Borehole extensometers consist of tensioned rods anchored at different points in a borehole as indicated in Figure 7. A change in the distance between the anchor and the rod head provides the displacement information for the rock mass.
Figure 7: Multi Point Borehole Extensometer
Extensometers measure the axial displacement between a number of reference points along same measurement axis. The wire extensometer is widely used and may be installed on the slope surface, or within a borehole (figure).
Figure 8: Wire Extensometer
A typical wire extensometers typically a measure baselines upto 80m in length with a precision of ±0.3mm per 30m of length (Gili et al., 2000). It offers high level of precision in the line of the measurement axis. The disadvantage, however, is that one dimensional displacement vector does not measure out-of-line displacements. The main sources of error in extensometers result from friction in the reference head and between the linkages, temperature induced sag, and the stress/strain characteristics between the linkages. All of these can have a significant impact on their use in a harsh mining environment. The extensometer must also be anchored outside the zone of deformation, which can be an issue if the deformation area is large.