Mechanics: The Science of Machinery

by A. Russell Bond

EXPLORING SUBSURFACE CONDITIONS

In all engineering work it is highly important to explore subsurface conditions before starting any construction or even drawing up plans. The ordinary pneumatic percussion drill will not serve for deep holes. Instead a rotary boring motion is requisite. The tool is mounted with black diamond cutters which cut through the hardest rock. Water is introduced into the hole to lubricate the tool and also to wash out the abraded material. By trapping this material the character of the rock penetrated may be determined. The material, however, is pulverized and does not furnish, as a general rule, a fair sample of the rock. When it is highly important to determine the exact nature of the rock or other material a core drill is used. In other words the drill is a hollow tube set with cutting crystals about its periphery. As the tool is revolved it cuts an annular slot in the rock leaving a central core standing. This core is broken off and drawn to the surface and furnishes a true sample of the material encountered by the drill.

There is an interesting type of core drill in which steel shot is used in place of diamonds. The cutting end of the drill is a collar with a notched edge. Steel balls are fed into the drill and under the cutting head. The balls are caught in the notches and rolled around against the rock surface. There is also a partial dragging action. The friction is sufficient to wear away the rock. As the balls wear away they are replaced with new ones. Very evidently such drilling costs less for upkeep than diamond drilling, but it is not as rapid as the latter.

LOCATING ROCK UNDER HUDSON RIVER

A notable illustration of exploration drilling was furnished by the surveys for the Hudson crossing of the Catskill Aqueduct. To bring water from the Catskill Mountains to New York City it was necessary to cross the Hudson River. In order to furnish a permanent conduit it was decided to carry the water in an inverted siphon bored through solid rock. It was necessary to obtain a profile of the rock at the point where a crossing seemed most feasible. Borings were therefore made from a barge anchored in the river, but it was impossible with such an unsteady working base to carry on the boring to any considerable depth. Tides, the wash of passing steamers, floating ice, all combined to obstruct the work. Drills were constantly broken. Finally it was decided to do the boring from opposite banks of the river at such an angle that bore holes would meet or pass each other under the middle of the river. The exploration boring from the barges indicated that solid rock lay at a considerable depth below the river bed and that the bore holes would have to be set at a sharp angle to keep from breaking through rock. The river at that point is about 3,000 feet wide and a boring from the surface at the middle of the river had been driven to a depth of 768 feet without striking solid rock. Two shafts were sunk to a depth of about 250 feet and from these diamond drill borings were started at such an angle that they would cross at a depth of 1,500 feet. Excellent rock was encountered throughout the boring. Then a second set of borings was made which crossed at a depth of 950 feet without encountering any appreciable amount of water. It was accordingly decided to carry the aqueduct in vertical shafts, one at each side of the river, at a depth of 1,100 feet below water level, and then connect these shafts with a horizontal tunnel. It was important to have a good solid rock cover over the tunnel because the aqueduct reaches the river with a head of 400 feet which added to the 1,100 feet of depth of the inverted siphon gave a total head of 1,500 feet or a hydraulic pressure of about forty-two tons per square inch.

Although the diamond drill borings just mentioned were remarkable because of their inclination and because of the cramped quarters from which they were driven, they do not begin to compare in depth with some of the borings made in search of water and of oil. The deepest boring in the world to date is near Fairmount, West Virginia, where a hole six inches in diameter was driven to a depth of 7,579 feet or nearly a mile and a half. At that point an earth slide stopped further borings.

BORING FOR OIL

The time-honored method of boring for oil known as the percussive system is to hammer through the earth and rock with a heavy steel drill. The drill really consists of a long string of parts measuring altogether as much as sixty feet in length. (See Figure 64.) The drill proper or bit has a cutting edge adapted for the character of the material it is to penetrate. The bit is attached to a steel bar known as the “auger stem” which may be from twelve to forty-five feet in length. Then come the “jars” or a link member which allows a play of about sixteen inches. The purpose of this is to assist in freeing the bit from the material it is penetrating by jarring it upward on the upstroke of the drill. Above the jars there is another bar known as the sinker, and this is provided with a rope socket to which is attached the cable that carries the string of drill parts. The cable passes over a pulley to a walking beam which gives the necessary up-and-down motion.

FIG. 64.—A STRING OF WELL-DRILLING TOOLS