BORING. The operations of deep boring are resorted to for ascertaining the nature, thickness and extent of the various geological formations underlying the surface of the earth. Among the purposes for which boring is specifically employed are: (1) prospecting or searching for mineral deposits; (2) sinking petroleum, natural gas, artesian or salt wells; (3) determining the depth below the surface of bed-rock or other firm substratum, together with the character of the overlying materials, preparatory to mining or civil engineering operations; (4) carrying on geological or other scientific explorations.
Prospecting by boring is practised most successfully in the case of mineral deposits of large area, which are nearly horizontal, or at least not highly inclined; e.g. deposits of coal, iron, lead and salt. Wide, flat beds of such minerals may be pierced at any desired number of points. The depth at which each hole enters the deposit and the thickness of the mineral itself are readily ascertained, so that a map may be constructed with some degree of accuracy. Samples of the mineral are also secured, furnishing data as to the value of the deposit. While boring is sometimes adopted for prospecting irregular and steeply inclined mineral deposits of small area, the results are obviously less trustworthy than under the conditions named above, and may be actually misleading unless a large number of holes are bored. Incidentally, bore-holes supply information as to the character and depth of the valueless depositions of earth or rock overlying the mineral deposit. Such data assist in deciding upon the appropriate method for, and in estimating the cost of, sinking shafts or driving tunnels for the development and exploitation of the deposit. In sinking petroleum wells, boring serves not only for discovering the oil-bearing strata but also for extracting the oil. This industry has become of great importance in many parts of the United States, in southern Russia and elsewhere. Rock salt deposits are sometimes worked through bore-holes, by introducing water and pumping out the solution of brine for further treatment. The sinking of artesian wells is another application of boring. They are often hundreds, and sometimes thousands, of feet in depth. A well in St Louis, Missouri, has a depth of 3843 ft.
Boring is useful in mines themselves for a variety of purposes, such as exploring the deposit ahead of the workings, searching for neighbouring veins, and sounding the ground on approaching dangerous inundated workings. In the coal regions of Pennsylvania, bore-holes are often sunk for carrying steam pipes and hoisting ropes underground at points remote from a shaft.
Several of the methods of boring in soft ground are employed in connexion with civil engineering operations; as for ascertaining the depth below the surface to solid rock, preparatory to excavating for and designing deep foundations for heavy structures, and for estimating the cost of large scale excavations in earth and rock.
Lastly, a number of deep holes have been bored for geological exploration or for observing the increase of temperature in depth in the earth’s crust; for example, at Paruschowitz, Silesia, about 6700 ft. deep; at Leipzig, Germany, 6265 ft.; near Pittsburg, Pennsylvania, 5532 ft.; and at Wheeling, West Virginia, nearly 5000 ft. The two last mentioned were intended to obtain as complete a knowledge as possible of the bituminous coal and oil-bearing formations.
There are five methods of boring, viz.: by (1) earth augers, (2) drive pipes, (3) long, jointed rods and drop drill, (4) the rope system, in which the rods are replaced by rope, (5) rotary drills. The first two methods are adapted to soft or earthy soils only; the others are for rock.
1. Earth augers comprise spiral and pod augers. The ordinary spiral auger resembles the wood auger commonly used by carpenters. It is attached to the rod or stem by a socket joint, successive sections of rod being added as the hole is deepened. The auger is rotated by means of horizontal levers, clamped to the rod—by hand for holes of small diameter (2 to 6 in.), the larger sizes (8 to 16 in.) by horse power. Clayey, cohesive soils, containing few stones, are readily bored; stony ground with difficulty. The operation of the auger is intermittent. After a few revolutions it is raised and emptied, the soil clinging between the spirals. Depths to 50 or 60 ft. are usually bored by hand; deeper holes by horse power. For sandy, non-cohesive soils, the auger may be encircled by a close-fitting sheet-iron cylinder to prevent the soil from falling out.
Pod augers generally vary in diameter from 8 to 20 in. A common form (fig. l) consists of two curved iron plates, one attached to the rod rigidly, the other by hinge and key. By being turned through a few revolutions the pod is filled, and is then raised and emptied. For boring in sandy soils, the open sides are closed by hinged plates. Fig. 2 shows another type of pod auger. For holes of large diameter earth augers are handled with the aid of a light derrick.
| Fig. 1. Fig. 2. Pod Auger. |