BORING is employed for : (I) prospecting or searching for mineral deposits; (2) sinking petroleum, natural gas, artesian or salt wells; (3) determining the depth below the surface to bed rock or other firm substratum, together with the character of the overlying soil, preparatory to mining or civil engineering opera tions; and (4) geological or other scientific explorations.
Prospecting by boring is most successful in the case of mineral deposits of large area, nearly horizontal, or at least not highly inclined; e.g., deposits of coal, iron, lead and salt. Beds of such minerals may be pierced at any desired number of points, and the depth at which each hole enters the deposit and the thickness of the bed itself are readily ascertained, so that a map can be con structed with some degree of accuracy. Samples of the mineral are secured also, furnishing data as to the value of the deposit. While boring is sometimes adopted for prospecting irregular and steeply inclined 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, boreholes supply information as to the character and depth of the valueless depositions of earth or rock overlying the useful mineral. Such data assist in deciding upon the appropriate method for, and estimating the cost of, sinking shafts or driving tunnels to develop and exploit the deposit.
Several of the methods of boring in soft ground are employed in connection with civil engineering operations ; as for ascertaining the depth below the surface to solid rock, preparatory to excavat ing deeply, and for estimating the cost of large-scale excavations.
In sinking petroleum wells, boring serves both for discovering the oil-bearing strata and for extracting the oil. Since about 1918 a number of the deepest borings in the world have been made for oil or natural gas; for example, two in West Virginia of 7,386 and 7,579ft., two in south-west Pennsylvania of 6,300 and 7,248ft., two in Texas of 5,40o and 5,9o8ft., four in California of 6,24o, 7,187, 7,212 and 7,319ft. and one in Spain of 5,285 feet. Rock salt deposits are sometimes worked through boreholes by introduc ing water and pumping out the brine for further treatment.
Boring is useful in mines for exploring the deposit ahead of the workings, searching for neighbouring veins and sounding the ground on approaching dangerous inundated workings. In the coal regions boreholes are sometimes used to carry steam pipes and hoisting ropes underground at points remote from a shaft.
Lastly, 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 Czuchow, Silesia, 7,348ft. deep; at Leipzig, Germany, 6,265ft.; near Pitts burgh, Pa., 5,532ft.; and at Wheeling, W. Va., nearly 5,000 feet. The two last mentioned were bored to obtain as complete knowledge as possible of the bi tuminous coal and oil-bearing formations.
(I) Earth augers comprise spiral and pod augers. The spiral resembles the wood auger used by carpenters. It is attached to a rod by a socket joint, successive lengths of rod being added as the hole is deepened. The auger is rotated by horizontal levers clamped to the rod—by hand for holes of small diameter (2 to 6in.), by horse power for the larger sizes (8 to i6in.). Cohesive soils containing few stones are readily bored ; stony ground, with difficulty. The operation is intermittent; after a few revolutions the auger is raised and cleared of the soil clinging between the spirals. Depths to 5o or 6oft. are usually bored by hand; deeper holes by horse power. For non-cohesive soil, the auger may be encircled by a close-fitting sheet-iron cylinder to retain the soil.
Pod augers are generally 8 to loin. in diameter. A common form (fig. 1) consists of two curved plates, one attached to the rod rigidly, the other by hinge and key. A few revolutions fill the pod. Figure 2 shows a type for sandy soils. For holes of large diameter the auger is handled with the aid of a light derrick.
(2) Drive pipes are widely used for testing the depth and char acter of soft material overlying solid rock and as a necessary pre liminary to rock boring when some thickness of surface soil must first be passed through. In its simplest form the drive pipe con sists of one or more lengths of wrought iron pipe, open at both ends and tin. to 6in. in diameter.
