10

HOLE SPACING STUDY

In-mine and field research into the
misfire problem was conducted in two
parts: in the Bureau of Mines Experimen-
tal Coal Mine during February 1984 and
at Dark Hollow Mine during April 1985.
The research conducted in the Bruceton
Experimental Coal Mine is shown in table
3. This work was not intended or expect-
ed to solve the entire misfire problem,
but merely to determine whether there was
a misfire problem as had been proposed,
At the time, 18-in spacing and 15-ms
delay time between holes were considered
the conditions that would most likely
lead to misfires. The delays reported in
table 3 were obtained using a combination
of instantaneous and coal mine type delay
detonators, Each shot consisted of a
pattern of three parallel holes, each
1.75-in diam by 7.5 ft deep, laid out in
a horizontal row. The spacing between
the holes and the delay time between the
center hole and the two outer holes were
varied to determine the conditions that
would lead to misfires. Two types of
misfires were observed for the 18-in
spacing. In some cases the explosive
failed to fire and the column was found
intact in the borehole following the
shot. In other cases an inspection of
the face following the shot revealed that
the explosive. was gone but the borehole
was still intact, indicating a lack of

detonation; it is suspected that the ex-
plosive may have burned in these cases.
The first type of misfire is indicated in
table 3 as "did not fire,” and the second
is indicated by “empty.” As indicated in
table 3, misfires were observed for all
of the shots at 18-in spacing, and no
misfires were observed for the holes at
27-in spacing. The delay time seemed
to have no effect on the occurrence of
misfires.

Additional research was conducted at
Dark Hollow Mine to verify the occurrence
of misfires at 18-in hole spacing. The
hole diameters and depths employed in
this part of the work were the same as
those used in the mine, but the layout
was changed in that some of the shots
were in a vertical row rather than a hor-
izontal row. No attempt was made to vary
the delay time between boreholes. A coal
mine type period 1 delay detonator was
used to prime the center hole and coal
mine type period 2 delay detonators were
used in the two outer holes. Hole spac-
ings of 18 and 24 in were used. As may
be seen in table 4, the 18-in spacing led
to misfires for most of the explosives.
(In the table, the terminology “2/4 M"
means that two out of a total of four
cartridges in the hole misfired.) A few
misfires were also observed at the 24-in
spacing, but the frequency was much lower
than that observed for the 18-in spacing.

TABLE 3. - Results of misfire research in Bruceton Experimental Coal Mine

(All holes were 1.75-in diam and 7.5 ft deep and were file loaded.

DNF means “did not fire."

SP means “shot properly.”

Empty means hole was empty.)

Permissible explosive Spacing, in Timing, ms Result

Right | Left] Right | Left] Left Right

EmuLSionsssecccccccccceeccvces 18 18 15 15 | DNF DNF

Granular B. eee cece eee ene 18 18 15 15 | SP DNF
Water gel Avseccsccsccccscvnee 18 18 15 15 Empty | Empty

Water gel B.... ee rcccccccee 18 18 15 15 DNF SP
Doweecvecece eee ecccccene 18 18 70 70 DNF Empty

Do. eee ecceeee eee ceeees 27 27 70 70 SP SP

DOecevccceccccccsceccseceves 27 27 85 15 SP SP



SHORT-DELAY BLASTING IN UNDERGROUND COAL MINES

By Richard J. Mainiero ' and Harry C. Verakis 2

ABSTRACT

The Bureau of Mines has conducted research to determine whether the
total elapsed delay time for blasting bituminous coal in underground
mines could be safely expanded beyond the present 500-ms limitation
without igniting a methane or methane-coal dust atmosphere. The results
indicated that the increase of total delay from 500 to 1,000 ms_ had no
detectable effect on safety relative to incendivity as long as permissi-
ble practices were observed in all other aspects,

Research was also conducted to evaluate the safety of 18-in hole spac-
ing for delay blasting in coal relative to misfires. For a variety of
permissible explosives, misfires were observed for about 50 pet of the
holes at 18-in spacing. Based on this observation, the Bureau has rec-
ommended that the new regulations prohibit hole spacings of less than 24
inches in underground coal mines. Misfires were also observed for 24-in
spacing, but the number of misfires and test shots was too small to sup-
port any firm conclusions.

