The TBM for Westerschelde tunnel’s west bore started work three months after that for the east bore, but ground problems there have resulted in it overtaking its twin. When T&TI visited the site at the end of October, the west bore TBM had progressed 2000m and the east bore 1600m. Current progress averages seven rings a day for both machines, or 14m, on a planned 6-day week, 24h a day.
The back-up system for each TBM, also put together by maker Herrenknecht, is 195m long. It carries the TBM control room, grout system pumps and control, segment handling equipment, hydraulic power systems, electrical transformers and switchgear. A 40m-long skid-roller on the invert is supported by a 155m long roller-mounted pantechnicon running on side-mounted brackets. The gap allows two to three days in the normal construction cycle for correction of any lining ring problems. To deal with the heat and dust, forced face ventilation at 0.5m/s is fed through Proteus flexible ducting, 2.4m in diameter using Korfmann fans. An additional ventilation system on each TBM directs cooling air to various parts of the machinery. Cooling is also provided by water circulation of the hydraulic and compressed air systems.
To compress the construction period, permanent services ducts and highway base layers are being installed simultaneous with tunnel boring. This demands close attention to installation of the lining before the road base is laid on the invert. Normally the use of construction rail transport would prevent road construction, but at Westerschelde tracks are run on top of the precast concrete utility ducts in the floor.
The early installation of utility ducts allows critical E&M services to go in before the completion of tunnelling.
Lining
The two TBMs are equipped with tunnel lining segment erectors, which lift and position the eight elements of each 2m-wide ring. Each 450mm-thick trapezoidal segment weighs 11t with the exception of the 4.5t key. The first 15 rings of each bore are fully bolted using curved bolts to provide a stable thrust block for the TBMs, with subsequent rings bolted to each other with straight, angled bolts.
Segments are assembled on the surface into ring sets, handled by two gantry cranes. Loco-hauled trains transport each set to the TBM back-up assembly where the segments are loaded by a Rowa vacuum suction hoist and monorail on to a conveyor in the correct order for installation.
Manufacture of the segments at a purpose-built plant at nearby Terneuzen greatly reduces the pressure on the local roads which already serve several industrial sites. And full quality control procedures achieve the tight tolerance of 0.3mm for optimum gasket water resistance. Moulds and segments are measured using a photogrammetry system supplied by VMT.
Further protection against water ingress is given by a three-compartment wire-brush grease filled tail seal between the last erected segment rings and the TBM tail-shield. The seal serves to keep out groundwater, lubricate the shield for forward movement, and to indicate possible leaks. Any water which does come in is directed to the permanent drains with gravel filter surrounds. Pumping stations have been installed at the lower levels of the tunnels.
The annulus behind the lining and directly behind the TBMs is injected with grout, mixed on the surface and transported underground in rail remixer cars. KMW continually optimises the mix with various proportions of fly-ash, filler, sand and additives, but little cement. The increase in overcut naturally increases the amount of grout required.
The exception to the general lining design is at the cross-passages. Here a single oversized steel segment is used to accommodate the passage opening.
Frozen passages
The large number of cross-passages necessary have caused construction and scheduling problems. According to Wim Van de Linde, executive manager for Westerscheldetunnel: “Originally the passages were to be 500m, but the government said this was not safe enough and wanted them every 100m, based on experience with immersed tube construction. A ministerial level review eventually decided on 250m spacing. The contract had already been awarded by then, and more passages means more construction risk.”
The interval stipulation was just one of the fire precautions demanded by the government in the face of incidents elsewhere. Another is a 45mm-thick layer of concrete on the lining to meet the national fire code.
KMW is using ground freezing methods for construction of the cross-passages within the high groundwater pressures. Specialists from the Geodelft engineering consultancy are currently conducting tests on the first two to determine freeze performance in the main two ground types. Holzmann has considerable experience in freezing water-bearing gravel and sand and these tests will determine whether this experience is valid. An article on the research and development work in tunnelling conducted by Geodelft and the ‘Delft Cluster’ co-operative group will appear in a later issue of T&T International.
