Blasting Competency Programme
Module 5: Drilling
1
Contents
Factors in Drilling Performance ........................................................................................... 2
Performance Parameters (Drillability Factors ) .................................................................... 2
Drill Selection ....................................................................................................................... 3
Blast Design ........................................................................................................................... 4
The Choice of Blasthole Size ............................................................................................... 4
Drilling Accuracy ................................................................................................................... 5
Sources of Error ................................................................................................................... 5
Reduction of Backbreak ....................................................................................................... 7
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
2
Drilling
Factors in Drilling Performance
A number of factors influence rock penetration and cuttings removal during drilling, which in
turn largely determine drill performance. there are four groups of these factors :-
• Operating Variables: These affect the components of the drilling system - drill, rod, bit
and fluid, and are mostly controllable and include two categories of factors :
o drill power, blow energy and frequency, rotary speed, thrust and rod design; and
o fluid properties and flow rate.
• Drillhole Factors: These include hole size, length, and inclination, and are dictated by
outside requirements and are thus independent variables.
• Rock Factors: These consist of the properties of the rock, geologic conditions, and the
state of stress acting on the drillhole. These determine the drilling strength of the rock
and the limit drill performance.
• Service Factors: These consist of labour and supervision, power supply, job site,
weather and so on, and have some effect on drill performance.
Performance Parameters (Drillability Factors )
In selecting the optimal drilling system or evaluating drill performance, four parameters are
measured or estimated.
• Process energy and power consumption
• Penetration Rate
• Bit Wear (life)
• Cost (ownership + operating)
Energy and power affect operating cost, but they are more important as determinants of the
penetration rate. Both penetration rate and bit wear are major criteria of performance, with
wear more critical in deep holes and hard rock. Cost is the ultimate measure of performance;
a drill can have excellent performance, but if it is not cost effective, then an alternative
system should be sought.
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
3
Drill Selection
The selection of a production drill follows a
well defined procedure, the steps in selection
being as follows :-
• Determine and specify the conditions
under which the machine will be used,
• State the objectives for the rock
breakage phases of production cycle of
operations in terms of tonnage,
fragmentation, throw, vibrations, loading
and haulage restrictions, pit slope
stability etc.
• Based on the blasting requirements,
design the drillhole pattern.
• Determine the drillability factors, and , for the kind of rock anticipated, identify the
drilling method most suitable.
• Specify the operating variables for each system under consideration, including drill,
rod, bit, and fluid factors.
• Estimate performance parameters, including machine availability and costs, and
compare.
• Select the drilling system that, in best satisfying all requirements, has the lowest overall
cost, commensurate with safe operation.
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
4
Blast Design
Once the desired drilling method has been chosen upon one must consider the most efficient
method of employment. Blasting results have a direct effect on safety, efficiency and cost of
a mining operation. Poorly designed blast layouts can result in toes and poor fragmentation
leading to increased secondary blasting costs and greater wear on loading, hauling and
crushing equipment.
There are a number of factors which affect the fragmentation achieved, blasthole size having
an influence on most of these factors.
The Choice of Blasthole Size
Good and efficient blasting requires that :-
• Sufficient energy is provided, in the right form, for the job in hand,
• All of that energy is released usefully,
• The explosives are correctly distributed through the rock mass,
• On detonation, flyrock, ground vibration and airblast are kept to a minimum,
• There should be minimal cost and maximum safety.
The choice of blasthole diameter therefore has a major influence on blasting design, quite
apart from the fact that larger holes produce more rock per metre drilled.
The important factors linking explosives and drilling are:
Drilling accuracy Inaccurate drilling causes variation of the burdens and spacing,
resulting in inconsistent breaking, the extent of the problems being
proportional to the deviation. With larger drills, and thus larger burdens.
the percentage deviation is reduced.
Hole closure Blockages are usually caused both by loose fragments from the
sidewall protruding into the hole, and by carelessness in dislodging
surface stones into the hole.
Blasting Blasthole diameter determines the burden and spacing. If the burden is
Geometry great enough to exceed half the bench height, then problems arise with
end effects at the top and bottom of the bench
Supervision By adopting a larger hole size, the number of holes required is reduced
and supervisory potential enhanced.
Production In smaller operations there is little point in using large hole diameters,
requirements as the drill would be poorly utilised. Real advantages accrue for large
scale - hard rock operations.
Explosives It is well worthwhile choosing a hole size large enough to permit the
selection efficient use of poured or pumped explosives.
Explosives Cartridged explosives are designed to deliver full explosive energy in
performance small diameter holes. There is a cost penalty for this ability.
