The second Coen Tunnel in the A10 orbital motorway around Amsterdam is part of the larger project ‘Capacity Expansion of Coen Tunnel’. As well as the construction of the Second Coen Tunnel, the scheme includes the renovation of the First Coen Tunnel, the expansion of seven existing structures, and the building of nine new structures. Some 11km of noise screen is also being realised. In combination with the construction of the Westrandweg four-lane motorway bypass, all this is intended to ensure better trafflc flow around Amsterdam towards the rest of North Holland province. Commissioned by the Rijkswaterstaat public works entity, the project is being designed, built, flnanced and maintained in the context of a public-private partnership (DBFM contract) by the Coentunnel Company, a consortium of Arcadis, Besix, CFE, Dredging International, Dura Vermeer, TBI Bouw and Vinci Grands Projects. The design and realisation of the works is being carried out by Coentunnel Construction, a conglomerate of the construction companies Besix, CFE, Dredging International, Dura Vermeer, TBI Bouw, Vinci Construction and Croon Electrotechniek.

Design of immersed tube tunnel

The main elements of the Second Coen Tunnel are:

  • Northern approach (183m): open tunnel tray;
  • Transition structure (50m): closed tunnel with service building and pump chamber;
  • Immersed tunnel (714m);
  • Transition structure (50m): closed tunnel with service building and pump chamber;
  • flSouthern approach (273m): open tunnel tray.

The realisation of the Second Coen Tunnel started in 2008 with the preparation of a design for the immersed tunnel and the approaches. The immersed tunnel consists of a number of tunnel elements that are built in the construction dock and transported by water to the site. The construction of the immersed tunnel elements took place at the construction dock in Barendrecht. This construction location immediately brought with it a number of prior conditions, such as restriction of the element dimensions in connection with transit through the sea locks at IJmuiden, and also requirements concerning transport over the open sea (the North Sea).

Due to its flnal location in the North Sea Canal, as well as the usual soil and water loadings, account also had to be taken of the possible sinking of seagoing vessels sailing through the said canal, and of anchors being dropped.

A 30m-high chimney was also built on to one of the segments for the extraction of exhaust gases.

The main dimensions of the tunnel elements were determined at an early stage. The immersed part was made as long as possible, in order to allow as large a part of the tunnel as possible to be prefabricated. To reduce the number of immersion operations, the tunnel elements had to be as long as possible. The length was however restricted by the dimensions of the construction dock in Barendrecht and the Noordersluis sea lock to be traversed at IJmuiden. Besides this, a greater element length leads to greater forces (bending and shear) during the sea transport and immersion. It was flnally opted for four tunnel elements of 178.5m, each assembled from seven segments 25.5m long. The tunnel cross-section comprises two vehicle tubes and one central tunnel channel, and is characterised by its asymmetrical shape (see Figure 2).

Basic construction principle

From a structural viewpoint, the Second Coen Tunnel is a chain of stiff structural elements (segments) on a flexible bed. The segments are separated from each other by three types of joint: immersed joints between the tunnel elements; segment expansion joints between the segments; and a closure joint where the tunnel is closed after the immersion of the last element.

The joints provide flexibility to the structure, so that bending moments remain limited. To avoid differential subsidence, teeth are introduced into the joints, to transfer both horizontal and vertical transverse forces. The tunnel is kept watertight by rubber seals. A steel-rubber seal strip is applied round the entire circumference of the segment joints, with the possibility to inject this further afterwards. An initial water seal in the immersion joints is ensured by the GINA gasket. After flnishing the joint structure, a permanent water seal is applied in the form of an Omega section (Figures 3 and 4).

Both the tunnel segments and the joints between them are dimensioned for two situations:

This complex and laborious design process demands great precision. A balanc

  1. The temporary building phase including floating off, transport and immersion;
  2. The usage phase, including incidents that may happen during the lifetime.

This complex and laborious design process demands great precision. A balance must continually be found between opposites that often concern differences between these two design situations.


With an eye on the floating transport, the tunnel elements must be light enough to be transported in both fresh and salt water with sufficient freeboard (the part that remains above water during transport). Too little freeboard makes the tunnel elements unstable. However, after immersion, the tunnel elements must remain solidly on the canal bed. This contradiction is bridged by designing the tunnel cross-section such that the relationship between concrete volume and water displacement comes out favourably for the freeboard, while still providing enough space to ballast the tunnel with water during immersion, and to provide it with a layer of concrete ballast in the final situation. A third criterion follows naturally from the requirement that the tunnel cross-section must not only be able to bear the soil and water pressures, but also emergency loadings such as an anchor being dropped or a sinking ship. The designers also had to take account of the actual specific weight of the concrete including reinforcement, the salt content of the water, and the actual concrete dimensions in relation to the concrete weight and water displacement.

