Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 560

The measuring principle of a rod extensometer is illustrated in Fig. 17.6 by means of
a construction pit which has been excavated to a particular depth (Wittke 1990). Just
above the temporary invert of the pit a single-point rod extensometer is installed into
a horizontal borehole. Then the construction pit is excavated to a lower level. If the
extensometer rod is long enough, the anchor remains fixed during excavation and the
total horizontal wall displacement

δ H can be measured (Fig. 17.6, upper). However, if
the extensometer anchor is located in an area which is still subject to deformation by
a further excavation of the pit, the anchor suffers a displacement

δ 2 and only a part
of the horizontal wall displacement can be measured as relative displacement δ 2

(Fig. 17.6, center). In such cases the extensometer head can be tied to a benchmark to
determine the absolute horizontal wall displacement.

δ H -

Figure 17.5 Plastic rod extensometers (Glötzl GmbH)