Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 149

connection to the measurement bolts, a precision of 0.01 to 0.1 mm can be achieved and
thus much more precise than optical or geodetic methods [25].
Figure 4-11 Diagram of
convergence measurements
in a tunnel (Interfels GmbH).
Convergence measurements are evaluated with a displacement-time diagram (Fig. 4-12),
whose coordinates start from the zero measurement and with the repeat measurements be-
ing entered as difference from the zero measurements. Optimisation of the support entails
the best possible adaptation to the final displacement, which means a (nearly) horizontal
curve in the diagram.
In the example shown here (Fig. 4-12), the crown settlement F S is approx. 100 mm. Accord-
ing to experience, the horizontal convergence is about 10 to 70 % of the vertical, although
it can also be zero or even negative depending on the ground conditions. Statements about
the absolute displacements of the individual measurement points can be made by providing
additional extensometers (see Section 4.3.1.5) or by optical surveying of displacements (see
Section 4.3.1.3), which enable the measurement of vertical displacements over longer dis-
tances from survey points on the perimeter of the tunnel or on the ground surface.
Figure 4-12 Graphical
display of convergence
measurement in a time-
displacement graph.
The primary sources of errors are incorrect handling of the wire or measuring tape - kinks
for example can lead to false readings - and failure to compensate the effect of temperature.
Obstruction. Since the zero measurement has to be undertaken as soon as possible after
excavation, this can easily obstruct tunnelling work. For this reason, other methods of