Civil Engineering Reference
In-Depth Information
length of the core runs.
the times specified for the camera operation. At
each time interval a photograph is taken of the
dip meter and compass, and the depth is recor-
ded. When the tool is recovered from the hole,
the film is developed to show the hole orientation
at the recorded depths. Other hole orientation
tools include the Tropari single shot instrument,
and gyroscopes for use in magnetic environments
(Australian Drilling Industry, 1996).
Most methods of orienting core involve mark-
ing a line down the core representing the top of the
hole. Since the orientation of this line is known
from the hole survey, the orientation of all dis-
continuities in the core can be measured relative
to this line, from which their dip and strike can
be calculated (Figure 3.13). Figure 3.13 shows
that a plane intersected by the core has the shape
of an ellipse, and the first step in the calcula-
tion process is to mark the down-hole end major
axis of this ellipse. The dip
(δ)
of this plane is
then measured relative to the core axis, and a
reference angle
(α)
is measured clockwise (look-
ing down-hole) around the circumference of the
core from the top-of-core line to the major axis
of the ellipse. The dip and dip direction of the
plane is calculated from the plunge and trend of
the hole and the measured angles
δ
and
α
. The
true dip and dip direction of a discontinuity in the
core can be determined by stereographic meth-
ods (Goodman, 1976), or by spherical/analytical
geometry methods (Lau, 1983).
In a few cases, the core may contain a dis-
tinct and consistent marker of known orientation,
number of natural discontinuities
0.5 m length of core
Fracture index
=
(3.23)
(c)
Core recovery
is a measure of how much
rock has been lost during drilling. Core loss
may result from weak zones being washed
out by the drilling water, or grinding of the
core during drilling, or the presence of an
open cavity. Drillers can often detect areas
of very weak rock or cavities where there is
a sudden increase in the advance rate; these
zones should be noted on the log. The core
recovery value is determined by measuring
the length of recovered core compared to the
total length drilled. If a length of lost core is
identified when logging the core, it is good
practice to install a wooden spacer at the
location of the lost core.
total length of recovered core
length of drill run
Recovery
=
×
100
(3.24)
On Figure 3.12, values for these three para-
meters are shown graphically so that areas of
weak or broken rock can be readily identified
when scanning down the log. When making RQD
and fracture index measurements it is important
that drilling breaks in the core are identified and
not included in the reported values.
3.6.4 Core orientation
For conditions where there is insufficient design
data on discontinuity orientation from surface
mapping, it may be necessary to obtain this data
from drill core. This will require that the core be
oriented.
The first step in orienting core is to deter-
mine the plunge and trend of the drill hole using
a down-hole survey tool. One such tool comprises
an aluminum (non-magnetic) drill rod that con-
tains a dip meter and a compass, both of which
can be photographed at specified time intervals.
The orientation tool is lowered down the hole on
the end of the drill string, and is held stationary at
Reference angle,
Discontinuity
surface (ellipse)
Dip,
Maximum
dip vector
Top of core (
in situ
)
Direction of drilling
Figure 3.13
Measurement of orientation of
discontinuity in oriented drill core.
