Plenty of space and safety in solid rock.
Essential to the functioning of Gripper TBMs are their drilling, bracing, support and safety systems. The drilling system, i.e. the cutterhead, is fitted with cutter rings (disks). The rotating cutterhead forces the disks against the tunnel face under high pressure. In this process the disks roll over the tunnel face, thereby loosening the native rock. The excavated rock, or "chips" as it is commonly known, is collected in muck bucket lips (openings in the cutterhead) and discharged via hoppers onto a conveyor belt.
The conveyor belt conveys the material along the length of the TBM to the transfer conveyor between the TBM and the back-up area. From there the excavated material is taken straight out of the tunnel either on conveyor belts or to the loading dock of a muck car. The TBM is braced radially with grippers (1) against the tunnel wall, with hydraulic cylinders pressing the cutterhead against the tunnel face to enable a further section of tunnel to be excavated. The maximum boring stroke is governed by the length of the pistons in the thrust cylinder. After completion of a boring stroke, the drilling process is interrupted and the machine moved forwards, with the Gripper TBM being stabilized by an additional support system.
The tunnelling performance of a Gripper TBM depends essentially on the time required to install rock supporting devices. The same safety measures are used as those in conventional tunnel building: rock anchors, meshes and shotcrete, together with the segments characteristic of TBMs and the steel beam supports which are particularly suitable for this purpose. The Gripper machine enables comprehensive rock support measures to be taken even right behind the cutterhead, in the so-called L1* work area. Ring erectors (2), anchor drilling devices (3) or wire-mesh erectors (4), for example, can be provided for installing the steel supports. Shotcrete is applied and segments are installed in the back-up area.
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