Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 162

4.3.3 Stress and strain measurements in the support layer
Comparison of the actual state with the assumptions in the design calculations, which has
already been mentioned in Table 4-2, requires not only the measurement of displacements
but also stress and strain in the support layer. This enables a complete mechanical descrip-
tion of the system of tunnel-support-ground. Fig. 4-29 shows the instrumentation normally
used for this purpose (“Standard instrumentation for tunnelling”) from [24].

1 Measurement of displacements of the supported tunnel perimeter

(convergence tape or geodetic distance measurement, see Section
4.3.1.1 or 4.3.1.2)

2 Measurement of displacements in the ground around the tunnel

(extensometers or inclinometers, see Section 4.3.1.5 or 4.3.1.6)

3 Monitoring of the anchors (anchor force sensor, measuring an-

chor, see Section 4.3.4)

Figure 4-29 Standard
instrumentation for
the monitoring of deep
tunnels; Interfels GmbH.

4 Monitoring of the shotcrete (pressure cells, vibrating wire sensors,

see Section 4.3.3.1)

4.3.3.1 Radial and tangential stress measurement in concrete
Stress measurements are mainly undertaken in the support, mostly in concrete. There are
two basic types:
1. Instruments using the hydraulic principle.
2. Instruments using the vibrating wire principle.
The latter type of system is used in many applications due to its universal measurement
principle, for example in extensometers, anchor force measurement and steel strain meas-
urement.
Principle of pressure cells working on the hydraulic principle. The investigated stress σ is
compensated by a hydraulic or pneumatic pressure that builds up independently in the sensor
and in the supply pipe (Fig. 4-30). This is possible because the sensor is designed as a pressure
relief valve, which is loaded and controlled with the investigated stress. The back-pressure p