Another design task arose from the desire that the tunnel elements should behave as a single unit during floating off, transport and immersion, but should lie like a flexible ‘vacuum-cleaner hose’ on the canal bed in the final situation. This is to avoid large forces in the structure. For this reason, the tunnel elements were provided with expansion joints between the segments at regular spacings, with the segments being held together by transport pre-stressing.

This pre-stress was then removed after immersion by cutting through the cables.

Relationship between transport and design

The realisation of a tunnel element that is suitable in both the final and the building phases is a supremely challenging balancing exercise.

This section describes the temporary design situation, namely the floating off, transport and immersion of the tunnel elements.

In order to carry out this assessment properly, all forces and stresses in the temporary building and transport phases must be correctly mapped out using a scheme that is followed meticulously during each building and transport action, including:

  1. The construction of the elements themselves
  2. The floating off of the elements in the construction dock
  3. The transportation of the elements to their final location
  4. The immersion of the elements
  5. The ballasting of the tunnel elements and the completion of the tunnel, taking into account the later maintenance regimes.

As well as the preconditions listed earlier for the dimensions of the tunnel elements, their stability (rotational too) during floating off, transport and immersion is of importance.

Next, the various temporary building and installation phases are described, to consider their influence on the design of the tunnel elements.

Floating off

The elements of the Second Coen Tunnel are constructed on a gravel bed in the construction dock (see photo, below). The height of the elements is 8.6m, so that the top of the roof is about 1.4m below NAP (Amsterdam ordnance datum). The water level during floating off (at low tide) varies between 0.5m above and below NAP.

The distance over which the element must be raised to have its roof protrude above the waterline with a freeboard of 0.20m (tunnel elements (TE) 1 and 4) is between 1.1 and 2.1m; that for a freeboard of 0.9m (TE2 and TE3) is 1.8 to 2.8m. When the ballast tanks are full, the elements are unstable as long as they are under water. Due to this, the risk exists that they will float up rotated about their longitudinal axis.

Because the elements are vertically curved, the position of their rotational axis is not flxed. The risk therefore exists that the element might turn about a vertical axis too, so that the primary or secondary end would be displaced far to one side, causing it to hit the adjacent element. This risk applies especially to the sharply curved TE2 and TE3. For this reason, the elements are floated off from either the primary or the secondary end flrst. To prevent the element being able to rotate around its longitudinal axis during this, a ground reaction of 500t is maintained. Once the length of the part protruding above the water is sufflciently great to ensure waterline stability, the tanks at the opposite end are emptied.

The exact weight of the tunnel element is equally crucial during the transport, immersion and flnal phases. The weight and centre of gravity of the tunnel elements are calculated based on the design drawings, and also verifled through accurate monitoring of the amounts of concrete and reinforcement used while building the elements. This can also be calculated exactly during floating off, by combining the measured freeboard with the density of the water.

At each phase of the floating off of the elements and the fllling of the construction dock, the tooth forces and the pre-stress forces are calculated and verifled. In this, the timing of the injection of the pre-stress channels is important: if this is done before floating off, the friction between the floor and the ground underneath could prevent the joints being completely sealed. This is therefore done after floating off, which reduces the effective pre-stress force. Account is taken of this when determining the phasing. After checking, the effect of this phenomenon proved to be negligible as it happens. The transport pre-stressing is gone into later in the article.


Firstly, the transverse stability of the element to be transported is calculated, to ascertain whether it will try to rotate about its longitudinal axis during the transport phase, and to what extent this phenomenon is sensitive to the variation of parameters. To this end, a calculation is performed for each element, in which the element’s reactive torque is calculated as a function of its rotation about its longitudinal axis (on the waterline):

  • Determine the weight and the centre of gravity of the element, based on the survey of the geometry realised, and its veriflcation during floating off;
  • Calculate the position of the centre of gravity with respect to the pressure point as a function of the element’s angular rotation;
  • The reactive torque is equal to the product of the water displacement times the horizontal distance between the centre of gravity and the pressure point.

It emerged from this calculation that all elements were rotationally stable during the transport phase.

Transport stresses

Secondly, the influence of wave loadings on the tunnel elements during transport is calculated. This is to investigate whether the number of pre-stressing cables present would be sufflcient for the sea transport.

