| 摘要 |
In order to carry out a non-emerging drilling in the wall 1, the bit 2 is advanced by means of jacks 8 along columns 7 while the bit is guided by a bar 14. Once the cut 3 produced by the bit is sufficiently deep for there to remain only a very thin film 1c between its front and the opposite surface 1b of the wall 1, the bit is withdrawn and the core 4 is torn off by the jack 15. <IMAGE>
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