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Motorway of the future

Roads - New design approach

A prefabricated piled road system from the Netherlands offers a new approach to cope with a changing environment. Bill Holdsworth reports.

Road building technology throughout Europe needs radical change according to Simon Poot, head of innovation at Dutch concrete manufacturer BetonSon. He has long believed that standard road construction is maintenance intensive and the prime reason for increasing traffic congestion.

This opinion is vindicated by a recent report from the UK's Institution of Civil Engineers stating that damaged and potholed roads in the UK require more than ú5bn ($7.7bn) of investment for repair.

Poot says road skills and technologies have hardly changed over the years. 'Old rural roads [in the Netherlands] that traced out the higher clay levels more or less floated. The construction methods used means that heavy motorised traffic has not only destroyed the roads, but has allowed the lower layers of peat to become saturated. Consequently, roads have become unsustainable and kept the 'maintenance mill' constantly turning.'

He is calling for a a new approach to road building throughout Europe. He believes that piling, in more or less unstable soils, is the only guarantee for sustainability and has advantages for many different ground conditions.

In Europe, discussions about a new road only seem to start after several years of traffic jams. In the Netherlands it takes 10 to 15 years before such a project gets the go ahead.

The economic cost of traffic congestion, coupled with a desire to reduce the annually rising maintenance bill while improving traffic safety and control, has led the Dutch Ministry of Transport, Public Works and Water Management to rethink outdated policies. The challenge is to design a 'road to the future' based on knowledge gathered over the past 30 years.

Since February 2002, the latest thinking in geotechnics, thermodynamics, solar engineering and transport communications have become part of the Netherlands' new approach to road design and are critical elements in the fully integrated motorway systems now being tested on the country's A50 motorway.

One of the main proposals is the ModieSlab concept, a innovation from concrete manufacturer BetonSon, international consultant Arcadis Sustainable and Heijmans, one of the largest road contractors in the Netherlands.

The ModieSlab system consists of prefabricated concrete 'layered' plates incorporating a number of systems. Starting at the top, the 380mm thick slab comprises a 3mm to 5mm fine porous top surface with 7dBA noise reduction, followed by a 11mm to 16mm porous concrete layer of 20% to 30% density which sits on a 50mm concrete layer with drainage grooves and conduits for electronic wiring, intelligent traffic controls, lane warning lights and a solar power collecting system.

All this rests upon a slab of 80MPa self-compacting concrete designed to take the heaviest traffic loads.

Roads built with large concrete slabs need good foundations and good connections for comfortable use. Precision fixing was critical with required tolerances of °0.2mm.

To ensure a design for maximum live loadings of 200kN/m 2(Dutch Class 60 Bridge Standard), a form of bridge construction was developed using concrete sleepers to span a series of deep piles, after tests proved that keeping to a minimum of 1mm tolerance between slabs over the pilot test length of 100m was unachievable using traditional construction.

BetonSon says that with the slabs laid on piled supported 'bridge sleepers', the level achieved is akin to that of a laserguided slipform paver.

Road sections can be laid at a rate of 100m/h from special container-style lifting and laying machines. Service-grid tunnels for water, gas, electricity, or transporter tunnels can be easily incorporated and serviced.

Although the area of the first 12-month pilot study using 5m by 3.5m slabs does not have any known soft soil conditions, it was discovered that a maximum of 40mm settlement occurred at the concentrated load points of the transverse sleepers.

Part of the ministry's brief for the pilot test was to seek a guarantee for maintenance between an unpiled and piled road over the 12 month monitoring period. For the unpiled example the ModieSlab team made two concrete filled trenches in a longitudinal direction and set down 400mm by 400mm beams in wet concrete.

Steel cups were then fitted as part of the adjustment items of the sleepers. After this adjustment the space between the underside of the sleepers and top of the beams - as well as the space under the beams and the soil - was filled with grout for maximum bearing. When there are no piles the space between the sleepers must be filled with compacted sand or soil to withstand vehicular brake forces.

Because settlement takes time, the differential measurements for traffic 'comfort impact' and surface level tolerances have yet to be fully evaluated. What is clear is that piled construction is proving to be cheaper with improved levelling.

Drainage tests indicate a surface water run-off time of 3s. Because the system is effectively a bridge structure, water flowing from embankments quickly courses away. The cross bridging elements also allow passage for wildlife.

Enough solar power can be collected from 1km of four lane motorway for conversion into thermal energy to heat/cool the equivalent of 400 family houses.

Water can flow through pipes integrated in the structure and stored in pipes within piles.

Accumulating evidence is proving that it works, is affordable, and an income resource.

Research at the Technical University of Delft found that with air temperatures of 12degreesC to 15degreesC, road surfaces temperatures rose to more than 50degreesC.

With aquifer or deep piled storage and with varying amounts of solar energy (estimates indicate a rapid rise in the next 20 years) annual energy yields of 300kWh/m 2were achieved. So as well as being fast to build, these modular roads can give some payback to clients and have the potential to provide a clean, sustainable energy source.

These 'solar roads' are already being demonstrated at a number of urban locations across the Netherlands. Demonstration projects are being planned for Frankfurt Airport in the coming months.

Poot is aware that within a generation the Netherlands and many other parts of the world will be flooded. 'Deep piling is already part of building construction with operable floors well above the projected rising sea levels, ' he says. 'Our new cities will be built upon piles, on the water.

'Deep piling technology will also bring modular roads that can ride one above the other and still gain the benefit of solar radiation while acting as bridges in a new water-world.'

After a second year of extended tests incorporating any design changes, there is a growing confidence the system will go nationwide in the Netherlands in 2004.

Including all the environmental, social and safety considerations, lower maintenance costs and generated energy payback, a conservative cost estimate is $44/m compared with a conventional asphalt road at $99/m 2-with an additional 20 year design life and minimal maintenance.

In addition to the German trial, two demonstration projects are planned near Turin, in Italy. In the UK, Lord Corbett of Castlevale is advising and helping the London-based energy company Icax to lobby government about interfacing these types of road systems with new energy systems for buildings, and a demonstration project seems likely later this year.

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