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Six of the best


The need for extensive tunnels and cuttings has led to some innovative ground engineering on the new Betuweroute freight line in The Netherlands.

Adrian Greeman reports.

The Dutch do not build bored tunnels. There is no need to go deep in a flat country lying mostly below sea level; instead, engineers have developed immersed tube methods.

But the increasing clutter of infrastructure in Europe's most densely populated country has changed that. To find space and minimise environmental impact, projects are going underground.

On the new Betuweroute (see box), from Rotterdam to the German border, there are six tunnels. Three - including the 8km Sophia tunnel - are bored, while two shorter sections are cut and cover. For the sixth, contractors are carrying out an unusual ground level 'tunnelisation' by putting the tracks inside a 1,500m long artificial dyke to protect an urban area from noise and accident risks.

The first significant project is the 3km long Botlek tunnel, now being fitted out. The Betuweroute runs through Rotterdam harbour and was formed by adding two new tracks to the Harbour Line.

These must pass under the Oude Maas River, a busy 11m deep inlet to the main harbour. As there was no room for another bridge, tunnelling proved to be cost-effective.

Work began in March 1999 under a Eu132M (£89.2M) design and build contract let to BTC, a Dutch-German consortium of Ballast Nedam, HBW, Stukton, Van Hattum en Blankvoort and Wayss & Freitag.

Betuweroute's project manager for tunnels, Jan Jonker, says: 'The contractor used an earth pressure balance machine to make the crossing instead of the bentonite slurry TBM we suggested in the reference design. That allowed the 1835m central bored section of the tunnel to be brought to a higher alignment by about 2.5m because of a reduced threat of blow-out.' Like much of the sea-reclaimed areas of The Netherlands, the ground is extremely soft, a mixture of fine sea silts and about one third peat. Beneath an initial 5m of dredged sand and fill there is at least 12m of this Holocene layer, which is typically underlain by a medium to coarse grained gravelly sand layer about 20m thick. The water table lies almost at the surface. The whole combination produces potentially strong buoyancy uplift effects and a risk of blow-out.

Using an earth pressure balance machine allowed a maximum 18m from sea level to tunnel crown, which meant it passed only 7.2m below the river bed, less than the diameter of the 9.87m TBM.

The route will carry heavy freight trains (see box) and therefore each of the twin single track bores on the Botlek section needs a 9.45m outside diameter with an internal diameter of 8.65m.

A second advantage of the earth pressure balance machine was that it did not require surface facilities such as a slurry cleaning plant. In the crowded port, filled not only with jetties and offloading points, but tank farms, refineries, industrial works and all their road and rail links, finding work space was a problem. A bentonite spoil system would also have been needed to clean the fine silts and clays from the 20m deep start and finish sections of the tunnel, which would have been expensive.

These difficult sections demanded ground treatment.

Interlocking 800mm diameter, 20m deep soil mix columns were used over 200m long stretches at the start and finish points.

'It worked, ' says Jonker, 'though we did not get quite as much strength as supposed, perhaps 5N/mm 2. But that proved enough.' To cope with the coarse sandy conditions for the bulk of the drive, where water pressure reached 3.5bar, a ring of foam injection nozzles was fitted just inside the circumference of the shield of the German Herrenknecht machine.

The TBM was fitted with both screw conveyor and slurry pumps to handle the soil/water mixture ifit became too liquid, another move that paid off at various points during the under river drive. The Botlek drives were completed in 2001, having made an average 10m to 12m daily.

Slightly further on, work continues on the 8km Sophia tunnel, claimed to be the first project in the world to make use of a continuous drive TBM, though in the event the machine was used more in its conventional mode.

The Sophia tunnel passes under twin channels of the River Noord, an arm of the Rhine estuary with a central island, the Sophia polder. The £203M crossing begins with open sections of trench and some cut and cover tunnel before trains enter a central 4.2km of bored tunnel. It passes under two motorways, local roads and several flood prevention dykes. As at Botlek, the ground comprises peats and silts overlying the Pleistocene sands.

The machine, devised and built by Herrenknecht, uses a series of 28 independently retracting jacks, each longer than standard hydraulic jacks on a TBM. A control system allows them to be withdrawn to allow insertion of one segment while the others continue to push the machine from the ring behind. Jack pressure is gradually transferred to the new ring as segments are inserted.

