Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 152

4.3.1.5 Extensometer measurements
Convergence measurements and levelling can only record the movement of a surface (tun-
nel walls or ground surface), but not deformation in the ground or the distribution of
deformation with depth. In order to record these effects, single or multiple extensometers
are installed in boreholes drilled from the tunnel (Fig. 4-16 left) or - for shallow tunnels -
above ground (Fig. 4-16 right).
Figure 4-16 Extensom-
eter to monitor deforma-
tions in the direction of a
borehole. Borehole drilled
from the tunnel (left) or
from above ground (right).
Single extensometer. A single extensometer consists in principle of a rebar, which is ce-
mented into a drilled hole (fixed point), the measuring rod and the extensometer head,
which is fixed into the opening of the hole with anchors (rod guide and measuring instru-
ment). The movement of the rock mass is shown by the change of length of the free length
of the measuring rod projecting from the extensometer head.
Multiple extensometer. Multiple extensometers (Fig. 4-16 and Fig. 4-17) or a number of
single extensometers of various lengths can be used to determine the movements of the
rock mass to give - presuming that the fixed point anchored at depth does not lie within
the loosened zone - a picture of the extent and deformation behaviour and the absolute
displacement of the perimeter of the cavity (Fig. 4-18). Details of the installation of a
multiple extensometer can be found in the relevant product information.
In addition to extensometers with measuring rods, telescopic tubes (rope) and wire ex-
tensometers are also used. The use of the latter type is recommended when simultaneous
horizontal displacements are to be expected.
The borehole diameters vary according to the number of fixed points from 25 to 100 mm.
Measurement accuracies of about 0.1 to 0.01 mm enable early recording of displacement
tendencies. The measured values can be read at the location from mechanical dial gauges,
but the installation of an electronic transducer at the extensometer head for fine measure-
ment and registration is often more practical.