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.

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 TodaySeries 4 I No 9

Armour rock or Riprap blastingMauritz Kotzé, Snr Explosives Engineer; Werner van Wyk, Explosive Engineer; Simon St J Tose, Group Consulting Mining Engineer

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Introduction

One of the more unusual applications of explosives is in the

blasting of large rock for the use in construction of ports

and breakwaters. The objective in this type of blasting is to

generate large boulders of that will be able to withstand the

onslaught of the elements, wind, water and temperature.

By its very nature the large rocks need to keep their shape

and form for this specific application.

Background

The ability to blast large rocks with explosives may be just

as difficult as or even more than blasting rocks to small

fragment sizes. To blast smaller rocks the simple solution

would be to add more energy or explosives to the given rock

volume to be blasted. The inverse is to remove explosives

to decrease the fragmentation size; this may work in theory

but not always in practice.

The nature of this type of blasting requires an extremely

high level of drilling accuracy. The same is needed for pre-

or post-splitting. The higher the quality of the blast holes,

the more predictable the outcome will be. The sizes of the

rocks that can be generated are directly related to the rock

mass or the rock quality that is being mined. Geology of the

rock may be the greatest obstacle on achieving the desired

results.

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Geological Considerations

Weathering can extent 6 to 18 meters below the original ground surface. Weathering breaks down the rock and weakens

existing planar features such as bedding, schistocyte, and jointing. In rocks such as limestone and dolomite, secondary deposits

of calcium carbonate can cement existing joints. Existing quarries, or quarries that have been in operation for many years,

probably will produce material with less waste because excavations are partly or completely through the zone of weathering.

New quarries, or quarries where rock production has been limited, must content with the weathered zone and will likely

produce a less desirable product.

The area where the blasting for riprap is proposed needs to be thoroughly investigated prior to any attempt to blast the riprap.

Areas where closely spaced fractures, bedding planes and the like occur should be avoided. The rock mass will tend to break

along these weaknesses and the blast may have undesired results.

This investigation will eliminate some benched in the operation or could render the whole mine as unsuitable for riprap blasting

(i.e. joint spacing and bedding planes less than 1m apart cannot produce rocks bigger than the dominant joints).

Should a suitable area be identified it must be considered where the riprap will be sorted, stored and transported. With this in

mind it could turn out to be a logistical nightmare, with provision for a sorting area, storage area and a loading area. Multiple

handling of the rock may result in a reduction in the quality of the riprap and a cost/revenue implication.

Basic Design Recommendations

The following rules of thumb have been found in the search for the methodology to blasting armour rock or riprap as it is

known.

Blast parameters:

Description Unit Lower value Upper value

Bench Height m 15 20

Hole Diameter (ø) mm 75 115

Sub-drill 10 x ø

Stemming height 15 x ø

Burden to Spacing ratio B:S 1.4:1 2:1

UCS > 100MPa UCS < 100MPa

Toe powder factor kg/m3 0.650 0.500

Column powder factor kg/m3 0.500 0.250

Explosive coupling ratio % 45 55

Explosives Today - Series 4, No 9

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Following the above guidelines the historical results achieved have been found to be as follow:

Block weight (kg) Percentage (%)

UCS < 100MPa UCS > 100MPa

> 3000 30 50

1000 – 3000 20 25

50 – 200 25 15

< 50 25 10

For the production of riprap the powder factor is reduced by as much as 60% compared to a conventional operation mining of the same rock type.

• The specific charges should be low, an initial estimate value of 0.2kg/m3 can be tested

• The spacing to burden ration should be between 0.5 - 0.7 : 1

• Single row instantaneous blasting should be adopted as opposed to short period delays as it reduces the tearing between blastholes. Where this is not possible a delay could be introduced by firing groups of 3 to 5 holes instantaneously, with a delay between the groups

This low charge may be sufficient to loosen the rock but will not necessarily move it forward. The explosive charge should be well distributed in the blasthole with a reasonable bottom charge. The reduced energy will result in uneven floors and will require corrective measures.

Design 1: Linear charge (Packaged explosives)

Assuming a bench height of 15m and 76mm diameter holes an initial design would be to use cartridge explosives taped onto a line of 10g/m detonating cord as detailed in table 1.

