Volume 9 Edition 2 2012 - The ASIA Miner

Drilling & Blasting
Playing it Straight
No matter what drilling method is selected, overall D&B performance
will suffer unless holes are drilled straight and according to plan, from
collar to bottom. When an operation “drills holes that look like
spaghetti,” according to Arne Lislerud, surface applications manager
for Sandvik, it can expect:
• Floor humps, hindering efficient loading due to uneven pit floors;
• Unstable pit walls and difficult first-row drilling;
• Safety concerns from flyrock;
• Stemming material blowouts that generate safety, excessive dust
and “bad toe” concerns;
• Poor blast direction, affecting quality of floors and walls;
• Misfires that produce safety hazards
The keys to achieving consistent straight hole drilling, said
Lislerud, are simple: Be aware of the numerous issues that lead to
drillhole deviation; operate with a technically sound drill rig, drill
string and instrumentation; and motivate drillers to strive for best results.
Good practice dictates only 2%-3% maximum drillhole deviation
in regular production drilling operations. For collar position
error control, Lislerud recommends:
• Using tape, optical squares or alignment lasers or GPS for measuring-in
collar positions;
• Marking collar positions using painted lines, not movable objects
such as rocks, etc.;
• Protecting completed drillholes with shothole plugs to prevent
holes from caving in (and filling up);
• Using GPS guided collar positioning devices, such as Sandvik’s
TIM3D drill rig navigation system.
Similarly, to control drill-hole deflection:
• Select bits less influenced by rock-mass discontinuities;
• Reduce drill string deflection by using guide tubes, etc.;
• Reduce drill string bending by using less feed force;
• Reduce feed foot slippage since this will cause a misalignment of
the feed and lead to excessive drill string bending;
• Avoid gravitational effects that lead to drill string sag when drilling
inclined shotholes (>15°);
• Avoid excessive bench heights.
Choosing the proper bit face design can enhance drill-hole straightness,
he also noted. When a percussion bit first starts to penetrate
through a rock-joint surface at the hole bottom, for example, the
gauge buttons tend to skid off this surface and thus deflect the bit.
More aggressively shaped gauge inserts (ballistic / chisel inserts) and
bit face gauge profiles (drop center) reduce this skidding effect by enabling
the gauge buttons to “cut” through the joint surface quickly,
thus resulting in less overall bit deflection.
The right bit-skirt design also helps: As the bit cuts through a joint
surface, an uneven bit face loading condition arises; resulting in bit
and drill string axial rotation that is proportional to bit impact force imbalance.
A rear bit skirt support (retrac type bits) reduces bit and string
axial rotation by “centralizing” the bit.
Other deviation countermeasures include using a longer bit body,
adding a pilot tube behind the bit, using lower impact energy, or employing
a drilling control system that can rapidly react to varying torque,
feed and percussion or pulldown demands based on hole conditions.
Additional information regarding the 2011 Mining F orum can be obtained
at www.theminingforum.com.
March/April 2012 | ASIA Miner | 65