April 1982 l TPR 118 

Adobe Charge Program 

BUREAU OF MINES TECHNICAL PROGRESS REPORT 

--- 

EVALUATIONF A SHEATHED 

PERMISSIBLE EXPLOSIVE CHARGE 

FOR OPEN SHOOTING 

IN FLAMMABLE ATMOSPHERES 
EVALUATION OF A SHEATHED PERMISSIBLE EXPLOSIVE CHARGE FOR OPEN SHOOTING 

IN FLAMMABLE ATMOSPHERES 

Richard J. Mainiero and J. Edmund Hay 

Bureau of Mines Adobe Charge Program 

Technical Progress Report 118 

April 1982 

U.S. DEPARTMENT OF THE INTERIOR 
CONTENTS 

Abstract ....................................................................... 1 

Introduction ................................................................... 2 

Experimental procedures and results ............................................ 2 

Charge design ............................................................. 2 

Incendivity tests ......................................................... 2 

Rock-breaking tests ....................................................... 4 

Test 1 ............................................................... 4 

Test 2 ............................................................... 4 

Test 3 ............................................................... 4 

Test 4 ............................................................... 4 

Conclusions .................................................................... 9 

1. Diagram of prototype nonincendive rock-breaker charge ...................... 3 

2. Photograph of prototype nonincendive rock-breaker charge ................... 3 

3. Limestone boulder broken in test 1 ......................................... 5 

4. Limestone boulder broken in test 2 ......................................... 6 

5. Limestone boulder fractured in test 3 ...................................... 7 

6. Limestone boulder broken in test 4 ......................................... 8 

ILLUSTRATIONS 

Page 
EVALUATION OF A SHEATHED PERMISSIBLE EXPLOSIVE CHARGE FOR OPEN 

SHOOTING IN FLAMMABLE ATMOSPHERES 

By Richard J. Mainiero ’ and J. Edmund Hay’ 

ABSTRACT 

The Bureau of Mines has developed a prototype nonincendive explosive rock-breaker charge that can be fired unconfined in underground 

bituminous coal mines without the danger of igniting a flammable 

atmosphere that might be present. At present, unconfined shooting in 

underground coal mines is prohibited, but there are situations where 

the use of such shots would yield an overall improvement in safety. 

The charge consists of l-1/2 lb of permissible water gel explosive in 

the form of a short cylinder 7 inches in diameter and 7/t? inches high, 

surrounded by a l/2-inch-thick layer of damp salt, and encased in latex 

rubber reinforced with cheese cloth. The latex rubber housing provides 

a charge package that is strong enough to resist rough handling yet is 

pliable enough to conform to an irregular stone surface. A charge of 

this shape was found to be more effective at breaking rock than charges 

with lined or unlined cavities. 

Incendivity tests in a gallery have shown that the prototype charge 

will not ignite a flammable methane-air atmosphere when fired. Rockbreaking tests made with limestone boulders indicate that the charge 

will satisfactorily break stone slabs weighing 1 to 2 tons, and that two 

or more charges might be effective for slabs weighing up to 10 tons. 

In cooperation with the Mine Safety and Health Administration 

(MSHA), a test protocol for the certification of these charges as "permissible" for use in flammable atmospheres is being developed. 

'Chemical engineer. 

2Supervisory research physicist. 

Both authors are with the Pittsburgh Research Center, Bureau of Mines, 

Pittsburgh, Pa. 
Because of the hazards associated 

with flammable methane gas and coal dust, 

the shooting of mudcaps (adobes) or other 

unconfined explosive charges in underground bituminous coal mines is prohibited; all explosives must be fired in 

stemmed boreholes. However, there are 

situations where it would be advantageous 

from a safety standpoint to fire unconfined shots. This would be in the areas 

of dislodging loose roof slabs, overhangs, rock-fall leveling, slab or 

boulder breaking, and crevice shooting. 

The Bureau of Mines has been developing 

an explosive charge that is nonincendive, 

meaning that detonation of the charge 

INTRODUCTION 

will not ignite a flammable atmosphere. 

This explosive charge could be applied to 

the blasting conditions mentioned 

earlier. The explosive charge has not 

yet been certified as "permissible" for 

use in flammable atmospheres, but the 

necessary protocol is being finalized 

with MSHA. 

