In the second type of puncture-resistance test, a stopping was inflated against a rusty roof bolt, which projected 7 inches from the roof of the wooden mine model entry directly into the contact area of the stopping. In a first trial, the stopping was inflated to 32 inches water gage and left for 1 day. A 1/2-inch cut resulted in the stopping, but the roof bolt did not force its way through. In a second trial, the stopping was inflated to 21 inches water gage and left for 1 day. The result was a pinhole-size air leak in the stopping, which probably occurred due to stretching of the internal Mylar layer.

In the third type of puncture-resistance test, a stopping inflated to 20 inches water gage was not punctured by tenpenny common nail points projecting 1 inch from the model entry walls or roof, but nail points projecting 2 inches or more did puncture it.

The final puncture resistance test consisted of inflating a stopping to 21 inches water gage directly against a 6-inch-diameter pipe flange projecting 1 foot from the roof of the mine model. The stopping was left inflated for 2 days without developing any leaks.

All of the punctures produced in these tests were readily patched internally through the inflation ports on the stoppings using silicone rubber sealant which adhered well to the Mylar.

RESTRAINING STOPPINGS DURING INFLATION

During the first trial with a stopping in the Safety Research Coal Mine, it became apparent that stoppings would have to be held in place during inflation. The mine location where the stopping was to be inflated had a cross section of 70 ft2 and an airflow of about 30,000 cfm. When the stopping was about one-third inflated, the moving airstream began to roll the stopping down the mine airway. Two men could hold the stopping in place, but continued inflation was difficult. Eventually, it became necessary to shut off the mine airflow to complete inflation. When the airflow was resumed, the differential pressure across the stopping was 1.5 inches water gage. It was obvious that if the pressure had been larger it would have been impossible for men to hold down the stopping.

From this it was clear that the stoppings would have to be held in place with straps, at least during inflation. Once a stopping establishes contact with the mine walls and roof, and develops some internal pressure, it remains in place. The problem was solved by using four nylon straps that were tied to a roof bolt or some similar projection (fig. 5). Trials conducted in the Safety Research Coal Mine with stoppings having straps attached showed that inflation was much easier. However, subsequent tests showed that the straps were not always trouble-free.

AIR SEALING CAPABILITIES OF STOPPINGS

Tests to measure the air sealing capabilities of the emergency inflatable stoppings were conducted in the model mine entry, in the Safety Research Coal Mine, and in two western metal mines. Table 1 clearly shows that the inflatable stoppings do not form effective air seals unless the corners of the mine airway are packed with rags or other material. These corner gaps can be seen in figure 4. A fairly large volume of packing material is needed to properly fill the gaps in the corners and around pipes and other obstructions, but even with the corners packed, there is still substantial air leakage across the stoppings. With packing, the percentage of air leaking past the stoppings ranged from about 4 to 16 percent. Without packing, the leakage was as high as 56 percent.

This suggests that if an airway does not have any major irregularities, such as large tubing or pipes along the wall and high riding tracks, the airflow could be reduced by at least 90 percent if packing were used. However, a 90-percent reduction in airflow would certainly not be acceptable if an attempt were being made to cut off the flow of toxic gases to trapped miners. In most cases, it would not be sufficient to cut off the oxygen flow to a mine fire. At best, the inflatable emergency stoppings would be used as shortterm partial seals, while more effective stoppings were being constructed of wood and fabric.

STOPPING TESTS IN WESTERN METAL MINES

The emergency inflatable stoppings were tested in two western metal mines offering widely differing conditions. The test site in the first mine (mine A) was a smooth-walled concrete entry with no obstructions such as piping or tracks. This 11- by 10-foot airway carried 100,000 cfm or air, but the static air pressure was only about 0.2 inch water gage. This static air pressure was low enough that no difficulty was encountered in inflating the stopping, which was held in place by four nylon straps. The stopping was inflated with the mine air supply to an internal pressure of 8 inches water gage and the gaps at the airway corners were carefully packed with sandbags. A ladder was used to reach the upper corners. The airflow was reduced from 100,000 cfm to 3,700 cfm. This was an encouraging reduction, but results in the second mine (mine B) indicated that the success achieved in mine A was due to the exceptionally good test conditions.

The test site in mine B was a 7.5- by 9-foot very rough-walled airway, with numerous pipes along one wall and tracks on the floor. There was about 3 inches of mud on the floor and a 16-inch-deep drainage ditch along one wall, with about 10 inches of flowing water. The measured airflow was 27,000 cfm, and the static pressure in the airway was 4.2 inches water gage. The stopping was inflated using the small blower. Numerous difficulties were encountered in erecting the stopping. To begin with, when the stopping was taken from its carrying bag and laid on the mine floor, the fitting to which the pressuresensitive switch is connected filled with mud. Another problem was trying to keep the blower, battery pack, and inflation port clean of mud and water.

The stopping was strapped in the upwind direction with four nylon straps tied to a steel cable along the mine wall. When the stopping had been inflated to about one-third of its total volume, it was caught up in the airflow where it hung from the four nylon straps, much like a horizontally inflated parachute. Although it was possible to complete the inflation, the 4.2-inch water gage differential pressure across the stopping exerted enough force on the straps to keep the stopping from fully expanding in the airway. As a result, the stopping did a very poor job of filling the airway corners, and there was considerable leakage past the pipes and along the tracks. inflation system shut off at an internal pressure of 5.2 inches water gage. Even with the aid of the reinflation system, an internal pressure between 3.8 and 5.2 inches water gage could be maintained for only about 3 days, whereas in the laboratory, it lasted more than 3 weeks. Leaks developed in the stping because of the pulling and tugging during inflation.

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The results of the trials in these two mines make it clear that inflatable stoppings could only be used as short-term partial seals during the erection of permanent seals. Even in mine A, where nearly ideal conditions were encountered, leakage past the stopping was still measurable. In mine B, in a more difficult and representative situation, the concept of a quickly erected, inflatable bulkhead, which is semipermanent and leaktight, proved to be unrealistic because of the problems of inflating the stopping in an airway where there is substantial static pressure, and because of the poor air seal obtained.

STOPPING COSTS

Table 2 gives the approximate cost of each of the four types of 12-footdiameter inflatable stoppings tested by the Bureau of Mines, and that of the blower and inflation-reinflation battery pack.

These stopping costs are for a very limited number; if the stoppings were effective enough to be mass-produced, then costs might be lower. These high costs led the Bureau to explore the possibility of using less expensive types of inflatables, such as inflatable cargo dunnage bags. (See appendix.)

CONCLUSIONS

Trials were conducted by the Bureau of Mines to evaluate inflatable stoppings as an emergency measure to control mine fires or to seal off toxic gases from trapped miners. Several types of 12-foot-diameter spheres showed the inflatable stoppings to be unacceptable for two major reasons. First, they are difficult to inflate in airways with high airflows and pressures. Second, they do not form a good air seal with the mine airway. Even in a smooth-walled concreted entry, the best seal that could be obtained still allowed about 4 percent leakage. Under less ideal conditions, the leakage. ranged as high as 56 percent.

Modifications on the stoppings might increase their inflation and sealing capabilities, but it appears that these changes would make the stoppings too complicated to serve as quickly erected emergency stoppings.

There is some possibility that these stoppings might serve as partial seals during the erection of permanent seals. However, parachute stoppings (1) developed by the Bureau of Mines offer a more efficient and less expensive tool for accomplishing this purpose.