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HomeMy WebLinkAboutRudenko Article re: Blasting Complaints '., 1" " AN ANALYTICAL APPROACH FOR DIAGNOSING AND SOLVING BLASTING COMPLAINTS by Douglas Rudenko Vibra- Tech Engineers, Inc. Hazleton, Pennsylvania, USA ABSTRACT Have you ever had a neighbor complain about a blast one day, but says the next day's blast was better, even though the Peak Particle Velocity (pPV) increased? How about neighbors that complain about a small trimming shot, but don't complain about a big production shot? Have you ever reduced the pounds/delay and gotten more complaints? Mine operators are often bewildered when trying to correlate blast design parameters to homeowner complaints. When complaints come, standard practice is to set up a seismograph in the neighbor's yard to monitor the blast. Immediately after the blast, the ~ighbor usually reappears from the house wanting to know the seismograph reading. When told that the PPV was 0.1 inIsec, well below damage levels, the homeowner explodes saying "Your machine is wrong". Perhaps he is . right. Understanding why people complain about blasting is not about PPV, it's all about structural response. People could care less about how much the ground vibrates; they only care about how much their house vibrates. In this paper we will examine the primary factors that control structural response. Through the use of a large array of digital seismographs, the effects of two production blasts on a community surrounding a quarry are studied. The predicted structural response resulting from each blast is calculated at 175 seismograph locations. The predicted response is then correlated to previous research on human perceptibility to vibrations. The results of this analysis and correlation give a better understanding of what the residents actually feel in their homes. The detonations of two single-hole signature blasts are also recorded by the array of seismographs revealing the dynamic characteristics of the geology surrounding the quarry. Once the influence of the geology was understood, blasts were configured to disrupt the natural rhythm of the site through the use of millisecond. delays. THE FAMILIAR PROBLEM' Consulting seismologists are often called upon to visit neighbors of quarries against whom complaints have been lodged. We try to gain an understanding of why the neighbors complain by listeniD.g to what they say. "At 12 noon when the shot went off, my husband and I were sitting eating lunch when all of a sudden it felt like the whole house was thrown off its foundation. My china closet danced across the room. And I heard plaster falling in the walls. I don't care what Yaw' seismograph said You can't tell me that didn't damage my house. Why can't you make all Yaw' blasts small like the one last week We were having lunch on the patio and hardly felt that one. " At the quarry however, the seismologist hears a different story. "We can't get enough rock for the crusher. The loaders and trucks don't work more than half the time. The blaster spends more time on the job than I do and we still don't produce enough. We're down to .five-hole shots with three decks per hole. I'm not making a dime and the regulators say I'm getting too many blasting complaints. My PPVs are a 1/4 of the state limit What am I suppose to do?" Addressing blasting complaints is more thail just looking at the peak par-dele velocity. For those who still think only peak particle velocity is important, are any of these situations familiar? . YourPPV is low, but you still get complaints. . One neighbor complains, even though closer residents do not. . One neighbor only complains about stripping shots, while another only complains about production shots. . A neighbor complains about a blast one day, but says the next day's blast was better even though PPV increased. . You reduced your lbs/delay by adding decks and got more complaints. All of these situations are common and are the result of how a particular structure responds to a particular blast. When a complaint comes, common practice in the industry has been to set up a seismograph in the neighbor's yard to monitor the blast. One of two scenarios inevitably follows The neighbor stands outside with the seismograph operator proclaiming after the shot, "That was .. nothing like the last one". Or the neighbor goes inside his house to wait for the blast. After the blast he immediately reapp.ears wanting to know the seismograph reading. When he's told the PPV was 0.10 - well below damage levels - the neighbor erupts, "W el~ your machine is wrong!" In some ways your neighbor is correct. FACTORS CONTROLLING STRUCTURAL RESPONSE Thereat cause of why people.complain about blasting is structural response. Your neighbors could care less about how fast a particle on the surface of the