Air Overpressure Prediction Equation for Construction Blasting

Air overpressure Prediction Equation for Construction Blasting Anthony J. Konya, Missouri S&T University University & Calvin J. Konya Ph.D., Precision Blasting Services Abstract State and Federal construction specifications require the Blasting Contractor to prepare a Master Blasting Plan which is reviewed by the Owner ’s Representative. The Master Blasting Plan requires the Contractor to submit the design of a typical blast. Most blasting specifications also also require the Blasting Contractor to submit Daily Blasting Blasting Plans. The Master Blasting Blasting Plan and the Daily Blasting Plan require that the Blasting Contractor also submit the types and quantities of explosives and initiators used. The specifications also request that the Blasting Contractor submit the the pounds of explosive used  per delay for both production and presplit blasts. The Blasting Contractor may also be required to  predict the anticipated ground vibration and air overpressure levels for the proposed blasts. There are equations that can be used to predict the ground vibration. There are also equations that can be used to to  predict the air overpressure levels. The data for these equations were primarily derived from quarry and surface coal mine and some construction blasts. Data from major construction projects were analyzed in this study to determine new air overpressure propagation equations for construction blasting. These new  prediction equations compare calculated results analyzed from field data with other existing air overpressure prediction equations. Production blasting air overpressure values are also compared to air overpressure from presplitting.

Background Air overpressure is an atmospheric pressure wave transmitted from the blast outward into the surrounding area. Air waves travel through the air similar to compressional waves in water. Shear waves are not transmitted through fluids such as air or water. This pressure wave consists of audible sound that can be heard, and concussion or sub-audible sound that cannot be heard. If the pressure of this wave is sufficient sufficient it can cause damage. Generally air overpressure is an annoyance type of problem that does not cause damage but causes unpleasant relations between the operator and those affected nearby. Air overpressure is generated by the the explosive gases being vented to the atmosphere as the rock ruptures, by stemming blow out, by displacement of the rock face, by displacement around the borehole and by ground vibration. Various combinations of these may exist for any given blast. blast. Overpressure and Decibels Air overpressure is most commonly measured in decibel (dB). It is also measured measured in pounds per square inch (psi). The decibel is defined in terms of the overpressure overpressure by the equation: dB = 20 LOG(P/P0) where: dB = P = P0 =

Sound levels in decibels (dB) 2 Overpressure in psi (lbs/in ) 2 Overpressure of the lowest sound that can be heard in psi (lbs/in )

-9

-9

2

P0 = 2.9 × 10  = 3 × 10  psi (lbs/in ). Some typical sound levels with values in both dB and psi are shown in Figure 1.

Air Overpressure and Wind Equivalents For residents around blasting areas the use of wind equivalents as units for air over pressure is preferred to decibels or pounds per square inch. Typical activities and damage criteria are expressed in Figure 1 in wind equivalents as well as pounds per square inch and decibels.

Figure 1. Typical Sound Levels (Konya, 2008)

Sound Levels Sound levels are measured on different weighting networks designated A, B, C, and Linear. These differ essentially in the ability to measure low frequency sound. The A-network corresponds most closely to the sound heard by the human ear and discriminates severely against the low frequencies. The B-network discriminates moderately against low frequencies and the C-network only slightly while the Linear network measures all frequencies. Sound produced by a blast is primarily low frequency energy and sound measuring devices should have a low frequency response capability to accurately represent the sound levels. A C-weighted network, or  preferably a linear-peak, should be used.

Spectral analysis of blast sounds was done by Siskind and Summers, 1974, which clearly showed the very low sub audible frequencies.

Scaled Distance for Air Overpressure Air overpressure is scaled according to the cube root of the charge weight similar to what is done with underwater blasting and not the square root of the charge weight used for ground vibration, that is: K = d/(W)

1/3

where: d W K

= = =

Distance (ft) Maximum charge weight per delay (lbs) 1/3 Scaled distance value for air overpressure (ft/lbs ).

Potential Damage for Air Overpressure There are two distinct regions of potential air overpressure damage which are referred to as Near Field and Far Field. Near Field This is the region around the blast site where there is direct transmission of the pressure pulse. The  potential for damage in the near field is considered small with reasonable blast design. The details of spacing, burden, stemming, explosive charge, delays, covering of detonating cord trunklines and use of cord with minimal core load can minimize air overpressure. Proper execution of the design ensures a very low probability of glass breakage. Far Field (Air Overpressure Focusing) This represents the region far from the blast site (from 4-20 miles, 6.4-32 km) where direct transmission cannot produce damage. It represents a concentration or focusing of sound waves in a narrow zone. These waves travel up into the atmosphere and are refracted back to the earth, producing a large overpressure in a narrow focal zone. The cause of air overpressure focusing is the presence of an atmospheric temperature inversion. The more severe the inversion, the more intense the focusing can be. Wind can also be a significant factor adding to the effect.

