Environmental Engineering Reference
In-Depth Information
Table 5.7.
Instrumentation and methods for the measurement of sur-
face and subsurface displacement (adapted from Dunnicliff,
1995 and Fell et al., 2000).
Parameter
Instruments/Methods
Surface deformation
Surveying methods, including GPS
Crack gauges/surface extensometers
Tiltmeters
Multi-point liquid level gauges
Photogrammetry
Satellite images
Remote video
Subsurface deformation
Inclinometers
Simple borehole deformation measurements
Fixed borehole extensometers
Slope extensometers
Shear pin indicators
In-place inclinometers
Multiple deflectometers
Acoustic emission monitoring
Time domain reflectometry (coaxial cables)
Pendulum
The results of repeat surveys of these marks should be recorded as amount of differen-
tial movement and plotted as vectors of vertical and horizontal displacement. These plots
enable a quick assessment of the amount and consistency of the movements that have
been recorded.
5.15.2
Borehole inclinometers
Subsurface movement can be measured using several different instruments as listed in
Table 5.7 and discussed in Chapter 20. These instruments are also used in the study of
landslide movements and are described by Dunnicliff (1995).
The most common method uses a borehole inclinometer, the principles of which are
shown in Figure 20.27. Successive readings on a borehole inclinometer provide informa-
tion on the amount and direction of deflection and the location of the point of rupture
( Figure 5.37 ) . A borehole extensometer uses magnets which can be installed outside the
PVC casing in soil or rock (Figure 20.23). Sliding couplings in the casing allow relative
movement which can be measured using a probe lowered down the borehole.
The installation of electrical shear strips within a slope or embankment when moni-
tored by a readout unit can indicate the location of an area of slope movement but not the
amount of movement.
5.16
COMMON ERRORS AND DEFICIENCIES IN GEOTECHNICAL
INVESTIGATION
Many problems during design and construction of dam structures are caused by poor
quality or inadequate geotechnical investigation. This is often due to the investigation not
following good engineering practice. Many of the problems are known to competent
practitioners. In some situations lack of available finance can force the adoption of poor
 
 
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