The purpose of this paper is to 

quickly disseminate preliminary information on the essential details of the 

research to the mining industry. More 

detailed information will be presented in 

subsequent publications. 

EXPERIMENTAL PROCEDURES AND RESULTS 

Charge Design 

Research conducted by the Bureau of 

Mines has shown that a properly designed, 

prepackaged explosive charge could be 

safely fired unconfined in a flammable 

atmosphere if the explosive was covered 

with a layer of flame-inhibiting material, such as salt, that would be dispersed as a fine cloud upon firing. For 

an explosive charge of l-l/Z lb, equivalent to two l-1/4- by 16-inch cartridges 

of permissible water gel explosive, a 

l/2-inch layer of salt proved to be 

adequate. 

One question to be answered was the 

proper shape for the explosive charge. 

Charges of various shapes with lined and 

unlined cavities were tested with the 

result that the complexity and cost of 

these charges was not justified in terms 

of rock-breaking efficiency. The most 

effective charge was one in the form of a 

short cylinder 7 inches in diameter and 

7/8 inch-high. This shape spread the 

explosive over a large surface area 

of the rock , yet provided a thickness 

adequate to provide for efficient 

detonation. 

Based on the research described 

above, the charge illustrated in 

figures 1 and 2 was developed. The 

charge is covered with a l/2-inch layer 

of damp salt (88% salt, 12% water), which 

in turn is encased in a housing consisting of latex rubber reinforced with 

cheesecloth. This configuration provides 

a charge package that is soft enough to 

conform to the irregular surface of a 

rock yet strong enough to withstand rough 

handling. 

Incendivity Tests 

The first step in the evaluation of 

the nonincendive charge was to determine 

whether it could be fired safely in a 

flammable atmosphere. To this end, incendivity testing was conducted in a 

"bombproof" bunker using a steel and 

plastic gallery 6 feet wide, 6 feet tall, 

and 8 feet long. Inside the gallery a 

3- by 3- by l-foot concrete block that 

served as a simulated boulder was set on 

edge, and the charge was taped to one of 

the vertical faces. Methane was added to 

the gallery, and the gas-air atmosphere 

was constantly mixed with a circulating 

fan and monitored with an infrared analyzer. When the gallery atmosphere stabilized at 9% methane, a sample was taken 

for analysis, the 

apparatus were shut 

was fired with an 

tor. Ignition of 

identified by the 

vibration, which 

nonignitions. 

mixing and sampling 

down, and the charge 

instantaneous detonathe methane could be 

intense noise and 

did not accompany 
3 

Latex rubber reinforced 

with cheesecloth 

Rubber tube serving as guide for 

insertion of detonator 

, . 

88% NaCI , 12% $0 

I ‘/2 lb permissible explosive 

88% NaCl , 12% H,O 

I ” -I 

FIGURE 1. - Diagram of prototype nonincendive rock-breaker charge. (Later charges incorporate 

polystyrene rather than cardboard for inner explosive housing and may have an additional detonator well on the side.) 

FIGURE 2. - Photograph of prototype nonincendive rock-breaker charge. 
4 

Ten of the prototype charges were 

tested for incendivity in the manner 

described above. All charges satisfactorily broke the concrete block without 

causing ignition. Most of the concrete 

fragments remained within several feet of 

the gallery. 

Since the charge was placed atop the 

boulder, no tape was necessary to hold it 

in place. As can be seen in the lower 

photograph of figure 4, the charge again 

broke up the boulder well. One large 

piece remained, but it had so many cracks 

in it that one blow with a sledge hammer 

would have shattered it. 

Rock-Breaking Tests 

Test 3 

A series of tests was conducted at 

the Bureau's Lake Lynn test facility to 

determine how well the prototype charge 

would break boulders. For these tests, 

limestone boulders were selectively 

chosen from among many scattered across 

the grounds of an abandoned limestone 

quarry. To begin the tests, two boulders 

were chosen that were approximately the 

same size as the concrete blocks broken 

in the incendivity tests. 

Test 1 

The upper photograph in figure 3 

shows the boulder used in the first test. 

The boulder was 54 inches wide, 32 inches 

high, and 18 inches thick, with an estimated weight of 2,700 lb. It was placed 

on edge with the charge firmly taped 

against the side. In all of the tests 

the charge was positioned on a fairly 

smooth and flat surface of the boulder, 

perpendicular to the shortest dimension. 

