Geokon GK-604D, User Manual

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APPENDIX A. Inclinometer Theory
A.1. Inclinometer Theory
In the geotechnical field inclinometers are used primarily to measure ground
movements such as might occur in unstable slopes (landslides) or in the lateral
movement of ground around on-going excavations. They are also used to monitor
the stability of embankments, slurry walls, the disposition and deviation of driven
piles or drilled boreholes and the settlement of ground in fills, embankments, and
beneath storage tanks.
In all these situations it is normal to either
install a casing in a borehole drilled in the
ground, to cast it inside a concrete structure,
to bury it beneath an embankment, or the
like. The inclinometer casing has four
orthogonal grooves (Figure 74) designed to fit
the wheels of a portable inclinometer probe
(Figure 75). This probe, suspended on the
end of a cable connected to a readout device,
is used to survey the inclination of the casing
with respect to vertical (or horizontal) and in
this way to detect any changes in inclination
caused by ground movements.
The probe itself contains two MEMS, (Micro
Electro-Mechanical Sensor), accelerometers, which flex when acted on by the force
of gravity. Since the output voltage is proportional to the sine of the angle of
inclination, the output is also proportional to horizontal deviation of the borehole (or
the vertical deviation of a horizontal borehole).
In order to obtain a complete survey of the ground around the installed inclinometer
casing it is necessary to take a series of tilt measurements along the casing.
Typically an inclinometer probe has 2 sets of wheels separated by a distance of 2
feet (English system) or .5 meter (Metric system). A casing survey would begin by
lowering the probe to the bottom of the casing and taking a reading. The probe
would then be raised at 2 foot (English system) or .5 meter (Metric system)
intervals and a reading taken at each interval until the top of the casing is reached.
The set of readings thus generated is called the A
+
readings. Marks on the cable at
2 foot (English) or .5 meter (Metric) spacing facilitate the process. The probe is then
removed from the casing, rotated through 180 ° , replaced in the casing, lowered to
the bottom of the borehole and a second set of readings (the A
−
set) obtained as
the probe is raised at the reading interval.
A+
B+
A-
B-
(reference direction)
Figure 74 - Inclinometer Casing (end view)