UNDERLYING CAUSES OF ACCIDENTS FROM EXPLOSIVES Flying material from blast.-The most frequent type of accident, as indicated by these summaries, is that in which persons are killed or injured by blasted material. Approximately 43 percent of the accidents listed in table 4 were of this type, and about the same proportion of those listed in table 5 appear to be of this nature. A great number of such accidents occurred when blasting was done in opencut mines without having everyone in a safe location. Some of the factors that cause injuries from flying blasted material are : 1. Staying too long at the face when spitting fuse. 2. Unguarded shots, improper warning methods, and not going to a safe 3. Returning too soon before all shots have fired. Premature shots.-The second greatest class of accidents in blasting was premature shots, causing approximately 28 percent of the accidents listed in table 4 and probably a greater proportion of those in table 5. The usual factors producing accidents of this type are: 1. Detonation of charge while loading hole due to impact of tamping rod 2. In electric blasting premature detonation of charged holes may be Missed holes.-Detonation of missed holes was twice as frequent underground as in surface mining in the accidents of table 4; the proportion is even greater in those of table 5. Although the cause of the detonation of misfired holes is not known in a number of cases, the majority of such accidents is due to the dangerous practices of picking or drilling in bootlegs or close to a missed hole. Asphyxiation from noxious gas.-Asphyxiation from fumes of explosives in underground mines occurred when men returned too soon after blasting or, through some mishap, were overcome by powder smoke before reaching a place in good ventilation. In the accidents listed for the period of 1925-35, 20 of the 167 underground accidents (12.0 percent) were from this cause; in the list of the period 1940-44, only 4 of 47 underground accidents (8.4 percent) were due to fumes. The lists in the 2 tables do not permit any but general comparisons; therefore, it can only be stated that some improvement is indicated, perhaps through use of more suitable explosives and better ventilating practices. These summaries fully support the generally accepted opinion that few accidents happen in the storage or transportation of explosives. In the list of table 4, only 1 accident in 23 occurred under these conditions; of those in table 5, the proportion was about 1 in 11. At mining operations, 4 cases were associated with storage and 3 with transportation. About 95 percent of the accidents in each of the tables were in the use or attempted use of explosives and detonators. The most common unsafe practices in the use of explosives were: Forcing or pounding charges in boreholes; overstaying the time for lighting fuse; accidental contact of flame or heat with detonators, fuse, or explosives; drilling into missed holes; and returning too soon after blasting, resulting in injuries or asphyxiation. Several fatal accident occurs so suddenly they cannot tell what happened, or they leg wires of electric detonators to charged circuits or by failing to make sure that all persons were in the clear before firing. The 50 accidents at underground mines listed in table 5 resulted in 46 fatalities, while the 36 at open-cut mines and quarries caused 72 deaths. Blasting in the course of construction work was also hazardous, with 20 accidents causing 26 deaths. The most prevalent types and causes of explosives accidents are clearly indicated by the foregoing summaries. From similar reviews of the actual unsafe conditions or practices that have caused repeated accidents there have been formulated standard practices and rules to prevent continued death, injury, and destruction. In arriving at these conclusions it is necessary to survey the circumstances of a sufficient number of unfortunate mishaps to obtain reliable information. In many explosives accidents determination of the cause is difficult, since the evidence of the origin is destroyed by the blast and frequently those involved are killed. Survivors are often of little help, as the accident occurs so suddenly they cannot tell what happened, or they may prefer not to divulge whatever information they have. In most explosives accidents the cause is a careless, misjudged, or foolhardy act of an individual; such acts are often the result of improper education and supervision in the handling and use of explosives. ACCIDENTS TO CHILDREN FROM BLASTING CAPS The number of children injured by blasting caps each year probably totals at least 125. Of these accidents, perhaps 40 percent are connected with the use of explosives in mining. These estimates are based on reports gathered by the Bureau of Mines and by the Institute of Makers of Explosives on this type of accident; in many cases the results of these accidents have been death, blindness, crippling, or disfigurement for some child. When explosives and, more especially, detonators come into the hands of children they are usually detonated in one manner or another, frequently causing injury to 2 or 3. Typical examples are the following: A 10-year-old boy lost the sight of one eye when he tried to extract the contents of a blasting cap with a nail. The boy found the cap in an old barn. Six boys were