Research Project This research project will consider only the overpressure in the near field. There are many potential equations that have been developed to predict air overpressure. The equations are given in Table 1.

Table 1. Equations for (Near Field) Prediction of Air Overpressure (ISEE Blasters Handbook 2011) Blast Type

Open Air (no confinement)

AP (mb) Metric Equation - . P=3589 × SD3

Coal Mines (parting)

P=2596 × SD3

Coal Mines (highwall)

P=5.37 × SD3

- .

P=0.162 × SD3

Quarry Face

P=37.1 × SD3

- .

P=1.32 × SD3

Metal Mine

P=14.3 × SD3

- .

P=0.401 × SD3

Construction (average)

P=24.8 × SD3

- .

P=1 × SD3

Construction (highly confined)

P=2.48 × SD3

- .

P=0.1 × SD3

Buried (total confinement)

P=1.73 × SD3

- .

P=0.061 × SD3

- .

AP (psi) US equation - . P=187 × SD3  

P=169 × SD3

- .

Source

Perkins USBM RI 8485

- .

- .

- .

- .

USBM RI 8485 USBM RI 8485 USBM RI 8485 Oriard (2005)

- .

Oriard (2005) - .

USBM RI 8485

Project Details Air overpressure measured on major construction projects, with the use of three to four inch (75 to 100 mm) diameter blastholes, and depths to 35 feet (10.7 meters), are different than the average overpressure results calculated from overpressure equations for construction. This size of blasthole is common in general construction for highways, locks and dams and site preparation. Since the authors must be able to predict the air overpressure values for these blast conditions, they evaluated air overpressure data and determined more site-specific equations for air overpressure prediction. Three cases were studied where researchers evaluated the results for a total of 386 blasts. The projects were in granites and sedimentary rock such as shale, sandstone and limestone. In general presplit blastholes are encouraged to vent to get a clean break to the collar of the holes. These  presplit blastholes were also evaluated to determine an equation for air overpressure from presplits and compare it to production blasts. In all cases the mean value equation was developed as well as the 95% confidence level equation. These equations were compared to the existing published equations given in Table 1. Figure 2 shows the air overpressure results for the same scaled distances for eight equations given in Table 1.

Comparisons of AP for Different Equations 10

1 Open Air Coal Parting    i    s    p    n    i

   P    A

0.1 Coal Highwall Quarry 0.01

Metal Cons Avg

0.001

Cons High Con Total Con

0.0001 1

10

100

1000

SD3 (feet/pounds1/3)

Figure 2. Comparison of Air Overpressure (psi) vs Scaled distance (d/W

1/3

)

Results Air overpressure data from three separate projects were compared using a regression analysis of the air 1/3 overpressure and scaled distance (d/W ). Figure 3 shows the graphical data as well as the mean value equation and the 95% confidence level equation in psi. Figure 4 shows the same data and equations when the air overpressure data is in Decibels (dB). Figure 5 shows the same data and equations in metric units of kPa, meters and kilograms. When this equation for construction blasts is compared to the results from the equations in Figure 2, the results are very close to the results of equations previously derived for quarries and metal mines. Figure 6 shows a graph of Figure 2 with the results of the new equation added to this graph.

Figure 3. Regression Analysis and Equations for Air Overpressure (psi) (US units)

Figure 4. Regression Analysis and Equations for Air Overpressure (dB) (US units)

Figure 5. Regression Analysis and Equations for Air Overpressure (kPa) (Metric)

Comparisons of AP for Different Equations 10 Open Air

1

Coal Parting    i    s    P    n    i

   P    A

Coal Highwall

0.1

Quarry 0.01

Metal Cons Avg

0.001

Cons High Con Total Con

0.0001 1

10

100

1000

Konya

SD3 (feet/pounds)

Figure 6. Comparison of AP for all equations including Konya

As an example, one can calculate a table of air overpressure values from the three equations and compare the results (Table 2). If the data is rounded three decimal places the results of the three equations are nearly identical. A graphical representation of this data is given in Figure 7. Table 2. Comparison of AP Values Scaled Distance Quarry (psi) Metal (psi) Konya (psi)

