Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 574

Pressure cells operating according to the vibrating wire principle measure the displace-
ment of the membrane by a change of length of a tensioned free vibrating wire leading
to a change in natural frequency. This is accomplished by a vibrating wire strain sensor.
Figure 17.19 shows two vibrating wire pressure cells.
In other types of pressure cells the displacement of the membrane is measured pneu-
matically or electrically using a strain gauge or a membrane switch (DGGT 2004a).

Figure 17.19 Vibrating wire pressure cells (Geokon Inc.)

The measuring accuracy that is attainable with pressure cells depends on type, normal
stiffness of flat jack, quality and conditions of installation, stiffness of the cell fluid and
the magnitude of measured stress changes. From experience, the pressure can be meas-
ured with an error of < 5% between 20% and 80% of the nominal value of the pressure
cell (DGGT 2004a).
A typical arrangement of pressure cells in a shotcrete membrane of a tunnel is illus-
trated in Fig. 17.20 (Wittke 1990). One cell is placed on the outside of the shotcrete
membrane to measure the stress

σ r at the interface between shotcrete and rock mass,
that is, the radial loading of the shotcrete membrane referred to as “radial stress” or