ground in their yard is moving. All blast vibration complaints are due to how much your neighbor's house shakes, not how much the ground shakes. There are three factors of ground vibrations that determine how much your neighbors' houses vibrate. They are ground vibration amplitude (pPV), ground vibration duration, and ground vibration frequency. In order to understand how these three factors control how a house vibrates, let's think of a more familiar model - a swing. A swing is a single-degree-of-freedom vibration model that behaves in a manner similar to a house. Think of ground vibration amplitude as how hard you push the swing. If you push it harder, it goes higher. Structural response therefore, is directly and linearly proportional to ground vibration amplitude. If you reduce your PPV by half, structural response will be cut in half. Ground vibration duration is an equally important parameter in considering structural response. Using the swing analogy, you can easily make a swing go higher without pushing it harder, by simply pushing it again. The more times you push the swing the higher it will go. Longer ground vibrations continue to shake the house causing a greater amplitude of structural response. Frequency is the most important of the three factors of ground vibration. Ground vibrations at the fundamental frequency of a house are like pushing a swing whenever it comes back to you. If you push the swing at any other time, you disrupt its rhythm. If a house is exposed to ground vibrations near its fundamental frequency, the house will amplify the vibration. Ground vibrations below the fundamental frequency of the house will still cause the house to vibrate at least as much as the ground. If the frequency of the ground vibration is more than 40% greater than the fundamental frequency of the house however, the house will vibrate less than the ground. DETERMINING THE FUNDAMENTAL FREOUENCY OF OUR NEIGHBORS' HOUSES In 1976, Ken Medearis published a report to the National Crushed Stone Association on the development of rational damage criteria. In this report, Medearis determined the fundamental frequency and damping ratio of 63 residential structures subjected to micro-vibration testing. He found that the height of the structure primarily determined its fundamental frequency. The fundamental frequencies of the structures in this study were found to range from 4 to 18 hertz (Hz), with an average fundamental frequency of9.6 Hz. A summary of the results from the Medearis study is shown in Figure 1. Figure 1. Fundamental frequencies of structures. Fundamental Frequencies of Residential Structures ~ 011o stci.,..~ ~ ... f 035 f 035 03 0.2 i 0.15 i 0.15 :i 0.1 j 0.1 ! 0.05 .E 0.05 0 5 10 15 20 0 5 10 15 20 Frequency, Hz Frequency, Hz ~ ~ AD~ 0.3 ... f 035 b 0.25 03 ] 0.2 i US I 0.15 i 0.1 i 0.1 ,1:1 ,1:1 ! 0.05 .E 0.05 0 0 6 10 15 20 0 5 10 15 20 Frequency, Hz Frequency, Hz In the 1970s, the United States Bureau of Mines (USBM) also studied the fundamental frequency of residential structures. The USBM determined the dynamic properties of 23 residential structures subjected to actual blasts rather than impulse loading as in the Medearis study. The values for frequency and damping computed from actual blasting records tended to be slightly lower than the Medearis study. The USBM also determined that while houses vibrate as a single-degree-of-freedom between 4 and 12 Hz, the natural frequency of the house's midwall tends to occur between 12 and 20 Hz. In order to control the response of a structure that has more than one fundamental frequency, you must control the two lowest fundamental frequencies. For residential structures this means minimizing ground vibrations between 4 and 20 Hz. THE HUMAN SEISMOGRAPH Structural response plays a critical role in your neighbor's perception of blast vibrations. The tolerance and reactions of humans to vibrations is another important aspect of understanding why people complain about blasting. Humans notice and react to vibration levels much lower than the levels established as structural damage thresholds. The authors of U.S. Bureau of Mines Report of Investigation 8507 (RI- 8507) state quite plainly that the "method of analyzing the damage potential of blasting vibrations has the disadvantage of possibly underestimating annoyance reactions." One study on human response to transient vibrations was the Wiss and Parmelee study. In this study the responses of 40 people to damped, sinusoidal pulses of 5-second duration were recorded. Figure 2 shows the threshold response (dashed line) of the most sensitive people along with the mean response (solid line) of the average subject. 