10

0.141

0.078

0.116

20

0.072

0.048

0.067

30

0.049

0.036

0.048

40

0.037

0.029

0.038

50

0.030

0.025

0.032

70

0.021

0.020

0.024

100

0.015

0.015

0.018

120

0.013

0.013

0.016

150

0.010

0.011

0.013

1

0.1    i    s    P    n    i

Quarry

   P    A

Metal 0.01

Konya

0.001 1

10

100

1000

SD3 (feet/pounds

Figure 7. Air Overpressure in psi for Quarry, Metal Mines, and Construction (Konya)

The question is commonly asked as to the relationship of air overpressure for production blasting verses  presplit blasts. An analysis of presplit data was done using air overpressure data regression analysis to determine the 95% confidence level for air overpressure for “precision  presplit”  blasts fired independently from the production blasts. The “precision presplit” holes were stemmed with drill cuttings and a stemming plug. The holes were, in all cases, lightly loaded with detonating cord and drilled on two foot centers. Figure 8 shows the results of the presplit blasts when compared to  production blast at the same site. The graph also shows the 95% confidence equation for the general equation of the merged three production blast data used in this study. Figure 8 shows the air overpressure for site F in granite. Data from site F was used to produce the 95% confidence level air overpressure equations for both production and presplit blast. The equations used to develop Table 3 are as follows: Presplit (USA-PSI) (95 % confidence level site F), PSI = 1.55*(SD3) -0.84 Production (USA-PSI) (95 % confidence level site F), PSI = 0.56*(SD3) -0.82. An analysis of the air overpressure data showed that presplit blasts will produce about on average about 254% higher air overpressure for the same scaled distance than would result from production blasts. The average value can be conservatively assumed to be near 250% or 2.5 times the air overpressure for  production blasts The equation in Granite for presplit (USA-psi) (95 % confidence level site F):  psi = 1.55*(SD3) -0.84. The equation in Granite for presplit (Metric kPa) (95 % confidence level site F): kPa = 4.93*(SD3) -0.84

1

   i    s    P    n    i

   e    r    u    s    s    e    r    p    r    e    v    O    r    i    A

0.1 "All Production" Presplit Site F 0.01

Production Site F

0.001 1

10

100

1000

SD3

Figure 8. Air Overpressure in psi for Site F Presplit and Production

Table 3. Comparison of AP (psi) in Presplit and Production Blasts

.

SD

Presplit

Production

Difference

% increase

10

0.224

0.085

0.139

263

20

0.125

0.048

0.077

260

30

0.089

0.034

0.055

262

40

0.070

0.027

0.043

259

50

0.058

0.023

0.035

252

60

0.050

0.020

0.030

250

70

0.044

0.017

0.027

258

80

0.039

0.015

0.024

260

90

0.035

0.014

0.021

250

100

0.032

0.013

0.020

246

120

0.028

0.011

0.017

254

140

0.024

0.010

0.015

240

160

0.022

0.009

0.013

244

180

0.020

0.008

0.012

250

200

0.018

0.007

0.011

257

Conclusions 1. Air overpressure, best fit, equations for 3-4 inch (76 to 102 mm) diameter blastholes, with depths to 35 feet (10.7 meters), in both granite and limestone are: US units (psi) 1/3

- 0.89

1/3

- 0.89

Mean value

AP(psi) = 0.14(d/(W ))

95% confidence

AP(psi) = 0.95(d/(W ))

US units (dB) 1/3

Mean value

AP(dB) = -17.81*LOG(d/(W ) +153.71

95% confidence

AP(dB) = -17.81*LOG(d/(W ) +170.34

1/3

where: d = Distance in feet W = Pounds/delay Metric units (kPa) 1/3

- 0.89

1/3

- 0.89

Mean value

AP(kPa) = 0.43(d/(W ))

95% confidence

AP(kPa) = 2.89(d/(W ))

where: d = Distance in meters W = kilograms/delay. 2. Presplit air overpressure is about 2.5 times greater than air overpressure from normal production  blasting for the same scaled distance in this study. The equation in Granite for presplit air -0.84. overpressure (USA-psi) (95 % confidence level site F) is psi = 1.55*(SD3) The equation in -0.84 Granite for presplit (Metric-kPa units) (95% confidence level site F) is kPa = 4.93*(SD3) .

References th

1. “International Society of Explosives Engineers Blasters Handbook – 18   Edition (2011) (pp 587), International Society of Explosives Engineers, Cleveland: ISEE. th 2. Konya C. J., (2008). Rock Blasting and Overbreak Control- 4  Edition, Montville: IDC Inc. 3. Siskind, D.E., and Summers, C.R., (1974). "Blast Noise Standards and Instrumentation," (Report  No. TPR 78), Washington, DC: U.S. Bureau of Mines.