‘supervisory chemical engineer, Pittsburgh Research Center, Bureau of Mines,
Pittsburgh, Pa.

Supervisory physical scientist, Approval and Certification Center, Mine Safety and
Health Administration, U.S. Department of Labor, Triadelphia, WV.



er

Mes el Se SS ;
ae ; Da ae NS Ps ie ore.
i pe! ee pawl ti he a

FIGURE 5. - The steel and plastic incendivity test gallery in place against the highwall face.

FIGURE 6. - Gallery in place against the highwall face. The instrumentation truck and connecting
lines can be seen in the background.


FIGURE 2, - Pit in which highwall shots were fired.



INTRODUCTION

, For the purpose of improving regula-

tions, the Mine Safety and Health Admin-
istration (MSHA), U.S. Department of
Labor, has made a comprehensive review of
the blasting and explosives standards for
underground coal mines contained in Part
75, Title 30, Code of Federal Regula-
tions. A concurrent review was also made
of the approval requirements contained in
Title 30, Parts 15 and 25, for permissi-
ble explosives, blasting machines, and
related blasting items. From these re~
views, MSHA completed development of pre-
proposal safety standards for the use of
explosives in underground coal mines and
related requirements for the approval of
permissible explosives and blasting
items. The preproposals are the first
stage inthe MSHA rulemaking process,
wherein public comments are solicited for
consideration, review, and evaluation
prior to an MSHA proposed rule.

The current approval standards, which
contain use requirements for explosives,
were reorganized, and the use require-
ments were consolidated into a Part 75
preproposal draft. The Part 75 pre-
proposal draft contains sections on the
use of explosives in bituminous, lignite,
and anthracite mines and a section on
compressed air blasting. The preproposed
approval standards cover approval re-
quirements for permissible explosives,
including newly prepared requirements for
sheathed explosives for use in unconfined

applications, approval requirements for
water stemming bags, new approval re-
quirements for detonators, and also re-

quirements for approval of blasting
machines. These preproposed explosive
use and approval requirements were an-

nounced in mid-1984 in the Federal Regis-
ter with a request for public comments.

During development of the preproposed
explosive use and approval standards,
MSHA asked that the Bureau of Mines con-
duct research on multiple short-delay
blasting of coal with permissible explo-
sives. The type of research requested
had not been performed since the work
done inthe Bureau's Experimental Coal
Mine at Bruceton, PA, over 30 years ago.
Accordingly, the Bureau, with the assist-
ance of MSHA, sought a mine site at which
to conduct experiments on the delay
blasting of coal with permissible explo-
sives. A site was found for the experi-
mental work and officially acquired in
early August 1984. The site, known as
Consolidation Coal Co.'s Dark Hollow
Mine, is located between St. Clairsville
and Cadiz, OH. It is a surface coal mine
site covering several acres of bituminous
coal, from part of which the overburden
has been removed, and a nearby 50-ft
highwall. The exposed coal seam and the
seam inthe base of the highwall (both
Pittsburgh No. 8) are about 5 ft thick
(figs. 1-2).

A main objective of the Bureau's exper-
iments at the acquired site was to study
and determine if the total elapsed delay
time for multiple short-delay blasting of
bituminous coal could be safely expanded
beyond 500 ms without igniting a methane
or methane-coal dust atmosphere. MSHA
presently limits the total elapsed time

to 500 ms. Secondly, there have been
many reports over the past few years
of misfires of permissible explosives.
These reports were not well documented,

but sufficient reports of this type were
received over a period of time to warrant
study. The experimental arrangements,
tests performed, and conclusions reached
are described herein.