'The difference in the threshold response to the mean response shows the subjective nature of human response to vibrations. The results of the Wiss and Parmelee study were reanalyzed in RI-8507 for duration-of-vibration effects. The results of this analysis showed that human tolerance to vibration decreases the longer the vibration continues. Fipe Z. Human response to transient l'ibrations of various frequency and duration (From Wiss & Parmelee, 1974). 10.000 1.000 ~ j € .!a 0.100 ~ ~ l ~ 0.010 Barely Perceptible --_...----- I--.:;:-~I 0.001 1 10 Frequency, Hz 100 A DIFFERENT APPROACH Based upon our understanding of what causes structural response and how people react to vibrations, let's examine a case history from a limestone quarry in the Midwest. The maps shown in Figure 3 are color contoured to coincide with the four threshold response levels from the Wiss and Parmelee study. Blue representing "barely perceptible" from 0.01 to 0.10 inIsec. Green representing "distinctly perceptible" from 0.10 to 0.40 inIsec. Yellow representing "strongly perceptible" from 0.40 to 1.0 in/sec. and red representing "severe" above 1.0 inIsec. The figure shows a comparison of peak particle velocities from two different production blasts. Production Blast l(on left) was located approximately 400 feet east of Production Blast 2 (on right). Production Blast 1, known as the "road stone" is located two levels lower in the pit than Production Blast 2 that is located in the "glass stone". The "glass stone" is more porous and less dense dolomite than the "road stone". Bench heights at this quarry are between 50 and 75 feet. The charge weights for each blast were 472 and 6361bs/delay respectively. During this study 170 digital seismographs were placed within a 1.5-mile radius of the quarry. As you can see from the contours of ground vibration, there doesn't seem to be any difference between the two blasts, except that the "glass stone" typically generates all the blasting complaints. Why? Figure 3. Comparison of recorded peak particle velocities from produmon shots at Midwest Stone Quarry to Wiss & Parmelee, 1974. Production Blast 1 "Road Stone" Production Blast 2 "Glass Stone" Response Velocity (IDIsec:) """~,- 0.01 0.10 0.40 1.0 2.0 Figure 4 is a comparison of some waveforms from the detonation of a single column of explosive in the "road stone" and the "glass stone". When an explosive charge is detonated in a borehole, the ground experiences a shock that lasts only a few milliseconds. The resulting ground motion is determined primarily by the dynamic properties of the geology, not the explosive. This ground motion represents the natural mode of vibration for that particular travel time path. These waveforms and spectra are a few examples from the "road" and "glass" stone. Notice that Single Hole 2 in the "glass stone" produces higher amplitude, lower frequency, longer duration vibrations than those of Single Hole 1 located in the "road stone". Are these just isolated events around the qua.qy or the tip of the proverbial iceberg? Figure 4. Comparison of single hole blasts from Midwest Stone Quarry. 0.02 iblsee 0.065 iDlsee ..0.5 0.5 1 1.5 I . Road Stone 2.5 3 3.5 4.5 4 o 2 . Glass Stone 10 II ~ 1 I =-- 10.1 lloo 0.01 1 10 100 10 II ~ 1 b I :> jO.1 0.01 1 10 100 10 I 1 b J 10.1 0.01 1 10 100 Frequencr, HI DIAGNOSING THE PROBLEM Figure 5 is a summary of the dominant frequencies observed from all the seismometers located around the quarry. This figure shows a comparison of the dominant frequencies observed from the two single hole blasts, one in the "road stone" and one in the "glass stone". The graph is plotting the number of occurrences of ground vibration energy within 20% of the spectral peak. For comparison, the data are shown with the probability density function for "all structures" from the Medearis study. The "glass stone" produces significantly more occurrences of vibration energy at. frequencies most likely to match those of the surrounding residential structures. The data show a large number of occurrences ata fundamental frequency of 10 and 11 Hz. If the frequency of the ground vibration matches the fundamental frequency of the house, there is a very efficient transfer of energy from the ground to the structure. Figure 5. Dominant frequencies of ground motion from signature blasts Midwest Stone Quarry. 1 All I a. 75 rIJ '; '#. ~ i .