Combined Technologies Allow for High Tunnelling Power in Changing Geological Conditions.

Double Shields are amongst the most technically sophisticated tunnel boring machines in use in tunnelling operations. Combining the Gripper principle and the installation of the segments in one perfectly coordinated process, Double Shields can easily be adapted to the particular geological conditions of any tunnel route. This type of machine is thus ideally suited for drilling long tunnels in hard rock where geological fault zones occur.

The Double Shield TBM owes its name to its special design, whose main feature is an extendable front shield (1) in the front part of the machine, which allows the cutterhead (2) to be extended. The reaction forces (torque and axial and longitudinal forces) arising during drilling are conducted into the rock by the extended gripper shoes (3), which are located in the middle section of the tunnel boring machine. Because these forces have been dissipated, the lining segments (5) can be installed during tunnelling, ensuring high tunnelling performance. This is not possible using the conventional method. On completion of a thrust stroke, the gripper shoes are retracted and the rear section of the machine is pushed against the front shield by the auxiliary thrust cylinders (4). This changeover phase only lasts a few minutes and then the next section of tunnel can be drilled.

However, continuous drilling like this can be carried out only in undisturbed sections of rock because the gripper shoes need the surrounding rock as an anchorage. When the Double Shield reaches a section of rock containing fault zones, the telescopic front shield is retracted. The entire boring machine is then driven forward for drilling only by the auxiliary thrust cylinders (4), which are supported on the tunnel lining (5). This type of tunnelling is referred to as "discontinuous" since in this process, as with a conventional shield, tunnelling with the thrust cylinders is not possible until a segment ring has been installed.

For further information, please request our brochures or our Double Shield animation from our
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