Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 556

If settlements are to be expected, resulting from a lowering of the groundwater table
due to tunneling, the zero reading should be carried out before the beginning of con-
struction. The measuring points on the tunnel contour should be installed close (

≤ 1 m)

behind the temporary face (Wittke et al. 2002).
Figure 17.2 (upper) gives an example of the representation of vertical displacements of
points on the tunnel contour measured during a vault excavation versus time. The lower
part of Fig. 17.2 shows the advance of the temporary face over time.

17.3 Monitoring of Vertical Displacements on the Ground Surface

Vertical displacements of the ground surface, such as settlements, subsidence or heav-
ing, can either be measured geodetically by the leveling of measuring points (Fig. 17.1)
or by using a liquid leveling system.
Vertical displacement measurements with the aid of liquid leveling, also referred to as
“hydrostatic leveling”, are based on the principle of communicating pipes. The meas-
uring principle can be explained by means of a traditional liquid leveling system which
was even used in ancient Egypt. It consists of two open standpipes that are connected
by a fl exible tube. This system is fi lled with water. Since according to the principle of
communicating pipes, the water level in both standpipes is the same, the relative vertical
displacement of the second pipe compared to the fi rst pipe can be read from the water
elevations z 2 and z 1 in both pipes as s = z 1 - z 2 (Fig. 17.3, upper).

Nowadays electronic or digital water leveling systems are used in which several sensors
and a reference station with reference sensor are interconnected by means of a pres-
sure line fi lled with liquid, an air compensation line and a digital data cable (Fig. 17.3,
lower). The elevation changes of individual sensors are then derived from gravitational
pressure differences of the liquid by comparing the pressure at each sensor with the
pressure at the reference sensor.
Although reading accuracies of 0.001 mm are specifi ed by manufacturers, a measure-
ment accuracy of 0.01 mm is considered to be more realistic, due to various sources
of error (Fecker 1997). Nevertheless, the measurement accuracy of liquid leveling is
considerably higher than of geodetic measurements.

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