Environmental Engineering Reference
In-Depth Information
The pipe must be protected from disturbance during construction. Plant growth and cor-
rosion within the pipe may in time inhibit torpedo movement.
4.3.3
Lateral Displacement
Inclinometer
Application
Continuous measurements of lateral deflections are made from the surface with the incli-
nometer. Commonly used in slope studies, this device may also be used to measure defor-
mations in soft soils beneath fills, or installed behind retaining structures or in pile
foundations.
Instruments
The inclinometer contains servo-accelerometers that can detect lateral movements of the
order of
0.0001 ft per 2 ft of casing (the normal depth increment at which readings are
taken). Since the voltage output is proportional to the sine of the angle of inclination of the
long axis of the sensor, from the vertical, it can be used to measure true deviations from
verticality.
Installation and Operation
The inclinometer is lowered and raised in specially grooved casing (3.38 or 2.79 in. O.D.)
installed in a borehole and extended to a depth below the anticipated movement zone. The
casing, the servo-type inclinometer, cable, and digital readout are shown in
Figure 4.16.
In
soil formations, it is advisable to install 6-in. casing to allow for packing sand between the
casing and the grooved casing, as the 6-in. casing is withdrawn to provide a sure contact
between the inclinometer casing and the borehole wall. The casing bottom is often grouted
into place to assure fixity. For monitoring pile and wall movements, the casing is attached
directly to the structural member.
The casing is grooved at 90
intervals to guide the inclinometer wheels and to allow
measurements along two axes. The relatively flexible casing deflects freely during ground
movements. From readings taken at regular depth intervals, a profile of the casing is con-
structed. Repeating the measurements periodically provides data on location, magnitude,
direction, and rate of movement. At some amount of casing deflection the inclinometer
will not pass the bend. Slope Indicator Co. give the shortest radius at which the probe can
be retrieved as 10 ft and the shortest radius curve at which the probe will pass for reading
as 17.6 ft.
Figure 4.17
is an example of inclinometer readings of a failure zone in a slope.
Constructing a vector from the readings at 90
°
provides the resultant of movement direc-
tion and amount. Plots of deflection vs. time are prepared; movement acceleration is an
indication of approaching failure.
°
In-place Inclinometer Sensors
Slope Indicator Co. provide an in-place inclinometer system consisting of a string
of inclinometer sensors permanently deployed in the casing
(
Figure 4.18)
.
The sensors
are set to span the zone where movement measurements are desired. The sensors can
be connected to a data acquisition system that continuously monitors movements and
can trigger an alarm when it detects a rate of change in deflection that exceeds a preset
value.
