Page 11 - TunnelTalk Annual Review 2010 Preview

Fehmarnbelt tunnel option
S. Lykke, Project Director, Tunnel, Femern Bælt A/S and W.P.S. Janssen, Sr. Project Manager, (Tunnel Engineering Consultants, Nijmegen, The Netherlands) for the Rambøll-Arup-TEC JV
consideration. The combination of the road and rail tubes in one cross section is an advantage in this respect as transverse escape to a number of safe non-incident tubes is possible. User safety will not only affect the design, but is also an important issue in the approval process and public acceptance of the tunnel solution.
The selected cross sectional arrangement of the two road and rail tubes side by side, with the road west and the railway east, coincide with the arrangement on the existing landfall infrastructure and requires no weaving to connect.
The dredged material from the trench will most likely be disposed on both sides off the shore. The portal and ramps structures are planned to be constructed in these reclaimed areas. They are intended to have a natural appearance and are supposed to become an area of natural beauty. The immersed section of the tunnel, with a length of about 18.5km, connects the two portal buildings at either side.
Immersed tunnel section
The immersed section of the link represents more than 90% of the total length and is the main driver for the project cost. As a result, paramount attention has been given to the development of the immersed tunnel cross section. The emphasis has been on reducing the construction time and optimizing the required internal space while maintaining an adequate safety level and self rescue ability for the users.
Based on minimum functional requirements, such as the traffic envelope and the basic space requirements for installations, a wide range of options for the cross section have been listed and evaluated applying multi-criteria analysis to compare aspects such as cost, safety, construction time, constructability and operation and maintenance. A first evaluation has been based mainly on qualitative aspects and experience gained during the construction of similar projects including the Øresund Fixed Link in Denmark, the Bosphorus Crossing in Istanbul, and the Busan Geoje Link in South Korea.
The generated cross sections can be divided into three arrangements;
• Sections which combine rail and road in one level with the objective to concentrate material and functions.
• Sections which combine rail and road in two levels to allow for a combined use of ventilation ducts and galleries for rail and road.
• Separate tunnels for rail and road with the objective to allow for separate alignments and independent construction.
In addition to these arrangements three different structural concepts and/or construction methods have been considered
Immersed tunnel cross section
Design of the standard immersed tube elements
Cross section of a special services element
(i) reinforced concrete constructed traditionally or by full sectional casting (ii) modified singular steel shell sections and (iii) steel concrete composite sections.
A quick assessment showed that the two level arrangements did not have real potential to generate sufficient added value because the sections complicate construction and extend the production time since for this cross section full sectional casting is not an option. The tall cross section also requires deeper dredging, a deeper casting basin, and deeper and longer access channels to the potential casting basin areas. The combined use of centrally placed ventilation ducts and galleries did not compensate for the extra cost involved.
The option of having separate alignments for road and rail creates more flexibility with regard to design and construction but is more costly. For example, separate tunnels have the advantage that the rail tunnel alignment is not constrained with respect to the road alignment and visa versa. However, the impact on land will most likely increase by having two different alignments. Another important drawback is related to safety as combined use of safety provisions in rail and road tubes is not possible.
The one level arrangement has been identified as having the best potential for final optimisation as the various arrangements of rail and road tubes in this one level layout allows for an effective use of the road tubes in case of an accident in the rail tube and vice versa. Furthermore it is expected that
the combination of rail and road tunnel in one level cross section will result in synergy during construction, transport and immersion. In addition, the dredging depth can be minimised.
From the palette of the one level sections, the arrangement with the rail tubes at the outside was initially favoured but was abandoned due to the impact of the space requirements for the installations. Space for the necessary transformers is created by implementing special wider elements every 1,800m. Apart from space for the transformers these special elements provide space for pump rooms, parking lay-bys for maintenance staff, ventilation niches in the roof, and other operational and maintenance facilities. The intention has been to concentrate all diversions from the standard element in these special units.
Elementconstruction
The construction of the elements will be a massive operation in which an element will be delivered to site for sinking every second week. In order to achieve this within the given time frame of about 2.5 years for the construction of the elements, a number of production facilities are needed.The length of the tunnel calls for an industrialized production process similar to that used for the Øresund Link. A high production rate at peak of about two 180m long elements in two months turned out to be realistic at the Øresund. The production under controlled ambient conditions proved to be beneficial
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