Westerschelde Tunnel Construction, Netherlands

In November 1997 building started on a 6.6km tunnel underneath the Westerschelde river in Zeeland, the Netherlands. Zeeland is made up of islands interconnected by numerous dykes and bridges. The only two areas that are still unconnected are Zeeuwsch-Vlaanderen and Zuid-Beveland, separated by the Westerschelde, the port entrance to Antwerp. The 16km motorway tunnel is due for completion in November 2003.

With an estimated cost (excluding tax) of €726 million (Guilders 1.6 billion), the Westerschelde Tunnel is important in that it is the first engineering structure designed in a fundamentally correct way on the basis of an expected service life of 100 years. It will also be unique in Western Europe in its depth, length, diameter and the fact it is constructed in soft soil.

TUNNEL BORING MACHINES (TBM)

The two 185m long boring machines were designed and built by Herrenknecht in Germany. Each TBM takes 15 months to construct, and three months to install. Travelling at 12m/day, the machines will work for 24 hours a day, six days a week to meet construction targets of 27 months of boring.

SOIL CONDITIONS

The soil in the area is made from soft Boom clay stratum and therefore the construction of tunnels is very difficult. This type of clay is very dense and impermeable, also the sand stratum, above and below the clay is very dense and contains glauconite (similar to mica). In the past, this tunnel boring technology has only ever been used successfully in Japan. To solve this problem the soil is mixed with water and bentonite (volcanic clay with swelling properties) before it is pumped to the surface, where the bentonite is separated and reused.

HYDRO SHIELD

To prevent water and soil from flowing into the tunnel, the hole being bored is supported by a steel hydro shield. This is an 8m cylinder, which places the tunnel lining as the shield proceeds. In a single operation they bored a whole cross-section of the tunnel. 53,000 segments of concrete lining, guaranteed for 100 years, will make up the tunnel tubes, which are simultaneously put in place by the TBM’s. After each tunnel section is completed the road is constructed, including the cable ducts, the drainage and thermal protection materials and the cross-sections.

At its deepest point, the tunnel is 65m below sea level, and machines are designed to work up to 8.5bar. At present they are working on pressures of 7bar, which is the highest ever sustained by a shielded TBM. The TBM has two pressure chambers to allow the transfer of divers by transport capsule from a pressurised habitat on the surface to the face for continuous operations.

In June/July 2000 the cutters on the west and east machines had to be replaced due to deformities caused by these immense pressures. A crew of highly experienced North Sea divers made the necessary works. This was the first time that a full series of saturation dives had been carried out in any tunnel, and redefined the limits of what can be achieved in the modern tunnelling situation with foresight, advanced equipment design and proper preparation.

TWIN TUNNELS

The two twin-tunnels run 12m apart, they have 11.3m mixshields in the outside diameter, and each will have two 3.5m wide lanes for traffic. Safety measures mean that the tunnels are linked every 250m by foot passages that are constructed by freezing the surrounding soil before excavating the connections. Due to financial constraints however, a separate evacuation tunnel for pedestrians will not be built.

OTHER SAFETY FEATURES

All efforts will be made to ensure that the Westerschelde tunnel is safe. The design incorporates many safety features:

• Control Building – will be staffed around the clock. If there is any problem, the control staff will intervene immediately.
• Protective Dykes – the soil from the excavation will be used to create dykes on each side of the tunnel accesses. These dykes will protect the tunnel against water in the surrounding polder and will protect the polder should the tunnel be flooded.
• Fire Prevention – the tunnels will be equipped with emergency stations at 50m intervals, with powder fire extinguishers, fire hose reels and intercoms.
• Lighting – the lighting level will be lower than in many other tunnels because counterbeam lighting is being used. It ensures that drivers can clearly see the contours of the vehicles in front of them.
• Air Quality – the carbon monoxide concentration and the visibility in the tunnel are measured automatically. In the event of a traffic jam or fire, the ventilation fans will switch on automatically to displace the fumes.

COSTS

To construct, maintain, operate and manage the tunnel operations, the government set up a public limited company called N.V.Westerscheldetunnel. The shareholders for the company are 95% central government and 5% Province of Zeeland. Therefore when the costs of the tunnel have been recovered after 30 years, travelling on the tunnel will be free and operating costs will be met by the government. Until that time the toll will average €5.5 (11.75 guilders) per vehicle.

LEAD CONTRACTORS

Primary contractors include Herrenknecht, who provided the tunnel boring machines and GeoDelft who carried out the surveying. Schöma have provided underground wagons and locomotives for transporting people and equipment to the site. The special exhaust-optimised engines are supplied by Deutz and Ceresola is providing project management for the facilities, including 6,600 pre-cast segment rings.

The KMW (Kombinatie Middelplaat Westerschelde) partnership are to design, construct and maintain the tunnel for the first ten years of operation. KMW consists of three Netherlands companies (BAM Intrabouw B.V., Heijmans N.V., and Voormolen Bouw B.V); two German companies (Philipp Holzmann A.G and Wayss & Froytag A.G.); and one company from Belgium (Franki N.V.).