Safety

Exploring technology-based blasting rules

During the last five years, electronic initiation systems have been challenging the currently accepted empirical blasting rules and guidelines. From the 1970s to early 1990s, much blasting research was conducted with the use of reliable delay detonators, both electric and non-electric.

These detonators introduced an improved delay timing accuracy (? 5% scatter) that was not available in the past and allowed new empirical rules to be developed linking particular delay timing to the reaction of rock or structures. This research work was conducted by government agencies, USA Bureau of Mines (USBM), explosive manufacturers ICI Explosives (now Orica) and also individual blasting consultants. There are two rules of thumb in literature:

1. An eight millisecond (8ms) firing window for consecutively initiating charges to minimise blast vibration.
2. Construction blasts produce high blast frequency and large mine blasts produce low frequency.

The widely accepted introduction of electronic blasting systems in the Australian quarry and construction industries during the past five years has made it possible to explore and test the accuracy of these previously mentioned empirical guidelines.

Blast vibration is an area where large advances have been made with the introduction of electronic blasting systems. Urban zones are encroaching on quarry leases and there is a limitation to reducing the blast vibration through simple quality control. When designing a vibration-compliant blast, most Australian quarries use8ms as their firing window for consecutively firing blast holes. At many quarry sites, a 8ms firing window will not be correct. Through understanding how to interpret blast waveforms and design blast initiation sequences that use alternative time envelopes in the same blast, the optimal firing window can be quickly identified.

Two South Australian quarries have utilised Orica?s advanced vibration analysis to determine their site-specific firing windows. This has enabled these sites to significantly improve blast vibration control. Both sites have also been able to increase blast sizes with an electronic initiation system. Understanding site-specific firing windows has led to improved blast economics and has also reduced the number of blasts required per year.

One quarry first trialled electronic initiation in 2007 using a 8ms firing window with the results only providing a five per cent improvement on vibration as comparedto the pyrotechnic initiation system. Analysis of the trial results were conducted to further refine future trials.
In 2008, the same site fired a modifiedelectronic blast, using an initiation sequencewith 28ms and 14ms firing windows. From this, it was found that the 14ms window gave distinct spikes in the vibration waveform but the 28ms waveform was much flatter. Further blasts and analysis at the site determined that a firing window between 15ms and 18ms suited the blast geometry and geologyat that site. Blast volumes have been successfully increased from 15,000 tonne to50,000 tonne shots, without compromising blast vibration limits.

In Australia, with the exception of Queensland, our regulations provide no allowances for channelling blast frequency into higher bands. The current Australian Standards3 make reference to frequency for blast limits at unoccupied structures, but do not consider occupied structures such as residential houses.

Sites where blast vibration measurements are part of a licence agreement generally are required to measure blast vibration in units of velocity (mms-1) or acceleration (mms-2). Using either of these measures, mathematical relationships can be used to convert them to a displacement. Actual particle displacement is what causes damage to structures or causes neighbours to feel the blast at a quarry. The magnitudeof both velocity and acceleration are directlyrelated to the magnitude of displacement.

The other factor that influences particle displacement is frequency. Higher blast frequencies will cause less displacement when comparing the same magnitude of velocity.

One commonly used blasting standard thatrecognises this mathematical relationshipis the DIN 4150 standard4. This is a Germanstandard and is used throughout Europe andin New Zealand. These types of standards originated from work documented in the RI 8507 report, which was conducted by the USBM.

The accuracy of electronic detonators allowsthe user to select a target frequency and then design the initiation sequence to achieve thedesired dominant frequency. A standard non-electric blast cannot manipulate blast frequency due to the delay scatter associatedwith the detonator. Sites that are currently using smaller diameter blast holes (= 89mm)to control blast vibration or decking could use a frequency-based vibration limit to increase the blast hole size or eliminate decking if the blast frequency can be manipulated into the higher bands (>30Hz).

The displacement (0.0796 mm) experienced with a PPV of 5mms-2 at a frequency of 10 Hz is the same as the displacement (0.0796 mm) experienced by a particle that has a PPV of 25mms-1 and a frequency 50 Hz.

Blast frequencies have been successfully manipulated in a range of blast hole diameters and geological settings. Quarries operating in close proximity to neighbours have the ability to improve the effects ofblasting on the community and surroundingenvironment by exploring the use of electronic detonators and tailoring initiationtechnology to their sites. Sites can reduce the blast vibration readings by using the correct firing window for their operation or maintain vibration levels and improve fragmentation even where timing below the 8ms window is required to achieve this.

The ability to control blast frequency at a quarry can provide many benefits. The quarry can reduce the displacement being experienced by their neighbours and eliminate conservative blast hole parameters (= 89mm) by channelling blast frequency into higher bands. The opportunity exists to fire multiple blast holes or larger blast holes, within the sites firing window, and cause less displacement at the monitoring locations.

To investigate a site?s actual firing windowor determine whether frequency channellingis possible at your quarry, a reliable cost-effective electronic initiation system is required. The Uni tronic Electronic Initiation System is the system that the author uses to investigate his customers? firing windows and ability to channel blast frequencies. The correct design parameters can lead to improved results and efficiencies, as well as more positive relationships with your neighbours and community. You may also be rewriting the blasting ?Rules of Thumb? for your site.

REFERENCES
1. Siskind DE. Vibrations From Blasting. International Society of Explosive Engineers (ISEE), Cleveland, 2005.
2. Dowding CH. Construction Vibration. Library of Congress Cataloging-in-Publication Data, USA, 2000. Committee CE-005, 2006.
3. AS 2187.2-2006 Australian Standard: Explosives – Storage and use – Part 2: Use of explosives.
4. DIN 4150-1 (2001-06): Structural vibration ? Parameters

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