ACCIDENTS FROM FALLS OF ROCK OR ORE.

BY EDWIN HIGGINS.

INTRODUCTION.

Accord

More men are killed and injured in the metal mines of the United States from falls of rock or ore than from any other cause. ing to figures published by the Bureau of Mines in Technical Paper 61, there were employed in and about metal mines during the calendar year 1912, a total of 169,199 men.

Of this number 110,056 worked underground and 59,143 on the surface. There were 661 fatal accidents, 522 underground and 139 on the surface. Of the fatal accidents 212 were caused by falls of rock or ore, this number representing 32.1 per cent of the total number of men killed on the surface and underground, and 40.6 per cent of the number killed underground.

Copper mines showed the largest percentage of men killed from falls of rock or ore; iron mines came second. Lead and zinc mines showed about the same percentage of fatalities as gold and miscellaneous metal mines, and were third in order. These figures indicate that the greatest number of accidents from falls of rock or ore occur in ore bodies of great width.

This circular is published by the Bureau of Mines to draw the attention of the miner to the chief causes of accidents from falls of rock or ore, and to set forth some of the ways in which he may protect himself from such accidents. For the sake of brevity, the word "falls" is used instead of "falls of rock or ore."

THE UNDERGROUND WORKER CAN PREVENT MANY ACCIDENTS

FROM FALLS.

Even if it were possible it would serve little purpose to point out just what proportion of accidents from falls is blamable to the management and what to the miner. However, the writer, who has been a worker underground, believes that the problem of preventing accidents from falls is almost wholly in the hands of the miner. A mine equipped with thousands of dollars worth of safety devices and worked by careless or indifferent miners will have more accidents than a mine

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without a single safety device but with miners who are always thinking of preventing accidents to themselves and their fellow workmen. A careful study of many accidents from falls indicates that a large proportion of the accidents might not have happened if the miner or the underground boss had used more judgment or care. Of course, many falls are unforeseen, such as those caused by the movement of a large block of ground, or by the falling of ground that even the most experienced miner would have no reason to believe unsafe. If a miner persists in taking chances and in disregarding his own judgment, or that of others who know, he is liable to be injured. If the underground boss fails in his duty as an inspector, or in compelling his men to obey orders promptly, he is responsible to that extent for accidents to men under his charge.

Ignorance of the nature of the ground or of the proper method of timbering is not a reasonable excuse for accidents from falls. If a miner does not know the ground where he is working, there are ways for him to find out about it. If he does not know how or where to place timber, he may readily find some one to show him. The experienced miner who is hurt by a fall will find, in most cases, that the accident would not have happened if he had been more careful or if he had followed his best judgment.

It lies within the power of the underground worker to reduce greatly the number of accidents from falls. The exercise of this power is a duty that every miner owes to himself and to his fellow workmen. The management of a mine is human, as is the miner, and therefore makes mistakes. Above all things the management needs the assistance and cooperation of the miner; without it little progress can be made toward lessening the number of accidents from falls.

VARIED CONDITIONS IN METAL MINES.

Ore deposits vary greatly in size, shape, and occurrence. They may occur in veins, beds, masses, shoots, or irregular pockets; they may be flat, sloping, or vertical. Gold-bearing quartz veins an inch wide are profitably mined in the western part of the United States; on the other hand, some of the iron-ore deposits of the Lake Superior district are 500 feet wide. Some classes of ore deposits are found only at or near the surface; others extend a mile or more underground.

In metal mines the ore may vary from ore that is as soft as mud to ore so hard that sparks will fly when it is drilled, and the same is true of the walls or rock surrounding the ore.

In many mining districts faults or breaks in the rocks are numerous, and at such places there is often a marked change in the nature of the ground. If the fault is sharp and well defined, there may

be no change, but where the rock has been crushed and ground, blocky and heavy ground may be expected. Sudden changes are met with when drifting into a dike, or through a contact. In either case the rock may change from hard to soft or from soft to hard rock.

In some metal mines work is carried on in ground that must be supported by rock walls several feet thick. In other mines, when drifting in wet and heavy ground, it is necessary to "spall" or "forepole" and place heavy timber sets with no space between them.

THE CAUSES AND PREVENTION OF ACCIDENTS FROM FALLS.

Accidents from falls are many and varied in their nature, depending on the kind of ground worked. They include accidents due to the dropping of small pieces of rock or ore, as well as those resulting

the entrance to the mine.

from the caving of large masses of ground. Miners have been killed by being struck on the head by a piece of rock no larger than a baseball. Falls may occur at faces, in timbered or untimbered workings, in raises or stopes, in short, at almost any point beneath or beyond Rock or ore falls because it lacks support. Although hard and firm ground is often self-supporting, there is a limit to the size of the excavation that may be made, even in the hardest ground, without providing support. The usual tendency, whether drifting or stoping, is to use too little timber, or to wait too long before placing the

timber.

that may occur in all of the varied types of ore deposits, the most Although it is impossible in this circular to consider in detail falls

common causes of falls that may occur in different classes of ore deposits and the precautions that should be taken to prevent such falls are discussed on the following pages.

DRIFTING OR CROSSCUTTING WITHOUT TIMBERING.

In driving without the use of timber it is taken for granted that the ground is hard and firm enough to support itself. In this class of work, falls occur when the opening is too large or is not of proper shape. Ground that will not stand if the drift is large may hold well if a small, well-arched opening be carried. If a soft rib of rock is encountered when driving in hard ground, the tendency is to drift through it without timbering; a fall sometimes results.

Many accidents are caused by the falling of loose rock. All loose rock should be removed from the face, back, and sides as driving progresses. This should be done immediately on returning to the face after blasting.

Figure 1 shows a mucker who is beginning to load ore at the face without first inspecting the back. Figure 2 shows the result of such carelessness. The accident might have been avoided had the mucker taken proper care in examining the place before he began work.

In driving in ground where timbering is required many varying conditions are met. In heavy ground, where the rock is soft, mudsills must be used, for without them the posts (legs) of the timber

sets, whether they be large enough or not, will be forced into the floor. If the timber is not large or strong enough the caps and posts will be crushed, provided they can not be forced into the floor. If the timber sets are placed too far apart, the weight of the ground may cause it to break through the lagging. Figures 3 and 4 show wrong and right methods of timbering loose ground.

In driving through heavy ground next to a firm hanging wall the danger is from the footwall side, and the cap and the footwall post will take most of the weight. If the footwall is firm and the ledge. and hanging wall weak, the pressure will be on the cap and the hanging-wall post. If both walls are hard and the ledge is soft, the greatest weight will be from the back. When the direction from

lines is loose.

which the greatest pressure is coming is known, timber of sufficient strength should be opposed to this pressure. The sets should be carefully placed and well blocked, and enough lagging should be used. It should be borne in mind always that timber sets or props are not meant to support tons of loose rock or dirt. The usefulness of timber is in taking the weight over a small area, thus relieving a weakness which might soon spread to a large block of ground.

A common cause of falls is the failure to use enough blocking about the timber. Where the ground is at all inclined to fall or slough off there should never be even the smallest space left over the timber sets or lagging. All loose rock should be pulled down and the remaining space filled in solidly with rock or tightly blocked with pieces of timber.

In mining deposits of hard ore, say 10 to 100 feet or more wide, the usual methods employed are some form of breast, underhand, or