Question: Would it be efficient to use concrete coring drills to cut a 3 1/2x8-foot opening in mass concrete?

The wall is 6 feet thick at this point, so sawing would not be possible.

Answer: Yes. ASC Corporation, a general contractor in Boise, Idaho recently used two 7 1/2-horsepower diamond core drill machines to cut a 50x90-inch opening in an 84-inch thickness of a dam wall at the Pelton Dam in Warm Springs, Oregon. The opening was 70 feet above ground, but there was a 20x20-foot pedestal just below it that served as a working platform.

On this job the concrete had to be cut and removed in two pieces to limit the weights to about 8 tons each, small enough to be handled by a 325-ton crane when its boom was extended 280 feet. A critical part of this job was the safe removal of the two large pieces of concrete.

This was accomplished by inserting into the core-drilled holes two pieces of greased 5 1/2-inch extra-heavy pipe to serve as tubular tracks on which to slide the top piece out. It was then removed readily with cables and two 400-Hertz electric winches.

The lower piece was cut and removed the same way. The tubular tracks may be seen in temporary storage above the lower piece of concrete in the picture.The job required 64 cuts 6 inches in diameter and 84 inches deep, a total of nearly 450 feet of drilling. The time required to make all cuts and remove the concrete was reported to be 80 man-hours. A 50x90-inch opening is cut in the 7-foot-thick dam wall.