The first cross-passage is situated in glauconitic sand, and the second in the Boom Clay. Both are still under land, whereas all but one of the later cross-passages will be below water. The planned 26 freeze holes are drilled in a circular pattern around the passage section. A portable refrigeration plant, with a calcium chloride solution as the refrigerant, should form a frozen mass about 2m thick. The influence of the saline groundwater and the behaviour of the Boom Clay are under particular scrutiny. Freezing time is expected to be about 30-45 days with a refrigerant temperature of -37°C.
Every crossover has an electrical connection cellar. At low points of the tunnel, drainage pumping stations are located under the roadway.
On the surface
The size and intensity of the project requires substantial surface installation chiefly for handling spoil and materials. The two TBMs were easier to service from one site.
Sites on both banks of the Westerschelde were considered. Van de Linde explained the choice. “More land was available at Terneuzen and negotiations with the farmers went better. It was also easier to make a tunnel ramp on this site, but it still needed 18 months to gain access. Nevertheless we could start work six months earlier at Terneuzen compared with the start at Ellewoutsdijk.”
The site covers 32ha and the main construction area includes what is claimed to be the largest narrow-gauge railway in Europe with 19.5km of track including that in the tunnels. The rail transport system includes eight Schöma haulage locomotives, 80 spoil wagons, two man-riding trains, a fire-fighting wagon and an ambulance wagon.
Betonwaren Fabriek Terneuzen’s concrete segment manufacturing plant is charged with making 53,000 concrete segments, requiring some 200,000m³: of concrete. Segments are stored in a large yard, along with other precast concrete components, mainly 2mx2m section utilities ducts which are bought in.
The site also houses a large spoil separation and slurry treatment plant to deal with a high proportion of fines.
Government directives on construction recycling and pollution called for careful consideration of what to do with the spoil. The plant, designed by KMW, generates graded material from the mixed spoil heaps. Some spoil has been used to build dykes to protect the tunnel installations and prevent flooding of the portals, but most is being separated into inert sand and clay. Van de Linde explained that sand can be dumped into the Westerschelde provided it does not contain more than 1% bentonite. The clay could affect fishing if dumped in the estuarine waters and is more difficult to dispose of. Attempts are being made to sell this material for industrial use or otherwise to recycle it. That thought to be saleable is put into temporary storage.
Electrical power is provided from the local grid supply which is already set up for several heavy industrial operations. This would not be sufficient, however, to operate a third TBM, even with the possibility of emergency supplies from Belgium.
Emergency facilities include precautions for flooding, fire, compressed air working incidents and the unusual need for alarm and evacuation procedures in the event of an emission of toxic chemicals from the neighbouring Dow processing plant.
Nearby is an information centre for visitors which has proved very popular. Part of a busy public relations campaign to inform and persuade the public and businesses of the benefits of the new fixed crossing, the centre presents the history of the Westerschelde crossings, the construction technology, environmental matters and likely future development.
On the north bank, near Ellewoutsdijk, present construction is limited to the rectangular reception shaft, installed by wet caisson sinking. Later construction, together with a new road to link the tunnel to the main highway, will include a toll plaza, bus station, restaurants, a fuel station and park-and-ride facilities. The client is budgeting for extra finance to make the site attractive and so help with marketing.
Project progress
For the first time in the Netherlands, both construction work and subsequent tunnel operation is governed by an integrated and comprehensive safety plan comprising six large volumes entitled ‘Working safe’, ‘Emergency incidents’, ‘Safety management’, ‘Incidents during operation’, ‘Maintenance – working safe’ and ‘Project dossier’. The safety plan ties not only the contractor and the client but also government agencies and the emergency services, who were all involved in its development. Included are emergency evacuation procedures during construction and of the public during operation. Van de Linde believes that the safety plan can be used as a model for other projects.
A completion delay of four months has been incorporated into a revised business plan to accommodate the safety upgrades, moving the opening date from 15 November 2002 to March 2003, according to Westerscheldetunnel general director Tin Buis. However, with construction, and in particular the tunnelling, running eight to nine months behind schedule, opening is now put back to November 2003. KMW’s Bernd Otten explained that tunnelling activities currently occupy 326 KMW workers plus 24 engineers. In addition there are 60 project staff and 220 workers in the segment.
Related Files
Figure 2
Figure 1
