Environmental Engineering Reference
In-Depth Information
The wires are housed in telescoping segments of pipe. Corrections for overall translation is
made by surveying the position of the terminal well. ANCOLD (1983) indicate the accuracy
to be
1.5 mm.
Soil Instruments Ltd. (1985), and Hanna (1985), describe other extensometers which
can be used to measure horizontal displacements and strains. These include devices which
use vibrating wire and resistance potentiometers to do the measurements.
Figure 20.25 shows the use of such devices to measure deformations in the earth core
of a dam parallel to the dam axis. In this case, several extensometers are installed with
extension rods connecting the anchor plate and the vibrating wire extensometers.
Whether it is wise to install such instruments is doubtful, because of the defect it gives in
the dam core and the information is not usually so important.
Lateral movements in dams can also be measured by use of borehole inclinometers. The
inclinometer casing may be installed during construction of the dam, or after, in the event
that excessive displacements are observed. If the casing is installed on a slope, e.g. along
the face slab of a concrete face rockfill dam, horizontal and vertical displacements can be
determined.
Most inclinometers are similar to that shown in
Figure 20.26
,
in that the inclinometer
is passed down casing which is grooved to maintain the orientation of the sensor probe.
The tilt of the sensor is measured and recorded by servo-accelerometers located within
the probe. Where two accelerometers are used they can sense the inclination of the tube in
two directions at right angles to each other. Processing of the signal at the surface allows
display of the angular rotation, or displacement normal to the casing in the two direc-
tions. Settlement gauge magnets may be installed around the casing to allow assessment
of axial deformations.
The casing may be plastic or aluminium and should be able to telescope, to allow for
differential settlement. Soil Instruments Ltd. (1985) indicate a resolution of
0.1 mm.
Hanna (1985) indicates that careful calibration and maintenance is necessary to achieve a
high precision.
Figure 20.25.
Use of extensometer to measure strain parallel to the dam axis (ICOLD, 1989).