Blasting The cost of the initiation system is therefore related to the number of
accessory usage. holes. With larger holes accessory costs can be reduced, and it
becomes more economical to adopt a more sophisticated system which
offers maximum control over blasting results.
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
5
Advantages of larger Holes Advantages of Small holes
• less susceptible to blocking • better fragmentation with same powder
• less to drill factor
• easier to supervise • better ability to fragment hard capping
• more accuracy in setting up • better control of backbreak
• less likely to omit a hole • better control of flyrock, vibration and
• less likely to omit to charge noise
• easier to time • better ability to drill inclined holes
• less time moving and setting up • suitable for low benches.
• Suitable for on-site explosives • Better distribution of explosives in rock
• explosives detonate more effectively
• less timing units needed
• better productivity
• more accurate
• better with high benches.
Drilling Accuracy
The best overall effect for general blasting operations is achieved by uniform fragmentation
and clean breaking at the bottom of the holes. This requires accurate drilling in order to
ensure that the explosives charges are located correctly relative to each other.
Accurate drilling can be obtained by adopting the largest possible hole size and drilling the
shortest hole length consistent with efficient blasting.
Sources of Error
! "
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
6
Marking and Collaring Errors
$
Normally the free face to the blast is created by the previous
blast and the profile is likely to be overbroken in the collar region
and irregular. This raises the problem of marking the first row of
holes in order to achieve the planned burden at the toe rather %&'
than the crest. Overburdening in this area will result in failure to
break to grade, even if the rest of the blast is marked and drilled
perfectly. ( )
This source of error can be overcome: *
• establish a reference line at a distance of twice the burden
behind the back row of holes. After blasting, the position of
the new front row of holes for the next blast is then taped
forward from the reference line.
• A theodolite can be employed to survey in the collaring
positions correctly.
• The horizontal set-back from the toe required to locate the
hole position on the bench top may be measured directly
using a measuring rod projecting over the bench with an
plumb line.
Inclination and Direction Errors.
An error in inclination at the collar of a hole causes
deviation of the hole at the toe. A 5° error with a 12m
high bench can reduce a 2m collar burden to 1m toe
burden resulting in excessive flyrock. If the hole
immediately behind this in the next row is drilled with
a 5° error in the opposite direction the resultant toe
burden will be 4m. Every drill rig should be equipped
with a reliable alignment device.
It is important that inclined holes be drilled in the
same direction as well as at the same angle. This
can result in severe spacing errors at the toe of the
hole.
Deflection Errors
Deflection is the source of error that is most difficult
to influence. Three major factors influence the
magnitude of deflection :-
• Structural geology. Pronounced jointing tends
to deflect the drill bit parallel to the jointing.
• Angle of Drilling. Holes drilled of the vertical
wander downwards under the pull of gravity.
• Stiffness of the drill string. Unlike the above factors, this is controllable and the
deflection can be reduced by the following means :
o Ensuring the drill boom is placed firmly against the drilling surface.
o Employing larger diameter drill rods or adopting In-the-hole bits.
o Using stabilisers
o Limiting the hole length.
© African Explosives Limited, 2009
Blasting Competency Programme
Module 5: Drilling
7
Hole Depth Errors.
In surface operations a method of level control should be adopted wherever slopes result in
height differences over 0.3m across the site.
Hole length gauges, which automatically stop the drill when the pre-set depth is reached can
be fitted to most rigs. Holes drilled too deep should be backfilled with drill chippings to the
correct depth.
Undergauge, omitted or lost holes
Undergauge holes result in reduced explosive charge or the inability to accept cartridged
explosives. As the mass per metre of a pourable explosive is proportional to the square of
the hole diameter, breaking is seriously affected by even a small loss of gauge.
Omitted holes are more likely to occur with small blastholes drilled on a close spacing, with
the result that it is not obvious when a hole is missing.
Lost holes are common in some geological conditions, and result in poor breaking owing to
the absence of explosive. To minimise this, all holes should be checked for depth prior to the
start of charging, and actions taken if a blocked hole is found.
Reduction of Backbreak
Poor face conditions due to backbreak often necessitate the repositioning of holes, resulting
in an introduced error. There are two methods for reducing backbreak :-
• Inclined Hole - these not only reduce backbreak but improve breaking at the toe and
give increased heave, resulting in a flatter muckpile.
• Controlled blasting techniques in the back row - normally a reduced charge mass
and/or decking will prevent backbreak on the new face.
It is good practice to fill any incorrectly drilled holes with drill chippings to prevent
them from being inadvertently charged. This is particularly so when blasting
overburden on top of coal otherwise coal damage and losses will occur.
© African Explosives Limited, 2009
