| 摘要 |
PROBLEM TO BE SOLVED: To easily conduct operation, in which an inner casing is inserted so that the normal of the arcuate recessed section of the inner casing is passed at the central section of the next position of drilling when preexcavation is completed and the inner casing is inserted to an excavation casing. SOLUTION: The ruler 6 for determining the inner-casing normal used for a drilling method in which boulder stones in a ground are cut and removed without pushing and letting-escape to the drilling section of previous excavation by using a special sectional-shaped inner casing 3, to which an arcuate recessed section 3a is formed, when a pit continued in a column row-shape is drilled and formed and excavation is conducted successively in order from the first pit to the second and third pits is composed of a cylindrical body 6a externally fitted under the state, in which the body 6 can be mounted and demounted to and from the upper end of an excavating casing 1 and phase can be adjusted around an axial center to the excavating casing, and a projecting section 6b inscribed with the arcuate recessed section 3a formed on the internal surface of the upper end section of the body 6a. The inner casing can be inserted only when the arcuate recessed section is positioned in phase coinciding with the projecting section.
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