Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 136

despite concreting of the inner lining in order to gain further valuable data. At least two
monitoring sections as shown in Fig. 4-5 should be provided in every tunnel. The locations
of principal monitoring sections used during the construction phase are generally still suit-
able as monitoring stations. Measurements in the completed state should generally be read
at ever increasing intervals until it is clear that deformation has fully subsided. It can be
assumed that instruments are read at least once a month during the construction period.
The same requirements for the measuring instruments apply as for the construction phase.
Figure 4-5 Principal
monitoring section for the
inner lining (W = piezom-
eter, D = pressure cell).
Additionally: continuation
of reading in the pressure
cells in the outer support
layer at the same monitor-
ing section (see Fig. 4-3).
4.2.3.2 Evaluation
The measured data is entered in a continuously updated graphical display, partly also as
a continuation of records from the construction state. Clearly unusual events should be
reported to the employer and his consultants immediately. The measured data is to be
presented in a clear form and interpreted at regular intervals. Readings should also be
compared to the original structural calculations. This comparison should be extensively
interpreted in writing.
Measured data, the associated comparative calculations and interpretations should be
summarised in a detailed report after the completion of measurements and delivered to
the employer.
4.2.4 Special features of shield drives
The measurement programme for a shield-driven tunnel is intended on the one hand to re-
cord the effects on the environment and surface building, and on the other to determine the
deformation of the segment lining and the loading on the lining. In addition, the monitor-
ing of tunnelling by recording and interpretation of machine data is particularly important
in shield tunnelling [43].
4.2.4.1 Instrumentation
Surface settlement. In order to monitor settlement, transverse chains of five to six survey
points are arranged as at a spacing of 25 m above the route of the tunnel to determine the
surface settlements (Fig. 4-6, top right).