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 11
Blast monitoring: Before, During and After Werner van Wyk, Explosives Engineer
When you try to get to the bottom of a dispute between two opposed parties you will realise that there are three sides to every story; yours, mine and the truth. The same applies to a bench for blasting. You have the top bench, the free face and the floor. For that reason we need to monitor what happens before, during and after a blasting event to get the whole picture.
the big picture. Each one of the items will in some way feed another until the results are achieved. a. Videographic/photographic surveillance of surrounding properties
Every event always has three things in common. What happened before the event, the event itself and the consequences of the event. In this publication we will highlight some key methods of what you can measure before, during and after a blasting event. Before The possible monitoring outlined in this section is described individually although each forms an integral part of the big picture. Therefore it is critical that the connections are made between the various items to form 1
Videographic or photographic surveillance of the properties in the vicinity of a mine is highly recommended. This will serve two functions. The first is getting to know the people who will most likely be affected by the mining operation. A specialist in public relations may help build a relationship with the neighbours. Secondly, it will give you evidence to measure against should the neighbour claim damage or the like from you. By having evidence of the structure and property the likelihood of the neighbours claiming damages is also minimised as they are aware that you have the before photo or video.
b. Acoustic recordings of ambient noise
Similar to the videographic or photographic surveillance
Explosives Today - Series 4, No 11
it is recommended that one has a solid base to compare to when dealing with airblast. To compare it to an everyday event e.g. if a vehicle should backfire at midnight in a quiet residential area, you will give a lot of people a fright. Should the same happen while you are looking at a noisy motorbike parade, no-one will give it a second thought.
include laser scanning and photographic stereo imaging.
It does not matter what method is used to capture the data as long as the data can be interpreted accurately by blast design software. The chosen software will aid in the graphic illustration of a blast design and must base calculations on accepted rules of thumb and the experience of the user.
The user will then be able to create a blast design appropriate for the conditions and the required results. This will include:
The only difference is the ambient noise that you compare the sudden load noise to. More about airblast can be found in Explosives Today Series 4 Volume 2 – Annoying your neighbours? (Managing airblast)
oo Accurate planned hole depths from the measured bench to the intended floor. oo Correctly applied burdening of all holes. oo Accurate placement of the intended hole position and orientation.
Office noise over 5 minutes open plan. Min 45.9, max 60.4dB c. Meteorological conditions
The wind speed and the general direction influence the direction into which the airblast will be pushed. Thus meaning that the effect of the airblast will be more severe in the direction of the wind.
Other meteorological conditions will influence the airblast and its effect e.g. temperature inversion layers, the altitude of the cloud base and the amount of cloud cover.
These conditions cannot be controlled and should be taken into consideration before any attempt is made to initiate a blast.
e. Borehole deviation recordings
Once holes are drilled it is desirable to make sure that we receive the best quality product, in this case the borehole. The quality of the borehole is extremely important. To compare it to an everyday event:
Should we have the latest, most technologically advanced vehicle and one wheel is flat, we cannot expect the vehicle to perform at its best. Yet we expect this where the borehole is the wheel and the explosives the engine of the vehicle.
All the holes should therefore be quality inspected. The criteria should include the length of the actual drilled hole compared to the designed depth, the angle at which it was drilled and the direction of the angle in relation to adjacent holes and the free face of the blast. Ideally this will be a feature of the blast design software to import blast hole deviation data, associate the data with specific blast holes and display the differences between planned and actual.
d. Survey of the intended blasting area
An additional survey should be conducted to capture the actual positioning of the drilled holes. This will enable the blaster to make informed decisions regarding the quality of the holes and ultimately the timing of the blast.
The best way to design a blast is to create a three dimensional representation of the intended blasting area. This can be achieved by various methods which
f. Explosive mass requirements 2
As there will always be a difference between the
Explosives Today - Series 4, No 11
planned and actual depth of a hole, we need to calculate the mass of explosives twice. Once the hole depths are planned and then after the actual depths were measured.
The initial calculation will give the blaster an indication of the amount of explosives that will be needed to charge the blast. The calculation based on the actual depth will be more accurate and this is the amount the blaster expects to see on the delivery note.
