Geotechnical and Materials Engineering Standards for Bridge ...

If the core is broken by handling or during drilling (i.e. the
fracture surfaces ere fresh irregular breaks rather than natural joint
surfaces); the fresh broken pieces should be fitted together and counted
as one piece. Some judgement is necessary in the case of thinly bedded
sedimentary rocks and foliated metamorphic rocks, and the method is not
so precise in these eases as it is for igneous rocks, thick-bedded
limestones, sandstones, etc. However, the system has been applied
successfully even for shales, although it 1s necessary t o log the cores
immediately upon removing them from the core barrel, before air-slaking
and cracking can begfn.
The procedure obviously penalizes rock masses where core recovery Ps
poor. Th5.s is appropriate because poor core recovery usually reflects
poor quality rock. Paar drilling equipment and techniques can also
cause poor recovery. Fox this reason, double-tube core barrels of at
least NX size (54 mm in diameter) must be used, snd proper supemision
of drilling is imperative.
As simple as the procedure appears, it has been found that, as an
indicator of general quality of rock for engtntering purposes, the
RQD-value I s more sensittve and consistent than grass percentage core
recovery.