Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 134

In addition to the standard monitoring sections, at least two principal monitoring sections
should be provided in every tunnel as shown in Fig. 4-3. It may also be appropriate to pro-
vide surface monitoring sections at the same location if it can be expected that deformation
could extend to the surface. As soon as the working sequence has become established after
driving the first 50 to 100 m of tunnel, the first principal monitoring section should be set
up as near as possible to the round. The second principal monitoring section then serves
to check and interpret data from the first. In longer tunnels, or when the geological condi-
tions are very variable, for example in fault zones, the provision of a number of principal
monitoring sections is to be recommended. The data from principal monitoring sections also
provides the basis for dimensioning and deciding when to install the inner lining.
Invert heave monitoring section. In areas with clay minerals susceptible to swelling,
invert heave is also measured. Sliding micrometers are only provided in special cases. If
a number of these measurements are also performed in the inner lining, they are generally
assigned to the same monitoring station.
Monitoring section to evaluate the time to install the inner lining. In sections of tunnel
where the inner lining is installed although the residual deflection rate is still above 2 mm/
month, internal monitoring sections are installed at a spacing of at least 500 m at the same
locations as the external monitoring sections. Measurements at the internal measurement
section are undertaken at least once per month.
4.2.2.3 Surface measurements
Surface measurements are necessary for shallow tunnels, particularly when there are
buildings in the area affected by the tunnel. This normally entails precision levelling to
observe the behaviour of settlement with time, supplemented when necessary with ex-
tensometer or sliding micrometer measurements to determine the movement vectors. All
measuring equipment should be set a sufficient time before the arrival of the tunnelling
works. Measurements are generally continued until a month after the completion of the in-
vert slab or invert arch. Such measurements are specified according to the local conditions.
4.2.2.4 Basic rules for implementation and evaluation
The installation of measuring instruments and data loggers should be undertaken with
great care in order to avoid damage. Projecting sensors should be provided with protec-
tion against blasting and vehicles. Data should be recorded at all measurement locations
as soon as they are installed and for as long as they are accessible. The reading intervals
should be adapted to suit the geological conditions, for example starting daily, then every
few days and finally one measurement per month, with shorter intervals if required.
Operational details. There are various procedures for purchase, installation and logging:
Instruments are installed by the contractor or by an appointed specialist. The contractor
also provides auxiliary equipment like ladders, lifting equipment, lighting and transport.
All measurements are undertaken by the contractor. The measurement programme speci-
fied in the contract serves as a guideline for the instrumentation and its arrangement and
installation as well as the reading of the data. The extent of measurements to be under-
taken is specified by the employer or his appointed experts with the agreement of the
structural engineer and the construction company.