Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 569

Figure 17.14 Trivec probe (Solexperts AG)
For a fl oating or inverted plumb line, the lower end of the wire is anchored in the rock
mass underneath the foundation level of the dam and the upper end loaded by a weight
swims freely movable within a container (Fig. 17.12, center). The lower end of the wire
must be anchored deep enough to be a fi xed point.
The measurement of the horizontal displacement of the dam at certain elevations can
be carried out optically or by distance measurement devices using lasers. With the aid of
plumb line devices a measuring accuracy of 0.05 mm can be achieved (Fecker 1997).
Figure 17.12 (right), for example, shows the arrangement of plumb line devices for the
arch dam El Cajon, located in Honduras. By means of a suspended and inverted plumb
line the horizontal displacements are measured at fi ve inspection galleries at six ele-
vations (17 m, 77 m, 137 m, 167 m, 197 m and 218 m above foundation level). These
displacements were used to derive a defl ection curve of the dam.
17.4.3 Three-Dimensional Monitoring of Displacements along Boreholes
Devices that measure the displacements both parallel and normal to the borehole axis
have also been developed. Figure 17.14, for example, shows the so-called Trivec probe
developed by ISETH, being a combination of a sliding micrometer and a probe in-