The blaster will then also have a better idea of the mass for each hole and should charge the holes accordingly. Should the hole require more than the calculated mass the blaster needs to intervene, assess the particular situation and take the appropriate action.
g. Blast hole timing simulation
The timing used in a blast is critical. The actual positions for the blast holes should be used to facilitate the timing of the blast. Specialist software is available to use the actual positions and create a timing pattern. The best software will be able to create timing using shock tube products and electronic delay detonator timing.
attention.
When filming the event the setup must correspond to the intended purpose of the recording. If recording is purely for record keeping, the blast area should be off-centre to either side horizontally and only occupy about Âź of either horizontal or vertical recording space. This method will allow the viewer a fair area to identify the amount of movement in the intended direction and upwards from the blast area. Velocity of detonation and timing The Velocity of Detonation (VoD) instrumentation requires a cable, deployed with the initiating system and booster down the hole, to measure a change in the cable. Two methods are used for this. The first is Time Domain Reflectometry (TDR) where the cable length is constantly measured and the shortening of the cable is graphically displayed against time. The second method relies on the change in resistance over time when the cable is shortened.
With this software the blaster will be able to: oo identify the particular area in the blast that may have an adverse effect on the muck pile shape oo identify an area where a possible increase in blasting-induced vibration, where multiple holes are initiated simultaneously, can be expected.
Both methods will display a distinctive and normally rapid change in length or resistance the instant the cable is broken by the booster. If multiple holes are measured, the accuracy of the initiation system can be calculated
Once this is identified the timing can be changed and simulated until the blaster is satisfied with the outcome.
During Video recording The video recording of a blasting event is not restricted to only the event itself but should also include a wide view of the location of the blast area. This will give the viewer an overview of the location and the prevailing conditions. Thereafter the focus will change to the blast area itself to record the initiation. Once the dust has settled it is always good to record the results, paying particular attention to the new high wall, sidewall, muck pile shape, amount of throw, power trough and any other areas that need 3
when the timing plan is consulted and the monitored holes identified. There is a substantial amount of work that needs to be done before and during the process of deploying the cable in the holes. This must be considered and planned for well in advance, and sufficient time allocated for it.
Explosives Today - Series 4, No 11
High-speed video capturing When considering the capturing of a blasting event with high speed video equipment we need to be very specific about the event we wish to capture. Each specific part of the event will have a specific method of setting the instrument up. For instance if the need is to measure stemming confinement, or the lack thereof, the camera needs to be positioned behind the blast, preferably at an angle. In contrast to this, if we need to capture face velocity the camera needs to be perpendicular to the intended face movement. It is therefore difficult, if not impossible sometimes, to address more than one specific result from a single high speed capture.
3. The Gas Release Pulse, (GRP) Results from blowouts e.g. the face bursting or cratering as a result of weak rock, geological features and/or under burdening of the front row of drilled holes (figure 3). 4. The Stemming Release Pulse, (SRP) Violent stemming ejection usually results from incorrect stemming lengths, as well as drill chippings rather than a graded stemming material (figure 4).
Figure 1 Air Pressure Pulse (APP)
Airblast and vibration monitoring The control of both airblast and vibration lies with the appointed blaster. To control airblast we need to identify what the source of the airblast is and eliminate the source in the design.
