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Good news for shipping

GEOSYNTHETICS

Hydraulic engineering products have helped secure the bed of the important Mittellandkanal in Germany.

The Mittellandkanal is one of the most important waterways in Germany, providing a vital east-west transport route.

But changes in the shipping using it are causing problems on the canal bed. Ships' draughts are getting deeper and can be up to 2.5m.

'Clearance between ships' propellers and the canal bed has become much smaller with the passage of time, ' says engineer Andreas Kagel, site manager at Minden West in central Germany where remedial work is being undertaken by contractor Josef Möbius.

The bed consisted of a clay seal overlain by a 700mm thick protective layer of a gravel, overburden and clay. This mixture was in danger of being stirred up by propellers, and the gradual removal of the small clay particles would mean the clay layer would eventually lose its sealing properties and the watertightness ofthe bed would be undermined.

Josef Möbius designed the project to remove about 400mm of the layer and replace it with a partially-grouted stone rubble lining.

The lining is made of 400mm of armourstone, class II (100300mm), placed on geotextile filtermats manufactured by Huesker.

Prevention of future fines loss was achieved by the use of a HaTe nonwoven, type E800/2 geotextile, tested by the German Federal Institute for Waterway Engineering (BAW). The material has a demanding role to play.

'On one hand, the nonwoven must be resistant and not tear and on the other it must have filtering properties appropriate to the soils, ' says Kagel.

'To prevent the underlying fine-grained particles from escaping through the nonwoven, the geotextile must also have good separation properties.'

About 230,000m2 of geotextile was installed and covered with 400mm of stone protection to protect 4km of the Mittellandkanal.

'The urban setting means the width of the canal varies greatly and there are numerous bridges, ' adds Kagel. Waterway traffic had to be maintained during the contract period, which ended in October 2003.

Defending the dykes

Great floods on the Oder in 1997 and Elbe (2002) saw the German emergency services struggling to seal dykes against the huge volumes of water, and concerns were expressed over the dykes' long-term efficiency.

As well as the obvious problem of overtopping, the structures could also become saturated, allowing water to simply flow through them.

Many of the dykes were built by residents long ago, using local building materials such as sand, earth and building rubble.

The Rotbach River at Dinslaken, near Essen in western Germany, was one area identified as a potential flood risk.

Site manager Markus Böhmer, from the river catchment water authority client the Lippeverband, says: 'There are active coal workings in this region.

Were an abandoned gallery to collapse somewhere, the whole area would subside and the beck could spread out to fill the depression.'

To prevent this, the Rotbach has to be kept within its channel. The difficulties being faced were similar, but not as extreme, as those on the banks of the Oder. But because the Rotbach dyke was built from colliery waste, it required additional protection against long-term water penetration.

A NaBento RL-N geosynthetic clay liner from Huesker was used in the works. Böhmer says: 'The liner seals off the colliery waste tailings so that they can no longer become saturated.'

About 15,000m2 of the liner were installed along almost 4km of the Rotbach, which flows around the town of Dinslaken and is mostly surrounded by fields and paddocks.

Foundation soils in the area of the embankment, prepared by local contractor Heijmans Bau had to remain smooth and level, so it was not possible to drive or walk on them. However, Huesker's liner dealt with this problem.

'You should think of it as a carpet, which needs to be laid on a perfectly flat floor, ' explains Böhmer.

Sealing the joints in the liner with adhesive or bentonite powder or paste after placing, would not be possible, so it has 500 mm of bentonite pre-applied along the edges.

'It is now established that a geosynthetic clay liner, in this case NaBento, can play its part in the protection against flooding catastrophes, whether in North RhineWestphalia or on the banks of the Oder and the Elbe, ' Böhmer says.

The work was completed at the end of last year.

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