just an (educated) guess
:)

Open-type TBMs have no shield, leaving the area behind the cutter head open for rock support. To advance, the machine uses a gripper system that pushes against the side walls of the tunnel. The machine can be continuously steered while gripper shoes push on the side-walls to react the machine's forward thrust. At the end of a stroke, the rear legs of the machine are lowered, the grippers and propel cylinders are retracted. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring begins again. The open-type, or Main Beam, TBM does not install concrete segments behind it as other machines do. Instead, the rock is held up using ground support methods such as ring beams, rock bolts, shotcrete, steel straps, and wire mesh (Stack, 1995).
http://en.wikipedia.org/wiki/Tunnel_boring_machine

. La reacción producida se transmite al hastial del túnel mediante los grippers (fuerza de anclaje). Los grippers también compensan el par producido por la cabeza de corte, que se transmite a éstos a través de la viga principal.

Cuando se ha terminado un ciclo de avance, se necesita reposicionar las zapatas de agarre (grippers), para la cual se apoya la viga principal en el apoyo trasero.
http://es.wikipedia.org/wiki/Tuneladora

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Note added at 58 mins (2011-08-31 17:12:38 GMT)
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Now, less of a guess
:)

http://www.google.com/search?q=tbm "reaction structure"&ie=u...

Trenchless installation of conduits beneath roadways - Google Books Result
books.google.com/books?isbn=0309060214...Tom Iseley, Sanjiv B. Gokhale, National Cooperative Highway Research Program - 1997 - Technology & Engineering - 76 pages
The shaft floor and thrust reaction structure must be designed to ... Place shield or TBM on the jacking tracks. 5. Mate jacking push plate to shield or TBM ...
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Development of an Extruded Tunnel Lining System - Volpe Center ...
volpedb.volpe.dot.gov/.../vntsc_outside.product_repository.display... - Cached
The facility consists of: a 10-foot diameter, 22 foot long steel "tunnel"; an ETLS; a "TBM" reaction structure; and concrete mixing and pumping equipment. ...
Trabajos realizados por Impulso - Arquitectura, Ingeniería y ...
www.impulso.es/trabajos.php?idioma=en&trabajo=55 - Cached
Business structure for the design and structural definition of the reaction structure, cradle, and countervault for the TBM for the extension of the FGC to Terrassa. ...

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Note added at 4 days (2011-09-05 07:14:41 GMT) Post-grading
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gracias a ambos
:)