| TABLE 57 | ||
|---|---|---|
| Rates of Rock Drilling | ||
| Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep. | ||
| (From Gillette) | ||
| Hard Adirondack granite | 48 | |
| Maine and Massachusetts granite | 45–50 | |
| Mica-schist of New York City. Possible | 60–70 | |
| Mica-schist of New York City. Average | 40–50 | |
| Hard, Hudson River trap rock 40 | ||
| Soft red sand stone of Northern New Jersey | 90 | |
| Hard limestone near Rochester, N. Y | 70 | |
| Limestone of Chicago Drainage Canal | 70–80 | |
| Douglass, Indiana, syenite. Difficult set ups | 36 | |
| Canadian granite on Grand Trunk R. R | 30 | |
| Windmill point, Ontario limestone: | ||
| 3⅝-inch drills | 75 | |
| 2¾-inch drills | 60 | |
| 2¼-inch drills | 37 | |
145. Steam or Air for Power.—The choice between steam or air is dependent on the conditions of the work. Steam is undesirable in tunnels on account of the heat produced. In open cut work it is at a disadvantage because of the loss of power due to radiation from the hose or pipe. The life of the hose is not so long as when air is used, escaping steam causes clouds of vapor which obscure the work, and serious burns may occur due to hot water thrown from the exhaust. It is advantageous since leaks may be easily discovered and remedied, it requires less machinery than air, and it is sometimes less expensive. With compressed air, gasoline or electric motors can be used for operating the compressors.
| TABLE 58 | ||||
|---|---|---|---|---|
| Rock Blasting | ||||
| (From Gillette) | ||||
| Character of Material | Powder Used per Hole | Depth of Hole, Feet | Distance Back of Face, feet | Distance Hole to Hole, feet |
| Limestone of Chicago Drainage Canal | 40 per cent dynamite | 12 | 8 | 8 |
| Sandstone | 200 pounds black powder | 20 | 18 | 14 |
| Granite | 2 pounds 60 per cent dynamite | 12 | 1½ | 4½ to 5 |
| Pit mining, Treadwell, Mine, Alaska | 12 | 2½ | 6 | |
146. Depth of Drill Hole.—The depth of the hole is dependent on the character of the work. The deepest holes can be used in open cut work where the shattered rock is to be removed by steam shovel. The face can be made 10 to 15 feet high. The depth of the hole in center cut tunnel facings are from 6 to 10 or even 12 feet. In the bench the depth is equal to the height of the bench. In narrow trenches where the rock is to be removed by derrick or thrown into a bucket by hand, the hole should be sufficiently deep to shatter the rock to a depth of at least 6 inches below the finished sewer. Frequently shooting to this depth at one shot cannot be done due to the built up condition of the neighborhood or other local factors. The depth of the hole in trench work should not much exceed the distance between holes. Deep holes are usually desirable as a matter of economy in saving frequent set ups, but the holes cannot be made much over 20 feet in depth without increasing the friction on the drill to a prohibitive amount.
147. Diameter of Drill Hole.—The diameter of the hole should be such as to take the desired size of explosive cartridge. The common sizes of dynamite cartridges are from ⅞ inch to 2 inches in diameter. In drilling, the diameter of the hole is reduced about one-eighth of an inch at a time as the drill begins to stick. This reduction should be allowed for, and experience is the best guide for the size of the hole at the start. In general the softer or more faulty or seamy the rock, the more frequent the necessary reductions in size of bit.[[90]] For hard homogeneous rock the holes can be drilled 10 feet or more without changing the size of the drill bit.
148. Spacing of Drill Holes.—The spacing of holes in open cut excavation is commonly equal to the depth of the hole. The character of the material being excavated has much to do with the spacing of the holes. The spacing, diameter and depth of holes used on some jobs is shown in Table 58. Gillette states:
It is obviously impossible to lay down any hard and fast rule for drill holes. In stratified rock that is friable, and in traps that are full of natural joints and seams, it is often possible to space the holes a distance apart somewhat greater than their depth, and still break the rock to comparatively small sizes upon blasting. In tough granite, gneiss, syenite, and in trap where joints are few and far between, the holes may have to be spaced 3 to 8 feet apart regardless of their depth for with wider spacing the blocks thrown down will be too large to handle with ordinary appliances. Since in shallow excavations the holes can seldom be much further apart than one to one and one-half times their depth we see that the cost of drilling per cubic yard increases very rapidly the shallower the excavation. Furthermore the cost of drilling a foot of hole is much increased where frequent shifting of the drill tripod is necessary.
The common practice in placing drill holes is to put down holes in pairs, one hole on each side of the proposed trench; and if the trench is wide one or more holes are drilled between these two side holes[[91]] but in narrow trench work, such as for a 12–inch pipe, one hole in the middle of the trench will usually prove sufficient.
The holes are spaced about 3 feet apart longitudinally. After the holes have been completed they should be plugged to keep out dirt and water.