Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 572

Figure 17.16 Principle of convergence measurements (Wittke 1990)

Figure 17.17 Example of convergence measurement sections in a tunnel

17.6 Pressure Monitoring

Stress and stress change monitoring using pressure cells is carried out for example to
measure the stresses in the shotcrete membrane and the internal lining of a tunnel. Pres-
sure cells operate according to different measuring principles. A detailed description of
pressure cells is given in Fecker (1997) and DGGT (2004a).
Figure 17.18 illustrates a hydraulic pressure cell consisting of a flat jack and a pressure
monitoring unit which are connected by a short line. The pressure monitoring unit com-
prises an inner and an outer chamber which are separated by a flexible membrane. Flat
jack, line and inner chamber are filled with oil or another pressure transferring hydraulic
fluid (cell fluid) and form a closed system. The outer chamber is connected to a pressure
line and a return line.