~ 50 I e c.I o 'S 25 t ~ e i 100 .. > .I(esii1~~tilil i:~s(fromMedeariJ) Signature Blasts . Signature 1 - Road Stone III Signature 2 - Glass Stone o 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Frequency, Hz. The aerial distribution of the fundamental frequencies observed from the two single hole blasts is shown in Figure 6. Low frequency ground vibrations from 1 to 13 Hz are shown in red and yellow. Green annotates higher frequencies from 13 to 23 Hz. The aerial extent of the ground vibrations from the two blasts was roughly the same. The area affected by low frequency ground vibrations however is significantly different. The area effected by low frequency ground.vibrations from Signature Blast 2 is 200% greater than that of Signature Blast 1. This area is approximately 4 square miles. When you consider structural response to low frequency ground vibrations, this data may explain why the quarry primarily receives complaints from blasts in the "glass stone". Figure 6. Fundamental frequency distribution at Midwest Stone Quarry. Fundamental Frequency (1&) 1 8 13 19 23 The Medearis study shows that the average fundamental frequency of residential houses is approximately 10 Hz. Using response spectra analysis it is possible to predict single-degree-of-freedom response to an applied ground vibration. Let us assume that the neighbors' houses around this quarry have a fundamental frequency of 10Hz. Figure 7 shows the predicted response from two different blasts recorded at the same location. Production Blast 1 was detonated in the "road stone" whereas Production Blast 2 was in the "glass stone". The PPV of the "road stone" blast was 1.5 times greater than that of the "glass stone" blast. The structural response of the "glass stone" however, was 3 times greater than that of the "road stone" blast. Considering this figure its easy to see why a neighbor complains about a blast one day but says the next day's blast was better, even though the PPV increased. 1\ il Figure 7. Comparison of structural response of production blasts from Midwest Stone Quarry. 0.09 iDlsec Structure Motion Ground Motion 0.084 inlsec Production Blast 1 0.058 in/see 0.26 iDlsec Production Blast 2 Figure 7 showed ground vibrations from the production blast fired in the "glass stone" were amplified by a 10 Hz structure by 4.5 times. The authors ofRI-8507 experienced amplifications of up to 4 times for racking motion and up to 8 times for midwall motion. The typical amplification experienced during racking motion was 1.5. Figure 8 shows the percent of recorded vibrations whose calculated structural response was greater than typical. Approximately 35% of the recorded vibrations from a production blast in the "glass stone" produced a structural response greater than 1.5 for a 10 Hz structure. Amplifications from the "road stone" for a 10Hz structure were less than 1%. Figure 8. Comparison of likelihood of structural response to production blast type from Midwest Stone Quarry. 80 1170 .. .~ U Eo< 60 ~J t b 50 I'- ~~ ].fl40 > ! 1 l30 !~ J .. 20 ....a Q 1.1 I! 10 If o 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Frequency, & USBM Typical Residential Strudures MidwaB Response Shear or Racldog Response Considering a 10Hz structure, Figure 9 shows the aerial distribution of peak response velocity. In other words, a prediction of what is possibly felt by the surrounding neighbors in their homes. The response velocities are color contoured to coincide with the four threshold response levels from the Wiss and Parmelee study on human perceptibility. Production Blast 2 in the "glass stone" subjects many more neighbors to vibrations considered to be distinctly to strongly perceptible. Vibrations in this range resulted in a 425% increase in area from Production 1 to Production Blast 2. Figure 9. Peak response velocity at 10 Hz for production blasts from Midwest Stone Quarry. Production Blast 1 "Road Stone" Prod uction Blast 2 "Glass Stone" Response Velocity (iDlsec) , ~'- ;),.:~-'-; - \ , ' ~t,.~, 0.01 0.10 0.40 1.0 2.0 Using the data from the Wiss and Parmelee study the authors ofRI-8507 showed that human tolerance to vibrations decreases as the duration of the vibration increases. Figure 10 shows the aerial distribution of response vibration duration assuming a 10Hz structure. In other words, ''how long the house is shaking". Comparing Production Blast 1 to Production Blast 2 we see that once again Production Blast 2 in the "glass stone" is less than desirable for the surrounding neighbors. Perceptible response velocities greater than 0.01 in/sec lasted for 2 to 3 seconds in many areas around the quarry. The area affected by response vibration duration greater than 2 seconds was 1400 % greater for Production Blast 2. Therefore, the only certain way to reduce the effect of blast vibrations on the surrounding neighborhood is to reduce structural response. : 'll Figure 10. Response vibration duration at 10 Hz for production blasts from Midwest Stone Quarry. Production Blast 1 "Road Stone" Production Blast 2 "Glass Stone" Ground Vibration Dundon (see:) LOO 2.00 3.00 4.00 &. ... .. A SOLUTION TO THE PROBLEM When operators start to receive many complaints about blasting they often require the blaster to use additional load decks to reduce the PPV. Other common cures are to shorten the delay interval or use high explosives rather than ANFO to increase the frequency. Often times the number of holes