The double gripper system
consists of two bracing planes, the front and the rear gripper unit. The main
beam, which is bearing the cutterhead and the drive, is supported by the
exterior shell and movable in longitudinal direction. Machines with double
gripper system are guided mainly by the gripper units, while in machines with
single gripper system also the thrust cylinders are used for steering.

Gripper TBMs are mostly
equipped with partial shields or hoods because also in stable rock conditions
rockfall is to be expected locally. The partial shields or hoods serve as
protection against falling rock and are often extended backwards by lamellae.
Such a lamella roof, also denoted as finger shield, bridges the unsupported
area between the shield and the area in which the support will be installed.

With the so-called partial
or cutterhead shields with radially movable segments, a supporting pressure on
the rock mass around the cutterhead can be applied (Maidl et al., 2001).
Partial shields also protect the cutterhead against falling rock and thus can
avoid a blockade of the cutterhead. With lateral, radially movable bracing
shields as well as with a vertically adjustable sliding shoe and invert shield,
respectively, the position of the cutterhead can be fixed. A steel dust shield
integrated in the cutterhead shield protects the working room from dust and
small rock particles.

For the support of the
excavation contour in the machine area of a gripper TBM, steel sets, steel
meshes, rock bolts and shotcrete can be used. The steel sets and meshes are
installed as a headgear normally immediately behind the cutterhead, using a
ring erector and a wire mesh erector. With the drilling equipment, boreholes
for rock bolts, exploration drillings and advancing support in form of
injections can be carried out. The support of the entire crosssection will
usually be completed in the backup area. The application of shotcrete in the
machine area is extremely difficult due to problems regarding space, dust and
rebound, and leads to a considerable reduction of the performance rates. 

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