EXPERIMENTAL PROGRAM

DELAY TIME STUDY

The first series of shots at the
Dark Hollow Mine was conducted in a pit
in which the overburden had been
stripped from the 5-ft-thick coal seam.
The resulting exposed coal block was

approximately 250 ft long by 100 ft wide.
This test area provided ideal research
conditions, since shooting could be con-
ducted in an actual coal seam while
avoiding many of the difficulties that
would be encountered in an underground
coal mine. For each shot, steel ingots


wi a

FIGURE 4, - Face loaded and ready for firing. Only 11 of the 12 holes are shown owing to the dif-

ficulty of taking photographs in the confined space of the gallery. The gas sampling tube can be seen

in the center.

Later in the research, agreement was
made with Consolidation Coal to use
another pit, which had a 5-ft-high coal
seam in a 50-ft highwall. At this loca-
tion shot holes were loaded to 3 1b. The
type of permissible explosive used, the
explosive loading per hole, and delay
timing were varied to determine the ef-
fect of these parameters on safety re-
lative to incendivity. In the test work
an attempt was made to use a sequential
blasting machine to control the firing
times for each of the holes in the shot
pattern, This technique proved unsatis—
factory because the multiple firing lines
needed were frequently broken. There-
fore, all but two of the shots were fired
using commercial, electric, delay detona~
tors connected in a single series toa
20-shot permissible blasting machine.

For this research, a gassy mine atmos-
phere was simulated by the positioning of
a  20- by 20- by 6-ft steel gallery
against the coal face. Care had to be
taken to provide a good seal between the
open front of the gallery and the coal
face. To accomplish this, a cutting ma-
chine was used to prepare a fairly smooth
coal face prior to positioning of the
gallery. The front opening of the gal-
lery had pieces of rubber affixed to it
in order to obtain a tight fit against
the face. Once the face had been loaded,
the open sides of the gallery were cov-
ered with 4-mil polyethylene. The gal-
lery is shown in figures 5 and 6. In
preparation for a shot, methane was in-
troduced into the gallery through a l-in
plastic tube connected to compressed gas
cylinders in an instrumentation truck



Information Circular 9064

Short-Delay Blasting in Underground
Coal Mines

By Richard J. Mainiero and Harry C. Verakis

UNITED STATES DEPARTMENT OF THE INTERIOR
Donald Paul Hodel, Secretary

BUREAU OF MINES
Robert C. Horton, Director


* . o o E x
FIGURE 8, - Planned ignition of incendivity test gallery in a shot against the highwall face.
Ignition was caused by loading the first hole beyond the collar without stemming.

FIGURE 9, - A later stage of the ignition shown in figure 8.


200 ft away. The gallery atmosphere was
constantly mixed by a squirrel-cage fan
connected to two corners of the gallery
by 8-in reinforced-plastic flexible duct.
The composition of the gas mixture was
continuously monitored by an infrared an-
alyzer. Gas samples were also taken for
laboratory analysis to verify the analyz-
er's calibration. When the gallery at-
mosphere had stabilized at 9 pct methane,
the mixing fan and analyzer were shut
down and the test shot was fired. All of
the shots were recorded on video tape by

cameras located in an area overlooking
the pit. An ignition inthe gallery
could be identified by the associated
bright orange fireball, which was totally
absent for nonignitions. The area of the
face with the broken coal resulting from
the firing of a slab round is shown in
figure 7. Figures 8 and 9 illustrate the
fireball produced by a planned ignition
of the gallery. Test results are summa-
rized in table l. Table 2 provides in-
formation on the permissible explosives
used.

FIGURE 7. - Coal broken by a typical shot in the uncovered coal seam.



top of the coal to provide burden and
minimize the tendency of the coal to re-
lieve in the upward direction. Prelimi-
nary shots indicated that 100 short tons
(st) of steel ingots placed over an area
measuring 20 by 10 ft was sufficient for
this purpose.
The delay
series of twor and

blasting tests began with a
three-hole shots to

provide information necessary for the
design of appropriate test-blast pat-
terns. The initial shots showed that

when two fully stemmed, 90-in-deep, 1.75-
in-diam holes, each loaded with 3 1b of
permissible explosive, were rear-primed
and fired at a 24-in horizontal spacing
with a 75-ms delay, partial wmisfires
(failure to propagate detonation through-
out the explosive column) were consist-
ently observed. Examination of the muck
pile showed the misfires to be the result
of cutoff holes. The misfired cartridges
of the second hole were no longer in
a column but were shifted up to 12 in.
to either side of the original column
location. These misfires were avoided if

T

the holes were placed 24 in. apart verti-
cally or diagonally or 30 in. apart hori-
zontally, or if the explosive load was
lowered to 1.5 to 2 1b per hole.

