Civil Engineering Reference
In-Depth Information
equipment and TT indicates a triple tube core bar-
rel. Also listed on the table are casing diameters;
casing is used in the upper part of holes, in soil
or weathered rock, to prevent caving. The cas-
ing and rod diameters are arranged such that N
rods fit inside N casing, and that B rods fit inside B
casing, for example. This allows holes to be “tele-
scoped” down to a smaller diameter, if necessary,
as the hole is advanced. It is usual in geotechnical
drilling to use NQTT rods, or HQTT if the rock
is highly broken because better core recovery is
achieved with the larger diameter.
For geotechnical drilling where one of the
objectives is to recover the weakest portion of the
rock, recommended practice is to use a triple tube
core barrel. The triple tube barrel comprises an
outer tube coupled to the drill rods and bit, which
turns with the rods while drilling. The middle tube
locks into a roller bearing at the top of the core
barrel such that it remains stationary while the
drill rods rotate. The inner tube is split into two
pieces longitudinally, and also remains stationary
during drilling.
The drilling procedure involves rotating the
rods and bit at high speed (up to about 1000 rpm),
while applying a steady thrust to the bit and
pumping water down the center of the drill rods.
In this way the bit is advanced into the rock while
the water cools the bit, and removes the cuttings
in the annular space outside the rods. The core fills
the core barrel as the bit advances, and the drilling
is stopped when the core barrel is full—the core
barrel is usually 3 m long. In holes with depths
less than about 20 m, it is possible to recover the
core at the end of each run by removing the rods
from the hole. For deeper holes the usual prac-
tice is to lower a wireline down the hole with a
core catcher on the end which locks into the core
barrel and releases it from the drill rods. The core
barrel is lifted from the hole without moving the
rods. The core is extracted from the core barrel by
pumping the inner split tube out of the barrel and
then removing the upper half of the tube. The core
can then be logged in the core barrel with minimal
disturbance. In contrast, if a double tube barrel
is used, the core has to be pumped or hammered
out of the tube, which inevitably results in some
damage and disturbance, especially to the weaker
portions of the rock that are the most important
to recover.
In very poor quality rock, it is possible to
modify the triple tube by inserting a clear plastic
liner inside the split inner tube. The core is
contained in the plastic tube so that it can be
logged and then stored with minimal disturbance.
3.6.2 Diamond drilling operations
The following are some of the factors that may
need to be considered in a diamond drilling oper-
ation. First, the drill rig should be set up on level
ground, or on a level platform if the ground surface
is irregular. Furthermore, the platform should be
robust and the rig should be securely attached to
the ground or platform. If the rig is able to move
during drilling, vibration in the rods will diminish
core quality and may damage the rods.
It is essential that the drill bit be continuously
flushed with a fluid, usually water, to cool the bit
and remove the cuttings. The circulating water
also lubricates the drill string to reduce the torque
required to turn the rods, and reduces vibration
of the rods. Water is usually supplied to the site
by pumping from a nearby river, or by a tanker
truck. Factors to consider in the supply of water
include the pumping head between the supply
and the site, freezing of the pipeline, and available
road access. It is usual that the return drill water
is collected in a settling tank at the site to remove
the cuttings, and is then recirculated down the
hole. This reduces the quantity of supplied water,
and eliminates environmental contamination by
silt-laden water.
The addition of certain chemicals and solids
to the drilling water can improve the properties
of the circulating fluid, and can be essential for
successfully drilling highly permeable or unstable
formations (Australian Drilling Industry, 1996).
The most common additive in diamond drilling
is organic, long chain polymers that have thixo-
tropic properties. That is, they have low viscosity
when they are stirred or pumped, but gel when
allowed to stand. The effect of these properties
is that the mud can be readily circulated in the