An instantaneous detonator was used to 

fire the charge. The lower photograph in 

figure 3 shows the rubble that remained 

after the charge was fired. As can be 

seen, the charge performed very well, 

breaking the boulder into pieces that 

could be conveniently handled. In addition, the firing of the charge did not 

produce any detectable flyrock. 

Test 2 

The boulder chosen for the second 

test was similar in shape to that used in 

the first test but was a little smaller 

(48 by 22 by 19 inches, 1,800 lb) and was 

placed flat rather than on edge. The 

boulder and charge are shown in figure 4. 

Because the Bureau's nonincendive 

charge had performed so well in tests 1 

and 2, it was next tested on a much 

larger boulder (60 by 80 by 48 inches, 

20,000 lb). The charge and boulder are 

shown in figure 5. The charge was not 

able to shatter the boulder. However, 

the charge did produce many cracks in the 

boulder and widened the cracks that were 

initially present. It appeared that firing another charge against the boulder 

would have shattered it. 

Test 4 

Test 4 was conducted to determine 

whether two of the nonincendive charges 

fired simultaneously could satisfactorily break a very large boulder similar 

to that in test 3. The boulder chosen 

was 91 inches long, 74 inches wide, and 

44 inches thick with an estimated weight 

of 25,000 pounds. The boulder and placement of the two nonincendive charges can 

be seen in the upper photograph of figure 6. The two charges, placed several 

feet apart on two flat faces, were fired 

simultaneously; the boulder was broken 

into several large pieces and many 

smaller pieces. The larger pieces were 

too large to conveniently handle manually, but inspection revealed that they 

were cracked to such an extent that it 

would have been a simple matter to break 

them with a sledge hammer. 

As noted for the incendivity tests, 

no significant flying debris was observed 

in any of the four tests. 
FIGURE 3. - Limestone boulder broken in test 1. Top: Boulder with positioning of nonincendive rock-breaker charge i I lustrated. Bottom: Remains of boulder after firing the 

charge. 
FIGURE 4. - Limestone boulder broken in test 2. Top: Boulder with nonincendiverock-breaker 

charge in place. Bottom: Remains of boulder after firing the charge. 
FIGURE 5. - Limestone boulder fractured in test 3. Top: Boulder with nonincendive 

rock-breaker charge in place. Bottom: Boulder following firing of the 

charge. This boulder was much larger than the charge was designed to 

break, yet the charge succeeded in cracking it. 
FIGURE 6. - Limestone boulder broken in test 4. Top: Boulder with two nonincendive 

rock-breaker charges in place. Bottom: Remains of the boulder after firing 

the charges. 
CONCLUSIONS 

Preliminary testing of the Bureau's 

nonincendive rock-breaker charge suggests 

that it is both safe and effective. It 

appears that the charge will not ignite a 

flammable methane-air atmosphere and is 

capable of breaking boulders weighing up 

to 2 tons into pieces that can be handled 

by one person. For boulders weighing up 

to 10 tons, several of the charges fired 

simultaneously might be effective. The 

charges also did not produce a flyrock 

hazard. 

The research reported here represents only a preliminary study of the 

nonincendive charge. Further questions 

must be answered before the charge could 

be approved for mine use, the most 

important being the question of incendivity. The charge proved nonincendive 

in 10 gallery tests, but more extensive 

testing mst be conducted to verify its 

safety in both gassy and dusty atmospheres. Furthermore, incendivity testing 

must be conducted to determine whether it 

is safe to fire multiple charges instantaneously or with delays in a flammable 

atmosphere. 

The question of the effectiveness of 

the nonincendive charge is secondary to 

safety but is still important. The 

charge appears to be very effective for 

breaking l- to 2-ton limestone boulders, 

but it may be too powerful for smaller 

boulders and is ineffective for larger 

ones. Therefore, it is necessary to 

develop and test charges of different 

sizes and further study the effectiveness 

of multiple charges fired instantaneously 

and with delays. 

INT.-BU.OF MINES,PGH.,PA. 26087 
UNITED STATES DEPARTMENT OF THE INTERIOR 

BUREAU OF MINES