13v101g

Investigation to Determine the Origin of Air Overpressure from Quarry Blasting W.J. Birch(1), L. Bermingham(1), C. Johnson(2), R. Farnfield(3) & S. Hosein(1) (1)Blast Log Ltd Charlbury Oxfordshire United Kingdom

(2) Blasting and Environmental Research Group, Mining & Quarry Programme, the University of Leeds United Kingdom

(3) Technical Services Manager EPC UK United Kingdom

Abstract Previous researchers have put forward two different theories as to the origin of air overpressure from quarry blasting. In 1980, Siskind et al postulated that the initial face movement gave rise to the displacement of air and that this resulted in the air overpressure pulse. However in 1990 McKenzie et al carried out research that indicated that the air overpressure pulse came into being as consequence of the shock wave created by the detention of the explosive charge in a blast hole, rapidly propagating through the rock and then impacting on the quarry face.

In an attempt to evaluate these opposing hypotheses, an investigation was undertaken over a 4 year period with the aim of determining the precise origin of air overpressure. The research was carried out at a Chalk Quarry in the North of England. The choice of this particular location was made on the basis that the rock type is very homogeneous with no discontinuities and the quarry operator carries out small full scale single row blast, (typically 5 holes per blast).

Thus a series of tests using single row fully instrumented full scale quarry blasts were carried out. The components that were assessed and investigated were the air blast arrival times in front of the face, the initial movement of the blast face, the precise initiation times of each blast hole, the speed of sound in both air and the rock host and an attempt was made at timing the development of the peak gas pressure within the blast holes.

The investigation indicated that the event (within the blast mechanism that is the source of air overpressure) always occurred sometime after the shockwave had impacted the face but on average before initial face movement. This lead to the conclusion that it was most likely that the transmission of gas through the face, created from the detonation of the explosive charge was the source of air overpressure. However it must be borne in mind that these air overpressure experiments have only been carried out on one specific rock type.

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1. Introduction There are two generally accepted theories of the nature of the exact phenomenon that generates air overpressure produced from a typical quarry or opencast blast. These are whether ground vibration along the free face (i.e. the initial shock wave) causes the pressure pulses seen in an air overpressure trace or whether it is attributable to the rock displacement.

The United States Bureau of Mines (USBM) conducted extensive research in the field of blasting induced air overpressure. The key research areas were determining the source of air overpressure pulses from blasting and structural response and damage caused by overpressure produced from surface blasting.

The USBM Reports of Investigations (RI) 8485 by Siskind et al defines four mechanisms during a blast which combine to form a single a root cause of air overpressure, these are; 1.

2.

3.

4.

Direct rock displacement at the face or collar of a blast hole. Known as Air pressure pulse (APP).

Vibrating ground, known as rock pressure pulse (RPP).

Escaping gas ejecting from the face through joints. Known as gas release pulse (GRP).

Ejected gases through the stemming, known as stemming release pulse (SRP).

The names given to these mechanisms were those attributed by Wiss and Linehan (1978). Siskind et al states in USBM RI 8485 that if a blast is properly designed and controlled, the air pressure pulse will be the dominant source, inferring that the rock displacement from the face is the main origin of air overpressure.

‘The air pressure pulse (APP) will dominate in a properly designed blast, and will only be absent for cases of total confinement (that is, underground blasts).’ Siskind et al. 1980.

The report also states that the rock pressure pulse (RPP) provides the least amplitude in air overpressure out of the four mechanisms listed above whilst the gas release pulse and stemming release pulse if they occur, can produce very high pressure levels, greatly exceeding levels from air pressure pulse.

‘RPP has the least amplitude of the air blast components; however, it is typically of higher frequency (identical to the Vv which spawns it), and enables us to predict the minimum air blast level expected (for example, 1.0 in/sec Vv will generate 0.0015 lb/in2, or 144 dB-peak).’ Siskind et al (1980).

A simplified relationship between ground vibration and rock pressure pulse was derived by Wiss and Linehan (1978); Where Vv is the vertical component of the ground vibration and measured in inches per second (in/s) and RPP is measured in pounds per square inch (lb/in2).

McKenzie (1990) acknowledges previous research conducted by the USBM but suggests that in fact that the peak air overpressure levels are generated not by rock displacement from the face but by the vibrations along the face (excluding when gas venting occurs through the face or stemming area).

‘The results of recent detailed studies suggest that after venting has been eliminated, the peak levels of overpressure are caused by ground vibration. The peak level is not generated by the ground vibration at the monitoring location, but instead by the peak vibration levels at the face.’ McKenzie (1990).

