The Westerschelde twin-bore highway tunnel project between Terneuzen and Zuid-Beveland in the south of the Netherlands is one of the first, and so far most challenging, soft ground TBM projects in the country. Not only does it feature twin 11.3m diameter bores, each 6.6km long, to cross under a major tidal river, but also a necessary depth of working which needs careful engineering to minimise project risk.

High ground and water pressures resulted in major problems for both TBMs last year, but fortunately the worst appears to have been passed. These conditions had been anticipated but not, perhaps, to the severity of the effects experienced. Both Herrenknecht Hydroshield TBMs are now making satisfactory progress but are not expected to recover much of the eight month delay in schedule.

Innovative aspects of the project include not only the TBM access and recovery operations but also the design-and-construct joint-venture contract awarded to KMW for the project, the structure and finance of the client NV Westerscheldetunnel, and the planned use of subsea groundfreezing to construct the cross-passages.

Method choice

Once the financial arguments for the fixed crossing had been established, the next step was the type of crossing. Hans-Willi Seidel, technical manager of contractor Phillip Holzmann, outlined the original plan as an immersed tube tunnel under the Pas van Terneuzen, 6m depth of fill on the Middelplaat mid-river bank and a 50m high bridge over the Everingen. "Until 1992 the belief was that it was not feasible to bore tunnels in soft and unstable soil" he said.

But the use of the Westerschelde as a major route for seagoing vessels caused problems for both types of crossing. An international treaty with Belgium to preserve the status of Antwerp as a major international port covers the river. The Belgians claimed that a bridge would cause an obstruction to ships so the Province of Zeeland requested alternative proposals. With a 28m streaming depth, fast tidal movements, scouring and deposition, plus temporary obstructions to navigation, the immesred tube tunnel was also a problematical method. River traffic would have to be stopped for around 35h for each installation and there is no possible detour. Also slope alignment would require extensive, and possibly environmentally unacceptable, excavation of both shores.

Even before the establishment of NV Westerscheldetunnel and the decision to utilise a tunnel of some sort, the client management had extensive discussions with those experienced in soft ground tunnelling at international conferences, and in Japan and Germany. This was part of a national move towards such technology with several projects planned in the Netherlands. The Second Heinenoord Tunnel, intended as a pilot project, was a major step in the process which produced a vast amount of useful data.

Executive manager of NV Westerscheldetunnel, Wim van der Linde, explained that environmental considerations were a big factor in favour of a bored tunnel, even if it might cost a bit more. Commenting on tunnelling advances since the construction of the Amsterdam Metro he says: "We thought that there was now a lot more experience in boring tunnels in soft ground, so we can do something more. Therefore engineers were looking more in that direction."


Tenders were sought from 1992. Four design and construct groups bid but only two were approved, both offering a bored tunnel for the first time in Netherlands transport projects. British engineering consultants Maunsell and Mott MacDonald were engaged for ‘second opinions’ on the tunnel design.

The successful contractor, KMW, offered a bored tunnel across the whole river as an alternative, providing the structural concept, job engineering and scheduling, equipment requirements, process specifications and engineering calculations for both the tunnels and approach ramps. KMW is led by German tunnelling contractor Philipp Holzmann with additional tunnelling expertise from Wayss & Freytag of Germany and the Dutch HBG group, its parent. Other participants in the joint venture are the Dutch companies Heijmans, Voormolen Bouw and BAM Infrabouw that are active mainly on the approach road construction and surface plant. The offer also included the E&M fit out and operation of the tunnel for ten years. The required start and end points of the tunnel had been established by the client but, according to Seidel, KMW had to calculate all other tunnel dimensions and grades to meet performance requirements.

The client required proof of a service life of 100 years and an audit of KMW procedures to ISO 9001. In order to speed construction the client provided input for design detailing after the contract was awarded. Subsequently there was close co-operation on all construction design matters. "When you have some problems, you always have to sit together," explained Van der Linde. "We discuss both small and big problems, such as where the money comes from!"


At the tender stage geological information was limited, with data from only four deep boreholes plus shallow seismic testing and bores. It was known that the predominant materials were glauconitic sand and Boom clay (Boomse Klei), but KMW negotiated for a closer spacing of exploratory holes. These revealed various lenses of clay and sand in mixed ground, and also that the lower glauconitic sands are connected to the upper sands hydraulically.

The tunnel slope, at a maximum of 4.5%, is planned to provide sufficient cover below the Pas Van Terneuzen and Everingen channels of 35m and 25m respectively. There is a tidal movement of 6m on the river which gives a maximum water pressure of about 6.5bar at the tunnel invert during high tides. This occurs at a depth of about 60m below mean sea level under the Pas van Terneuzen. Here an invert depth of 60m is necessary to prevent buoyancy of the tube.

