Inspecting the Inside of the Earth
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��N m i n i n g for coal or
���metals, opera- tors must know a number of things about their claims in advance unless they are out- and-out gam- blers. Before
starting operations at a mine the thickness, extent and richness of the vein must be estimated in order to determine whether the mine can be worked profitably. The depth of the vein from the surface, the dip or angle at which it lies and the nature of the ma- terials that will be encountered before reaching paying values, are also factors of the greatest importance. In a word, the mine operator must have a good idea of the "lay of the land" in ad- vance, or he may be doomed to failure from the start.
All of these questions are eas- ily answered in advance by means of core drills. Think of the way a corer takes out the heart of an apple and you have the main idea of the core drill. These drills have been used for taking samples out of the earth at varying depths from a few yards to several thou- sand feet. The speed of drilling, of course, depends upon the size of the core and the hardness of the rock, but the average is probably between two and
���How the drill samples the earth through which the bor- ing is made
���Piles of cores from the drill. Here is a record of the contents of the earth for hun- dreds of feet below the surface
��four feet per hour. Several typical cores are illustrated.
Figure 1 illustrates,
in section, a core drill
penetrating loose ma-
t e r i a 1 composed of
soft rock and earth.
Here the cutting bit is
shown with several
sharp cutting edges,
and the core barrel is
about three-quarters filled with
the different kinds of rock that
have been penetrated.
Figure 2 shows a core drill employing a steel shot bit, which type is used for cutting hai^d, solid rock. The rod F extend- ing to the surface of the ground imparts a rotary motion to the cutting tool. As the drill sinks deeper and deeper, this rod is extended correspondingly by screwing pieces into it at the top. The rod is hollow and through it are fed water and very hard small steel shot. The shot set- tles, entering the diagonal slot near the bottom of the bit which feeds it beneath the rotating bit, as shown at L. Here the weight of the drill, combined with the abrasive qualities of the shot, rapidly wears away the rock and permits the cutter to settle around the core.
While the core is being made, the cut- tings are washed upwards by the stream of water and settle in the receptacle B, which is known as a calyx. This gives an additional record, in inverse order, of the rock and earth penetrated, the mater- ials being in pulverized form, suitable for assay purposes. Figure 3 illustrates this point and also shows how the core is broken preparatory to extracting a piece. For this purpose, pebbles are fed into the drill in place of the shot. They jam around the core near the bottom and break it off as the drill is rotated. This wedged material also holds the core in place while the drill is being raised to the surface.
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