injured, four of them permanently, when they put a lighted match into the open end of a blasting cap. The boys had found several caps hidden near a prospect pit. Children should be warned of the danger of playing with blasting caps or of handling them. Users of explosives have the duty to see that they are properly stored where they may not be lost or forgotten and not where they can be picked up by children.8 SELECTION OF EXPLOSIVES The choice of a suitable type of explosive for the work it is to do is of real importance, not only in the blasting results achieved but in the effect on the health and safety of those who use it. Some ores are harder to break than others, and an explosive that works well at one mine may not do so at another where the formation is different. Also, the amount and type of gases produced must be considered in selecting an explosive for use in confined underground workings. 8 Harrington, D., and Warncke, R. G., Accidents to Children from Blasting Caps: Bureau of Mines Inf. Circ. 7275, 1944, 13 pp. Because of the numerous types and brands of explosives on the market, the selection of an explosive for the job in hand may necessarily be the result of trial and experience. Successful use of a certain kind of explosive at one mine may be a favorable guide to its possible use at a similar operation, but the technical representatives of explosives manufacturers will advise and assist in introducing and perfecting the use of an explosive that will give the best results. In general terms, a good blasting explosive should be stable at normal temperatures, not detonated by mechanical shock, easily handled, not injurious to health, and have sufficient disruptive power for the work required of it. In metal mining, blasting is generally done by the use of high explosives; that is, detonating explosives which are fired by shock from an intermediate agent called a detonator as compared to black blasting powder which acts by rapid burning initiated by an igniting agency. Detonating explosives decompose almost instantaneously while "low explosives" burn progressively from the point of ignition over a very brief but sustained period. High explosives used in commercial work are the dynamites, of which various types, grades, and strengths are made, such as straight nitroglycerin dynamite, ammonia or "extra" dynamite, gelatin dynamite, ammonia-gelatin dynamite, and permissible explosives. Dynamites are usually prepared in stick or cartridge form, but some low-velocity types are also made in granular form, known as free-running dynamite or "blasting powder." In this form they have been used in place of black powder in open-cut or quarry work. In selecting high explosives, especially for underground work, the principal factors to be taken into consideration are strength, velocity or shattering effect, water resistance, density, fumes, temperature of freezing, and stability in hot climates. From the standpoint of safety, the most critical factors are the stability in changing temperatures and moisture, the volume of poisonous gases evolved when the explosive is detonated or burns rather than detonates, and the freezing properties. STRENGTH Strength is the force developed by the explosive. The straight dynamites are rated on the percentage, by weight, of nitroglyercin which they contain. The percent-strength grading of any other kind of dynamite means that it will release as much force as the same grade of straight dynamite measured by weight. Although a 60percent straight dynamite contains three times as much nitroglycerin as a 20-percent straight dynamite, the other ingredients are correspondingly less; and since all of them enter into the explosion, the 60-percent dynamite is approximately one and one-half times as strong as 20 percent rather than three times as strong. Two dynamites of the same strength may not necessarily produce the same blasting action; this is due to the fact that other properties, particularly density and velocity, also influence performance. VELOCITY The rate of exploding or detonating is called the velocity of an explosive. Explosives of greater speed of action generally have more shattering effect than those of lower velocity, but strength and density also have an influence on the shattering power of an explosive. The three qualities should be considered together in selecting a suitable explosive to secure desired fragmentation. RESISTANCE TO MOISTURE Water-resisting qualities of high explosives vary greatly and are important where blasting is done in wet ground. If blasts are fired soon after loading in wet holes, an explosive having moderate resistance to water can be used; where the explosive may be left under water for a length of time, a gelatin or semigelatin dynamite, or one of similar high water resistance, should be used. The permissibles and the lowdensity ammonia dynamites have relatively little resistance to water. DENSITY The density of high explosives may be indicated by the number of 114- by 8-inch cartridges to 50 pounds. In very hard ore and rock it is desirable to concentrate the explosive force in the bottom of the hole, and a high-density explosive is required. In well-drill holes, highdensity gelatin is often used in the bottom of the hole and a bulkier explosive with less density above to bring the charge up enough in