Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 154

Figure 4-19 Extensometer measurements at the construction of the Stadtbahn, Mülheim an der Ruhr
[69]. Settlement in transverse profile (left) and behaviour above the crown with time (right).
Layout and installation. The positioning of extensometers should consider the following
points:
- In stratified or foliated geology with anisotropic stress-strain behaviour, extensometers
should be installed as far as possible both normal and parallel to the bedding or foliation
in order to investigate the effect of deformation in different directions.
- Extensometers should generally be installed before the tunnel advance or, if this is not
possible, immediately after the tunnel advance. If there is already an investigation head-
ing in a planned rock cavity, initial displacements can be recorded with extensometers
that can be disconnected. When the excavation approaches, the endangered components
can then be removed, leaving the remaining components unaffected in the borehole.
- For the arrangement of fixed points at various depths of the borehole, it is sensible to take
into account the prevailing bedding conditions from the borehole logs (if possible from
cores).
Since the behaviour of the fixed points can no longer be controlled in a filled borehole,
measurements are subject to uncertainty whether the actual behaviour corresponds to the
assumed behaviour, i. e. whether the fixed points remain at the intended locations or come
loose from the rock mass together with the mortar or are pulled out of the mortar.
Length measuring anchors. Another proposal for the layout of extensometers is made by
F. Schuermann [214] under the description “length measuring anchor process”. Four sin-
gle rock extensometers are set in a monitoring section as shown in Fig. 4-20, in pairs with
two horizontal and two vertical. In order to record the formation of a load-bearing ring in