Considering this information, the
blasting pattern illustrated in figures 3
and 4 was developed. This pattern, known
as an “off-the-solid" slab round, is
commonly used for conventional mining of
coal in Appalachia. Two-inch-diameter
holes made by a hand-held drill were
used in this phase of the research. In
some of the earlier work, 1.75-in holes
made with a drilling rig were used. How-
ever, the drilling rig was not available
throughout the test program. The slab-
round pattern permitted the research work
to be conducted with total elapsed delay
times up to 5,500 ms. Emphasis in the
test work was placed on experiments using
the slab pattern with a total elapsed de-
lay time of 1,000 ms. In the experi-
mental work, the explosive columns were
rear-primed and file-loaded, and clay was
used for stemmed shots.

we
we

8
3
we
oe

oe

90"
|
woo” _|95°__f90°}_|as° /s0° /75°__/70°__/es° e_f55°_f50°_ 5°

18"
: oy F
+30: 24"

be
Ne

18”

|

FIGURE 3. - Layout of the 12-hole, slab-round, shot pattern used in delay blasting research.


ft

in

1b

ms

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

foot pet percent
inch s second
pound st short ton
millisecond



ll
TABLE 4. ~ Results of misfire research at Dark Hollow Mine
Permissible explosive Result!
Ist hole? I 2d_hole3
18-in-SPACING
Horizontal orientation:
EMULSION. eeeeeecccecccccccecsces | 4/5 Messecsccccccccseee | 4/5 Me

DOs eseececcccsceeve 4/5 M.
Gelatinous Ascccccccseee 2/6 Moses eee »| Shot properly.

DOwscecccecvccecsccece Shot properly.... . Do.
Gelatinous B.... oe dOsseeee seeeeeeee Do.

DOs see eevecccccccccssvcccceeee | oe edOeenee ee eeeee Do.
Granular A. oe dO ccccccecscccevens Do.
Granular B. AIT Meceoeeee eeecee| 6/7 Me

Dovecesee Shot properly..........| Shot properly.
Water gel A...... B/A Mececccccccceccesee| 1/4 Me

Dos ssevecccccccccceeecccceeese | Shot properly.s..eeeeeee| Shot properly.

Vertical orientation:
Emulsion... 4/5 Mececcscececeececes| 4/5 Me
Gelatinous A 2/6 Meee seeceeeee| Shot properly.
Gelatinous B 516 Messcvccececcsecees | 2/6 Me
Granular A. Shot properly..........| Shot properly.
Granular B. ee dOeececcecccccevcvee | 1/7 Me
Water gel As. ssesesescccescecees | 2/4 Messoeccccecccceces| 1/4 Me
24-in-SPACING
Horizontal orientation:
Emulsion... .csessccscececevcecece | 1/5 Mecvevcscevcecscese | 2/5 Me
Gelatinous A........ seeeeeeee | Shot properly..........| Shot properly.
Gelatinous B..... tee eeeecccee | ce dOeesccceevecccesecs Do.
Granular Avsssccscceeceeee we edOseeee oe cece Do.
Granular Beeseeeceeee . ooedow. eee ee eeeee Do.
Water gel Assessccccecccccvevece | sesdOsesscccccccscccece Do.
Vertical orientation:
EmuLSion. ..ceeeeseeevccecccecees 4/5 M.
Gelatinous A oe eeeee Shot properly.
Gelatinous B.. ee eeee eee Do.
Granular A..... eee eeeee| Shot properly... Do.
Granular B. eee eee seeee | oe dOsceeee wee Do.
Water gel Asseceeccccececcceccee | seedOnsccecccccens 1/4 M.
14/5 M indicates 4 of 5 cartridges misfired (etc.).
21st hole is left hole for horizontal orientation, top hole for vertical
orientation.
32d hole is right hole for horizontal, bottom hole for vertical
orientation.
DISCUSSION AND CONCLUSION
As shown in table 1, no ignitions were long as permissible practices are uti-
obtained for any of the shots of 12-hole lized for all other aspects of the shot.

