Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 570

clinometer. The Trivec probe allows the displacements in three orthogonal directions
to be measured at the same time (Köppel et al. 1984). To compensate for elongations
due to temperature changes, the probe is also equipped with a temperature sensor.
The sensitivity of the instrument for displacements parallel to the borehole axis is
0.001 mm/m. The measuring accuracy for displacements normal to the borehole axis
is specifi ed as

0.05 mm/m (Fecker 1997).

17.5 Monitoring of Relative Displacements between Rock Surfaces

Relative displacements - that is changes in spacing - of points located at opposed rock
surfaces over the course of time are monitored by means of so-called “convergence
measuring devices” which measure the distance between pins anchored in the rock mass.
These pins are connected by a tensioned measuring tape or wire. A detailed description
of convergence measuring devices is given in DGGT (1996).

Figure 17.15 (upper) shows a convergence measuring device with steel tape. The tape is
tensioned by a constant force between a pin and the instrument which is itself attached
to a second pin at the rock surface. Then the spacing between the pins is measured using
a displacement gauge. The changes in spacing of the pins are determined as differences
of measured pin distances in consecutive measurements.
The so-called “distometer” developed by ISETH, illustrated in Fig. 17.15 (lower), is a
convergence measuring device using a tensioned invar wire instead of a tape (Kovari et
al. 1974, Brady & Brown 2006).
Figure 17.16 illustrates the principle of convergence measurements by means of the
example of two construction stages of a construction pit. In construction stage 1 the
measurement pins l and r are installed at opposed rock walls directly above the pit's
temporary invert (Fig. 17.16, left). Then the zero reading is carried out. After the pit
is excavated up to construction stage 2 a consecutive convergence measurement is con-
ducted. The relative displacement

δ r between pins l and r due to excavation is then
calculated from the difference of measured pin distances between measurements (Fig.
17.16, right).
Figure 17.17 shows examples for the arrangement of convergence measurement sec-
tions for full face, vault and sidewall adit excavations of tunnels (Section 20.2.1). The
measuring points on the tunnel contour should be installed closely (

δ l +

1 m) behind the
temporary face as in the case of geodetic measurements. By this means, a major portion
of the displacements due to excavation can be monitored.
The accuracy of convergence measurements mainly depends on the type of measure-
ment device and the length of the measuring section. A measuring accuracy ranging from
0.1 mm to 1 mm is specifi ed for convergence measuring devices with steel tapes
and measuring section lengths of 15 to 30 m. An even higher measuring accuracy of
0.02 mm to 0.03 mm is specifi ed for convergence measuring devices with invar wire
for corresponding measuring section lengths (DGGT 1996).

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