Explosives Today 2

Figure 3

Explosives Today Series 4 No 2

Airblast Propagation The airblast level for any given blast can be expected to decrease with distance in accordance with the following relationship: L = a – blog10 3

D �E

Where: L = Airblast level (dB) D = Distance from blast (m) E = Explosive charge mass per

delay (kg) D 3 �E = Cube root scaled distance

(m/kg1/3) ‘a’ & ‘b’ = Propagation constants The parameters a & b are influenced by the following factors and may be calibrated to some extent by field measurements: • Charge mass/delay • Delay intervals • Drilling pattern • Stemming length • Direction of initiation • Type of initiation system Extensive field work that has been conducted by AEL Mining Services on various customer sites indicates that the following values can be used to

Figure 4 estimate the airblast levels with an expected variance of ±10dB:

a = Confined Blasts 165±20

= Unconfined blasts 195

b = Decay factor ≈ 24 The maximum expected airblast would come from the detonation of unconfined charges, whereas in bench blasting the minimum expected level would be that provided by the vertical component of ground vibration.

The graph based on work carried out

by D E Siskind, of the US Bureau of Mines shows the upper and lower levels of airblast derived from measurements taken at 25 sites.

(Figure 3.)

Hence, if it is required to destroy, unconfined, a 25kg case of explosives by detonation without risk of incurring damage; the minimum distance of the site chosen from the nearest dwelling is calculated as follows: 1

1

D =320E 3 =320 x 25 3 =936m On the other hand, should 128dB be the limiting airblast level, the corresponding value for =600 and the minimum distance then becomes: 1

There is scatter of data between these two lines as a result of blast design and weather conditions which significantly influence airblast generation and propagation.

It can also be seen from the graph that doubling of the distance for a given charge mass can be expected to reduce the airblast level by 6.6 dB. (Figure 4.)

AEL Mining Services Explosive Engineers based at the regional offices are available to help and advise on any problems related to airblast.

Reference to the upper line for unconfined charges, shows that for airblast to be limited to 134dB, the corresponding value of the ratio,

This document replaces all previous Explosives Today on this subject including Series 2. No 24: June 1981 (R C Espley-Jones/A F Goetzsche)

D 3�E

“Annoying your neighbours?”

(Managing airblast)

1

D =600E 3 =600 x 25 3 =1754m

Simon Tose, Group Consulting Mining Engineer

A mining or construction site can be a major source of annoyance to the local community, particularly if not managed and controlled properly.

Usually the major economic benefits of the mine or project are accrued elsewhere to the larger population, whilst the local community have to “suffer” the irritation and disruption.

= 320.

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: 1.1 warrant 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 a diffusion 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 in terms 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.

AEL Mining Service Limited (PTY) Ltd 1 Platinum Drive, Longmeadow Business Estate North Modderfontein, 1645 Tel: +27 11 606 0000 www.aelminingservices.com

Noise, dust, heavy mine or construction traffic, local disruption to traffic flows, perceived devaluation of property and the potential for damage are some of the many

challenges that have to be managed.

Add a need for blasting to the equation and the perception of the local community is in a word “disaster”.

Airblast and blasting vibrations are undesirable by-products from the use of explosives to fragment rock for mining, quarrying and construction.

When a blast is detonated part of the explosive energy is not utilised in breaking and moving rock, but is lost to the atmosphere and through the ground in all directions as wave

motion, similar to the ripple created in a pond when a stone hits the water.

In the Explosives Today, Series 4.

No1. “Good Vibrations” we looked at the impact of these waves through the ground. Now as these waves pass a given position in the air, the pressure of the air rises very rapidly then falls more slowly then returns to the ambient value after a number of oscillations. The maximum excess pressure in this wave is known as the peak air overpressure, generally measured in decibels linear (dB).

Unit of Airblast measurement – Decibel (dB) Airblast constitutes an impulsive air pressure over and above atmospheric.

