The Mittellandkanal is one of the most important navigable waterways in Germany, providing an east-west route for inland waterborne transportation. Ships' draughts continue to increase and are currently anything up to 2.50 metres. This has created a problem for the Mittellandkanal at Minden West, "The clearance between the ships' propellers and the canal bed has become much smaller with the passage of time," says civil engineer Andreas Kagel. The bed consisted of a clay seal overlain by a 700 mm thick protective layer of a gravel, overburden and clay mixture. This mixture was in danger of being stirred up by propellers resulting in the clay layer eventually losing its sealing properties by gradual removal of the small clay particles, thus the watertightness of the bed would no longer be guaranteed.

The works undertaken by contractor Josef Möbius, with Andreas Kagel being their site manager, are designed to remove about 400 of the 700 mm and replace this with a partially-grouted stone rubble lining to act as a bed protection layer. This lining consists of 400 mm of armourstone, class II (100-300 mm), placed on geotextile filtermats manufactured by HUESKER. Prevention of future fines loss was achieved by the use of a HaTe nonwoven, type E800/2, tested by the German Federal Institute for Waterway Engineering (BAW). The geotextile used has a demanding role to play. "On the one hand, the nonwoven must be resistant and not tear and on the other it must have filtering properties appropriate to the soils." stated Andreas Kagel. Also to prevent the underlying fine-grained particles from escaping through the nonwoven, the geotextile must have good separation properties. By preventing this migration from taking place, HUESKER's HaTe nonwoven will ensure that the stability of the canal bed is not compromised. Approximately 230,000 square metres of geotextile were installed and covered with 400 mm of stone protection.

A four kilometres length of the Mittellandkanal within the town of Minden was protected in water up to four metres deep. "The urban setting means the width of the canal varies greatly and there are numerous bridges," says site manager Andreas Kagel. In addition, there was a requirement to maintain waterway traffic during the contract period, which ended in October 2003.

BU MLK: The HaTe nonwoven is placed on the bed of the Mittellandkanal in Minden and then protected with a layer of stone. Once rolled out, the nonwoven is lowered on to the bed, where it prevents the washing out of fines from the underlying clay.

BU MLK: The work is carried out in water depths of up to four metres. With the length of canal lying within an urban area, the width varies continuously and there are numerous bridges, making it all the more important that a flexible material is used.