McKenzie’s work describes an experiment which evaluates the relationship between air overpressure and ground vibration. To do this, a cement wall was struck with a sledgehammer and the vibrations along the wall were recorded by vibration gauges. These were bonded to the opposite side of the wall.

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The experiment was to simulate the ground vibrations along a blast face prior to the fragmentation of the rock. Overpressure gauges were positioned at various distances from the wall. The results showed a linear relationship between air overpressure and vibration levels at various distances. These results can be seen in figure.1.

Figure.1 Results published by McKenzie (1990) Correlation between peak air overpressure and vibration levels It must be made clear that the ground vibration along the face is different to that which Wiss and Linehan refer to as ‘rock pressure pulse’. The term rock pressure pulse is given to the piston-like affect the ground vibrations give which produces low amplitude pressure pulses at the monitoring location; whereas McKenzie is suggesting that the vibration along the face creates a high magnitude pulse which then produces the main overpressure pulse. Thus the results of these two different research groups provide conflicting conclusions.

Siskind and Stagg (1997) state that the overpressure amplitudes, created by the air pressure pulses are proportional to the initial face velocity. This has been reinforced by the work conducted by Birch et al (2008) which suggests that the peak air overpressure levels, particularly in front of the blast, are related to the initial face velocities.

Morlock and Daemen (1983) stated that to accurately predict air overpressure levels, it is necessary to know the precise time each hole in a blast will go off relative to the others, the air and ground parameters influencing propagation and the detailed geometry of the blast. Further research was carried out by Birch et al (2012) at a chalk quarry in northern England to gain a greater understanding of the key parameters that control the peak air overpressure pulse generated by quarry blasting. They concluded that it was the interaction between successively detonated blast holes (either positive or negative) that makes air overpressure so difficult to predict. Their research work has established that the air overpressure pulse for a quarry blast from a single hole, when monitored in front of the quarry face being blasted, is dependent on the total mass of explosive, the duration of the detonation, the distance from the origin of the detonation to the observation point and the burden of rock in front of the blast hole at the initial point

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of detonation. In addition to these parameters detailed above, for a multi hole quarry blast, the geometry of the blast with respect to the observation point must also be taken into consideration. The resulting air overpressure pulse will be depend on the spacing of the holes, the orientation of the blast pattern, the timing between successive detonations and the speed of sound in air at the precise time of the blast.

2. Air overpressure monitoring at Melton Ross Quarry In order to establish the root source of air overpressure, extensive blast monitoring was carried out at Melton Ross chalk quarry. Typically each of the production blasts consisted of only five blast holes and initiated by non-electric detonators.

The optic fibre system data logger trigger system developed by Birch et al (2011) was used for all of the production blasts to record the exact firing times of each of the blast holes. The face movement incurred by the blast was also recorded by lowering a piezoelectric sensor in front of the face and connected to a high speed data logger. In addition the optic fibre system together with two low frequency microphones that were positioned at two known distances perpendicular to the face, directly in front of the first blast hole e.g. 40m and 80m from the first blast hole were also connected to the data logger.

The data recorded by the high speed data logger, provided the opportunity to calculate the exact moment the air overpressure is created during a typical quarry blast. The data logger recorded precisely the time the first blast hole was fired, initial face movement on the face and the time the air overpressure wave travels to two known points in front of the face, all on a common time base. In addition to this, the speed of sound in the rock for each blast was attained by connecting two seismographs together so that they also record on the same time basis as each other. The acquisition of the data using the methodology detailed above makes it possible to calculate the precise time the air overpressure wave was created and then to attempt to correlate it with the two possible sources. i.e. the shockwave, created by the detonation of the charge in the hole (after McKenzie) or the initial face movement that occurs afterwards (after Siskind).

3. Example calculation An example of how the source of the air overpressure for a given blast is calculated is provided below.

The data in this example was recorded from a five hole blast at Melton Ross chalk quarry on 25/05/2010.

Figure 2 Typical trace showing the precise firing times of the surface delay relay detonators and then the long lead time in hole detonators for a 5 hole blast at Melton Ross Quarry

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486ms

Figure 3 Optical fibre interface unit’s output used to determine the precise time the first blast hole was fired The optic fibre interface unit, (developed and described by Birch et al 2011) was used to trigger the monitoring system. The first group of readings in Figure 2 show the light impulse detected by the optic fibre sensors from the flame front proceeding along the shock tube, towards the detonators which was attached to the primer and loaded at the bottom of the blast hole. The second group of readings (circled in red) show each of the five blast holes firing. To calculate the time at which the air overpressure wave is generated during a blast, the time at which the first blast hole detonated at 486ms after the system triggered (see fig 3) must be set as t0 and the time of the air overpressure traces must be relative to this point.

