Innovative approaches are needed to construct a rail line over ground that is little more than 'solid water'. Lisa Russell reports on the Netherlands' High Speed Line.
Building a long lived high speed rail system gives the opportunity to take technology forward by introducing new techniques, believes deputy project director of Holland's new High Speed Line Jan Ochtman.
'It is important to be open for innovation where you can gain from it,' says Ochtman. This applies right across the project: in technical matters, procurement policies and also in the way environmental issues are handled, he says.
'It sounds idealistic, but we feel we ought to do this because we have very little by way of these large scale projects,' says Ochtman. 'Some of these techniques can only be proven if you take one giant step ahead. You have an obligation to use that opportunity to try out new things. If you take proven techniques now, you know they are going to be outdated by the time the project comes in service, and certainly by the time you are half way through the life.' It is a challenge for everyone to judge what should be provided, he adds.
The 100km line is due to come into operation in 2005, and its infrastructure manager, dubbed 'infraprovider', will have a concession to 2030 (see box below).
Engineering innovations being looked at include a ballast-free track and new trackbed technology to cope with the ground conditions. 'We do a lot of development work ourselves,' says Ochtman.
At first glance, the Netherlands' topography might appear ideal for building a railway. There are no inconvenient mountains in the way, although waterways and built-up areas provide obstacles of a different kind.
But the flat landscape masks problems underground. 'We have to build the rail line on solid water,' jokes design engineer Hans van Ammers, who is part of the HSL project organisation and has worked for DHV since 1976.
Put another way, the line has to be founded on a 30m depth of peat, clay and silt, close to sea level, with groundwater just below the surface.
Other countries, such as France for the TGV, have had problems with marshy grounds - but only for a few hundred metres, says van Ammers. In such cases, the poor soil can be excavated and replaced by rock. 'Here we have 60km of poor soil and we have no rock, so we have to find another solution.' The ground is difficult even to walk on, and impassable to conventional vehicles.
Much of the line was already designated to be on structures where is crosses built-up areas, over waterways and an environmentally sensitive region (see box page 35). Only a few sections of conventional embankments were needed, and there were potential problems in terms of settlement, especially differential settlements at transitions.
'We decided to get rid of all the embankments, and construct the entire line on a concrete slab with piled foundations,' explains van Ammers. The overall percentage increase in cost was relatively low.
The typical cross-section is now a double U track, a continuous low prestressed concrete viaduct which could be built insitu or prefabricated. Piled foundations are an effective solution for the poor ground conditions, but very costly at up to 30m deep.
HSL expects the design and build contractors to offer alternatives and its project organisation is already carrying out trials, including vertical drains, soil stabilisation by gypsum and cement, and foam concrete piles.
The chosen solution has to be sure of overcoming another potential problem. Extra deformation is predicted when a train on these soft wet soils reaches a critical speed of 250km/h to 300km/h, faster than the pressure wave it creates.
The rails would effectively be pushed up, giving an uncomfortable ride, wear and eventual danger. 'It is like an aeroplane breaking the sound barrier,' says van Ammers.
A further development is the selection of ballastless track. Traditional ballast track can cause problems when used in conjunction with high speed and concrete surfaces, he says, damaging both the ballast and the rails.
Ballast-free systems in Japan and Germany are based on conventional rail fastenings, which need maintenance.
HSL is developing an embedded system where the rail is 'glued' into a slot in the concrete slab using Corkelast, an elastomer with cork. The rail is continuously supported, and can be smaller, adds van Ammers, reducing the visual intrusion of the line. The disadvantage, of course, is that any subsequent correction or replacement is difficult.
Such a system has been used successfully in the Netherlands for more than 20 years, on a freight line and on level crossings. The problem is to extrapolate that experience to the high speed line, and tests and numerical modelling are being carried out to gauge its viability.