| 摘要 |
PROBLEM TO BE SOLVED: To efficiently drill continuous holes in the ground by maintaining continuity. SOLUTION: A large-diameter hole drilled by main drill rods 1A, 1B rotatably provided at the lower end of a drive device and provided with stirring blades 9 on the periphery, a small-diameter single hole drilled by a first auxiliary drill rod 13 rotatably provided on one side of the main drill rods 1A, 1B and provided with stirring blades 17 partially lapped on the rotating loci of the stirring blades 9 of the main drill rods 1A, 1B and having the rotating radii smaller than those of the stirring blades 9 on the periphery, and two small- diameter double holes drilled by two auxiliary drill rods 14 rotatably provided on the other side of the main drill rods 1A, 1B and provided with stirring blades 18 partially lapped on the rotating loci of the stirring blades of the main drill rods 1A, 1B and having the rotating radii smaller than those of the stirring blades 9 on the periphery and arranged at an interval in the direction perpendicular to the continuous hole execution direction are concurrently drilled in a first excavation. A second excavation is conducted to drill continuous hole lines so that a new small-diameter single hole is partially lapped on the existing large-diameter hole and two small-diameter double holes.
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