Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 361

Wire line core barrels (Fig. 13.6, right) are double tube core barrels for continuous
drilling in all rock formations. The inner tube containing the core has a special design
to be removed using a winch cable with a core lifter fixed on a steel wire. The outer tube
remains in the borehole during removal of the core, saving time.
During drilling, penetration rate and bit pressure is recorded continuously. Alterations
in the penetration rate may indicate rock boundaries, zones of different degree of joint-
ing, weak layers, cavities and so on. If penetration rate and bit pressure is known, in
most cases it can be found out whether core losses are due to weak layers and cavities
or whether the drilling has been carried out incorrectly (DIN EN ISO 22475-1 2007).
Flushing pressure and losses of drilling fluid should be recorded as well. The loss of
drilling fluid may indicate high rock mass permeability.
After removal, the cores must be placed in the correct position into storage boxes (Fig.
13.7). Each core or storage box may be permanently marked with an arrow in the direc-
tion of drilling. The designation of the drilling and the depths from which the cores have
been extracted should be stated on the side of the storage box (Fig. 13.7). Where the core
has disintegrated it should be arranged as best as possible in its original position. Since
the precise depth of core losses is usually not known, the lost length is stated at the end of
the core on the storage box. If cavities in the rock mass are encountered the correspond-
ing section of the storage box should be left blank and marked appropriately.
Figure 13.7 Core boxes of drilling BK 5.2/12 from 100 m to 105 m, Stuttgart 21 (Germany)
The positions from which rock samples are taken for laboratory testing should be indi-
cated and a substitute such as a piece of wood should be placed at the corresponding
section. The samples must be protected from drying. An effective protection against
changes of moisture content is possible using a metal box or a metal foil coated with
plastic (Cristescu & Hunsche 1998). Another possibility is coating the samples with
wax.
Further details regarding handling, storage and transport of rock cores and the suita-
bility of individual drilling systems can be found in DIN EN ISO 22475-1 (2007).