Civil Engineering Reference
In-Depth Information
Step 1: Drilling
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The diameter of the drill hole is partially determ-
ined by the available drilling equipment, but must
also meet certain design requirements. The hole
diameter should be large enough to allow the
anchor to be inserted in the hole without driv-
ing or hammering, and allow full embedment
in a continuous column of grout. A hole dia-
meter significantly larger than the anchor will not
materially improve the design and will result in
unnecessary drilling costs and possibly excessive
grout shrinkage. As a guideline, the diameter of
the drill hole should be about 1.5-2 times the dia-
meter of the full anchor assembly, with corrosion
protection, if any.
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4
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Figure 12.6
Tubex drill bit for advancing casing
through soil and weathered rock (courtesy: Sandvik
Drilling) 1, Shoulder; 2, Bit tube; 3, Guide;
4, Reamer; 5, Pilot bit.
Percussion drilling.
Holes for rock anchors
are usually drilled with percussion equipment that
utilizes a combination of impact and rotation of
a tungsten carbide drill bit to crush the rock and
advance the hole. The cuttings are removed by
compressed air that is pumped down a hole in the
center of the rods and is exhausted up the annulus
between the rods and the wall of the hole. Per-
cussion drills are either pneumatic or hydraulic
powered, and the hammer is either at the surface
or down the hole (DTH drill). The advantages
of percussion drills are their high penetration
rates, good availability and the slightly rough
wall that is produced. Precautions that must be
taken include minimizing hole deviation by con-
trolling the down pressure on the rods, and avoid
loosing the drill string in zones of weathered and
broken rock.
Where holes are to be drilled through an upper
layer of soil, or intermediate zones of weathered
rock in which collapse of the hole is possible,
equipment is available that installs casing as the
hole is advanced. Equipment manufactured by
Tubex
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uses a bit that expands, when torque
is applied, to ream out the hole to a diameter
slightly larger than the casing (Figure 12.6). At
the completion of drilling, the drill rods and
contracted bit can be withdrawn inside the cas-
ing. The maximum hole diameter for Tubex drills
is 356 mm. Drills manufactured by Klemm and
Barber advance casing during drilling by apply-
ing thrust and torque to the casing, which is
independent of the thrust and torque on the
drill rods.
For holes drilled with hand-held percussion
drills, the limits for efficient operation are a
maximum hole diameter of about 60 mm and a
maximum length is about 6 m. For track-mounted
percussion drills the hole diameters range
between about 75 and 150 mm, and the max-
imum hole length for top hammer drills is about
60 m, with the main limitation being excessive
hole deviation. For DTH drills the maximum hole
length is several hundred meters. For hole diamet-
ers larger than about 150 mm in hard rock, there
is a substantial increase in the size of the drilling
equipment, and this equipment is usually used in
vertical holes rather than near-horizontal hole.
Rotary drilling.
For drilling in weak rock
such as chalk and some shales it is possible to use
rotary drilling methods that include augers, drag
bits and tri-cone bits; drill hole diameters range
1 Manufacturer's name are given as example only, and are
not intended as endorsements of their products.
