Environmental Engineering Reference
In-Depth Information
Lifting cable
Outer barrel
(extends to surface)
Overshot assembly
Lifting dogs
Spearhead
Latch retracting case
Latches
Inner barrel swivel
Inner barrel
Reaming shell
Core catcher box
Bit
Figure 5.21.
Principal components of a wireline drilling system (Clayton et al., 1995).
The ratio between natural fragment size in the rock to be sampled and core diameter is
important. It is almost impossible to core conglomerate with 50 mm pebbles with N size
equipment (76 mm hole, 52 mm NMLC core, or 45 mm NQ3 core). As a general rule
coarse-grained fractured rock requires large diameter core. N size drilling is commonly
used for high-strength rocks, but H size (96 mm hole, 63 mm HMLC core, or 61 mm HQ3
core) is significantly more effective in lower strength materials and only costs about 10%
more than N size. On many dam sites defects which control strength and permeability in
the dam foundation are close to vertical. Vertical drill holes are unsuitable for sampling
these features and in the assessment of their effect on the foundation permeability.
The ability to drill angled holes at selected azimuths enables the choice of hole direction
to provide maximum information including the orientation of rock defects and to interact
defects which control the rock mass permeability. Angled holes are commonly slower to
drill and slightly more expensive (contract rates are usually 10% to 15% higher than for
vertical drilling). Drill holes at angles of less than 45° to the horizontal require anchoring
of the rig to provide thrust.
 
 
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