| 摘要 |
PROBLEM TO BE SOLVED: To provide a sheath pipe structure of a material for drilling a hole in natural ground, capable of minimizing a frictional force working between the material and the wall surface of a drilled hole during drilling, and securing sufficient strengths of a leading rod and members and parts to be attached thereto. SOLUTION: A material for drilling a hole in natural ground comprises rods 3 each having a cavity 35 therein and inserted into sheath pipes 1, 2, the sets of plural rods 3 and sheath pipes connected together to extend the material. A drilling bit 38 is mounted at the end of at least the leading rod 31, and striking forces are applied as the bit 38 is rotated, with water ejected through the cavity 35 in the rod 3 to drill a hole in the natural ground. Leading pipes 11, 21 serving as the front sheath pipes are formed to increase in diameter at their front ends so that their front ends 11a, 21a are greater in outside diameter than their rear ends 11b, 21b. The outside diameters of the other sheath pipes 12, 13, 22, 23 are set to be smaller than the outside diameters of the front ends and about equal to the outside diameters of the rear ends of the leading pipes 11, 21.
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