| 摘要 |
PROBLEM TO BE SOLVED: To minimize the amount of unexcavation in a planned excavation area. SOLUTION: Supporting portions 9 are movably provided along the longitudinal direction of each of two guides 8, in the two guides 8 which are arranged in a cutter frame 4, respectively. A portion, on the longitudinal inside with respect to a longitudinal end, of a beam-like variable beam 12 is rotatably supported on each of the supporting portions 9; a plurality of bits 18 are provided at the longitudinal end of the variable beam 12; either of the supporting portions 9 is moved along either of the guides 8; and the other supporting portion 9 is moved along the other guide 8, so that the longitudinal end of the variable beam 12 can be protruded to the radial outside of a rotary cutter 2. The width W1 of the bit 18a, which is arranged on the outermost periphery, among the plurality of bits 18 provided in the variable beam 12 is set greater than the widths W2 and W3 of the bits 18b and 18c which are arranged on the side of an inner periphery with respect to the bit 18a. When the cutter frame 4 is rotated once, the outermost periphery of an excavated hole is excavated by means of the bit 18a which is arranged on the outermost periphery. COPYRIGHT: (C)2008,JPO&INPIT |
