The origins of “Explosives Today” Claude Cunningham
Then... The 1960’s was a time of rapid evolution for blasting technology in South Africa. The sole explosives supplier, African Explosives and Chemical Industries Limited (AE&CI), operated the world’s two biggest explosives factories (at Modderfontein and Somerset West) and thrived chiefly on delivering dynamite and capped fuse systems to the nation’s huge underground gold, platinum and coal mines. Working methods were mature, but there was burgeoning pressure to increase productivity, whether by harnessing the ability to drill large diameter holes or by increasing the yield per blast from limited face length. Explosives technologies were also developing and offered great promise for moving toward these goals. The marketing group of the Explosives Division, under the General Manager Peter Lambooy, ran a team of explosives service engineers, mainly drawn from the mining industry, which worked with the company and its customers to implement efficient and safe blasting both in the country and in Africa. The demand for help in applying the new technologies and implementing safe and efficient blasting was more than the service engineers could handle; the result was “Explosives Today”, a technical bulletin written by these engineers, which would become a globally recognized source of useful blasting information. The first issue, “ANBA and Inclined Drilling”, was published in August 1965. Series Two started in September 1976 with “Selection of Explosives for Narrow Reef Blasting”, and finished with “Safety in Surface Blasting” in 1988. The first issue of the third series, launched in September 1988, was “The Historical Development of Commercial Explosives”, and the series ended with the tenth issue, “Destruction of Explosives Accessories” in December 1990.
The series was such an acclaimed resource for blasting information because it was entirely written by the engineers engaged in real blasting activities, subjected to intense peer review, with the meticulous attention of senior engineers. For most of this time, AECI Explosives (which changed its name progressively) was either the only explosives supplier, or by far the majority supplier, and had not yet encountered the disciplines and pressures of working in a highly competitive market. Long after the series was discontinued, old copies have been treasured and used as vital reference by personnel concerned with safe and efficient blasting around the world. And now... In line with our value proposition of Thought Leadership and a move to revive the publication, AEL’s Mining Optimisation Team has replaced these treasured copies with a new series. The first two issues were produced in the first quarter of 2013. This technically driven customer publication authored and tailored by our Mining Optimisation team is now available to customers in the form of an A4 folder equipped with a CD and flash stick containing all issues of the publication.
For more information and to order your copy, contact the Mining Optimisation Team c/o Simon Tose, Tel: +27 11 606 3960 Email: simon.tose@aelms.com
Explosives Today Series 4 I No 12
Drilling Accuracy Werner van Wyk, Explosives Engineer Introduction Irrespective of your individual application of drilling and
needed for the drill bit size. Equally important is the hole
blasting techniques, a uniform fragmentation and clean
length accuracy. All plans require holes to be drilled to a
breaking to the bottom of a blast hole is of the utmost
predetermined depth to create a flat or at least smooth
importance. This requires accurate drilling enabling you to
floor condition. Failure to address inaccuracies can result in
distribute explosive charges to achieve your desired results.
downstream costs rising for no apparent reason.
Drilling accuracy is a relative term, as a 1m deviation from
In theory all this can be minimised by drilling the largest
planned on a 2m x 2m pattern can be catastrophic, the
possible hole diameter for the shortest possible length.
impact of a 1m deviation for a 10m x 10m pattern will not
This in practice is impossible and for this reason the onus
be as severe. Keeping in mind that large patterns generally
of ensuring accurate drilling is a function of the appointed
require large diameter holes which exhibit little deviation
blaster and his production team.
owing to the size and rigidity of the drilling equipment 1
Explosives Today - Series 4, No 12
Drilling 1. Sources of error
Figure 1
Figure 1 depicts and lists the most common sources of drilling errors and is discussed further. 2. Marking and collaring Various methods are used for marking the intended collar positions of holes to be drilled. The chosen method is more than likely the easiest and quickest to implement and to use, no distinction is made between the blasting applications e.g. surface or underground applications.
Unless perimeter blasting techniques has been used, it is normal for any free face to have over break at the collar region. Figure 2 depicts the effect of over break on a surface operation and the positioning of the next intended hole. Two methods to reduce back break include the use of inclined holes or employing controlled blasting techniques. This raises the problem of marking the first row of hole in order to achieve the planned burden at the toe rather than the crest of a bench.
Figure 2 2
Explosives Today - Series 4, No 11
Accurate placement of the first row of hole is paramount as it will affect the intended result. Overburdening in this area may result in failure to break to grade and unintended fragmentation uniformity. Under burdening may result in unintended and unwanted fly rock. One of the following methods could be employed to overcome the source of error: •
Establishing a reference line at a minimum of twice the burden behind the back row of a blast. After blasting the position of the front row of holes can be marked by measuring forward from the reference line.
•
Using survey instrumentation to determine the face conditions and the intended collar position based on measurements and calculations.
