Work is rapidly gearing up to the construction stage for the Netherlands' first high speed passenger rail link, running south from Amsterdam, through Rotterdam, Breda and onto Antwerp in Belgium.
Settlement is always an issue in the Netherlands, but the settlement requirements of a high speed rail link pose a particular engineering challenge. This is especially so because the intention is to construct the 160km route using an innovative precast ballast-free track system. This removes the usual (but costly) option of reballasting to correct for any settlement.
As such, engineers have set about the task of designing a virtually settlement-free construction - a very tall order given the Netherlands' notoriously soft ground conditions, which along most of the route comprises up to 10m of Holocene clays and peat overlying denser Pleistocene sand.
Preliminary design assumes the entire line will be constructed on a concrete slab founded with end bearing piles in the sand. However client and design teams remain open-minded to new technology and last year set up a full-scale field trials at S'Gravendeel in the south of the country to assess alternative new technologies.
Companies were invited to propose and construct geotechnical alternatives to the piled slab option. The aim was to devise an embankment ramp, rising from 1m to 5m over a 100m long (ie modelling a bridge approach), that 24 months after the start of construction would have zero settlement at the top of the ramp (ie where the track might pass from embankment to piled bridge) and no more than 30mm at its lower end.
Other target criteria for participating companies was that the embankment itself must be built within 18 months, costs must be less than the piled slab, the construction technique must be previously untried in the Netherlands and the resulting embankment must have a dynamic stiffness better than a 5m sand layer. This is because when trains reach a speed of about 250km/h they overtake the pressure wave created at their front. Low dynamic stiffness in the track and the subsoil results in wave-like movement of rails as the train passes over, giving an uncomfortable ride and increasing wear. Finally the construction must not adversely affect the groundwater regime.
Work started on construction of five different embankments in spring last year, including a reference embankment (see box) and monitoring will continue for another year.
The decision has not yet been made whether any of the solutions will be incorporated into the project, however the project team is certainly positive toward the idea. 'It is important to be open for innovation where you can gain from it,' says Jan Ochtman, deputy project director for the High Speed Line. 'It sounds idealistic but some of these techniques can only be proven if you take one giant step. We see a large scale project of this nature to be an opportunity to try out new things.'
Time will tell, but even if none of the novel embankment solutions are adopted, the interest and experience generated by the trial will surely help acceptance of the techniques in other projects in the Netherlands. An obvious application would be motorway widening where differential settlement between existing and new carriageway is a very important consideration.