the hole to break the top rock, or the hole may be sprung and a large quantity of explosive may be used of a density suitable to the material. Bulky, low-density explosives may be used to distribute the charge along the length of a drill hole in material that is easily broken. FUMES PRODUCED Varying amounts of gas or fumes are given off when explosives are fired, depending on the kind of explosive, the completeness of the detonation, and the kind of material being blasted. For surface work the nature of these fumes is not so important, but for underground work an explosive that produces large amounts of objectionable or dangerous fumes is unsafe. Ordinarily, gelatin dynamite gives off the least quantity of poisonous fumes, although certain other types of explosives may be of the same order. Burning dynamite produces the most poisonous fumes, and incompletely detonated dynamite follows closely. Complete detonation is obtained ordinarily from an explosive which is in good condition and which is thoroughly confined by the elimination of air spaces in the charge and by tight tamping of the hole, and through the use of a strong detonator pointing toward the bulk of the charge. STABILITY IN VARIABLE TEMPERATURES High explosives now made are low-freezing, except when ordered to the contrary. They will not freeze ordinarily under exposure to such normal atmospheric temperatures as occur in the United States. This important development in explosives manufacture in recent years makes blasting possible in all seasons, with little or no need for the hazardous process of thawing explosives. The composition of the explosive should be such that it does not become unreliable or oversensitive through necessary exposure to extreme changes in temperature or from moist to dry atmospheres. Such alterations in the composition of an explosive will affect its efficiency, the fumes produced, and the safety of handling. STORAGE EXPLOSIVES AND DETONATORS The storage of explosives has a much deeper relation to safety in their use than is commonly realized. Improper storage of explosives and detonators leads directly to misfires, to incomplete detonation, and to the burning of charges in the borehole. The handling of misfires and the existence of undiscovered misfires are two of the chief sources of accidents from explosives. Incomplete detonation often leaves unexploded dynamite in the drill holes or scattered through the blasted material; numerous accidents, most of which cause serious injuries, are directly due to such unexploded dynamite. Explosives and detonators which have deteriorated to some extent because of improper storage or other cause may detonate and partly burn, producing an excess of noxious gases and inefficient results; such conditions may cause fatal accidents from the dangerous gases given off. For these reasons, it is imperative to prevent deterioration of explosives and detonators by storing them in dry, well-ventilated, cool magazines. This is essential to safety in the use of explosives. A leak in the roof, wet floors in a magazine, or any condition of storage that exposes ammonium nitrate explosives or blasting caps to moisture is likely to result in some or all of these troubles. Inadequate ventilation of magazines may also lead to misfires, incomplete detonation, or burning charges; for unless air circulates freely through a magazine, the atmosphere may become hot and humid, and long exposure to such atmosphere has essentially the same final effect as dampness upon ammonia explosives and blasting caps. Heat and humidity may affect nitroglycerin explosives by causing separation of the nitroglycerin from the other ingredients or a leakiness that makes the explosive much more sensitive and dangerous to handle. Cases and other containers of explosives should not be stacked against the walls of the magazine so as to stop or reduce ventilation. Stacks of explosives cases should be separated to allow access to all parts of the magazine, and ordinarily stacks should not be more than six cases in height. A steel magazine without some protection from the direct rays of the sun will absorb so much heat, at least in some climates, that the explosives inside may become hard and insensitive. This is particularly likely to occur in a climate where the days are hot and the nights are cold and the explosives are subjected successively to extremes of high and low temperatures. A number of cases of misfires and partial detonation due to this cause are on record. A steel magazine exposed to direct rays of the sun should be protected by a wooden or other nonconducting roof supported on posts, with enough clearance to permit free circulation of air between it and the magazine, and the magazine should be covered with a coat of aluminum paint. Tests conducted by the du Pont Powder Co. showed that certain magazines covered with aluminum paint were 14° cooler than magazines painted black. Storage of explosives and detonators should be arranged so that the oldest stocks will be used first, thereby avoiding accumulations of deteriorated supplies. Cases of high explosives should be stored so that cartridges are lying flat and not standing on end. Explosives retained for extended periods should be turned over at intervals of 3 months to counteract the possible cencentration of nitroglycerin in one part |