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TIMBY, New Construction Process for Immersed Tunnels

TIMBY, New Construction Process for Immersed TunnelsTIMBY, which stands for Tunnel IMmersed BY Bouygues, is a new process for construction of immersed tunnels. It is based on a machine designed to place under the bed of a waterway or sea channel successive rings made up of prestressed, precast concrete segments. This process is cheaper and reduces the environmental impact of construction because it needs no dry dock and does not disrupt ship traffic. TIMBY is particularly suited to tunnels with a cross section for a 2 x 2-lane road. The machine is designed for re-use.The traditional construction method for immersed tube tunnels imposes demanding technical conditions. Among those is a dry dock; extensive storage space for large precast concrete caissons; and disruptions to shipping when the caissons are floated out and sunk at their final location. Bouygues Travaux Publics has developed a new construction concept for immersed tunnels aiming at simplifying these technical conditions and reducing construction costs. This last objective is fulfilled thanks to significant cost savings on preparatory works of the tunnel bed; shallower and shorter access structures; and no need for a dry dock. Lastly, the new method is also attractive in terms of reduction of construction time to around six months for a 1 km-long tunnel.This new technology, patented by Bouygues TP, is very innovative but it is based on tried and tested methods as used in soft ground and water-bearing shield tunnelling technology, and prestressed concrete techniques. It combines the features of TBM tunnel construction with the advantages of the traditional immersed tube method. Herrenknecht collaborated to the development of TIMBY under a partnership agreement with Bouygues TP, and supplies its engineering know-how for the tunnel construction machine. Visit www.bouygues-construction.com and www.herrenknecht.comTIMBY can cross a river or sea channel from one side to the other, without marine intervention after completion of the trench. It is a construction method that is less dependent on dredging and other marine works required for tunnel construction, but the dredging and soil levelling is similar to the classical immersed tunnel method. The machine prepares the tunnel bed platform on finely levelled gravel material, which can be reinforced if necessary. The immersed tunnel lining is composed of precast concrete segments forming rings. The machine is equipped to erect the lining and use it as support to progress. The watertightness of the segmental lining is ensured in similar conditions as for a classical bored tunnel under the groundwater table in permeable soil. Nevertheless, two compartimented elastomeric compressible gasket barriers are included in the design to avoid water ingress.A temporary longitudinal prestressing is installed for segment stability during the lining erection process. A permanent transverse prestressing follows, to withstand bending moments in the lining to minimize tensions in the segments. Provision for additional permanent longitudinal prestressing is possible if needed for specific project loading. The TIMBY submarine machine consists of a double O unit comparable to a shield machine. Separable equipment is provided for each tunnel side, consisting of the erector and the back-up system with the power packs and supply logistics. After trench dredging and launching the cofferdam construction, the machine is assembled. The cofferdam is then flooded and opened so that the machine can advance towards the target shaft on the opposite side of the river or sea channel, stroke by stroke. After that, one segment ring after the other is installed and prestressed. The double shield has two roadheader arms levelling the gravel previously laid by means of a barge on the bottom of the trench to shape a proper bed for TBM advance. Any excess gravel is either removed or pumped to the side. But the machine does not bore. The rings are installed in a predefined sequence. First, the Y segment is placed in the invert by the right-hand erector. Then the next segments are installed, and the first half of the ring is completed. The left-hand erector then installs the segments on the other half in the same way. After the installation of the upper Y segment, the vertical separation wall is pushed in. Then the transversal prestressing system of four cables can be installed. To balance the buoyancy force, the tunnel has to be backfilled. Every third ring, a transverse precast L stone is installed to form a retaining wall for the next backfilling. Material is supplied to the rear part of the back-up system by truck, allowing a conveyor belt to supply the transverse conveyor belt which is filling the gap between the L stones. After that, the inner overhead travelling crane can install new rails and the completed back-up system can be pulled forward by three rings. Now, one operating cycle is completed. This procedure is repeated until TIMBY reaches the target shaft. The trench is then backfilled to cover the tunnel.TIMBY is therefore a new concept but issued from existing and improved techniques. It offers numerous advantages: reduced construction time, no navigation disturbance, no dry dock, reduction of ramp length and only with open cut, cheaper solution compared to classical immersed tunnel solution and re-usable machine for similar projects. View a  (RealPlayer) or  (Windows Media Player). For more, contact Pierre Longchamp, technical director for underground works, e-mail p.longchamp@bouygues-construction.com or mob. +33 660224639, and Werner Burger, burger.werner@herrenknecht.de or tel. ++49 (0)7824 302 222. 39/06.