3.1 Calculating air overpressure velocity Two low frequency air overpressure microphones were positioned in a line with the first blast hole as well as in front of and perpendicular to the quarry face to be blasted. For the case of the blast monitored in this example, the microphones were deployed 40.17m and 80.17m from the first blast hole.

Each of the microphones was connected to the data logger and so their output was recorded on the same time basis as the detonator firing times in Figure 3. As the pressure wave arrival times at each microphone is known, it is therefore possible to calculate the speed of sound in the air at the exact moment of the blast.

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613.37ms

Figure 4 Air overpressure trace recorded by the first microphone at 40.17m Figure 4 shows the air overpressure trace recorded by the first microphone. The time, at which the pressure level rises from zero, indicate the arrival of the pressure wave. For this blast the graph shows that the wave arrived 613.37ms after the data logger was triggered. However from Figure 3 it has been established that the first blast hole did not detonate until 486.0 ms after the data logger was triggered which, for the basis of this analysis, is deemed to be "time zero". This means that the blast wave arrives at the first microphone at 127.37ms (613.37-486.0) after the first hole detonated.

733.27ms

Figure 5 Air overpressure trace recorded by the second microphone at 80.17m

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The arrival of the pressure wave at the second microphone, deployed at 40m from the first microphone and 80.17m from the first blast hole was recorded 247.27ms (733.27 – 486) after the first blast hole was detonated.

Using the arrival times, the air overpressure velocity can now be calculated.

Knowing the velocity allows for the moment at which the air overpressure pulse emerges from the blast.

Air overpressure travel time from the face to the first microphone;

Therefore the time at which the observed air overpressure wave is generated is Now that the time at which the air overpressure leaves the face, has been calculated, it can be correlated to a possible source of its generation.

3.2 Determining the source of air overpressure To determine the source of air overpressure from a typical bench blast, the time at which the shockwave from the blast arrives at the free face and also the time at which initial face movement occurs must be known. Seismographs were deployed in the field to calculate the speed of sound in the rock and hence the time at which the resulting shockwave from the in-hole detonation, arrives at the free face. A piezoelectric sensor was created to detect the movement of the face. This was lowered down the face, in front of the first blast hole whilst ensuring the sensor was in contact with the rock at all times.

A seismograph was deployed directly below each of the two air overpressure microphones and connected together so that they both recorded on the same time basis. Once the ‘master’ seismograph triggered, the ‘slave’ seismograph also triggers. The arrival time of the shockwave in the rock recorded by each seismograph unit can be utilised to calculate the speed of sound in this particular rock for this given blast. The seismograph detects the vibration with a tri-axial array, therefore providing displacement along three axis, vertical, longitudinal and transverse. For the case of the blast in this example, the data recorded on the longitudinal channel of each seismograph was used in calculating the velocity. The first vibration recorded by each of the seismographs was in the longitudinal channel (component to register first arrival by the seismographs) and so indicates the arrival of the shockwave.

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Longitudinal Channel 10 8 4 2 0 -2 -4

-0.0020 0.0195 0.0410 0.0625 0.0840 0.1055 0.1270 0.1484 0.1699 0.1914 0.2129 0.2344 0.2559 0.2773 0.2988 0.3203 0.3418 0.3633 0.3848 0.4063 0.4277 0.4492 0.4707 0.4922 0.5137 0.5352 0.5566 0.5781 0.5996

Amplitude (mm)

6

-6 -8

Time (s) 4220

4228

Figure 6 the recorded vibration on the vertical channel of both linked seismographs Figure 6 shows the displacement in the longitudinal plane of both seismographs with unit ‘4220’ being the closer to the blast.

Longitudinal Channel Magnified 8

0.012695s

-0.00195s

4 2 0 -2 -4

-0.00195 0.00195 0.00586 0.00977 0.01367 0.01758 0.02148 0.02539 0.02930 0.03320 0.03711 0.04102 0.04492 0.04883 0.05273 0.05664 0.06055 0.06445 0.06836 0.07227 0.07617 0.08008 0.08398 0.08789 0.09180 0.09570 0.09961

Amplitude (mm)

6

-6

Time (s) 4220

4228

Figure.7 - Figure 6 magnified to display the arrival of vibration to the slave seismograph.

