100

2D
a
a .
: 10 Nat
9 hal
& &
;. 3
6 in Si
e le NH
+
Al
1 s
10 100 1000

Distance, ft

mA-DRIFT ¢B-DRIFT  4C-DRIFT
@P1-P8 -- P1/(R/46).9 — P1/(R/46)

Figure 13. Overpressures for the shot of 28.8 Ib of C4 at the face of
A-drift as determined from time of arrival.



be arriving at P6 and P7 with time shifts on the order of milliseconds leading to relatively complex
wave shapes.

Data from the underground and surface shots (Figures 9 and 10) were studied to determine
how the peak overpressure decreases with distance. To properly conduct this analysis in an
underground entry, data should have been collected for a straight section with no crosscuts.
This was not done here because we were originally hoping to determine the effect of crosscuts
on pressure decay. The data does, however, allow us to evaluate the distance effect if we look
only at transducer data that is in direct line of sight of the detonation and we assume that the
presence of crosscuts do not have a significant affect on the peak overpressure. This
assumption may not be too far off since Peterson, et. al. showed that peak overpressure
decreased by only about 10% on passing a side tunnel [3].

Figure 11 illustrates a log-log plot of peak overpressure versus scaled distance for the four
underground shots. Carrying out a regression analysis to fit a straight line to the data yields a
straight line with a slope of -0.85. For comparison, Figure 12 is a log-log plot of peak
overpressure versus scaled distance for the two shots in the surface quarry. For the surface
shots the data is fit by a straight line with a slope of -1.65.

As mentioned above, the data from the strain gauge type transducers permanently mounted
in the entries were considered to be too slow to give valid data on the peak overpressures.
These transducers did, however, yield valid data on the arrival time of the overpressure pulse.
An attempt was made to determine the peak overpressures down A-, B-, and C-drifts from the
arrival times using a technique detailed by Kinney [6]. Figure 13 illustrates this type of
analysis for Shot 4, detonation of 28.8 Ib of C4 at the face of A-drift. For comparison, the data
from the PCB Piezotronic transducers is included, as well as lines through the P1 transducer with
slopes of -0.9 and -1. The peak overpressures deduced from the arrival times agree with the
data from the PCB Piezotronic transducers reasonably well at shorter distances and seem to
follow a slope in the neighborhood of -0.9. At longer distances the agreement does not hold
and the slope appears to become increasingly more negative. This behavior could be an effect
of the crosscuts or a change in the mechanism for peak overpressure decay with distance; the
explanation for this behavior is unknown at this time. This analysis was conducted in the
expectation that it would yield information on the effect of path geometry on peak overpressure;
the pressure pulse traveled directly down A-drift, but had to go around two corners to travel
down B- and C-drifts. We would have expected that for a given distance, the pressure for A-drift
would be higher than those for B- or C-drift; this is not the case. The pressures in A- and B-drift
are about the same while those in C-drift were significantly higher.

CONCLUSIONS

Peak overpressure from an explosive blast decays much more slowly with distance
underground than is the case on the surface. In a typical underground entry the peak
Overpressure decays proportional to distance to the -0.9 power. Data collected in the present
study was insufficient to determine the effect of crosscuts but Peterson, et.al [3] found that the


Wi 7.2 1B C4 AT FACE. Pi W2: 7:2 LB C4 AT FACE, PZ
20.0
10.0
10.0 5. 0+ jean eee
0.0 pg 0.0
25.0 30.0 35.0 40.0 65.0 79.0 75.0 BO. 0
Ws: EN A = W477 UB C4 AY FACE, Pa >
4.0 T T T
2.0
2.0 a ral A ea ee aks
0.0 + t { 9.0 i
2: Q. 195,90. 115.0 159, 0 160, 0 170. 0
(WEr-7Z-UE-C4_AT- FACE PS
T n i
6.0 4.0
4.0 a 2.0 7 as saa
38 it f 0.0 |
135.0 14: fe) 155.0 179.90 180.0 190.0
We TB C4 AT FACE P7 WE: 7.2 LB C4 AT FACE, PS.
4.0
2. es ra] | 2.0 epee
0.04 0.90
220.0 230.0 240.0 15.90 25.0 235.0
Figure 5. Overpressure recorded by the 8 PCB Piezotronic pressure
transducers for the shot of 7.2 lb of C4 explosive at the face of
A-drift.. (Absissa is ms and ordinate is psig).
14.4 1B C4 AT FACE. Pi W2: 14.4 TB C4 AT FACE, PZ
20.0 20.0
fn n 10.0
Oe wi 00 Kini ae ee
“20:0 25.0 30.0 35.0 55.0 60.0 65.0 70. 0
+O
0
. 0
29,9 109, 0 110. 0 129, 0 149, 0 150, 0 160.0
ee a NT ee a ee
a 6.0
8.0 = s-° 7 Suse
4.0 _ 2.0 ind
0.0 $ 9.0 1
125.90 135.0 145.9 160.0 170.9 180.0
_W7: 14.4 LB C4 AT FACE. P7 We: 14.4 1B C4 AT FACE, PS.
1 Ty | heal
20 ahaa at || 20 Se,
0.90 t 0.0 Rover
210.0 220.0 230.0 200. 0 210.0 220.90

