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A New Age solution

Underground Groenehart

The big wheel that has just started to move must, as the song has it, 'keep on turning' for the next two years, writes Adrian Greeman.

The wheel at Groenehart is not a river boat paddle, but the cutterhead for the world's largest tunnel boring machine, 14.87m in diameter and just starting excavation for the 7km tunnel in the Netherlands.

Groenehart is part of the Brussels to Amsterdam high speed rail link, part of Europe's growing network. The little country wants to join the new railway age but this is made complicated by having the continent's densest population.

For the twin-track tunnel, just north of the Hague, passing through an area of reclaimed polders and drainage canals, a surface line would be perfectly feasible. But the Dutch government would like to to retain as much farmland and green space as possible within the Rotterdam-to-Amsterdam 'super-city' area and has opted for a tunnel.

Environmental concerns are also the main reason for the huge diameter of the tunnel. As part of reducing construction and operational impact, French contractor Bouygues proposed a single bore alternative to the twin bore bid-call design, with a dividing wall to separate tracks.

'The advantage is that less spoil is produced, and the dangers of settlement are less, ' explains Joseph Harnois, project director for the design and construct joint venture of Bouygues Travaux Publics and Dutch firm Koop. Contract conditions from client, the Netherlands transport ministry, call for no more than 20mm settlement along the 7,160m long tunnel line.

The divided tunnel also satisfies the increasingly stringent safety concerns for modern tunnels. Openings to balance air pressure waves from 350km/h trains are simply provided in the wall and can easily be closed in the event of fire.

Fewer sealable cross passage tunnel connectors could have been built between single bores and would have needed expensive ground freezing.

An additional problem would have followed from another environmental requirement. The sea-won polders have two water tables, one freshwater sitting close to the surface, and a deeper saline horizon in the sand.

'We are not allowed to disturb that one and create any polluting mixing of the water, ' Harnois explains.

But the single bore solution means a huge machine, 500mm greater in diameter than the giant which has just completed the Hamburg harbour road crossing in Germany. And the machine must operate in very different ground conditions.

The ground comprises around 12m of relatively soft peat and clay over a 25m to 30m layer of originally seabed sand. Below is a 2m clay layer and then more sand.

'It will pass through the upper sand layer primarily, ' says Harnois, 'keeping beneath the peat'. Some clay may be found because the layer is not consistent.

Rather than turn to a manufacturer, Bouygues designed the bentonite slurry TBM in-house working with France's NFM Framatome Technologie to manufacture the giant machine.

'One of the problems has simply been machining the parts, which are among the biggest pieces of steel ever milled, ' says Harnois. For example, the central toothed drive wheel for the cutterhead is 7m in diameter, machined from a single 45t steel element. It draws together the 3,500kW power input from some 14 electric motors with a torque of 30,100N/m More than 86 truckloads of elements travelled from the Creusot works in central France to barges on the Rhône. From there, they were taken by water to the site, where assembly and testing began late last summer.

Behind the big cutterhead is a 12.4m long shield for the motors and the machine's massive hydraulic segment erector, which lifts and positions the 15t, 2m long concrete units. Ten segments and a key make up a ring.

Segments are brought in from a plant in Belgium because the Netherlands does not have the aggregates for making the required concrete mix. A long life chloride resistant mix design is used to resist salinity.

Rather than a conventional back-up train behind the segment placer, there is a steel girder 'bridge' running 60m back to a rail mounted rear section. This can bring forward segments or precast concrete culvert elements, as well as other materials, delivered from the 35m diameter starter shaft using specially developed pneumatic tyred transporters. These are also a Bouygues design.

The bridge also carries the spoil removal pipes for the bentonite/spoil suspension pumped from the front of the machine to the huge separator plant housed close to the starter pit.

The reason for the unusual TBM configuration is the need to backfill the tunnel base as fast as possible after segment erection is complete. The tunnellers face a major problem with uplift in trying to form such a large void in soft, water saturated ground.

The tunnel design therefore includes a very lean, sandy concrete backfill for the bottom 25% of the internal bore.

The fill is compacted around a central service culvert formed from square precast concrete elements. This is installed behind the newly formed ring, leaving an elevated flat tunnel bed for the rear section of the TBM to move up and start ballasting the new section of tunnel.

But the ballast operation begins around 30m back from segment erection. There is still a major problem coping with the forces on the lining in the intermediate section, explains Harnois. If the rings move too far out of position, it could cause cracking in the segments.

To deal with this, Bouygues is applying for the first time a new ring grouting system developed at the firm's headquarters.

Harnois is reluctant to spell out in detail how this works, saying only that it uses a material with a viscosity which varies according the pressure put on it.

Alongside the use of the special annular void backfill grout will be an accurate system for monitoring exactly where each newly placed ring is sitting in relation to the already formed tunnel bore and the TBM.

Bouygues has installed its own system, PIXIS, which Harnois declares to be 'five years ahead' of anything else for TBM control and monitoring.

Some 600 parameters on the machine are measured continuously and adjustments are made as necessary. Altogether, 2,000 parameters are recorded.

'It monitors the front and rear centre points of the bore and the centre of the rings very accurately, ' says Harnois. 'We know where we are, where we have been and where we are going.'

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