Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 138

Monitoring sections. Special monitoring sections are laid out in advance or when re-
quired at passages that are especially sensitive to settlement. The number of points in
longitudinal and transverse directions is increased, with a spacing of only 5 m along the
alignment. Extensometers can also be installed in the ground, sometimes supplemented
by inclinometers. These measurement points are installed in advance of the passage of the
arrival of the shield machine around the location of the tunnel (Fig. 4-6, top left).
Based on the results of these measurements in combination with the data recorded at the
machine, the effect of support pressure, shield progress and annular gap filling on the sur-
rounding ground can be analysed.
In addition to the described geotechnical measurement programme, further measurements
can also be made on the tunnel lining as the drive advances.
Convergence measurement. Convergences of tunnel tubes are measured at specified
spacings. Starting with the first reading directly after the completion of ring building and
still within the protection of the shield skin, further measurements are made as the lining
leaves the shield tail, the annular gap is grouted, the annular gap grout hardens and the
backup passes by. Particularly in case of defects, the causes of structural damage to the
ring in the critical construction phase can be determined from the convergences.
The measurements can be made with inclinometers, theodolite or also distometers. Re-
garding the measured data, inclinometer measurements have the advantage that they re-
cord deformations continuously.
Crown point measurements. In order to provide further information about the deformation
behaviour of the latest segment ring, crown points are measured. This records the vertical
movement of the tunnel crown in the freshly grouted annular gap. Based on this data in
combination with other investigations, conclusions can be drawn about the drainage and
hydration behaviour of the annular gap grout and any necessary adaptation of the grout mix
can be made.
The behaviour of the ring as it leaves the shield has a particular potential for damage to the
segments due to the lack of solid bedding and the flexibility of the fresh grout.
Measurement of joint offset and joint openings. In order to analyse the behaviour of
the segments in more detail, the offsets and openings in the ring joints as well as the radial
joints can be measured in addition to the convergence and crown point measurements.
This makes it possible to record the course of ring deformation and the appearance of
structural deformation and analyse the causes.
Pressure cells. In order to check the resultant grouting pressure in the annular gap, pres-
sure cells can be installed in the segments at the monitoring section to record the curve
of pressure of annular gap grouting from the time of injection to its hardening. This can
detect pressure losses in the grouting pipes and also the drainage and hydration behaviour
of the annular gap grout.
4.2.4.2 Recording and evaluation of machine data
Numerous technical innovations [134, 141, 253] have developed shield tunnelling in loose
ground to a high-performance construction process in recent years. The scope of applica-