Tunnel structure

The twin bore, two lane highway tunnels of 10.1m id each are connected by cross-passages every 250m. These serve as escape routes and are also the location for electrical switchgear and other service equipment sited under the road. The lining is mainly of 450mm thick concrete segments cast on site but in the junctions with the cross-passages one concrete segment is replaced with a steel segment. Local authorities and the Ministry of Internal Affairs have required a number of fire precautions including the closer spacing of the cross passages, and a fire resistant inner lining tested to 1350°C. Each of the 26 cross-passages measures 2.50m by 2.70m in section and is around 12m long each.

The road basal layer includes precast concrete utility ducts. The client plans to run an electric service vehicle in one for easier inspection and maintenance.

Permanent tunnel ventilation will be provided by bi-directional jet fans in a system designed for fire smoke control. Each will be 1640mm in diameter.

Both tunnels are being driven simultaneously from the Terneuzen work site complex on the south bank of the river. The portal at this end is built with a ramp approach within a ‘polder construction’ incorporating permanent groundwater control. The rectangular reception shaft on the north bank at Ellewoutsdijk Polder has been formed by wet caisson sinking. The portal and ramp at this end is being anchored to resist buoyancy.

The two TBMs are Herrenknecht Hydroshields of 11.34m od, 11.55m long, and equipped with VMT guidance equipment. The cutting wheel rotates at 0-4rev/min with a 15MNm maximum torque. A special requirement is a separate centre cutterhead of 2.5m diameter which has bidirectional operation with up to 500mm forward movement.

The initial overcut was 10mm but following the stoppage and change of peripheral cutters, this was increased to 28mm. There is bentonite injection across the whole face. The TBMs’ total maximum forward thrust is 11,200t from 56 jacks. Between the last segment rings and the shield tail are three grease filled wirebrush seals.

Although the Boom clay is quite hard its basic particle size is close to that of the bentonite slurry and special separation measures were necessary. After extensive tests the separation plant design included a three stage hydrocyclone with a cut point of 10µm. Total flow of spoil up to 4000m³/h has been anticipated, to a project total of around 1.6M.m³. Pipes of 450mm diameter carry the slurry to the surface under power of large Warman centrifugal pumps; one on each shield machine, one on the surface, and four or five booster on the full run.

Herrenknecht Nederland director, Ing H M Gehlen, pointed out that there are always Herrenknecht TBM experts on site, with additional personnel for the first 200-300m of tunnelling. This was included in Herrenknecht’s supply contract.

As the TBMs approached the lowest point of the drives difficulties were experienced with squeezing ground and slow progress until both machines came to a halt in the lower glauconitic sand. The mineral glauconite tends to swell like clay, hence the squeeze on the shield. The sand is also highly abrasive, resulting in substantial cutter wear. Ironically the Boom clay, as well as having low permeability, is stiff and comparatively hard and so is relatively self-supporting.

Distortion of the TBM shield in the east bore has not greatly affected performance say site engineers. The cause has yet to be determined and is the subject of an investigation by Herrenknecht, Prof Maidl at the Univer-sity of Bochum, Prof Schmidt and other German experts. Commented Gehlen, "It’s hard to say what is the main reason for the distortion but it’s clear geological conditions were playing a big role."

Use of compressed air at the face for cutter changing is not an option due to the danger of blowout in the soft ground, hence the use of divers. At the low point the saturation diving team from Hyperbaric Tunnel Diving and Construction vof gained access to the face through two, 2m diameter, twin-compartment pressure locks designed for 6.5bar maximum pressure. There is also a third, smaller, lock for materials. The divers were carried from the surface to the face in a special capsule. This can be carried along the main deck of the backup system on a special track to interlock with the TBM airlocks. An artificial breathing mixture of helium, oxygen and nitrogen is needed at pressures above 6bar. The work resulted in a world record of 7.0-7.5bar for the highest pressure TBM face access (T&TI September 2000). The diving joint venture comprises Nordsee Taucher GmbH and Noordhoek Diving Co BV.

Although the recovery of the TBMs and the replacement of peripheral disc cutters were eventually successful, KMW is proposing to make a planned halt of both TBMs once well into the Boom clay. This will enable the work to be carried out in better ground before re-entering the deeper sands. Cutter changing is made easier by TBMs’ ability to withdraw each cutter 500mm and the security of five hydraulic plates within the cutterhead to support the face.

Part two

The February issue of T&TI will conclude this article with decriptions of the tunnel lining, cross-passage construction, surface installations and the integrated safety plan for construction, maintenance and operation.

Related Files
Principle features of the Terneuzen construction site
Location of the Westerscheldetunnel and new connecting roads compared to the two ferries to be replaced