The ear responds logarithmically to changes in overpressure and hence it is usual to express airblast in decibels as defined by the equation: L = 20 log10 (PP0 ) Where: L = Airblast level (dB) P = Overpressure (kPa) P0 = Reference or audibility threshold pressure

= 2 x 10-8 kPa or:

L = 20 (log10 P)+154 Airblast travels at the speed of sound in air, approximately 330 metres per second depending upon altitude and air temperature, with levels decreasing rapidly as the distance from the blast increases. However the levels can be greatly influenced by wind direction, speed and by atmospheric temperature inversions and clouds that can bend it back toward the earth and focus its energy kilometres away depending on the mass of explosives detonated per delay. The topography of the site and the direction of blasting can also exaggerate these effects.

The pressure waves consist of energy over a wide range of frequencies, some of which are audible and hence may be sensed in the form of noise, but most are at inaudible frequencies of less than 20Hz. This relatively low frequency component can be sensed by people in the form of a pressure wave known as concussion. The noise and concussion together is known as airblast or air overpressure.

As each site and surrounding area are unique in the way they respond to blasting, a suitable set of site specific limits need to be developed through a continued cycle of planning, well researched guidelines and design, suitable explosive and initiation systems, implementation, on site drill and blast practise, measurement and assessment.

A significant part of airblast is “human perception”, which can be addressed to a certain degree with community participation and understanding of the research. The rest is the attempt to keep the disruption and annoyance to a minimum. On the condition that there is no damage then we should be looking to keep the number of blasts to a minimum, to reduce the stress, annoyance and aggravation to the local community.

Individuals are sensitive to airblast (noise) and vibrations and can feel blasting events that are as little as 2% of acceptable vibration guidelines and/or limits. These events cause alarm owing to their abrupt and unexpected nature, people suddenly see “damage”, such as plaster cracks that they have previously walked past unnoticed on a daily basis, yet now perceive as blast damage to their homes. We experience heavy traffic everyday on our roads and travelling past our homes that also generates noise and significant vibration but little or no alarm or concern due to its familiarity.

There are no legislated limits for the control of airblast in South Africa, it is left to the “experts in the field” and the use of suitable formulae and guidelines to develop safe blast designs when blasting closer than 500m to structure(s) of concern.

The management of the blast design for such construction and mining projects is based on the well established guidelines. AEL Mining Services, AEL, use the work by the United States Bureau of Mines, USBM, RI8485 as an acceptable guideline in planning and designing blasts for surface mining, Table 1.

Table 1. AEL Guide for Airblast criteria Level 100dB 110dB

128dB

134dB

Description Barely noticeable Readily acceptable Currently accepted

by South African authorities as being a reasonable level (50.2Pa) for public concern.

(No more than10% of measurements should exceed this value.)

Currently accepted

by South African authorities that damage will not occur below (100.2Pa) this level. (No measurements should exceed this value outside of the mining boundaries.)

(2.0Pa) (6.3Pa)

However there is growing evidence that these measures do not necessarily address the public’s concern around noise and explosives.

This is because the average person forms a judgement based largely on his or her psychoacoustic responses and is usually unaware of the important distinction between the characteristics of the wave motion alone, the sound effects that accompany it and the potential for damage.

In fact these guidelines are based on the long term studies by the USBM, where 140dB was considered as a “safe” limit. The air overpressure was halved to create the 134dB limit we now recognise in an effort to bring control over the noise component of the airblast.

Human response to airblast from blasting is difficult to quantify. Airblast can be felt at levels well below those required to produce damage to structures. The duration of the event has an effect on human response, as does the frequency. Events are of a short duration, typically the longer the event, the higher the frequency the more adverse the effect on human response. Research has shown that longer duration blasts are preferable to many blasts with shorter durations, i.e. there are fewer times of perceived disturbance.

The response of an individual to any such event is dependent upon the same factors as that of ground borne vibration with the understanding of the phenomenon through public relations and the attitude of the operators being of utmost importance.