is reduced in an attempt to reduce vibration duration. None of these traditional techniques solve structural response problems. If fact, some actually make it worse. The only certain effect these traditional techniques have is smaller, more frequent, more costly blasts. ' Being a good neighbor doesn't mean you have to sacrifice your bottom line. The solution lies in understanding what causes high amplitude, low frequency, long duration blast events. Through the use of isoseismic techniques we have found that the biggest cause of blast vibration complaints is in the geology of the travel time path from the quarry to the house. Mapping this dynamic response around the quarry is the key to gaining control of blast vibrations. Once the influence of the geology is understood, blasts can be configured to disrupt the natural rhythm of the site through the use of millisecond delays. Let us consider the .design of an S-hole production blast using standard delay intervals as shown in Figure 11. This particular site had a fundamental frequency of 10Hz. By time lagging the single hole waveform by the delay interval and adding the waveforms we can predict what the production blast might look like. We intuitively know that a delay interval of 100 ms is bad for a 10 Hz site. It turns out thatSms is equally bad as this tends to reinforce the low fundamental frequency of the site. The best delay interval for this site appears to be 25 or 50 ms (1/4 or 1/2 the wavelength). Choosing these delay intervals causes the peaks and troughs of successive charges to be out-of-phase with each other, thus disrupting the ground's natural rhythm. The result is that the ground vibrates at a lower amplitude, for a shorter duration and with less low frequency energy to shake the neighbors' homes. Figure 11. Comparison of synthetic waveforms for 8-hole production blast using standard delay intervals. 2 1 o 8ms ... 2 1 o 17 ms ... 2 ~1 - 0 25 ms 8... j2 .1 'is 0 34 ms iH :'2 ..1 II 0 42 ms :.... 2 1 o 50 ms .1 2 1 o 100 ms ... ..z o 0.2 OA o.a o.a 1 1.2 1A D...,. I 34 (III.) 8 17 15 41 ~ 100 PPV (1..> 1.64 0.84 0.61 0.50 0.50 1L'IIl 0.63 RMll (Ips) 0.21 0.10 1l.043 0.046 0.044 Il.ro~ 0.144 PwcIpIlbIe DaraIlrJll 1.63 1.35 0.96 1.10 1.11 0.91 1.61 (1llC) When should my 8 charges ~~ ..P U3h th e <". . U'J ~ ~Vlllg r ._.~._.._~__._.....L_ ~ -. .. Figure 12 shows the reduction in energy at the fundamental frequency of the site. The spectra on this figure are plotted as a percentage of the energy produced by a single explosive column. 1000.10 meaning that the production blast would produce as much energy as the single hole. From the figure, we see that a delay interval of 8 ms produces 10 times the energy of the single hole. Whereas, 100 ms produces approximately 4.5 times the energy of the single hole. By comparison, 25 or 50 ms only produces 1/4 of the energy of the single hole. From this fundamental example, you can see a mine or quarry may never be able to eliminate all complaints but it can decrease complaints without increasing costs or reducing production. Figure 12. Comparison of spectral energy for S-hole production blast using standard delay intervals. REFERENCES Dowding, C.H., Murray, P.O. & Atmatzidis, D.K. (1981). Dynamic Properties of Residential Structures Subjected to Blasting Vibrations. Journal of the Structural Division. Proceedings of the American Society of Civil Engineers. 107(ST7). 1233-1249. M.....A......:s, K. r19"7'7't. Tl.e D-'el~-""e"'" a~ D ..+:~_..1 D......a...e r<":+.....;a ~~.. T ~n' 'D :"0 8+-...+.....0& ................u .I: \.1. "J..LU "'Y .l.V}I.I.U.I..1.1. .I. ~".l.VUQ.I. .......... IS ~.l.n....I..I. .I.V.I. .LJVY1'-.L"""'" &..I.U,","LU,", Subjected to Blasting Vibrations. Proceedings of the 18th U. S. Sym,posium on Rock Mechanics, 1-16. Reil, J.W., Anderson, D.A, Ritter, AP., Clark, D.A, Winzer, S.R., & Petro, AJ. (1985). Geologic Factors Affecting Vibration from Surface Mine Blasting (Mining Research Contract Report H0222009). U.S. Bureau of Mines. Rudenko, D. (1998, August). Understanding Blast Vibrations. Pit and Ouany. 30-33. Siskind, D.E., Stagg, MS., Kopp, J.W. & Dowding, C.R (1980). Structure Response and Damage Produced by Ground Vibration from Surface Mine Blasting (Report of Investigation 8507). U.S. Bureau of Mines. Siskind, D.E., Stachura, V.J., Stagg, M.S. & Kopp, J.W. (1980). Structure Response and Damage Produced by Airblast from Surface Mining (Report of Investigation 8485). U.S. Bureau of Mines. .~ Stagg, M.S., Siskind, D.E., Stevens, MG. & Dowding, C.H. (1984). Effects of Repeated Blasting on a Wood-Frame House (Report of Investigation 8896). U.S. Bureau of Mines. Wiss, J.F. & Parmelee, M (1974). Human Perception of Transient Vibrations. Journal of the S1JUctural Division. Proceedings of the American Society of Civil Engineers. 100(ST4). 773-787.