| 摘要 |
PCT No. PCT/FI95/00678 Sec. 371 Date Aug. 12, 1997 Sec. 102(e) Date Aug. 12, 1997 PCT Filed Dec. 13, 1995 PCT Pub. No. WO96/18798 PCT Pub. Date Jun. 20, 1996A method to drill a hole in the soil by a bit assembly and a bit assembly comprising a ring-shaped bit (2) to drill the outer circle of the hole and to which a protection tube system (4), mounted in the hole while drilling, is coupled, and a cylindric inner bit (1) to drill the center portion of the hole, from which bit at least the rotative motion is transmitted to the outer bit (2). Drilling by the outer bit (2) and pulling out the protection tube (4) are disconnected upon need by reverse rotation of the inner bit, whereafter drilling is continued with the inner bit (1) only.
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