Figure 3
Figure 4: Undulating ground conditions
3
Once the front row has been accurately marked the rest of the holes must be marked as accurately as possible. On level ground the minimum equipment required is a string line and two measuring tapes. The methodology is depicted in figure 2 & 3. The most common error occurring when faced with undulations is “crabbing”. “Crabbing” can be described as “measurement by eye”. It is extremely easy to make a judgement error of 10o and after that it compounds to the end. In undulating conditions survey instrumentation is the recommended method. Should this not be an option for the environment the use of knotted strings and simple trigonometry can be used. The problem is depicted in figure 4. Using painted stones to indicate hole positions create another potential error source. The stones could be kicked out of position or moved by the drill rig operator or assistant. 3. Inclination and directional errors Incorrect collaring of a hole causes unplanned toe positions. This effect is illustrated in figure 5 & 6. In the illustration a 2m planned burden is realised in a 1m actual burden with a 5o error. The second row was drilled by overcompensating for the error in a -5o error. This realised a total toe burden of double the planned burden. The same method can be used for spacing. We have to remember that we are working in a 3 dimensional world. Special attention must be given to the drilling when inclined holes are the norm or planned. Both the angle of the hole and the direction must be clearly indicated to the driller.
Explosives Today - Series 4, No 12
Figure 5: Inclination error
4. Deflection errors Three major factors influencing deflection or “wandering” are discussed. • • •
Structural geology: pronounced jointing tends to deflect the drill parallel to the jointing. Angle of drilling: Holes drilled off the vertical deflect downward under the force of gravity. Stiffness of the drill string: Unlike the above factors, this is controllable and the deflection can be reduced by the following: a) Ensuring the drill boom is firmly placed against the drilling surface b) Employing larger diameter drill rods c) Adopting down-the-hole drills d) Using stabilisers e) Limiting hole lengths
Drilling equipment manufacturers can best advise on the limitation of the equipment and to best mitigate deviation. 5. Hole depth errors Undulating or sloped surfaces requires the individual
Figure 6. Incorrect set up
3.5m or 14% of bench height
calculation of hole depths. This can be achieved by the use of survey instrumentation or laser levelling equipment. 6. Under gauge, omitted or lost holes Under gauged holes result in a reduced explosives charge or the inability of the hole to accept cartridge explosives. Any rock with a high silica content causes rapid gauge loss and require frequent measurement of the gauge loss during drilling. Omitted holes can be difficult to identify in closely spaced patterns. Good supervision, clear marking and systematic working are required to eliminate this problem. Lost holes are a common occurrence in some geological formations. The result is normally poor breaking owing to the absence of explosives. A blocked hole should be clearly marked and steps initiated to rectify it. Ideally this hole should be charged with explosives first to reduce the risk of another collapse. One of the causes of a collapsed hole is carelessness at the collar of a hole. Debris can be dislodged into the hole after drilling and prior to charging. Excessive fill material or the inability to clean the bench properly adds to the probability of hole being lost or measured short of planned depth. Discipline is the chief solution here, whether it is the preparation, drilling or charging operation.
4
Explosives Today - Series 4, No 12
Impact of drilling inaccuracies
Burden (m)
Spacing (m)
Powder factor (kg/m3)
All impacts are calculated using 2 hole diameters as a reference.
2.3
2.76
1.18
2.7
2.76
1.00
An assumption is made that all holes are drilled to precisely 10m and has a consistent diameter of 108mm. The effect on the powder factor (kg/m3) is calculated as = Mass of explosive per hole / Burden (B) x Spacing (S) x Hole length (H).
2.7
3.24
0.85
2.3
3.24
1.00
2.5
3.0
1.00
The mass of explosives per hole is assumed to be 75kg.
4. Effect of inclination errors
1. Effect of marking and collaring (burden)
Localised powder factor calculation for inconsistent burden is: Burden (m)
Powder factor (kg/m3)
2.3
1.08
2.5
1.00
2.7
0.93
Collar Burden (m)
Collar Spacing (m)
Volume @ 12m bench height (m3)
Toe Toe Volume @ Burden Spacing 12m bench (m) (m) height (m3)
2.5
3.0
90
2.3
2.6
72
2.5
3.0
90
2.5
2.7
78
2.5
3.0
90
2.7
3.0
97
2.5
3.0
90
2.5
3.2
96
2.5
3.0
90
2.1
3.0
75
2. Effect of marking and collaring (spacing) Localised powder factor calculation for inconsistent spacing is: Spacing (m)
Powder factor (kg/m3)
2.76
1.08
3.0
1.00
3.24
0.93
AEL Mining Services Explosive Engineers based at the regional offices are available to help and advise on matters arising from this publication. 3. Effect of marking and collaring combined Localised powder factor calculation for a combined inconsistent burden and spacing is:
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This document replaces all previous Explosives Today on this subject including Series 2. No 36: 2nd Quarter 1984 (CVB Cunningham)
Explosives Today - Series 4, No 12
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