Figure 6. Overpressure recorded by the 8 PCB Piezotronic pressure
transducers for the shot of 14.4 Ib of C4 explosive at the face of
A-drift. Absissa is ms and ordinate is psig.)


Table 1. Test summary

Number Shot Location Explosive Comments

1 End of A-Drift 7.2 Ib C4 . Equivalent to
10 Ib of TNT

2 End of A-Drift 14.4 Ib C4 Equivalent to
20 Ib of TNT

3 End of A-Drift 28.8 Ib C4 Equivalent to
40 Ib of TNT

4 Intersection’ 14.4 Ib C4 Equivalent to
20 Ib of TNT

5 Surface Quarry 10 Ib TNT

6 Surface Quarry 7.2 Ib C4 Equivalent to

"Intersection of A- and E-drifts



peak pressure after a side tunnel was 90 pct of that before the crosscut, i.e. the crosscut causes
a 10 pct loss of peak pressure.

In underground blasting it is typical for a blaster to get around at least one corner to protect
himself from the blast. This practice provides protection from flyrock but provides little protection
from airblast. In normal shooting there is no significant airblast since the explosives are confined
in boreholes, but airblast may become significant in cases of blown out holes, accidental initiation
of explosives, or unconfined blasting; in these cases precautions to protect personnel from
airblast must be taken.

REFERENCES

1. Hanna, N.E. and M.G. Zabetakis, Pressure Pulses Produced by Underground Blasts,
Bureau of Mines Report of Investigation 7147, 1968.

2. Olson, J.J. and L.R. Fletcher, Airblast-Overpressure Levels from Confined Underground
Production Blasts, Bureau of Mines Report of Investigation 7574, 1971.

3. Peterson, C.F., J.R. Barthel, and S. Peyton, Blast Wave Propagation in Mines, Bureau
of Mines Contract No. JO199003, Final Report, December 19, 1980.

. ineering Design Handbook, Principles of losive Behavior, AMC Pamphlet AMCP
706-180, Headquarters, U.S. Army Material Command, April, 1972, p. 3-5.

5. Dobratz, B.M. and P.C. Crawford, LLNL Explosives Handbook: Properties of Chemical

Explosives and Explosive Simulants, Lawrence Livermore National Laboratory, Livermore, CA,
January 31, 1985.

6. Kinney, G.F., Explosive Shocks in Air, The MacMillan Company, New York, 1962,
Example 5.2, pp 85-86.


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SINIANAOM INIW GNNOYIOYFIGNNA 10 MIA NY Td


BLAST WAVE PROPAGATION IN UNDERGROUND MINES
by

Richard J. Mainiero and Eric S. Weiss
U.S. Department of the Interior
Bureau of Mines, Pittsburgh Research Center
P.O. Box 18070, Pittsburgh, PA, USA

ABSTRACT

This project investigates the behavior of blast waves from the detonation of high explosives
in an underground mine. A series of explosive tests was conducted in the underground and
surface facilities at the Bureau of Mines’ Lake Lynn Laboratory to evaluate the potentially
dangerous effects of blast waves produced by open shooting, blown out holes, or accidental
detonations of explosives. Shots of C4 and TNT were fired at the dead end of a 6 ft high by
20 ft wide entry, at the intersection of two entries, and on the surface. C4 and TNT were chosen
for this research because they exhibit very consistent performance. Behavior of the blast wave

was evaluated through recording of the pressure as a function of distance from the explosive
detonation.

For both explosives, C4 and TNT, peak pressure decay with travel down the entry was
inversely proportional to distance to the 0.9 power. When comparing pressure measurements
at 200 versus 100 feet from the detonation; at 200 feet the pressure would be 1/2°° or
54 percent of that measured at 100 feet. By comparison, peak pressure for a surface shot
exhibits a decay with distance to the -1.6 power, yielding a pressure at 200 feet 1/2'-° or
33 percent of that measured at 100 feet. This result confirms that the blast wave from detonation

of an explosive in an underground mine maintains its force for a greater distance than would be
the case on the surface. :

Table and figures follow the text.