'A secondary segment erector is used which holds the first new section in line while the next ones are placed with the main erector, ' explains Jonker. An extension to the shield to cover the extra equipment means that it has to be articulated.

The whole arrangement should virtually double the pace at which a drive can be made, with a 40m daily average anticipated for these 8.45m internal diameter tunnels.

But though the stability from the secondary erector should combine with the sensitivity of the jack control mechanism to ensure steady progress, in practice the TBM tended to go off line after about six segments.

Jonker says: 'Complex software uses a three-point reference system to check the machine bias as it launches off, but there were some teething troubles with that.' To create the skill levels needed to control everything would have taken much longer, especially as this was the first time the Dutch members of the Tubecon consortium had built a bored tunnel.

Though the consortium brings experience from Germany with Hochtief and Philipp Holzmann joining Koninkijke Boskalis Westminster, Heijmans and NBM Anstelland, it decided to play safe and reverted to discrete drive phases followed by pauses for erection of the 1.5m wide segments.

'We found we could make good enough progress in standard mode, ' Jonker says. Despite the problems, he believes the experience gained from this project will see the continuous mode coming into its own in the future.

The first bore must be ready at the end of August this year and the second at the end of October.

This should be achieved.

Further issues on the tunnel include the need for automated sliding steel water doors, one at each end, a requirement peculiar to a country which is largely below sea level. To control risks at this point the tunnel walls are heavily reinforced, with additional concrete over a 200m length virtually filling the inside profile, allowing a minimum envelope for train passage.

A third bored tunnel only 10km from the Dutch-German border has also used an unusual method, this time to cross a 200m wide disused brickwork quarry. The contractor built a sand embankment through the 20m deep pit, compacting the sand with vibrator units to create a suitable medium for the TBM to pass.

The main purpose of this 2.58km long tunnel is to cross the Pannerdensch, a canalised section of the Rhine. For the central bored section, the French/Dutch Comol consortium of Vinci/CFE/ Welling and TBI achieved an astonishing average drive speed of 22m daily, Jonker says. The £94.6M tunnel has also had to deal with high loads from 4m high material mounds in the nearby brickworks.

Unusual works of another type are under way at Barendrecht, just south of Rotterdam. Here Betuweroute will add to an existing four-track rail corridor of two conventional lines and two port freight lines.

But the conventional train lines are also to be doubled to four tracks, and the new Amsterdam to Antwerp high speed rail tracks will also run through here, bringing the total to nine tracks, three of them for freight.

Not surprisingly, the local citizens think this a bit much and the whole complex is going underground along a 1.5km stretch.

Over five years, a phased sequence of track changes and diversions is needed to widen the corridor, with all the tracks to be enclosed in concrete boxes, each handling a line.

Within the new 'tunnels', tracks will be supplied with automatic fire prevention sprinkler systems fed by a new reservoir being created alongside. All the lines will be enclosed in a new landscaped embankment.

A joint Anglo-Dutch joint venture of Besix, Hegeman and Mowlem has been working for two years on the £54M project which so far has put two lines under cover and built a new underground station.

'We are installing about 12,000 piles 18m long along this section, ' says Peter Franke of Mowlem.

'That has given us some interesting experience because early driving work was affected by suction from a top clay layer. Now, we are first of all boring the top section before driving the piles.'

Rotterdam reigns supreme Betuweroute is a twin track heavy freight railway intended to consolidate the pre-eminent position of Rotterdam as Europe's largest port, by providing a modern 160km long connection to Germany.

Most is new build, though in the harbour an older line is being upgraded and rerouted. A huge marshalling yard is also being rebuilt.

Ten trains will run each way at 120km/hour, carrying a full range of goods from double stacked containers to hazardous chemicals, oil and gas. That means a clearance of 7.1m height is needed for structures and tunnels.

The power system will be 25000kV and safety and signalling will be to common European standards. The cost of the project is Eu4715M at 2001 prices.

The Betuweroute Project Organisation, working within the Dutch public railways body Railinfrabeer, is responsible for overseeing the project.

Railinfrabeer lets the contracts for the Dutch Transport, Public Works and Water Management ministry, the Rijkswaterstaat.

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