Description Unit Value

Burden * m 3.2

Spacing * m 1.6

Stemming m 3.0

Charge

Toe

4 cartridges of size 50x560kg 5.0

Column

20 cartridges of size 32x560kg 11.0

Number of initiators per hole 1

Specific charge kg/m3 0.21

*-Burden to Spacing ration of 1:0.5

Explosives Today - Series 4, No 9

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Explosives Today - Series 4, No 9

Design 2: Localised charges (Bulk explosives)

Assuming a bench height of 22m and 165mm diameter holes an initial design would be to use bulk explosives & cartridge toe charge in table 1.

Description Unit Value

Burden * m 4.3

Spacing * m 3.0

Stemming m 4.3

Charge

Top deck

Stemming deck m 4.3

Anfo charge & booster kg 10

Mid column deck

Air deck m 2.7

Anfo charge & booster kg 15

Toe deck

Air deck m 9.6

2 cartridges of size 110 x 600 Pentolite booster (largest possible unit available)

kg 12

Anfo charge kg 39

Number of initiators per hole 3

Specific charge (powder factor) kg/m3 0.26

The design in both design 1 and 2 should be considered as the starting point for the successful blasting of riprap stone. These recommendations are only applicable once a suitable area has been identified.

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Explosives Today - Series 4, No 9

5. Conclusion

The use of cartridge explosives being taped to a down line of detonating cord will be labour intensive but may produce a more uniform results due to the linear charge. The use of an air deck with bulk explosives although being less labour intensive might produce a higher percentage of smaller material due to the concentration of energy in specific areas of the blast hole.

It must be noted that these designs are given in good faith to provide a starting point for the customer and are not intended to be definitive designs. It is envisaged that a large amount of experimenting will be required before an acceptable result is achieved. It must also be remembered that the production of armour stone/riprap is highly dependent upon geology as such the joint spacing and weathering of the rock mass must be assessed prior to the commencement of blasting to ensure that the site is suitable for the production of armour stone/riprap.

AEL Mining Services Explosive Engineers based at the regional offices are available to help and advise on the use of electronic detonators for all blasting scenarios.

References:

1. Stemtek Limited, …for cost effective blasting, www.stemtek.co.uk

2. US Department of transportation, Federal Highway administration, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition, Design Guideline 4 Riprap Revetment.

3. Drilling and blasting of rock, E. Lopez Jimeno, C Lopez Jimino, Ayala Carcedo, (page 195)

4. DDME16.doc, Blasting guidelines for the Coega Project

5. DDM5, Coega Kop Quarry Blast Audit, Sean Rodgers AEL Blast Consult, 2004

6. Chapter 18, Engineering Geology Field Manual, second edition Volume ii, 2001

7. Guidelines for Stabilising Waterways 6 Rock Riprap, 1991

This document is a new addition to the Explosives Today series.

AEL Mining Services Limited (PTY) Ltd1 Platinum Drive, Longmeadow Business Estate NorthModderfontein, 1645Tel: +27 11 606 0000www.aelminingservices.com

DISCLAIMER AND INDEMNITYAny recommendations given by AEL Mining Services (AEL) in respect of this document are given in good faith based on information provided. AEL does not however warrant that particular results or effects will be achieved if the recommendations are implemented, due to potentially unknown aspects and/or conditions. AEL further does not accept liability for any losses or damages that may be suffered, as a result of the customer acting, or failing to act, on the recommendations given.

Explosives Today, Series 2 No 30 Surface and 31 Underground by M Beattie December 1982 and KS Ireland March 1983 respectively are replaced by this series.

COPYRIGHTAll copyright that subsists in this publication together with any and all diagrams and annexures contained herein, which shall include all and/or any ideas, plans, models and/or intellectual property contained in this document vests in AEL. Any unauthorised reproduction, adaptation, alteration, translation, publication, distribution or dissemination (including, but not limited to, broadcasting and causing the work to be transmitted in adiffusion service) of the whole or any part of this document in any manner, form or medium (including, but not limited to, electronic, oral, aural, visual and tactile media) whatsoever, will constitute an act of copyright infringement interms of the Copyright Act No.98 of 1978 and will render the transgressor liable to civil action and may in certain circumstances render the transgressor liable to criminal prosecution. This document remains the intellectual property of AEL. Intellectual Property: All ideas, concepts, know-how and designs forming part of this publicationbelong to AEL, save for where it is clearly indicated to the contrary.

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