PURPOSE:To excavate hard ground with good efficiency by guiding an excavation rod in such a way as to keep the verticality of the excavation direction of the rod in excavating the hard ground by the formation of pit wall of cement mortar in the surface soft ground. CONSTITUTION:A pit 2 is excavated in the soft ground by the first auger screw 1, and while pulling out the auger screw 1, cement mortar 3 is injected from the tip of the auger screw 1. After the cement mortar 3 hardens, the second auger screw 4 is erected concentrically with the pit 2 to excavate a pit 5. The auger screw 4 is pulled out, an excavation rod 8 having a rotary bit 6 on its tip and a weight 7 is inserted into the pit 5 to excavate the hard ground below the depth excavated in the precious process. In this case, by the pit 5 wall of cement mortar 3, the verticality of the excavation rod 8 in the excavating direction is well kept.
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