patterns of
the-solid

of 1.0, 2.0, and 5.5 s.
dicates that increasing the
from 0.5 to 1.0 s has no detectable ef-
fect on safety relative to incendivity so

stemmed holes fired as off-
slab rounds with total delays
The research in-
total delay

Although a few

gest trends or

shots

were performed
with total elapsed delay times of 2.0 and
5.5 8, the data were too limited
conclusions

to sug-
relative to

these long delays.


CONTENTS

ADStractecceseoee
Introduction.
Experimental program.

Delay time study..

Hole spacing study. eve ce eeees
Discussion and conclusion..
Bibliography... ...eeceseeees

ILLUSTRATIONS

1. Uncovered coal seam in which initial research was conducted....
2. Pit in which highwall shots were fired .
3. Layout of the 12-hole, slab-round, shot pattern used in

LESCALCHe ceceeeecccececcsecceees
4. Face loaded and ready for firing.. Seer eeeeeecee
5. The steel and plastic incendivity test gallery in place against

WALL LACCr eee eeeeeeer renner erect een e renee ene e renee eee ee eee e eee neeeseeeeees
6. Gallery in place against the highwall face..cccseccccecccceseccees
7. Coal broken by a typical shot in the uncovered coal seam.......e0-

8. Planned ignition of incendivity test gallery in a shot against the highwall

LACC re cece eee rece eeer ee eee nneceeresereeeeeseeeeee

9. <A later stage of the ignition shown in Figure B..ccsesseceeecceees

TABLES

1. Summary of 12-hole gallery shots stemmed with 2 ft of clay and rear primed.
2. Permissible explosives used in delay blasting research..cecccccsccnccceeeee
3. Results of misfire research in Bruceton Experimental Coal Mine....
4, Results of misfire research at Dark Hollow Mine.....cccseecseecees

10
ll



Library of Congress Cataloging in Publication Data:

Mainiero, Richard J

Short-delay blasting in underground coal mines,

(aformation circular ; 9064)
Bibliography: ps 12.
Supt. of Docs. nos: 1 28.27: 906-4,

1. Blasting—Safety measures, 2. Bituminous coal. 3. Coal mines
and mining-Safety measures. 1, Verakis, Harry (., 1942. I
Tide. IH, Series: Information circular (United States, Bureau of
Mines) ; 9064,

TN295.U4 [TN279] 622s |622°.334]

600 287



9
TABLE 1. - Summary of 12-hole gallery shots stemmed with 2 ft of clay
and rear primed
(Shots 18-22 were fired in the highwall. Shots 1-17 were fired in the
uncovered coal. All shots were off the solid slab rounds using
2-in-diam holes, rear priming, and 2 ft of clay stemming.
No ignitions were observed.)
Shot Permissible explosive Explosive, Hole Delay, s | Misfire
; 1b per hole| depth, in
Leceeccecceese | Emulsion. ...cccecceeecees 1.74 60 1 No.
~ 2. eoeee o | eeedO.ee 1.74 60 2 Yes.
I Beescccecsccee | ceedOveceee 1.74 60 1 No.
Aecccccces 0 dOecccccceece 1.74 60 1 No.
Gelatinous A.... 2.0 60 1 No.
i Granular A...ssseeeee . 1.6 60 1 No.
[ . 1.4 60 1 No.
Granular B 1.65 60 1 No.
| we dOecceecesceces 1.65 60 1 No.
weedOv eee 1.65 60 1 No.
Granular B... 1.65 60 2 Yes.
Water gel A 1.54 60 1 No.
we edOceeee 1.5 56 1 No.
oeedOeee 1.5 60 1 No.
ooedow. 3.0 88 1 No.
1l6...000- ooedow. 1.5 60 5.5 Yes.
L]eseeeee oe dOseeee 3.0 84 5.5 No.
3.0 96 5.5 No.
2.9 84 1 No.
Gelatinous A. . 3.0 72 1 No.
Water gel A.. . 3.0 84 1 No.
22ecnevececceee | oe dOscecccececseccccccce 3.0 84 1 No.

