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  A non-conventional system of recovering the TBM

  Sometimes the retrieval mode of the TBM after finishing the drive cannot be the typical vertical shaft extraction; the boundary conditions of the project (schedule, overlapping of activities and/or lack of space among others) may require a different solution. The case presented describes the solution adopted in a metro project where the TBM shield is left buried and forms part of the tunnel final lining. To make this possible, a chamber is excavated to disassemble the cutter head and to recover the main drive, while the rest of the TBM and backup system are brought back to the launch point through the already built tunnel. Besides, the TBM does not make a conventional breakthrough, but the rotary head and part of the shield are embedded in a block of ground that has previously been improved with ultrafine cement injections. By means of conventional excavation carried out from one side of the chamber, the rotary head is reached and released to proceed with its removal.

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