Geology Reference
In-Depth Information
tube, which does not rotate; the
flows between the inner
tube and an outer tube without touching the core. Such equipment has
low manufacturing tolerances so must be bought off the shelf, and the
bits are very expensive and only last perhaps 8 to 12m of coring before
they need to be replaced, which precludes its use on many projects.
Usually, the larger the diameter of the core barrel, the better the
recovery and quality of sample, and it is prudent to start using a
large diameter and reduce diameter as necessary with depth. The wide
range of casing, core barrel and drill rod sizes are listed in ASTM(1999),
which also discusses good practice. When there is good-quality rock
overlying soil material, retrieving the softer material can be a problem.
As for soil boring, the hole may need to be cased temporarily during
drilling to prevent it collapsing. Drillers generally try to recover about
1.5m of core per run before pulling all the drill string back to the ground
surface and dismantling it all. If recovery is low, then the driller might try
to reduce the core run to 1m or even less, but this does not always
produce better results. Other parameters such as thrust, torque and
flushing medium
uence on recovery, and much
depends on the experience, knowledge and attitude of the drilling crew.
Wire line drilling employs large-diameter rods, which effectively
support the hole as it advances. After each core run, the core barrel is
pulled up the centre of the drill rods, the core extracted, then dropped
back down the hole to lock into the bottom of the hole, ready to start
drilling again. The cutting bit stays at the bottom of the drill rods and is
not extracted with the core barrel. To change the cutting bit, however,
the whole drill string has to be removed.
A system that is very commonly used in Hong Kong and elsewhere for
sampling weathered rock and mixed rock and soil is aMazier core barrel.
This has a soil cutting shoe which is spring loaded and extrudes in
advance of an outer rock cutting bit when cutting through relatively
weak soil-like material (Figures 4.27 and 4.28). As conditions get
harder, the soil cutter is pushed back and the outer coring bit takes
over. This system, especially where combined with polymer foam
flushing medium may have more in
ush,
has been shown to produce good recovery of material in weathered and
mixed materials (Phillipson & Chipp, 1982). The sample is taken in a
plastic tube, which is later cut open so that the sample can be examined,
described and tested ( Figure 4.29). Drilling contractors will not open
tubed samples without instruction to do so, and, in practice, geotechnical
engineers sometimes order Mazier samples (from the of
ce) but then
never get round to opening and examining the samples, which is poor
practice. The author was recently involved in an arbitration where 20
boreholes had been put down with alternate Mazier sampling in soft
clays and then SPTs. The project was then designed on the basis of the
SPT data alone and went badly wrong, ending in arbitration. The sam-
ples had not been opened up for examination or testing. A similar system
to the Mazier, used in the USA, is the Dennison sampler (Hunt, 2005).
 
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