PURPOSE:To make excavation in the bottom of a pile hole executed easily and to make the bottom nearly flat by providing excavating bits on the external wall of a shaft rod to which a through hole and a connecting part are provided respectively on its middle and upper parts, and also by providing two-staged, expandable excavating bits to the lower part of the shaft rod. CONSTITUTION:A through hole 2 is drilled in the center of a shaft rod 1 and a connecting part 3 is provided on the outer upper side of the shaft rod 1. In the middle and lower parts of the shaft rod 1, expandable upper and lower bits 4 and 5 are provided in two stages. The upper bits 4 are so installed that they are supported with arms 7 and 7a that are rotatably hinged to vertical shafts 8 and 8a at their base ends. The bits 5 are formed in such that each one pair of fixed bid edges 11a-11d are installed protruding 1 downward in different radius, and the bits 5 are fixed with a horizontal shaft 13 so that the bits can make rotary motion. In normal rotation of an auger rod 16, a pile hole is excavated with the expanded lower bits 5, and in reverse rotation, an expanded hole bottom is excavated therewith afterward. Thereby the pile hole can be excavated with its bottom made nearly flat and kept always in maximum diameter.
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