Figure 2 Rock Pressure Pulse (RPP)
There are five main sources of Airblast:
Figure 3 Gas Release Pulse (GRP)
1. The Air Pressure Pulse, (APP) The piston-like effect as air is moved by direct rock displacement. Typically this is as a result of the forward movement of the free face and the heave on the top surface of the blast (figure 1). 2. The Rock Pressure Pulse, (RPP) The vertical component of ground vibration travelling along the surface. This pulse is the first signal to reach the airblast transducer and can be likened to a drum skin effect on the surface of the earth (figure 2). Figure 4 Stemming release pulse (SRP) 4
Explosives Today - Series 4, No 11
5. Airblast due to the initiation system products such as surface lines of detonating cord The same applies to the control of blasting induced vibrations. The blaster will be able to reduce the generation of vibrations by paying close attention to the following: Charge mass limits By limiting the charge mass per hole the amount of energy is limited and therefore the energy available to generate vibrations. As an example we can compare the difference when dropping a pebble in a pond of water and dropping a brick in the same pond. Which one generates the biggest waves? Establishing local scaled distance No two mining sites are the same. There are differences in topography, geology and the proximity of the community around the mine, to name a few. Thus each mine should establish its own set of constants for airblast and ground vibrations. In the absence of local constants the following can be used as a guide. For airblast: L = Airblast level in decibel (dB) D = Distance from blast (m) E = Mass of explosives per delay (kg) a = 165 +/- 20 for confined blasts 195 for unconfined blasts b = 24 as a decay factor
Blast timing All blasts should be timed and simulated on appropriate software. This gives the blaster an indication of the mass of explosives that may fire instantaneously. This can then be used in the prediction calculations as described above. Once the timing has been approved by the blaster it can be implemented on the bench without any last minute changes. The same software used to simulate the blast timing can be used to program the electronic delay detonator control equipment for use in the field thereby minimising the possibility of human error when programming timing on the control equipment. Having a software simulation and the log of the control equipment will serve you well in a situation where proof of what was done is needed. General blast design Good record keeping is imperative to good blast design. All records will aid in the transfer of knowledge from one person to the other. It is especially true for instances where blast design parameters fall outside the rules of thumb. A question of why it was done can then be easily answered with documentation. This will also prevent repetition of work already done and the results documented, whether the results were good or bad. After Visual inspection
For ground vibration we can use two options. The first is a very conservative means of controlling ground vibrations. Option 1: D = Distance from blast (m) E = Mass of explosives per delay (kg)
Option 2: D = Distance from blast (m) E = Mass of explosives per delay (kg) a = 1676788990876 b = -1.67 5
This is compulsory for all blasting areas. The blaster is required to inspect the blasting area after the blast and to declare it safe before continuing with the mining operation. Should areas of concern be identified these must be addressed and/or demarcated with visual barriers and signage where personnel are allowed or prohibited from entering. Photographic Photographic recording of the results of a blast is underestimated by most people. This is your only proof of what the results were before it will be loaded and moved from the blast area. A good photographic record of the results can be used to identify potential areas of improvement and later will serve as a reference to measure against when changes are made. The photographic record goes hand-in-hand with the physical measurements and the documentation of the measurements.
Explosives Today - Series 4, No 11
When photographs of the muck pile are taken, it is best to take them with a scale object, like a soccer ball for surface operations and a tennis ball for underground operations, for fragmentation analysis or as a visual reference of the fragmentation size achieved. Without a scaling object as a reference the photographs cannot be compared from one blast to the next.
• Power trough depth • Amount of cast achieved • Width and length of blast (for comparison to drilled pattern) Documentation As with most things in life, no job is done until the paper work is done. This means the writing and compiling of a detailed report for each blast. All the reports need to conform to legal requirements as a minimum, not as a maximum. With the exception of 3.1 and 3.2 (Videographic/ photographic surveillance of surrounding properties and acoustic recordings of ambient noise) all the other mentioned measurements and recordings should form part of the final blast report. Conclusion
Survey of the excavated blasting area Once the blasted area has been excavated it should be surveyed again. This survey will serve a double purpose. It is very easy to calculate the excavated area for volumetric calculations and therefore production masses. Secondly it is required for the next blast in the same area as a before blasting monitor. Comparison to expected volumes will show extra volumes that can be accounted for in over break beyond the drilling pattern (back and/or side break).
We should all strive to be the leaders in our field of expertise. Failing to document test results, experimental results and for that matter blasting-related results will adversely affect your ability to claim to be a leader in the field. The author would like all who read this publication to audit their own records, identify and document shortcomings and rectify these. This should be done from the point of view taken by an auditor, an inspector from the Department of Mineral Resources or the Chief Inspector of Explosives.
Measurements Physical measurements need to be documented and will complement any photographic records. Measurements that should be included are:
This document is a new addition to the Explosives Today series.
• Back damage • Side damage
Disclaimer: Any advice and/or recommendations given by AEL Mining Services Limited (“AEL”) in this publication, is given by AEL in good faith in order to provide assistance to the reader. AEL does not however: 1.1warrant the correctness of its advice and/or recommendations; 1.2 warrant that particular results or effects will be achieved if AEL’s advice and/or recommendations are implemented; 1.3 accept liability for any losses or damages that may be suffered, as a result of a party acting, or failing to act, on the advice and/or recommendations given by AEL; 1.4 accept liability for any acts or omissions of its employees. representatives and/or agents, whether negligent or otherwise. Copyright: All 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 publication belong to AEL, save for where it is clearly indicated to the contrary.
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