BLAST PHOTOGRAPHED AT THE MOMENT OF DETONATION. NOTE THE "SNAKE-HOLE" SHOT, IN LOWER RIGHT-HAND CORNER.

conditions for efficient loading is that each cut shall be the maximum width the shovel is capable of handling. Therefore it is important that the mass to be removed is of such width that it may be taken in full cuts. Usually the mass is removed in two or three cuts. The actual number of cuts varies because it depends on the size of the shovel and the magnitude of the mass that may be shot down efficiently at one time, but it is important that the mass shall not require a narrow cut to complete its removal. For example, if the mass to be removed is 2 cuts wide, the expense of moving the track and the number of moves for the shovel are the same for the half cut as for a full cut, consequently the cost of loading the half cut is excessive. Therefore the blasting foreman should so judge the magnitude of the charges that the mass of shattered rock resulting from the blast shall when lying at the maximum angle of repose be of such a width that it may be removed at one, two, or more full cuts. On the other hand if it is found that a blast of a certain size gives greater efficiency than those of larger or smaller size, then the size of the steam shovel should be such that it can load the entire mass in full cuts. The proper adjustment may therefore be made by varying either the size of the blast or the size of the steam shovel, the method of adjustment being governed by quarry conditions.

SECONDARY BLASTING.

DEFINITION OF SECONDARY BLASTING.

The term secondary blasting, as previously stated, is applied to the process of breaking with explosives the larger pieces resulting from the primary shots into sizes convenient for loading. The process is known locally as "blistering" or "bulldozing." Secondary shots are also occasionally termed "subsequent" shots.

CONDITIONS GOVERNING THE EXTENT OF SECONDARY BLASTING.

The amount of secondary blasting required depends on three main factors: (1) The nature of the rock, (2) the efficiency of the primary shot, and (3) the method of loading rock.

Rock that breaks easily and splits readily parallel with the bedding, like much of the cement rock of the Lehigh district, requires little secondary blasting, as the primary shots break the rock into small pieces. However, many limestone ledges break with difficulty, and many of the fragments resulting from the primary shot may be of large size. An excessive number of large blocks may also result from the presence of open joints which intersect the rock ledge and divide it into numerous independent masses. The masses in which the holes for the charges are drilled will probably be shattered sufficiently,

but those having no drill hole in them will probably be only slightly shattered, because the seams act as buffers and tend to check the force of the explosion. As a result, such masses may be merely pushed out and may not be broken. As a rule, the drill holes for primary shots should be more closely spaced in seamy than in sound rock, for such an arrangement promotes shattering and decreases the amount of secondary blasting necessary.

Also, light charges tend to throw the rock down in large fragments, whereas heavier charges would probably break it up much better. Furthermore, a low-grade explosive breaks the rock less efficiently than a high-grade explosive. In a number of quarries, "60 per cent" dynamite is used for all primary shots, for it is claimed that such an explosive breaks the rock much better than "40 per cent" dynamite. In quarries where the rock is difficult to break into fragments of convenient size, the quarryman should aim to use explosives that have a high disruptive effect rather than those the effect of which is mainly propulsive.

Secondary blasting is influenced greatly by the method of loading the rock. Rock loaded by hand must be broken into sizes that one man can handle, or can easily be reduced by sledging to the desired size, whereas steam shovels can load fragments of large size. Thus the expense for secondary blasting is usually much higher where hand loading is employed than where steam shovels are used.

METHODS OF SECONDARY BLASTING.

Two methods are in common use in secondary blasting, known as the " 'mud-capping" or "adobe" method, and the "block holing" method. The mud-capping method is known locally by other names such as "dobeying," "blistering," or "bulldozing," though some of these terms are applied to any method of secondary blasting. The method consists in placing a stick of dynamite, with a fuse attached, on the surface of the rock to be broken, and covering the dynamite with a mass of clay which tends to confine the explosion and direct it toward the rock. In the block-holing method, holes several inches deep are drilled in the blocks with hammer drills, and a stick, or part of a stick, of dynamite is placed in each hole. The rock dust that results from drilling is commonly used for stemming. Comparatively low-grade explosive is employed, "30 per cent" dynamite being commonly used. Whatever method is employed, a number of blasts are prepared, the fuses lighted, and the blasts thus discharged in rapid succession. In one quarry in the Lehigh district, electric detonators are placed in the charges, connected by wires, and fired simultaneously with an electric current.