TABLE 2. - Permissible explosives used in delay blasting research |

(All cartridges were 1.25-in diam)

Permissible explosive Cartridge | Cartridge |] Average detonation
length, in| weight, g rate, ft/s
Emulsion.....ees 12 264 16,540
Gelatinous A. 8 226 18,440
Gelatinous B. 8 212 16,010
Granular A. 8 187 8,200
a Granular B... 8 189 9,840
7 Water gel A 15 350 11,650
Water gel B.. 15 361 12,630



12

Three misfires were observed--in test
shots 2, 11, and 16. The misfire in shot
ll is not significant because the shot
was fired using a sequential blasting
machine and involved the failure of holes
10, 11, and 12 to fire due to the cutting

of the firing line connected to these
holes. Blasted-out coal from holes 1-9
apparently cut this firing line before

the sequential blaster could deliver a
firing pulse to holes 10-12. This is a
serious problem where multiple firing
lines are run close to the face. Three
firing lines had been used in both shots

2 and ll: one connected to holes 1-6,
one to holes 7-9, and one to holes 10-
12.

The cause of the misfire in shot 2 is

not known. While a sequential blasting
machine was used to fire this shot as
well, the misfire was different from that
in shot 11, in that the explosive column
and intact detonator appeared to have

been ejected from a single hole. The two
other holes connected in series with the
misfired hole, as well as all the other
holes in the pattern, fired properly;
thus the misfire could not be attributed
to a broken firing line, as for shot ll.

The misfire in shot 16 consisted of
half a cartridge of permissible water gel
explosive being found in the muckpile;
this could possibly be attributed to the
cutoff of an explosive column.

The use of 18-in hole spacing led to
misfires in 50 pct of the holes for the
variety of permissible explosives stud-
ied. This indicates that the use of 18-
in hole spacings in delay blasting in un-
derground coal mines is unsafe and should
be prohibited. A few misfires were also
observed at the 24-in spacing, but the
frequency was much lower than that ob-
served for the 18-in spacing and the sig-
nificance of these misfires is uncertain
at this point.

BIBLIOGRAPHY

1. Greenwald, H. P., H. C. Howarth,
J. Nagy, and I. Hartmann. Tests on the
Ignition of Natural Gas-Air Mixtures by
Permissible Explosives in the Experimen-
tal Coal Mine. BuMines Tech. Paper 716,
1949, 38 pp.

2. Hartmann, I., J. Nagy, and H. C.
Howarth. Experiments on Multiple Short-
Delay Blasting of Coal (in Two Parts).
Part I. BuMines RI 4868, 1952, 16 pp.

3. Hartmann, I., J. Nagy, E. B. McGib-
beny, and F. P. Christofel. Ignition of
Coal Dust by Permissible Explosives.
BuMines RI 4873, 1952, 18 pp.

4. Nagy, J., I. Hartmann, F. P. Chris-
tofel, and E. C. Seiler. Experiments on

Multiple Short-Delay Blasting of Coal (in
Two Parts). Part II. BuMines RI 4875,
1952, 22 pp.

5. Hartmann, I., and B. Lewis. Exper-
iments on Short-Delay Blasting in the Ex-
perimental Coal Mine. BuMines RI 5026,

1954, 6 pp.
6. Nagy, J., I. Hartmann, and R. W.
Van Dolah. Special Precautions for Mul-

tiple Short-Delay Blasting in Coal Mines.
BuMines IC 7895, 1959, 8 pp.
7. Nagy, J., I. Hartmann, E. N. Kawen-

ski, and R. W. Van Dolah. Hazards of
Cutoff Explosive Charges in Multiple
Blasting of Coal. BuMines RI 5613, 1960,
23 pp.

INT.-BU.OF MINES, PGH.,PA. 28189