access tube,
then the actual elevation of the borehole anchors can be computed by subtracting the measured
distances D
, D
, D
, etc. from the surveyed elevation of the top of the access tube. The
1
2
3
settlement of each anchor is then the observed difference of elevation from one survey to the
next.
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