A CORE DRILL----ITS LIFE AND WORKS

By H. S. Ladd,
Regional Geologist.

Many workers in the Regional Office and in the field who handle job applications have run across the trail of the Regional core drill and the name of its operator, V. C. Mickle. Since core drills are not common-place objects, it is not surprising to learn that many people have a rather vague idea of just what a core drill is and what it is supposed to do for the good of the Service. I shall attempt to show that there is nothing fundamentally mysterious about the apparatus though it is a complicated machine, calling for skillful operation and capable of working wonders underground.

A core drill is a highly specialized piece of equipment patterned on the much larger rigs that are used in drilling deep oil wells. The modern core drill, such as the Failing drill that we have, can bore a hole to a depth of over 1,000 feet and can bring up unbroken cylindrical sections (cores) of the rocks through which it passes. These cores give the geologist and the engineer a true picture of conditions underground and aid them in planning dams and other structures where accurate knowledge of foundation conditions is of the utmost importance. In between jobs of this type, our core drill is kept busy drilling and testing wells and assisting in grouting operations. In the relatively "soft" rocks of the coastal plain areas, the core drill is a highly efficient well drilling rig. Before considering specific jobs, however, it may be desirable to describe the drill briefly and show how it operates.

Some idea of the general appearance of the drill may be gained from the illustration. Its power is supplied by a Ford V-8 motor unit and is used to rotate a string of hollow rods or drill pipe. A bit is attached to the end of the drill pipe and rotates with it. A pump that forms a very important part of the rig forces a stream of thin mud down through the center of the drill pipe and the whole string thus rotates in a mud bath. The mud is forced upward outside the drill pipe, spills over into a pit and is picked up again by the pump. The mud carries up the rock fragments ground off by the bit and they later settle out in the pit, but the most important function of the mud is to seal off water seepage from the walls of the hole and thus prevent caving.

Drilling at Dam Site, Little Ocmulgee State Park, Georgia.

In coring operations, a special apparatus known as a core barrel is attached to the end of the drill pipe. This "barrel" is a double-tube several feet in length. The inner tube is suspended on ball bearings so that it is free to revolve independently of the outer tube. A ring-shaped bit is attached to the lower end of the core barrel. As it cuts an annular hole downward the core is passed upward into the inner tube by a "core lifter." When the core barrel is filled, the drill pipe is pulled out of the hole and the core removed intact.

When cores are not needed----as, for example, in most well drilling operations---the core barrel is laid aside and a "fish tail" bit of some sort is attached to the end of the drill pipe. This speeds up drilling operations for it is then no longer necessary to "come out of the hole" to empty the core barrel at five-foot intervals. Instead, the bit remains in the hole and 10-foot sections of drill pipe are swung upon the mast and added from above as the hole is deepened. If unusually hard layers are encountered, the string is pulled out of the hole and the "fish tail" bit is replaced with a special rock bit.

The Regional Office core drill was purchased over two years ago for the specific purpose of obtaining cores from dam sites and it has been used successfully in coring 110 holes on 10 sites in various parts of the Region. Many dam sites, however, do not require the use of a core drill as they can be examined satisfactorily with an auger, sand pump or by simply digging pits. In order, therefore, to keep the drill busy and thus retain the services of our driller, we started drilling wells in the intervals in between dam-testing jobs. To date a total of 28 wells have been completed successfully in 15 different parks. The total footage drilled, counting all types of holes, exceeds three miles. Five of the completed wells are 800 feet or more in depth, all five of them flowing freely at the surface. In drilling a well at LeRoy Percy State Park, in Mississippi, an eight-inch hole was carried to a depth of 500 feet. When the well was completed, it was tested by pumping its water into the empty swimming pool nearby. It filled the 200,000-gallon pool in 10-1/2 hours. An even larger supply of water was obtained from a six-inch well at Gold Head Branch State Park, in Florida, where tests showed 480 gallons per minute. A survey of all expenditures over a two-year period---allowance even being made for depreciation on the equipment--shows that the drill has been successfully operated at less than $1 per foot. Contract prices on coastal plain wells are at least double that figure.

Results at Fort Clinch State Park, Florida.

We were exceedingly fortunate from the start in obtaining the services of V. C Mickle, an unusually capable driller who had long experience drilling oil wells and who operated a diamond drill on the Boulder Dam for several years. Under ideal conditions the operation of a core drill is comparatively simple, for one who is mechanically minded, but 57 varieties of difficulties can--and do--arise to tax the ingenuity of the driller. The writer has watched our drill on many of its jobs--including one all-night session when, due to caving, the entire string of tools became firmly locked in the hole---and he has been constantly amazed at the number of tricks that Mickle has up his sleeve. When a chosen method fails there seems always to be some other way to solve the difficulty.

Mr. Mickle now has an Assistant Driller, William Sasser, formerly a CCC enrollee at Chehaw, GA-SP-9. Mr. Sasser was assigned to the drill temporarily while it was engaged in drilling a deep well at Chehaw State Park. He liked the work so well that he resigned from the CCC and has been steadily at work with the drill ever since. When, as sometimes happens in drilling deep wells, it is necessary to run the drill continuously for more than 24 hours, Sasser has run one 12-hour shift, Mickle, the other. Recently at Ft. Clinch, when it was necessary for Mickle to make a trip to Georgia, Sasser brought in an 800-foot flowing well unaided.

Plans for obtaining a second drill for the Region are now under way.