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Double Shield TBMs in operation
Double Shield TBMs are among the most technically sophisticated tunnel boring machines. They combine the functional principles of Gripper and Single Shield TBMs in one machine. In stable geologies, combining these methods permits the installation of concrete segments in parallel with the advance, resulting in very high advance rates. The powerful technology is therefore predestined for driving long tunnels in hard rock.
250 meters per week of bored and lined tunnel for the Palomino project, Dominican Republic.
Advancing with power
In tunnelling with a Double Shield TBM, a rotating cutterhead equipped with disc cutters is pressed against the tunnel face. High contact forces are required to work efficiently in hard rock with localized rock strengths of over 250 megapascals. Under this pressure, the disc cutters overcome the compressive and tensile strengths of the rock mass ahead and loosen so-called chips from the face. Water jets reduce the formation of dust and can cool the cutting tools. Buckets attached to the cutterhead collect the cuttings. As the cutter head rotates, thanks to gravity they slide through integrated chutes to the center of the machine and fall through the funnel-shaped muck ring onto the machine belt. At the end of the machine belt, the rock chips are transferred to belt conveyors or transport vehicles and transported out of the tunnel.
The disc cutters are mounted in the cutterhead in such a way that they can be changed from the rear under the protection of the structural steelwork of the cutterhead, without maintenance personnel having to enter the unprotected area in front of the cutterhead.
Functional principle
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High speed thanks to continuous tunnelling mode
Double Shield TBMs consist of two main components: a front shield, including the outer telescopic shield, with cutterhead, main bearing and drive, and a gripper shield including the inner telescopic shield with gripper unit, auxiliary thrust cylinders and tailskin. The main thrust cylinders connect the two shield sections with each other. They are located in the protection of the telescopic shield, where the shield skins of the outer and inner telescopic shields overlap. Double Shield TBMs are therefore also called telescopic shields. In stable rock, the machine braces itself radially in the tunnel with the gripper shoes. This allows the front shield to be advanced independently of the gripper shield using the main thrust cylinders.
The reaction forces during the boring process are dissipated into the rock by the extended gripper shoes. At the same time as the tunnel is being driven forward, the segments are installed in the tailskin area. The auxiliary thrust cylinders are only used to secure the position of the installed concrete segments. Once an advance cycle is complete, the gripper shoes are released and the gripper shield is pushed toward the front shield by the auxiliary thrust cylinders. This repositioning phase only takes a few minutes. This enables almost continuous tunnelling. In ideal rock formations, Double Shield TBMs can also work without segment support.
Operating principle at a glance:
Disc cutters use high contact pressure to break chips out of the face.
Buckets, chutes and muck ring ensure efficient removal of the cuttings onto a central belt conveyor.
Main thrust cylinders or auxiliary thrust cylinders push the machine forward.
Segmental lining
On Double Shield TBMs, a displaceable telescopic shield enables access to the tunnel wall for cleaning work.
Safe in geological fault zones
In fault zones or areas with low rock strength, radial bracing via the gripper shoes is not always possible. In these sections, the telescopic shield can optionally be completely retracted, with the front and gripper shields then forming a single unit. The forces required for the advance are applied via the auxiliary thrust cylinders. As with Single Shield TBMs, the last segment ring built serves as an abutment to brace against. Advancing and ring building can no longer be carried out simultaneously. However, this discontinuous mode offers greater tunnelling safety in difficult sections.
Application-related special solutions for difficult cases
Double Shield TBMs have a comparatively long shield skin. In the event of rock convergence, there is an increased risk of the machine becoming jammed. Herrenknecht engineers counteract this with various design solutions. These include, for example, graduated shield diameters, the option of increasing the boring diameter and optional high-pressure hydraulics to increase the thrust force. Break-outs and cuttings can block the telescopic joint and the annular gap. This hinders the advancement of the gripper shield. With the help of additional displacement cylinders arranged between the gripper shield and the inner telescope, access to the tunnel wall can be opened for cleaning or safety measures if required. Additional drilling equipment and drilling channels in the rear shield area allow the open telescopic area to be secured with a pipe screen above the TBM.
Torque box drives enable a local overcut and precise horizontal and vertical control options.
The right drive for every diameter range
For Double Shield TBMs in the diameter range from 3.8 to 8.2 meters, fixed drives with straight main thrust cylinders or lattice cylinders arranged in a V shape are used. During the advance, steering takes place directly via the thrust cylinders. For diameters of over 8.2 meters, however, displaceable drives with additional longitudinal displacement cylinders are usually used. The cylinders can be used to directly measure the contact force of the drive and thus make optimal use of the available power. The so-called torque box cylinders allow the drive to be moved horizontally and vertically. This allows a local overcut to be created that can support the steering of the TBM. In addition, this system makes it possible to increase the boring diameter by shifting the caliber rollers and optionally installing additional overcut housings. With all drive types, the cutterhead can be retracted for direct access to the tunnel face.
Meters of tunnel non-stop
Double Shield TBMs are mainly used in tunnel projects with changing rock formations. In stable rock, they achieve very high advance rates thanks to the continuous tunnelling mode. The combination of the Gripper and Single Shield TBM methods is therefore often the fastest and most economical solution for long drives in hard rock.
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Steffen Dubé
President and General Manager Herrenknecht Tunnelling Systems USA Inc.
Gerhard Goisser
Commercial Manager Herrenknecht Tunnelling Systems USA, Inc.