2. Drive pipes are widely used, both for testing the depth and character of soft material overlying solid rock and as a necessary preliminary to rock boring, when some thickness of surface soil must first be passed through. In its simplest form the drive pipe consists of one or more lengths of wrought iron pipe, open at both ends and from ½ in. to 6 in. diameter. When of small size the pipe is driven by a heavy hammer; for deep and large holes, a light pile-driver becomes necessary. The lower end of the pipe is provided with an annular steel shoe; the upper end has a drivehead for receiving the blows of the hammer. Successive lengths are screwed on as required. For shallow holes the pipe is cleaned out by a “bailer” or “sand-pump”—a cylinder 4 to 6 ft. long, with a valve in the lower end. It is lowered 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, another pipe of small diameter may be inserted and the driving continued. Drive pipes are often sunk by applying weights at the surface and slowly rotating by a lever. Two pipes are then used, one inside the other. Water is pumped down the inner pipe, thus loosening the soil, raising the debris and increasing the speed of driving. The “driven well” for water supply is an adaptation of the drive pipe and put down in the same way.
3. Drill and Rods.—This method has long been used in Europe and elsewhere for deep boring. In the United States it is rarely employed for depths greater than 200 or 300 ft. The usual form of cutting tool or drill is shown in fig. 3. The iron rods are from 1 to 2 in. square, in long lengths with screw joints (fig. 4). Wooden rods are occasionally used. For shallow holes (50 to 75 ft.) the work is done by hand, one or two cross-bars being clamped to the rod. The men alternately raise and drop the drill, meanwhile slowly walking around and around to rotate the bit and so keep the hole true. The cuttings are cleaned out by a bailer, as for drive pipes.
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| Fig. 3 Drill Bit. | Fig. 4. Rod Joint. |
| Fig. 5. Sliding Link. |
In boring by hand, the practical limit of depth is soon reached, on account of the increasing weight of the rods. For going deeper a “spring-pole” may be used. This is a tapering pole, say 30 ft. long and 5 or 6 in. diameter at the small end. It rests in an inclined position on a fulcrum set about 10 ft. from the butt, the latter being firmly fixed. The rods are 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 on the part of the driller. Average speeds of 6 to 10 ft. per 10 hours are easily made, to depths of 200 to 250 ft.
For deep boring the rod system requires a more elaborate plant. The rods are suspended from a heavy “walking beam” or lever, usually oscillated by a steam engine. By means of a screw-feed device, the rods, which are rotated slightly after every stroke, are gradually fed down as the hole is deepened, length after length being added. A tall derrick carries the sheaves and ropes by which the rods and tools are manipulated. The drill bit cannot be attached rigidly to the rods as in shallow boring, because the momentum of the heavy moving parts, transmitted directly to the bit as the blow is struck, would cause excessive vibration and breakage. It becomes necessary, therefore, to introduce a sliding-link joint between the rods and bit. One form of link is shown in fig. 5. On striking its blow, the bit comes to rest, while the rods continue to descend to the end of the stroke, the upper member of the link sliding down upon the lower. Then, on the up stroke the lower link, with the bit, is raised for delivering another blow. For large holes the striking weight is, say, 800 to 1000 ℔, length of stroke 2½ to 5 ft., and speed from 20 to 30 strokes per minute.
| Fig. 6. Kind Free-Falling Tool. |
By using the sliding link the cross-section and weight of the rods may be greatly reduced, the only strain being that of tension. To deliver a sharp, effective blow, however, the rods must drop with a quick stroke, which brings a heavy strain upon the operating machinery. For overcoming this difficulty, various “free-falling tools” have been devised. By these the bit is allowed to fall by gravity; the rod follows on its measured down stroke, and picks up the bit. Free-falling tools are of two classes: (1) those by which the bit is released automatically; (2) those operated by a sudden twist imparted to the rod by the drillman. One of the best known of the first class is the Kind free-fall (fig. 6). The shank of the bit is gripped and released by the jaws J, J, worked through a toggle joint by movements of the disk D. When the rod begins its downward stroke, the resistance of the water in the hole slightly raises D, thus opening the jaws and releasing the bit, which falls by gravity. On reaching the end of the stroke the jaws again catch the shank of the bit and raise it for delivering another blow. The Fabian free-fall may be noted as an example of the second class (see Köhler, Lehrbuch der Bergbaukunde, p. 57). Tools are sometimes used for cutting an annular groove in the bottom of the hole, and raising to the surface the core so formed, for observing the character of the rock.
4. Rope and Drop Tools.—This method was long ago used in China. Because of its extensive application in the oil-fields it is generally designated in the United States as the “oil-well system.” In its various modifications it is often employed also in general prospecting of mineral deposits and in sinking artesian, natural gas and salt wells. One of its forms is known in England as the Mather & Platt system.