The master unit began recording at -0.00195s which signifies the arrival of the shockwave through the rock. The initial reading was not of a sufficient magnitude to trigger the unit and so was recorded in the pre-trigger window of the seismograph. The wave arrival at the slave unit occurred at 0.012695s. Figures 6 and 7 are graphical representations, however analysis of the raw data was used to determine the arrival times as this can be performed more precisely. The speed of sound in rock was therefore

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Determining the speed of sound in the rock, allows for the time at which the shockwave from the blast hole arrives at the free face, which in turn can be compared to the time at which the air overpressure leaves the face.

The burden for this particular blast was 4.5m.

Figure 8 Face profile data of the first blast hole The time taken for the shockwave to travel from the blast hole to the free face is

The shockwave arrives at the face 1.64ms after the charge is detonated in the first blast hole.

The first movement along the face was monitored by a piezoelectric sensor lowered down the face in a position where the sensor is in line with the primer and detonators within the blast hole and also remaining in contact with the rock at all times.

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Figure 9 Output from the piezoelectric sensor in front the of face

First detected movement

Figure 10 The first movement detected by the piezoelectric sensor The first movement along the face was detected by the sensor 13.23ms (499.23 – 486) after the blast was initiated. The safest method of placing the sensor in front of the face was by lowering it down from on top of the bench with a member of the monitoring team directing the positioning from the quarry floor at

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a safe distance from the face. As a consequence of this method, the sensor was not positioned in the exact same position relative to the location of the base charge for every blast; however this variation was kept to a minimum.

4. Origins of air overpressure at Melton Ross quarry AOP Time AOP First Face Speed in Shockwave Velocity leave face Movement rock Arrival at Burden Date (m/s) (ms) (ms) (m/s) face (ms) (m) 25/05/2010 333.6 6.90 13.23 2740 1.64 4.5 12/03/2010 335.0 7.00 10.14 2719 2.76 7.5 07/05/2010 328.0 6.86 18.36 2574 2.70 7.0 04/06/2010 339.6 3.20 4.31 2547 2.50 6.4 14/05/2010 327.5 3.0 5.23 2734 2.85 7.8 18/05/2010 340.0 8.10 6.73 2731 2.05 5.6 30/09/2010 344.0 9.94 5.65 2181 3.70 8.1 19/10/2010 332.3 5.91 n/a 2728 1.90 5.3 04/01/2012 340.7 11.84 14.27 2049 2.34 4.8 20/01/2012 333.1 20.09 n/a 2323 1.89 4.4 04/04/2012 341.3 11.72 n/a 2240 3.70 4.8 08/08/2012 333.0 6.74 4.75 2454 1.55 3.80 09/08/2012 333.0 3.48 4.70 2307 1.78 4.10 13/08/2012 333.0 9.00 6.60 3000 2.60 7.80 Table 1 Derived times of shockwave arrival, face movement and generation of air overpressure Table 1 lists the calculated times of the shockwave arrival at the face, the time at which the air overpressure wave is generated and the first face movement detected on the face by the piezoelectric sensor.

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Time difference Time difference between the between the AOP shockwave arrival leaving the face and and AOP leaving the initial face movement Date face (ms) (ms) 25/05/2010 5.25 6.33 12/03/2010 4.24 3.14 07/05/2010 4.16 11.5 04/06/2010 0.70 1.11 14/05/2010 0.15 2.23 18/05/2010 6.05 -1.37 30/09/2010 6.24 -4.29 19/10/2010 4.01 No value 9.50 2.43 04/01/2012 18.20 No value 20/01/2012 8.02 No value 04/04/2012 5.19 -1.99 08/08/2012 1.70 1.22 09/08/2012 6.40 -2.40 13/08/2012 Table 2. Comparison of time difference between shockwave and face movement in relation to the air overpressure pulses leaving the quarry face Based on the fourteen results attained from Melton Ross (table 2) it is clearly evident that the time at which the air overpressure pulse exits the face, does not coincide with either the calculated arrival of the shockwave at the free face or the initial face movement. The air overpressure pulse always exited the face after the shockwave has hit the face. It is also clear that the initial face movement usually occurs after the air overpressure is released with the exception of four of the eleven blasts [with relevant data] and therefore is unlikely to be the root cause. The four results that suggest the air overpressure exits the face after the initial face movement may well be attributable to the placement of the piezoelectric sensor.

The remaining possible source for the generation of the air overpressure pulse is the gas phase during the rock fragmentation process. The release of rapidly expanding gases, produced from the explosive detonation, fragments and displaces the rock from the bench, through the face area, into the working void. The gaseous product, upon arrival, exits the face through geological planes of weakness such as joints and through cracks created by the shockwave. In the case of Chalk rock whilst it is known that there are few major joints, the rock itself is highly permeable due to the nature of its original formation.

