Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 563

mass are transmitted directly to the measurement marks. Their spacing and change in
spacing, are measured using a probe that is inserted into the casing and moved via cable
in a step-by-step procedure between the measurement marks that defi ne the measuring
points. By rotating the probe by 45° the probe's two heads are tensioned between two
adjacent measurement marks and brought into the measurement position.

Figure 17.8 Monitoring of the vertical displacements of the temporary invert due to excavation
by disconnecting the extensometer head (Wittke 1990)

The measurement of the spacing of adjacent measurement marks is carried out by
means of a displacement transducer inside the head. The measured data is transmitted
via cable to a digital data controller (Fig. 17.9, left). From the measured differential
displacements the rock mass deformation along the measuring line can be derived. With
the sliding micrometer the rock mass deformation parallel to the borehole axis can be
monitored over a length of up to 100 m. The measurement accuracy of the instrument
is specifi ed as better than 0.002 mm/m (Solexperts AG).
A similar probe extensometer is the so-called “sliding deformeter” also developed by
ISETH. The displacements measured by this probe are transmitted mechanically by
a precision measuring rod to the displacement sensor. The measuring accuracy of this
instrument is specifi ed as better than 0.02 mm (Solexperts AG).
Other probe extensometers, such as the incremental extensometer and the extensofor
probe, where the measurement of displacements is based on magnetic induction be-
tween the probe and measurement rings mounted on the casing, are less sensitive. The
measuring accuracy of these probes is 0.1 mm under favorable conditions (Fecker

1997).