The main problems to airblast “noise” are the subjective reactions, although

annoyance surveys have been made for sonic booms, shown in Figure 1 and 2. The main problem is to define just what is objectionable to noise and separate these from factors from other psychological and physiological reactions. (Table 2.)

As shown by the dotted line, when 50% of the impulses exceed 125dB and 95% are less than 130dB, 5% of the affected are still very annoyed.

Table 2. Human response Average Human response Barely to distinctly perceptible Distinctly to strongly perceptible Strongly perceptible to mildly unpleasant Mildly to distinctly unpleasant Distinctly unpleasant to intolerable

Airblast (dB) 50 – 70 70 – 90 90 – 120 120 – 140 140 – 170

In reviewing the above responses, one must distinguish between the average individual and those who may reside at either end of the human response spectrum.

Figure 1. Mining airblast compared to sonic boom, C Dowding

At one end are persons who might perceive some financial benefit or common good for the project.

Although they may incur the inconvenience of the blasting, unless they are physically damaged in some manner, they may not complain.

At the other end of the spectrum, individuals who do not want the project to take place may be disturbed by the slightest inconvenience and will generally make their feelings known.

“Best Practise” – Control of airblast Airblast should be controlled with regards to not only potential damage, but also adverse human response.

Airblast control, in practical terms, can generally be addressed by the blaster in the field by adhering to accepted guidelines in the following areas: • Assure that all holes have suitable and adequate stemming to provide good confinement of all the explosives • Drill fines, while readily available, do not make good stemming material. The use of graded angular chippings or stemming cartridges of clay or other suitable material depending on the hole diameter, are recommended • A minimum stemming height of thirty borehole diameters is usually sufficient to prevent “blowouts” and resultant airblast • Assure proper front row burden or well designed cut to provide “free” breaking surface • Precautions to be taken in areas known to exhibit geological weaknesses • No overburdened holes • Thus preventing blown-out holes • Do not allow any exposed explosives including detonating cord or surface connectors • Use of Electronic Detonators recommended • Assure proper timing sequence • If possible, keep the rate of detonation across the face less than the speed of sound.

(330m/s) • Limit the mass (kg) of explosives per delay.

• Single hole firing, controlled by the use of Electronic Detonators • When possible, time the blast to direct movement away from critical areas

• It has been shown that a 5 to 10dB difference in airblast levels can be expected between the front and back of the blast • Avoid adverse environmental conditions such as blasting when wind is blowing toward residential areas, or under an atmospheric temperature inversion • Wind can influence the propagation of airblast by +20dB • Avoid blasting early morning or late afternoon as temperature inversion can reflect back + 10dB •

Do not blast more often than necessary and time blasts to coincide with peak ambient noise levels

• Blast cover. e.g. sand and/or blasting mats • +300mm of sifted soil over detonating cord surface lines can reduce the acoustic noise by 26dB • Use pop-holes or mechanical breakers in the place of lay-on or Mud blasting charges

•

Maintain good public relations by notifying neighbours before blasting operations start and before each blast

Measuring Instruments Because of the impulsive nature of airblast, suitable equipment must be used to record the levels. A typical airblast monitoring system requires the following 3 components: • A mounted detector, which enables the sensor to be placed in a stable position above the ground. The meter should have adequate low frequency sensitivity preferably down to 2 Hz • Airblast detector system, which includes the microphone and associated signal conditioning amplifiers. The windscreen covers the microphone and is required to reduce noise due to wind • Data recorder and analyser (i.e. computer and appropriate software for linear frequency response rather than the normal weighted scales.)

Figure 2. Human response to sonic boom, dBlin, Siskind

The microphone or airblast sensor is placed near to the geophone or accelerometer, with wind screen/ sock/foam attached. If practical the microphone should not be shielded from the blast by buildings, vehicles etc. and be 0.3 to 1.0m from ground level. If not possible, the horizontal distance of the microphone from such object should be greater than the height of such object.