Measurement for monitoring, probing and recording evidence - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 163

can be measured at the end of the hose, i. e. outside the concrete, and is equal to the stress σ ,
apart from a slight pressure less in the supply pipe of about 0.01 N/mm 2 per 100 m pipe length.
Figure 4-30 Diagram of
the principle of a hydraulic
pressure cell; Glötzl
System, Gesellschaft für
Baumesstechnik mbH.
Principle of electrical vibrating wire sensors. Changes of a measured quantity (for ex-
ample travel, pressure, strain, force) cause changes of strain and thus the resonance fre-
quency of a measurement wire, which is fixed in the sensor but capable of vibrating (Fig.
4-31). The wire vibrates in the magnetic field of an electro-magnet system and induces an
electrical oscillation of the same frequency, which is transmitted through a cable to the
receiver and there processed to give the measured value. The measuring wire is stimulated
by the same electro-magnet system as the receiver. The wire frequency can be calculated
according to the formula
f = C · ε
with measurement constant C determined by length, mass and elasticity of the wire. The
change of strain ε of the wire is proportional to the change of the measured value and thus
determines the frequency change of a constructively fixed sensor.
Figure 4-31 Basic system of a vibrating wire sensor, System Maihak AG.
Measurements. The two types of measuring instrument can be used both to measure
radial ground pressure acting on the concrete layer (contact stress ground-concrete) and
to determine the tangential pressure distribution in the concrete layer itself as pressure
cell. Pressure cells (vibrating wire process) are installed for this purpose before spraying
the shotcrete and are thus lost. Fig. 4-32 shows a typical arrangement of pressure cells for
radial and tangential measurements in tunnelling.