Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 363

Figure 13.8 Mapping of a drill core (Wittke 1990)

Sometimes “frequency” F, defined as the number of natural discontinuities intersecting
a unit length of recovered core, also serves as a measure of the disintegration of cores
(ISRM 1978e).

A further quantity used to characterize the measure of disintegration of cores is the
RQD introduced in Section 12.4. In the example represented in Fig. 13.8, an RQD of
67% was evaluated corresponding to class “fair” according to Table 12.1.
Like the RQD (Section 12.5) also R and F depend on a number of influencing factors
and thus give a biased view of the natural degree of separation of a rock mass. One of
the main problems in finding out the natural degree of separation from the mapping of
drill cores is that artificial fractures caused by the drilling process often cannot clearly be
distinguished from natural discontinuities. In this respect, better results can be achieved
using observation methods of borehole walls that will be described in Section 13.5.2.

Discontinuity orientations can be determined on oriented, vertically drilled cores. Dif-
ferent methods are available to orient cores. One method described by Heitfeld et al.
(1982) is illustrated in Fig. 13.9. Before the core drilling is carried out a drilling with
small diameter is sunk at the perimeter of the planned exploration borehole to mark the
direction of north. The strike and dip angles

can then be measured at the core
using an angle meter and a tape, respectively. For an inclined borehole these angles are
apparent strike and dip angles that can be converted into true angles of strike and dip,
for example using graphical or mathematical methods (Lau 1982).

α and β