(From Gillette) ────────────────────┬────────────────────┬─────────┬─────────┬───────── Character of │Powder Used per Hole│ │Distance │Distance Material │ │Depth of │ Back of │ Hole to │ │ Hole, │ Face, │ Hole, │ │ Feet │ feet │ feet ────────────────────┼────────────────────┼─────────┼─────────┼───────── Limestone of Chicago│40 per cent dynamite│ │ │ Drainage Canal │ │ 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.