These calculations comprise the following steps:

  • The wave picture at sea is represented by a wave spectrum built up from a large number of regular waves each with its own height and period.
  • Via Fourier transformation, the time series of the water level is converted into a spectrum. The energy of each frequency band is given in this spectrum (though the disadvantage of this is that the phase relationship among the waves is lost).
  • It has been established for the North Sea that a spectrum is generally described by the peak period of the spectrum and the significant wave period.
  • The spectra can be normalised to a wave height of 1m. These spectra are therefore solely dependent on the peak period. There is thus a relationship between the spectrum and the significant wave period.
  • Responses: in a mental model, all forces and movements are determined for regular waves, each with a different wave direction and period, but with a fixed significant wave height of 1m. In this way, the responses for all relevant wave periods are found. Because the responses are assumed to be linear to the wave height, the transfer function from wave to response is found. Fourier transformation of the responses now yields the response spectrum, analogously to the wave spectrum. The computational model takes account of all the phenomena arising that determine the loading, such as the throwing up and reflection of the waves, the movement of the element, and the effect of the solid seabed.
  • The workability spectrum (sailing criterion) kept to for sea transport is defined by a significant wave height Hsig = 2.0 m with peak period Tpeak = 6.0 s; the survival spectrum by Hsig = 3.0 m and Tpeak = 9.0 s.

Transport pre-stress

To these wave loadings is added what is known as sailing loading: the effect that a tunnel element pitches over into the ‘potential well’, the water level drop as a result of flow around the element. The transport stresses calculated in this way are scaled up based on a comparison with a project for which both calculations and model tests have been done.

In order to keep the tunnel segments together during floating off, transport and immersion, a transport pre-stress is applied. This pre-stress is designed such that under operational sea conditions, no joint gaps arise (criterion: minimum pressure of 0.2N/ m m2), and that in the ‘survival situation’, no cable failure occurs. The pre-stress is applied to the floor and roof of the tunnel. In order to give the tunnel the opportunity to ‘settle’ on the canal bed, the pre-stress cables are cut. Because the pre-stress channels are injected, the cutting of the pre-stress cables means that the entire pre-stress force at the start and end of the segment will be transferred to the concrete (just as in the long-bed system in the prefab industry). Of course, this must be taken into account when specifying the reinforcement in floor and roof; once again an example of the effect of the transport conditions on the design of the concrete structure. At transportation time, following the weather report (forecast) was therefore of crucial importance, to assess whether it fitted with the sailing criterion. Before departure from the construction dock, it was decided, based on the weather forecast for the coming 48 hours, whether the transport could proceed. Another forecast update followed just before venturing into the North Sea, to determine whether the voyage could continue.


Each element has four ballast tanks each with a volume of 1000m3. The capacity of each of the four lifting points on the ballast tanks is 100 tons. The corner bitts, which are sited on the same cast-in foundations as the towing points for the sea tow, have a working load of 100t.

For the elements, two immersion pontoons with a waterline area of 200m2 were used. The pontoons were fitted with immersion winches. The tanks were filled by workers in the element itself. During slacking off, the elements were subjected to an upwards force that increased with depth, as a result of the increasing salt content of the water. To guarantee sufficiently-high immersion loadings, more ballast water was allowed in when needed. The shear forces in the elements corresponding to such phasing were also calculated and verified. For the two boundary elements, the immersion pontoons were sited on the secondary end. After the element had been shifted along the immersion trench to over its desired position, container pontoons were placed on the deck at the primary extremity. During immersion, the freeboard was first reduced by letting water into the ballast tanks.

The container pontoons were filled with water to keep them weighted down on top of the element with sufficient pressure during the immersion process. The entire deck was taken under water by letting water into the tanks.

The primary end was lowered in steps by letting water into the tanks. The secondary end was lowered using the lifting system on the immersion pontoons. For the two elements in the middle of the North Sea Canal, the immersion pontoons were placed at the primary extremity, while the floating sheerlegs was made fast to the secondary extremity.

After each tunnel element was immersed, it was supported temporarily at one end by a projection on the previous element, and at the other by two adjuster feet, so that a gap of about 0.5m was present beneath it. Shortly after immersion, this space was injected with a sand-water mixture through openings that had been made on the underside of the element, staggered to left and right. A ‘sand pancake’ was made through each opening. This flowing-in is a meticulous process, in which the viscosity of the flush and the flow rate of the water play important roles. The final quality of the individual pancakes differed, so that the foundation’s stiffness was not uniform.

It is clear that the building phase, including the transportation of the tunnel elements, has an enormous impact on the design of an immersed tube tunnel, and the design task is a combination of complex considerations and verifications.