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Cinders, you shall go to the ball

CONTRACTS

A NOISE BUND alongside the A12 motorway in The Netherlands is being stabilised and its contaminated fill sealed off by geosynthetics supplied by German manufacturer Naue Fasertechnik.

The road is one of the country's busiest, running between Utrecht and the Hague. A housing estate is being built alongside it near De Meern and the city of Vleuten/De Meern decided to build a barrier to reduce noise levels for future residents.

Designed by Nieuwegein-based consultant Enviro, it is a 1.5km long, 10m high embankment with a 3m high wall along its crest.

The embankment core comprises 650,000t of cinders from waste incineration plants. Dutch law classes the cinders as a contaminated 'secondary material' which means their use is governed by strict environmental requirements - they cannot come into contact with groundwater.

Enviro had to ensure that after final consolidation, the cinders were not within 0.5m of the highest groundwater level and that rainfall did not enter the embankment core. All installation had to be carried out by a qualified contractor using approved sealing materials.

Sealing and reinforcement work was carried out by Amsterdam-based contractor Cofra.

First, a Bentofix BFG 7000 geosynthetic clay liner and a 2mm thick Carbofol geomembrane made from chemically resistant high density polyethylene were placed directly on the compacted cinders to seal the fill.

The Carbofol has 'friction structures' on both sides to ensure there is enough shear resistance between it and the materials above and below.

A Secutex polypropylene nonwoven protection geotextile was laid on top of the sealing system. This has two functions.

Primarily, it protects the underlying geomembrane against mechanical damage, but it also works to drain surface water permeating through the topsoil layer above.

Finally, a Secugrid 120/40 R6 polyester geogrid was installed to stabilise the 1.5m thick topsoil layer. This has a low creep factor and its monolithic extruded flat strips have high tensile properties, with less than 2% strain, ensuring protection against construction loads. The geogrid was embedded into the fill crest to anchor it in place. Cofra installed an average of 1,000m 2of geosynthetic a day.

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