When of small size the pipe is driven by a heavy hammer; for deep and large holes, a light pile driver is necessary. The lower end of the pipe has an annular steel shoe; the upper end, a drive head for receiving the blows of the hammer. Successive lengths are screwed on as required. For shallow holes the pipe may be cleaned out by a "bailer"—a cylinder 4 to 6ft. long, with a valve in the lower end. It is low ered at intervals, filled by being dashed up and down, and then raised and emptied. If, after reaching some depth, the external frictional resistance prevents the pipe from sinking farther, an other pipe of smaller diameter is inserted and driving continued. Drive pipes are often sunk by applying weights at the surface and slowly rotating by levers. Two pipes are then used, one inside the other. Water is pumped down the inner pipe, to loosen the soil and raise the debris ; this is sometimes called "wash-boring." In a vari ation of this, for tough soil or hard-pan, a drill bit on a hollow rod is raised and dropped inside the pipe. The water, passing down the rod and through holes in the bit, raises the debris. The well known "empire drill" is an example. The "driven well" for water supply is an adaptation of the drive pipe.
(3) Drop drill and rod has long been used in Europe and else where for deep boring. It is now rarely employed for depths greater than 200 tO 3oo feet. The usual cutting tool or bit is shown in fig. 3. The rod is from to zin. square in long lengths with screw joints (fig. 4). Wooden rods are occasionally used. For shallow holes (5o to 75ft.) the work is done by hand, one or two cross-bars being clamped to the rod. The men alternately raise and drop the drill, while walking round and round to rotate the bit and so keep the hole true. The cuttings are cleaned out by a bailer, as for drive pipes.
In boring by hand the practical limit of depth is soon reached, due to the increasing weight of the rod. For going deeper (roo to 3ooft.) a "spring-pole" may be used—a tapering pole perhaps 3oft. long and 5 or 6in. in diameter at the small end. It rests in an inclined position on a fulcrum set about I oft. from the butt, the latter being firmly fixed. The rod is suspended from the end of the pole, which extends at a height of several feet over the mouth of the hole. With the aid of the spring of the pole the strokes are produced by a slight effort of the driller. Speeds of 6 to loft. per o hours are easily made, to depths of 200 tO 250 feet.
For deep boring the rod system (nearly obsolete) requires a more elaborate plant. The rod is suspended from a walking-beam actuated by a steam engine. By a screw-feed, the rod is fed down as the hole is deepened. A derrick carries the sheaves and ropes by which the rod and tools are manipulated. The drill bit cannot be attached rigidly to the rod as in shallow boring, because the momentum of the heavy parts would cause excessive breakage. A pair of sliding links between the rods and bit is therefore neces sary. On striking, the bit comes to rest, while the rod descends to the end of the stroke, the links closing up. On the up-stroke the lower link, with the bit, is raised for another blow. For large holes the striking weight may be 800 to i,000 lb.; length of stroke, 21- to 5ft.; and speed, from 20 tO 3o strokes per minute. By. using the sliding links the cross-section and weight of the rod may be greatly reduced. To deliver an effective blow, however, the rod and bit must drop with a quick stroke, which brings a heavy strain upon the operating machinery. To over come this difficulty, "free-falling tools" have been devised to cause the bit to fall by gravity ; the rod follows on its meas ured down-stroke and picks up the bit.
Free-falling tools comprise: (r) those by which the bit is released automatically; (2) those operated by a sudden twist im parted to the rod by the drillman. They are not now of sufficient importance to be detailed here. Tools are sometimes used for cutting an annular groove around the bottom of the hole and raising the core so formed for observing the character of the rock.
