Monitoring - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 566

wall of a vertical borehole and lowered via cable in stages of 0.5 - 1 m to the bottom of
the borehole. The tube has two orthogonal pairs of keyways that serve as guidance for
the probe. Displacements of the rock mass are transmitted directly to the guide tube.
Measurements of tilt and probe depth at each stage are used to compute deviations
of the guide tube from its original vertical position. From differences between succes-
sive measurements horizontal displacements can be calculated as a function of depth
(Fig. 17.11, right).
Figure 17.11 Vertical probe inclinometer
Probe inclinometers that are applied in vertical and horizontal boreholes denoted as
“vertical inclinometers” and “horizontal inclinometers”, respectively, are equipped with
tilt sensors measuring the angular deviation with respect to the vertical, according to
the pendulum principle. In vertical inclinometers the tilt sensors are thus installed par-
allel to the borehole axis. For an inclinometer equipped with only one tilt sensor the
probe is lowered with its guide wheels in each pair of keyways. From both measuring
traverses the horizontal tilt vector can be determined as a function of depth. Inclinom-
eters that are equipped with two orthogonally arranged tilt sensors allow the horizontal
tilt vector versus depth to be determined in only one measuring traverse. In horizontal
inclinometers only one tilt sensor is installed normal to borehole axis measuring vertical
displacements of the rock mass (DGGT 2002b).
Probe inclinometers are applicable in depths to 200 m and available for different diame-
ters of guide tubes of 35 - 75 mm. For large borehole diameters up to 150 mm the guide
tube optionally can be fixed in the borehole using spacers.