Constructing a high speed rail line across the flat Dutch polders may seem straightforward. But, as Adrian Greeman reports, the reality is very different.
Those unfamiliar with the Netherlands may be astonished to learn that the country's first high speed rail project involves no less than 170 bridges, box cuttings and viaducts on its 100km length, along with two 'flexible' immersed tube river crossings, a 1,200m long multi-span bridge and the world's largest diameter machine bored tunnels. Altogether construction, divided into five major sectors, will cost some US$5.6bn at 2000 prices.
Linking Amsterdam with Antwerp in Belgium via Schipol airport, Rotterdam and Breda, the curving route will help create a complete northern European network of 300km/h railways. Belgium is building on to Brussels, which is already linked with Lille, Paris and the Channel Tunnel.
Environmental factors have driven the design, according to client body HogesnelheidslijnZuid (HSL) - High Speed Line South. The company is part of the government's Rijkswaterstaat infrastructure directorate.
The giant 13.8m internal diameter 7.2km twin track tunnel for the Groenehart is the major example of this policy, chosen to save rare traditional farmstead and windmill landscape within the 'ring-city' formed by Amsterdam, Rotterdam and other towns.
Similar concerns led to the use of immersed tubes under two major waterways further south, and an architect designed 'signature' bridge over the Hollandsche Diep waterway, part of the intertwined Rhine and Maas river estuary. These latter schemes are both in one of the five contract sections for the line.
High speed tracks will also go underground at Barendrecht as part of a scheme putting a major new freight line and conventional track into a nine line landscaped box. HSL has seconded its work here to Betuweroute.
Construction of HSL South began in 1999 with completion due in 2005 though some slippage means a first section from Rotterdam to Antwerp is now due to open in October 2006, with the remainder in spring 2007.
Underground activity Consortium Bouwcombinatie HSL Drechtse Steden is creating two immersed tube tunnels as part of the US$530M Holland South section. Work also includes 30km of piled slab concrete line at grade.
Both tunnels are 2.5km in length, and while the Dutch are well experienced in immersed techniques the high speed line imposes some difficult features.
HSL engineer Peter Hoogen explains: 'First, the tunnels have to have a large cross-section because of the air pressure effects from 300km/h trains.
Connecting apertures between each side of the tunnel box as well as providing safety escape passages will also help reduce pressure waves. A large 7m diameter ventilation shaft will be used partly for this purpose.'
Even so the tunnel segments, now under construction in a dry dock on the Oude Maas river, will be unusually high at 8m. Fourteen 18m wide units jostle for space in the yard; most are 134m long although two centre units for each tunnel are 150m.
'The extra height means the units will have an unusually high freeboard of 1m when we float them early next year, ' says Martin Jansen for the contractor.
'This makes them harder to control in high winds.' They will be taken to site by tugs and positioned with winches.
To save time overall the usual practice of building cut paver approaches to the immersed tube section has been abandoned.
Instead, at each site the sides of the river are being excavated to take immersion units, which will be backfilled later.
Settlement is a major issue.
Like most of the Netherlands' reclaimed polders, the ground here has a minimum 12m thick soft peat layer with medium dense sand lying beneath.
'Ideally, as with all the track, we should pile but it would be too expensive, ' says Hoogen.
Ground load will be high on the side elements that are backfilled.
A 'flexible' tube is the answer, able to deflect slightly to absorb any initial movement without leaking. Tunnel units are being cast in 25m long sections with projecting shear keys and rubber seals between them to allow each to articulate slightly without excessive vertical movement.
Unit to unit connections will be standard, though fairly large, Gina and omega gaskets.
'It means that an overall 300mm settlement can be accommodated for the tube, while sticking to a stringent 2mm differential between any two elements, ' says Hoogen.
But flexible units would be awkward during float out so each element is stressed to the next in the dry dock. Strand will be cut on site to release the flexibility.
Settlement will also be tackled with special articulating entry plates between the tunnel and more rigid piled abutment sections.
Tunnels will also need to have special flood doors at one end because they penetrate sea protection dykes.
Part of the landscape
Unlike most of HSL the 1,200m Hollandsche Diep crossing in the south will be highly visible, sitting between an existing rail bridge and the main Belgium Rotterdam A16 motorway crossing.
The overall 2km long structure was subject to an international 'signature design' competition in 1998 won by Dutch firm Van Bentham Crouwel Architekten with Arup.
Constraints were tight because to maintain shipping channels the bridge must match the piers of the pre-war rail bridge. There is significant barge traffic, which also means piers must resist impact from a grouping of six standard barges each 78m long and 11m wide.
Gaby Schouten, client HSL's project leader, says that the winning design for hammerhead piers to support 10 main 105m long spans, 'was simple, which we liked, not fussy'. The design build contractor agreed, opting to stick with the competition winner.
Bridge work also includes 70m steel spans at each side, and 350m long concrete side ramp approaches and viaducts.
The gradient will be at the maximum permitted 2.5degrees because the track runs from a 24m high central shipping clearance down into an immersed tube tunnel just 2km north.
A 5.5m deep steel box structure for the main superstructure gives the stiffness required to handle the high speed train accelerations and cope with complex resonance effects. This box is made up from the steel pierheads, fabricated in one of three waterside yards and brought by barge for lifting in. Sections are hauled into place by strand jacks and linked together.
The box is actually a composite, with a concrete top deck. This is cast insitu for the hammerheads, although intermediate sections are precast. Pierheads were too heavy to lift in as one because the biggest floating sheerleg crane that could pass the motorway bridge downstream has a 500t maximum capacity, explains Gert Nederend, bridge project leader for the contractor.
Consortium Bouwcombinatie HSL Drechtse Steden is building the project; members are Ballast Nedam, Van Hattem & Blankevoort, Strukton Betonbouw, Van Oord ACZ, Waterbouw, HBG Civiel, and Maasdiep VOF.
Various techniques keep as much work onshore in the windswept estuary as possible.
Reinforcement cages for the hammerheads were made up in a special jig onshore before the 250t of steel was lifted in. And pilecap construction began with concrete caissons precast onshore and floated out into position.
'Each of these had five holes in the base, sealed with a steel plate, ' Nederend explains.
The 2.5m diameter holes matched the tops of 25m long driven tubular steel piles and were positioned over them. Divers cut away the seals to allow the caissons to be sunk over the piles.
These had been previously excavated to a 10m depth inside and they and the caisson bases were then filled with concrete.
Work is more than half way complete at present, with final deck sections due to be lifted in May during the third of three fortnight long river diversions.