(4) Rope and drop tools, also known as the "churn" or "oil-well" drill, are widely used for prospecting and for oil, artesian and natural gas wells. The chief difference from rod-boring is in using rope instead of jointed rods. For deep boring this saves the large amount of time consumed in raising and lowering the rod, when the hole is to be cleaned out or a dull bit replaced, since the tools are rapidly run up or down by means of the rope with which they are operated while drilling. The speed of boring is therefore but little affected by increase of depth, while with rod-boring it falls off rapidly. In its simplest form the "string of tools," suspended from the rope, consists of the bit, jars and rope-socket. The jars are a pair of sliding links, similar to those for rod-boring, but serving a different purpose, viz., to produce a sharp shock on the up-stroke, as the jars come together, for loosening the bit if it tends to stick fast. A heavy bar (auger stem) is generally inserted between the jars and bit to increase the force of the blow. The weight of another bar (sinker-bar) above the jars keeps the rope taut. The length of stroke and the feed are regulated by a "temper-screw" (fig. 5). Clamped to it is the drill rope, which is let out at intervals as the hole is deepened. The bits are usually 3 to 8in. in diameter; speed of boring, gen erally between 20 tO 4oft. per 24 hours, according to the kind of rock. A great variety of "fishing tools" is used in case of breakage of parts in the hole or other accident. For convenience in moving from place to place, especially in prospecting, the entire drilling machinery may be mounted on a truck.
(5) Diamond drill.—By methods (I) to (4) holes can be bored vertically downward only. By the diamond drill, holes may be bored at any angle, from vertically downward to vertically up ward. It makes an annular hole, the core of which furnishes a practically complete cross-section of the strata penetrated; the thickness and character of each stratum are revealed, with its depth below the sur face. Thus, the diamond drill is well adapted to prospecting deposits from which samples are desired.
The apparatus consists of a line of hol low rods, coupled by screw joints, an an nular steel bit, set with diamonds, being attached to the lower end; by a small en gine on the surface the rod is rapidly rotated and fed down automatically as the hole is deepened. The speed of rotation is from 3oo to 800 revolutions per minute, depending on the kind of rock and diame ter of the bit. While the boring is in progress water is forced down the hollow rod by a pump, returning to the surface through the annular space between the rod and the wails of the hole. The cuttings are thus carried to the surface, leaving the bottom of the hole clean and unobstructed. To recover the core and inspect the bit and diamonds, the rod is raised at every 3 to 8ft. of depth by a small drum and rope operated by the driving engine.
Diamond drills of standard designs (fig. 6) bore holes from 1.A-to in diameter, yielding cores of to IN in. diameter and are capable of reaching depths of a few hundred to 4,000ft. or more. In the South African gold-fields several holes from 4,5oo to 5,2ooft. deep have been bored. Diamond drills require 8 to 3o boiler h.p. Large machines will bore shallower holes of 6, 9 or even I2in. diameter. For operating in underground workings of mines, small machines are sometimes mounted on columns (fig. 7) . They bore 1 to i in. holes to depths of 30o to 400ft., cores beings to 'in. in diameter. Hand-power and truck-mounted drills are also built. Rates of advance for core-drilling to moderate depths range usu ally from 2 to 3 f t. per hour, including ordi nary delays, though in favourable rock higher speeds are attainable. In deep holes the speeds diminish, due to time consumed in raising and lowering the rod. If no core is desired a "solid bit" is used. The drilling then proceeds faster, as it is necessary to raise the rod only occasionally for examin ing the condition of the bit.
The driving engine has two cylinders, coupled to a crank-shaft, by which, through gearing, the drill rod is rotated. The rod is of wrought iron or steel tubes, usually in 5 to 'oft. lengths. There are two feed devices, the differential screw and hydraulic cylinder. For the dif ferential feed (fig. 6, 7) the engine has a hollow left-hand-threaded screw-shaft to which the rod is coupled. This shaft is driven by a spline and bevel gearing and is supported by a threaded feed-nut in the lower bearing. Geared to the screw-shaft is a light counter-shaft. The number of teeth in the gear-wheels is such that the feed-nut revolves a little faster than the screw-shaft, so that the drill-rod is fed downward a fraction of an inch for each revolution. To vary the rate of feed for different rocks, there are three pairs of gears with different ratios of teeth. The screw-shaft and gearing are carried by a swivel-head, which can be rotated in a vertical plane, for boring holes at any angle.
With the hydraulic feed (fig. 8) the rate of feed is independent of the rotative speed of the rod and can be adjusted with precision. There are either one or two feed cylinders, C, supplied with water from the pump. The rod, R, while rotating freely, is supported by the feed cylinder piston, P, and caused to move slowly downward by allowing the water to pass from the lower to the upper part of the cylinder. S is a ball-bearing and K a bevel gear, driven by a companion gear, L, on the engine shaft. Valves regulate the passage of the water and hence the rate of feed.