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ADVANTAGES OF BLOCK-HOLING OVER MUD-CAPPING METHOD.

Mud-capping is usually regarded as an inefficient and expensive method of secondary blasting. It is well known that black blasting powder discharged on a rock surface has little or no effect on the rock, whereas dynamite discharged in the same manner may shatter the rock to a considerable extent. For this reason it is a common belief among quarrymen that the explosive force of black blasting powder is directed upward, whereas that of dynamite is directed downward. It should be clearly understood, however, that all explosives exert equal force in all directions. The different results obtained from black blasting powder and dynamite are due to the different rates of combustion of the two explosives. Gases are evolved at a comparatively low rate of speed from black blasting powder and, consequently, they can readily push aside the overlying air and escape. When dynamite is discharged, however, the gases are evolved so rapidly that they can not readily escape and, consequently, exert a violent shattering force upon any body with which the dynamite is in contact. As this force is the same in all directions, it may be readily understood that when a stick of dynamite is discharged on the surface of a rock, a large proportion of the energy of the explosion is wasted and does no useful work. On the other hand, if the dynamite is discharged in a drill hole, a much greater proportion of the energy is expended on the rock.

As a result of tests made by the Bureau of Mines," it was found that the block-holing method is at least 10 times as effective in breaking limestone as the adobe method for a given quantity of explosive. Similar conclusions have been reached by many quarrymen as a result of practical experience. As a consequence, the block-holing method is commonly employed for a major part of secondary blasting, the adobe method being used only under special conditions. In some places, however, the latter method is employed extensively and with low efficiency.

METHOD OF BLOCK-HOLE BLASTING.

In drilling for secondary shots, lines of hose are usually employed to conduct compressed air to the hammer drills from the main air lines on the quarry bank. Other methods have also been noted. In one quarry a motor-driven air compressor is mounted in a box car and can thus be readily shifted to the place where drilling is required, making long air lines unnecessary. In another quarry the steam

See Snelling, W. O., and Hall, Clarence, The effect of stemming on the efficiency of explosives: Tech. Paper 17, Bureau of Mines, 1912, p. 15; Hall, Clarence, and Howell, S. P., The selection of explosives used in engineering and mining operations: Bull. 48, Bureau of Mines, 1913, pp. 37-39.

shovel is equipped with a compressor of the Westinghouse air-brake type. Where secondary blasting is carried on within a short distance of the steam shovel, this is a very convenient method.

If many large blocks are visible in the mass of rock thrown down by the primary shot, it is customary in some quarries to drill and blast these blocks ahead of the steam shovel. Where the mass is broken up more thoroughly by the primary shot such preliminary blasting is usually omitted. Even where the primary blasting is highly efficient blocks of large size are frequently encountered in loading with a steam shovel. Some of the blocks may be too large to be lifted with the dipper, or blocks that the steam shovel could handle may be too large for the crusher. In either event, it is customary to push such blocks to one side. As the steam shovel is advanced these large masses are left between the steam-shovel track and the face. They are later drilled and blasted and the resulting fragments are loaded with the next cut.

Where hand loading is employed the larger masses exposed at the surface are first blasted and other large masses are blasted subsequently as they are encountered in loading.

CIRCUMSTANCES THAT JUSTIFY THE MUD-CAPPING METHOD.

Occasionally in steam-shovel loading masses of rock too large to be moved with the dipper may effectively block further progress. For such blocks "mud-capping" is probably preferable to "block holing" to save time. The drilling might require only a few minutes, but during this time the steam-shovel gang and the train crew would be idle. The use of a few more pounds of explosive is cheaper than to keep 10 or 12 men, a locomotive, and a steam shovel idle for even as short a space as 20 minutes or one-half hour.

Another circumstance which may justify mud-capping is the obstruction of tracks with blocks of stone. As a result of a blast or a rock slide, large masses of stone may roll onto the tracks, preventing the passage of cars or locomotives. Stalling of loaded or empty cars may not only hinder the train crew, but also keep the steam-shovel gang or the hand loaders idle for lack of cars. As in the preceding case, time is of more value that a few pounds of dynamite. Therefore, mud-capping, which requires only a few minutes, is more economical than block-holing.

Where the large fragments are thin and slablike in form it may be difficult to obtain drill holes of effective depth, and mud-capping may give better results than the block-holing method.

The amount of blasting under the special conditions that may justify mud-capping, as outlined in the preceding paragraphs, probably represents a very small proportion of the total secondary blasting