Wi 10 1B INT N_QUARRY, SFT W2: 10 UB TNT_N QUARRY, 10 FT
40.0 : 1
th 30.0 1
nu in 20.0 1
10.0
00 BN
f a8 8 1 um
0 5.0 70.0 15.0 0.0 5.0 10.0 15.0
TNT _N_QUARRY, 15 FT Wa: 10 T5 TINT _N_QUARRY, 20 FT
14.0
12:0
10.90
8.0 "
&:9
“ a
J. “A 2.9 n
+t . Boe . t +
o 2 10.0 15.0 oo 5.0 76:0 15:0
TNT_N QUARRY, 30 FT W610 LB TNT QUARRY, 50 FT
T
6.0 4.0
3.0
40 2.0
2.0 1.0 th
0.0 ‘ta 0.0 cn!
0.0 10.0 20.0 oo 20 20.0 30.0 40:0

Figure 9. Overpressure recorded by the 6 PCB Piezotronic pressure
transducers for the shot of 10 lb of TNT in the surface quarry.
(Absissa is ms and ordinate is psig).

WE 72 18 C4 N QUARRY, 5 FT W2: 7.218 C4 W_QUARRY, 0 FT
25.0 tt
150.0 30.0
100.0 15.0
50.0 i 3-8
ar) rt A 00 +
0.0 5.0 10.0 15:0 oo 5.0 10,0 15:90
Ws: 72 18 C4 N QUARRY, © FT War 72 1B C4 “N “QUARRY, 20 FT.
25.0
20.0 f 15.0
15.0 r 10.0
10.0 5.0 -
5.0
&:8 n SN 0.0 +
a) 5.0 10.0 15.0 oo 5.0 10.0 15:0
WS! 7215 C4 _N QUARRY, 30-FT _Wer 72 1B CaN GUARRY, 50 FT
8.0 4,
6.0 3. -
4.0 2. 7 |
2.0 1.
0.0 rah °. FI
oro 10.0 20.0 70 20.0 30.0 40.0

Figure 10. Overpressure recorded by the 6 PCB Piezotronic
pressure transducers for the shot of 7.2 Ib of C4 explosive in the
surface quarry. (Absissa is ms and ordinate is psig.)



i 28.5 (5 C4 AT FACE, PI [W2: 285 1S C4 AT FACE, P:
38:8 20.0
20.0
18:8 pid FEES *Ol0 wy —*
20.0 25.0 30.0 35.0 590.0 55.0 60.0 65.0
Ws: 28.8 (8 C4 AT FACE, PS W4: 285 LS C4 AT FACE, Pa)
5.0 1 5.0 t
0.0 eee 0.0
85.0 90.90 95.0 199.0 130, 0 135. 0 149, 9 145. 0
(WS: 25S 1S C4 AT FACE PS) We: x
15.
5.0 8.0 nalied ae eea
0.90 0.0
11s 150.0 160.0 170.0
WE: 265 LB C4 AT FACE, PS
5.0 310 a
9.0 S18 —
195. 9 295.9 15.0 185.9 195.0 205. 0
Figure 7. Overpressure recorded by the 8 PCB Piezotronic pressure
transducers for the shot of 28.8 Ib of C4 explosive at the face of
A-drift. (Absissa is ms and ordinate is psig).
Wi 14.4 TS C4 AT CROSSCUT, Pi W2: 14.4 15 C4 AT CROSSCUT, PZ
20.0 : 738: 8 1
10.0 pasar 1 39. 3
°.

6
4.0 .
fe rents, Nel 4.

2.0 a 2.
0.0 _ oO.
490.0 150.0 160.90 1

0.0 740. 0 150.0

Figure 8. Overpressure recorded by the 8 PCB Piezotronic pressure
transducers for the shot of 14.4 lb of C4 explosive at the intersection
of A- and E-drifts. (Absissa is ms and ordinate is psig.)


