Immersed Tube Tunnels Research Articles

THE DETAILED DESIGN OF THE MEDWAY TUNNEL PROJECT.

Immersed tube tunnels may be considered to be particularly suited to a design-and-construct form of contract, in that there is considerable influence on the permanent structure by the temporary works required for the float-out and immersion of the tunnel elements. The scope of the reference design produced by the engineer for the Medway Tunnel project, which allowed tenders to be sought, afforded flexibility for the tenderers in their options for constructing the immersed tunnel, the cut-and-cover tunnels, and their approaches. This paper discusses the designs carried out by Mott MacDonald for the various forms of construction employed on the project by the contractor. These works include the cut-and-cover tunnels which were constructed using permanent diaphragm walls or with cast-in-place concrete within a temporary combi-pile wall arrangement, the piled slabs and gravity structures which were employed on the open approaches, the innovative sand-placing method used for the embankment built on very soft ground, and the immersed tunnel itself. The paper also addresses the coordination required in the design-and-construct process between the designer and contractor, and also between the civil and mechanical and electrical works. Finally, the paper discusses some of the technical complications encountered during the construction of the project, and how the design-and-construct form of contract was beneficial in achieving a rapid and satisfactory solution.

The detailed design of the Medway Tunnel project

Immersed tube tunnels may be considered to be particularly suited to a design-and-construct form of contract, in that there is considerable influence on the permanent structure by the temporary works required for the float-out and immersion of the tunnel elements. The scope of the reference design produced by the engineer for the Medway Tunnel project, which allowed tenders to be sought, afforded flexibility for the tenderers in their options for constructing the immersed tunnel, the cut-and-cover tunnels, and their approaches. This paper discusses the designs carried out by Mott MacDonald for the various forms of construction employed on the project by the contractor. These works include the cut-and-cover tunnels which were constructed using permanent diaphragm walls or with cast-in-place concrete within a temporary combi-pile wall arrangement, the piled slabs and gravity structures which were employed on the open approaches, the innovative sand-placing method used for the embankment built on very soft ground, and the immersed tunnel itself. The paper also addresses the coordination required in the design-and-construct process between the designer and contractor, and also between the civil and mechanical and electrical works. Finally, the paper discusses some of the technical complications encountered during the construction of the project, and how the design-and-construct form of contract was beneficial in achieving a rapid and satisfactory solution.

Immersed tube sandwich saves construction costs: M. Page, Tunnels & Tunnelling, 27(9), 1995, p 39

A novel steel-concrete-steel sandwich design for immersed tube tunnel construction is described, which it is claimed could offer cost savings of some 10 per cent compared with reinforced concrete or all- steel methods. The system, which involves no formwork or bar reinforcement, uses fabricated steel inner and outer shells, generally of 8mm to 10mm steel plate acting as formers. The outer skin automatically provides a watertight covering whereas a steel skin which does not contribute to the load bearing capacity is required for conventional concrete immersed tube construction. Also, the overall weight of elements is lighter. A series of steel stud connectors pre- welded to the plate sides and encapsulated in the concrete assure structural integrity. It is envisaged that the system would be suitable for structures such as immersed tube tunnels, caissons, storage vessels (gas and liquid), retaining walls and offshore structures.

Fort Point Channel: Concrete Immersed-Tube and Ventilation Building Design

The Central Artery/Third Harbor Tunnel Project in Boston, Massachusetts, is one of the largest highway projects over undertaken in the country. It requires the replacement of the existing elevated artery, I-93, with an underground tunnel extending through downtown Boston and an extension of the Massachusetts Turnpike Authority (MTA) I-90 from its existing termination at the I-93 interchange to Boston's Logan International Airport. The I-90 extension tunnels east under the existing South Station intercity and commuter railroad tracks, under historic Fort Point Channel while crossing above the 1915 twin subway tunnels, and continues through industrial South Boston with ramps surfacing in a new South Boston interchange, the heart of tremendous growth in Boston. From there the tunnel connects to the recently completed Ted Williams Tunnel harbor crossing to East Boston and Logan International Airport. The unique design challenges and solutions relating to the Fort Point Channel crossing, particularly the use of in-the-wet construction with concrete immersed-tube tunnels and the design interface to the ventilation structures, are presented. Structures required for the I-90 extension are concrete immersed tubes and jacked tunnels, as well as more conventional cut-and-cover tunnels, bridges, surface roads, and ancillary buildings. The geometric and physical restraints of the alignment initially required the placement of the ventilation building, which serves the tunnels, on a cut-and-cover tunnel transition section between the jacked tunnels and the concrete immersed tubes. Ultimately, placement of the ventilation building on the immersed tubes created a substantial cost and schedule benefit.

Immersed steel tube tunnels: An American experience

The idea of immersing a prefabricated tube tunnel was first put into practice in the United States in 1896, for a water oine crossing Boston Harbor. Over the past century, immersed tube technology has continued to evolve in the U.S. After introducing and discussing advantages and disadvantages of immersed tube tunnels, the paper describes the development of basic advances in the design and construction of immersed tube tunnels within the last 60 years in the United States. Design and construction features of selected major projects also are described.

Privately financed immersed tube tunnel projects

Based upon experience with many immersed tube tunnels, several of which were privately financed, the author discusses a number of aspects of privately financed projects. These aspects concern the legal and financial structure in terms of the roles of the parties involved, as well as technical matters related to design and execution. The time schedule is of major importance in limiting financing costs. Good insurance coverage of risks, through policies especially prepared for the particular structure, is also extremely important. En s'appuyant sur son expérience dans le domaine des tunnels immergés, et principalement dans le secteur privé, l'auteur examine certains aspects du financement privé des projets de construction. Ces aspects concernent la structure légale et financière en termes de rôle des parties concernées, ainsi que les considérations techniques relatives à la conception et à l'exécution des travaux. Le calendrier est d'une importance majeure quant à la limitation des coûts de financement. Il est également extrêmement important de couvrir les risques par une police d'assurance spécialement adaptée à la structure concernée.

Design criteria for immersed tube tunnels: Experience from the autobahn tunnel under the river Ems

The design criteria for immersed tube tunnels are illustrated in the construction of the 1453-m-long, six-lane highway-tunnel under the Ems River, near the town of Leer in northern Germany. The five tunnel tubes were immersed between November 1986 and June 1987. Floating, immersing and bedding require elaborate planning and in-situ monitoring to ensure the safety and quality of the structure because very small variations in large numbers govern the situation. Les critères de conception des tunnels à “tube sous l'eau” ont été mis en valeur lors de la construction d'un tunnel d'autoroute à six voies, long de 1453 m, et sous le fleuve Ems près de la ville de Leer en Allemagne du nord. Les cinq tubes du tunnel ont été submergés entre novembre 1986 et juin 1987. La mise sous l'eau et la pose au fond requièrent une préparation détaillée et un suivi sur place qui assurent la sécurité et la qualité de la structure lors des opérations, et ce grâce aux très petites variations des données qui contrôlent le problème.