DOUBLE SPIRAL CONCRETE CASING

PROBLEM TO BE SOLVED: To provide a double spiral concrete casing having a strong structure capable of withstanding stress concentration, for facilitating concrete placing work and reinforcement assembling work. SOLUTION: A casing is provided with a cylindrical shell-like steel frame arranged on the inner periphery of an opening part provided on a concrete casing upper floor plate, and two passages of an inner passage and an outer passage spirally extending around the cylindrical shell-like frame, whose peripheries are formed of walls, and partitioned by a steel partition wall 1. A first reinforcing material and a second reinforcing material 22 are arranged in a space formed of the outer surface of the cylindrical shell-like steel frame 4 and a cut-off 23 of the steel partition wall 1, and a space formed on a passage wall projecting part 10 in which a steel inner passage frame 5 as a wall of the inner passage meets a steel outer passage frame 4 as a wall of the outer passage, so as to be fixed to the steel partition wall 1 and respective frames.