100

a
°

Overpressure, psig

10
Scaled Distance, ft/Ib*.333

m 1,020-1
4 240-Ib

Ib TNT charge  2,000-Ib Amatol charge
@ 60-Ib TNT charge

TNT charge

Figure 1. Data of Hanna and Zabetakis from shots in an abandoned
underground limestone mine. (Ref. 1)

2 10
&
a
2
a
Fe 1
nL
®
FA
g 0.1
<5
oO
=>
6
% 0.01
®D
a

10

100
Scaled Distance, ft/Ib*.333

1000)

m Shot 1, 77.9 Ib AN-FO
a Shot 3, 77.9 Ib AN-FO

@ Shot 2, 75.1 Ib Dynamite

Figure 2. Data of Olson and Fletcher from shots in an underground
copper mine. (Ref. 2)


INSTRUMENTATION

Pressure pulses were recorded using PCB Piezotronic' Model 102A04 pressure transducers
having a range of 0-1000 psi with a rise time of 1 microsecond (resonant frequency 500kHz),
and strain-gauge type pressure transducers permanently mounted in the underground facilities
for the study of gas explosions. The strain gauge type pressure transducers were Genisco
Model SP500 (now Patriot Sensors) and Dynisco Model APT380DV-1C-C29. The location and
identification of the PCB Piezotronic pressure transducers are illustrated in Figure 4 for the shots
at the face of A-drift. The setup for the shots at the intersection of A- and E-drifts was similar
with the exception that P1 was placed 46 feet from the face and P2 was placed 5 feet closer to
the face, i.e., 5 feet from the shot. The Genisco and Dynisco transducers were connected to
the computer room in the surface control building where the data was digitized and stored. The
PCB Piezotronic pressure transducers were connected to data collection instrumentation and
a computer in the instrumentation room. The slow response time of the Genisco and Dynisco
pressure transducers (in the range of 0.2 to 0.5 ms) and the 1500 samples/sec rate were too
slow to be useful in measuring overpressure but they still yielded useful information on blast
wave arrival times.

RESULTS

Figures 5 through 8 illustrate the peak overpressures recorded by the PCB Piezotronic
pressure transducers for the four underground shots. The transducers closest to the detonation,
P1 (46 ft) and P2' (105 ft), recorded initial pressure pulses that were narrow and well defined
(<1 ms width). Some character is present in the traces as the transducers detect pressure
pulses reflected by the roof and rib. Further from the detonation at P5 (205 ft) and P8 (305 ft)
the pressure pulses broaden.

The pressures recorded by transducer P3 in the shots at the face of A-drift are difficult to
explain; the trace appears to be an overlap of two relatively strong pulses spaced 4-5 ms apart.
Where the two pulses could be coming from is not clear. A time difference of 4 ms at P3
represents a difference in travel time of about 6 feet. There do not appear to be two paths to
P3 that differ by 6 feet. In addition, if the multiple pulses detected by P3 were real, why didn’t
they show up at P4? Lacking a good explanation for the multiple pulses at P3, we must
consider the possibility that they might be artifacts of the transducer and its mount, possibly
vibrations of a loose transducer.

Transducers P6 and P7 also appear to be detecting multiple pressure pulses spaced close
together in time. In these cases the origin of the multiple pulses is easier to explain. Between
the detonation and transducers P6 and P7 there are multiple paths that differ by only a few feet
in travel distance, with corresponding time displacements of several ms. Pressure pulses will

"Reference to specific brand names is made for identification only and does not imply
endorsement by the U.S. Bureau of Mines.


LAKE LYNN LABORATORY

The Bureau’s Lake Lynn Laboratory is a unique mining research laboratory designed to
provide a modern, full-scale mining environment for the testing and evaluation of mine health and
safety technology. Although the facility was developed with mining research in mind, it also
serves as an ideal facility for the study of a wide range of explosion or fire phenomena in
underground facilities. This state-of-the-art laboratory is located in the rural foothills of the
Allegheny Mountains of Pennsylvania, approximately 60 miles southeast of Pittsburgh.

Lake Lynn Laboratory consists of both surface and underground test sites and is sufficiently
isolated from residents to allow mine/tunnel fire research and large-scale explosion testing of
gases, dusts, and chemicals (see Figure 3). It was built at an abandoned commercial limestone
quarry, where underground entries 50 ft wide by 30 ft high were developed when surface mining
ceased in the late 1960’s. From these old workings, the Bureau developed a total of 7,500 ft of
20 ft wide by 6 ft high entries. These entries, in conjunction with the novel use of two explosion-
proof bulkhead doors that can be positioned to open or close an entry, can be made to simulate
room-and-pillar and longwall configurations.

EXPERIMENTAL

Tests were conducted in the room-and-pillar part of the simulated coal mine at Lake Lynn
Laboratory. Explosives were detonated at two locations: the dead-end of A-drift and the
intersection of A- and E-drifts. These locations were chosen because they both allowed us to
observe propagation of the blast wave through the branching tunnels in the room-and-pillar
section of the facility and also allowed us to observe the difference between a detonation in the
dead end of a tunnel and at an intersection where the blast wave can propagate in three
directions.