It is known that on average 1kg of ANFO produces a detonation pressure of 1.7GPa, an energy output of 3.8GJ and 970 litres of gas with the correct oxygen balance and under normal temperature and pressure (SMME 1990). At Melton Ross where typically 65kg of ANFO was loaded into each blast hole, the approximate energy output would be 245.6GJ whilst producing 63,050 litres of gas. The release of such large volumes of gas from the blast hole is the last phase of the rock fragmentation mechanism and therefore occurs after the shockwave’s arrival at the free face and before the initial face movement. It is therefore possible that the high volume of gas under such high pressure can be the cause of air overpressure leaving the face. However tests using in hole disposable pressure sensors has so far proved inconclusive.

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The investigation into determining the source of air overpressure has provided a practical and effective methodology for collecting the required data. In spite of this, caution must be taken whilst regarding the results provided.

Only eleven results containing all the relevant data were possible during the investigation, of which all have been recorded from blasts at Melton Ross quarry. The piezoelectric sensor, placed in front of the face also proved to have certain limitations. The piezoelectric wafer only detects movement along one plane of direction and therefore the first detected movement is dependent on the orientation of the wafer.

However in the field, it was impossible to control the exact orientation of the sensor when placing it on the front of the face. In addition to this, it was not always possible to position the sensor at the estimated same level as the primer cartridge in the blast hole. This would also lead to a small time lag between the detected first movement and actual face movement.

Due to the nature of the rock and the techniques used to blast it, the burden was found to fluctuate quite markedly from crest to toe in each blast. With such a large variation, determining the precise burden "in line" with the sensor and primer was not always practically possible. As a result, the arrival time calculations will be degraded. .

6. Conclusion The methodology established to examine the origin of air overpressure has proven to be practical and effective in the collecting the required data in a fully working quarry operation and therefore can be repeated at various other quarry blasting operations.

Within the bounds of experimental error, in general the derived time that the air overpressure leaves the face has been found to lie between the derived arrival time of the shockwave at the face and the initial face movement. It can therefore be postulated that it is more likely that in the case of quarry blasting in chalk, the air overpressure is associated with the gas phase, as the gaseous product from the explosives rapidly pours out of the quarry face under high pressure. However, further work is required to prove this beyond reasonable doubt.

References Birch W.J, Bermingham L & Farnfield R. 2008 " Relationship between Air Overpressure & Face Velocity" [ISEE] Proceedings of the thirty fourth annual conference on explosives and blasting techniques, New Orleans, Louisiana, USA, January 27th-30th 2008 Birch W.J, Hosein S, Bermingham L & Rangel-Sharp G 2011 "Low Cost Method for monitoring ShockTube Detonator and Explosive Product performance in Blast holes" Proceedings of 37th Annual Conference on Explosives & Blasting Techniques, San Diego, California, USA, February 6-9th 2011, (International Society of Explosives Engineers, 2011) Birch W.J, Bermingham L., Farnfield, R. & Hosein S 2011 "Control of air overpressure from quarry blasting? - It's about time" Proceedings of 38th Annual Conference on Explosives & Blasting Techniques, Nashville, Tennessee, USA, February 12-15th 2012, (International Society of Explosives Engineers, 2012) McKenzie C.K, Heilig J.H, Hickey S.M & LeJuge G.E 1990 "Ground Vibration and Overpressure Generation: How much do we really know?" Institute of Quarrying (Australia Division) 34th Annual Conference, Hobart, November

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Morlock C.R & Daemen J.J.K 1983 "Keeping Airblasts under Control" Proceedings of the ninth annual conference on explosives and blasting techniques, Dallas, Texas, USA, January 31st – February 4th 1983 Siskind D.E & Stagg M.S. 1997 "Environmental effects of blasting and their control." [ISEE] Proceedings of the twenty third annual conference on explosives and blasting techniques, Las Vegas, Nevada, USA February 2-5 (ISEE).

Siskind D.E, Stachura V.J, Stagg M.S & Kopp J.W. 1980 "Structure response and damage produced by airblast from surface mining." USBM Report of Investigations 8485.

Society for Mining, Metallurgy and Exploration (SMME) 1990 "Surface Mining 2nd Edition pp 543-544." Society for Mining, Metallurgy and Exploration Inc. Littleton, Colorado, USA. Wiss, J.R & Linehan P, 1978 "Control of Vibration and Blast Noise from Surface Coal Mining".

United States Bureau of Mines, Open Files Report, 103(4)-79.

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