| 摘要 |
PURPOSE:To reduce excavation resistance and to perform excavation by means of a small excavation system, by a method wherein, after a small hole is excavated by a means of a first excavating tool and the hole is filled with sand, the hole is excavated for an increased in size by means of a second excavating tool having size larger than that of the first excavating tool. CONSTITUTION:After a small hole is excavated by means of a drill pipe 3 provided at its forward end with a small first excavating tool 1, the small hole is filled with sand 11. A second excavating tool 12, having a final outer size, is connected to the outer periphery of the first excavating tool 1 for mounting, and re-excavation is progressed for an increase in the size of the hole. In this case, since the hole excavated in a first stage is filled with the sand 11, the hole is prevented from collpasing, and since the portion of the sand 11 is decreased in excavation resistance, excavation for an increase in size can be effected without needing high excavation torque. This enables excavation of a large hole through a small and low-cost excavating mechanism. |
