Hydraulic Structures: Fourth Edition

INSTRUMENTATION 299

audible signal as probe

passes magnet stations

rigid PVC access

tube, 25 mm I.D.

c. 3.0 m

Installation in

fill during

construction

grout

borehole

Installation in

original ground

or existing fill

top of fill

reed-switch

probe

annular plate

magnet station

differential

movement

joint

natural ground level

annular ‘spider’-type

magnet stations

Fig. 7.4 Vertical settlement: magnet extensometer

ciple can be extended to existing fills and foundations as shown, using
special ‘snap-out’ spider-type magnet stations designed for installation at
suitable horizons within boreholes. Individual point-settlement gauges,
generally operated on the manometer principle and illustrated in Fig. 7.5,
are sometimes preferable for the more plastic fill materials, e.g. in cores.
Horizontal components of internal deformation may be determined
by the borehole inclinometer referred to previously or by adaptation of
the principle of the vertical extensometer gauge. In the latter instance, the
tubes are laid in trenches within the shoulder fill, with the magnet stations
or collars installed at suitable horizontal intervals. A special duct-motor
device can then be employed to ‘walk’ the probe through the tube and
back, determining the location of each station as it passes.
It is customary to provide fixed-length rock extensometers (not illus-
trated), to monitor the abutment response of arch dams. Similar exten-
someters may also be appropriate for other types of concrete dams if
foundation deformability is a critical parameter.