The bit, in fig. 9, is of soft steel, set with 6 to 8 or more diamonds, according to its diameter. The diamonds, usually from I 2 to 21 carats in size, are carefully set in the bit, projecting but slightly from its surface. Two kinds of diamonds are used, "carbons" and "borts." The carbons are opaque, dark in colour, and being tougher than the brilliant, with no cleavage planes, are suitable for hard rock. Botts are im perfect brilliants, and are best used for softer rocks. As the bit wears, the stones must be reset from time to time. The wear of carbons in a well-set bit is small though extremely variable. Above the bit are the core-lifter and core-barrel. The core-lifter, A (fig. Io), grips and breaks off the core, and raises it to the surface. The barrel, 3 to 'oft. long, fits closely in the hole and may be spirally grooved for the passage of water and debris. It serves partly as a guide, tending to keep the hole straight, partly for holding and protecting the core.
Diamond drills do not work satisfactorily in broken, fissured rock, as the carbons are liable to be injured or torn from their settings. In these circumstances and for soft rocks, the diamond bit may be replaced by a steel-toothed bit. Another core-drill is the Davis Calyx drill. For hard rock it has an annular bit, accom panied by a quantity of chilled-steel shot ("shot-boring") ; for soft rock a toothed bit is used.
Diamond-drill holes are rarely straight, usually deviating con siderably from the direction in which they are started. Very deep holes may vary as much as 45° or even 6o° from their true direc tion. This is because the rod does not fit closely in the hole and therefore bends. Deviation is likely to occur also in drilling through inclined strata, especially when the layers are of different degrees of hardness. By using a long and closely fitting core-barrel deviation is reduced but cannot be wholly prevented. Nearly horizontal holes always deflect upward, because the sag of the rod tilts up the bit. Diamond-drill holes should therefore always be surveyed. This is done by lowering into the hole instruments for observing at a number of successive points the direction and degree of deviation. If accurately surveyed a crooked hole may be quite as useful as a straight one. For details see Peele's Mining Engineers' Handbook, sec. 9, 2nd edition.
BIBLIOGRAPHY.-W. H. Jeffrey, Deep Well Drilling (1921) ; Victor Bibliography.-W. H. Jeffrey, Deep Well Drilling (1921) ; Victor Ziegler, Oil Well Drilling Methods (1923) ; L. C. Tassart, Exploita tion du Petrole; A. B. Thompson, Petroleum Mining (chapters on drilling) ; L. C. Uren, Petroleum Production Engineering; Foster, Textbook of Ore Mining; "Rod-Boring with Spring-Pole," School of Mines Quart., vol. xvi., p. i.; "Churn-drilling," Eng. and Min. Jour. (June 3, June 25, 1916, July 2, 1921) ; Trans. Amer. Inst. Min. Engs., vol. liv., p. 216 ; "Oil Well Drilling," U.S. Bureau of Mines Bul letins 134, 182, 207, and Tech. Paper 66; Eng. and Min. Jour. (July 5, Aug. 2, Aug. 9, 1919 and March 31, 1923) ; Trans. Amer. Inst. Min. Engs., vol. li., p. 62o ; Robert Peele, Mining Engineers' Handbook. On diamond drilling see Denny, Diamond Drilling; Robert Peele, Min. Engs.' Handbook, sec. 9; Eng. and Min. Jour. (Nov. 14, 1914, Jan. 7 add July 22, 1922, March 31, 1923) ; "Bore-hole Surveying," Coll. Guardian (July 3o, 1926) ; Instn. Min. and Met., vol. xiv., p. vol. xvii. p. 87, vol. xxi., p. 481, vol. xxxv., p. 153 ; Instn. Min. Engs. (1926) ; Eng. and Min. Jour. (April 26, 1924) . (R. PE.)