Precast concrete liners for blind-drilled shafts

CIM Bulletin, Vol. 74, No. 830, 1981

ERIC R. SKONBERG, Design Engineer, Santa Fe Engineering & Construction Co. Orange, California

Technology exists today to blind drill a mine shaft 20 feet in diameter, unlined, over 3,000 feet deep. These shafts must be lined in the most economical, safe and expedient manner possible. A new technique of stacking precast concrete cylindrical sections in the unlined shaft has been developed to yield a dry and hydrostatically lined shaft.These sections must withstand an external radial pressure of 3,000 feet of water yet take up as little room as possible in the shaft. Using high-strength concrete, wall thicknesses may be kept at 2 feet. The outside diameter of the sections has been chosen to be 18 feet to allow for some annular space between the liner and shaft wall. The concrete sections are produced in segments 10 feet long weighing 75 tons each.During drilling operations, the concrete sections are cast and cured. After drilling operations are complete, a head-frame is situated over the shaft. The unlined shaft remains full of fluid. A special liner running tool is mated with the section in three steel ports at the bottom of the section. The section and running tool are lowered into the hole, and the tool is disconnected and returned to the surface. Subsequent sections are lowered and stacked using the same operation. Grout is placed behind the liner to result in a dry shaft.
Keywords: Shaft sinking, Shaft drilling, Blind drilled shafts, Liners, Concrete liners, Precast concrete, Hydrostatic pressures.