Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 377

The equipment for conventional mapping, also referred to as “compass method”
(ISRM 1978e), consists of
- a Clar compass to measure the orientations of discontinuities,

- a measuring tape to record the locations, spacing values and trace lengths of

discontinuities,

- a geologist's hammer to make discontinuities visible that are, for example, covered

by a thin weathered layer, to loosen rock samples and to sound the rock,

- a notepad or a prepared mapping form in which a sketch of the mapping and

comments on additional geological features can be entered,

- a camera to take pictures for documentation.

Discontinuity orientation
Discontinuity orientations are measured using a Clar compass as shown in Fig. 13.20.
The compass needs to be leveled horizontally by means of a bubble level and then the
lid is leant on the discontinuity (Fig. 13.21). When the magnetic needle is unlocked it
aligns itself with the earth's magnetic field after a few seconds. Once the needle has
been locked again, the discontinuity's dip direction

α d (Section 2.7.2) can be read off

the horizontal circle and the dip angle on the vertical circle (Figs. 13.20 and 13.21).

The magnetic needle possesses a red and a black end, whereas the vertical circle has
red and black sectors. If the index mark of the vertical circle lies within the red sector,
the dip direction must be read off at the red end of the magnetic needle. If the index
mark of the vertical circle lies within one of the black sectors, the dip direction must
be read off at the black end (Fig. 13.21). The dip angle is read off the index mark of
the vertical circle.

β

Figure 13.20 Clar compass