Two explosives, C4 and TNT, were employed in the research. We initially planned to employ
TNT in all tests since there is extensive information available in the literature on the propagation
of blast waves from TNT in surface detonations. Conducting the experiments using TNT proved
to be a problem, however, because we were unable to cast TNT charges within the time frame
available for the research and we felt that shooting packages of flaked TNT might not yield
reproducible results. We therefore decided to substitute C4 for TNT. C4 yields a higher peak
Pressure than TNT so the weight of C4 that yields a peak pressure equivalent to a given weight
of TNT was determined based on the relationship that peak pressure is proportional to heat of
detonation [4]. The heat of detonation for C4 is 1.40 kcal/g and the heat of detonation for
TNT is 1.02 kcal/g, yielding a ratio of 1.02/1.40=0.72 [5]. Thus, 3.6 Ib of C4 was substituted
for 5 lb of TNT, 7.2 lb of C4 was substituted for 10 Ib of TNT, etc. In practice we found that the
flaked TNT yielded reproducible results and used it in some of the shots.

A number of explosive shots were also conducted in the surface quarry area of Lake Lynn
Laboratory to serve as a comparison to the results of the underground shots. The surface and
underground tests are summarized in Table 1.


INTRODUCTION

This report describes a study of the propagation of the blast from an unconfined explosive
charge in an underground mine. Since World War Two there have been numerous investigations
of the propagation of air blast on the surface but few studies have been done on the
propagation of air blast in an underground environment. An early study of this topic was
conducted in 1968 by Hanna and Zabetakis of the Bureau of Mines, who studied the blast wave
propagation from unconfined TNT charges and modified amatol charges in an abandoned
underground limestone mine. The authors found that the peak pressures followed the cube root
(Sachs) scaling law within one tunnel diameter of the detonation; at greater distances the
pressures were generally higher than the scaling law would indicate. Overpressure data
recorded by Hanna and Zabetakis are presented in Figure 1 as a plot of overpressure versus
scaled distance. The plot indicates that the overpressure decays as scaled distance to the -0.6
to -0.85 power [7]. This small negative slope indicates that the peak overpressure decayed
very slowly with distance.

Another study of airblast overpressure from explosives was conducted in 1971 by Olson and
Fletcher, who studied the propagation of blast waves produced by production blasts in an
underground copper mine [2]. The authors found that for both dynamite and AN-FO blasts,
the peak overpressure could be expressed as the equation

P = 4.9 X 103 (D/W")-218,

where D is the distance from the blast and W is the weight of explosive. Here the slope of the
log-log overpressure versus scaled distance plot is much more negative than that observed by
Hanna and Zabetakis (see Figure 2), indicating that the peak overpressure fell off much faster
with scaled distance.

In 1980 Systems, Science, and Software conducted a study of blast wave propagation in
mines under contract to the Bureau of Mines [3]. Laboratory studies of blast wave
propagation in an array of pipes and instrumented explosive shots in two gold mines were
conducted in an effort to determine how the blast wave is affected by the presence of crosscuts,
side branches, splits in the entry, and narrowing and widening of the entry. The study was not
directly intended to determine how the peak overpressure decayed with distance down the entry,
but analysis of the results for shots in a gold mine indicates that peak overpressure decays
approximately as distance to the -0.9 power; the section of the mine employed for the study
contained no crosscuts but there were a couple sharp corners and variations in mine dimensions
of a foot or two. This result is in reasonable agreement with the work of Hanna and Zabetakis.

The current study was conducted in an effort to better understand how explosive blast waves
propagate in underground mines. Blast waves in underground mines travel much further than
is the case in open air and a better understanding of their behavior will contribute to safer
blasting.


1000

ee on
7 1 a t it.
Ul ; i i, { oT:
2 400 =< 7 ine
Ea — = =
o u —~* : =
> x To
Q t~-— fe i
3 10 ivi A!
o = = == i. :
— =
TT os TT iva iT
1 jee ee pe aera
1 10 100 1000;
Scaled Distance, ft/ib*.333
a Shot 1-7.2 Ib C4 at face + Shot 2-14.4 Ib C4 at face
a Shot 3-28.8 Ib C4 at face w Shot 4-14.4 Ib C4 at C.C.
Figure 11. Plot of overpressure versus scaled distance
(distance/explosive weight 1%) for the underground shots.
1000
2 —
Q i
= 100 =
@ =
Fe =I
2 in
2
=
S 10
oO —
1 jl
1 10 100
Scaled Distance, ft/Ib*.333
ma Shot 5-10 LB TNT ¢ Shot 6-7.2 Ib C4
— Shot 5 -- Shot 6
Figure 12. Plot of overpressure versus scaled distance for the

surface shots.