The Rivierenland Water Board invited two groups (Boskalis/Cofra and Van den Herik) to develop geotextile screen technologies that would be specially designed to prevent piping under levees. Piping is a phenomenon that occurs in the case of abnormally high water levels, wherein water seeps under a levee and causes the migration of fines. This action can weaken a levee and contribute to failure.
Both companies were given a 500 m section of levee in which to install a vertical geotextile screen system to stop sand transport but allow seepage water to flow through.
On 8 July, Boskalis/Cofra launched the world’s first anti-piping technology based on these geotextile screens. Van den Herik will follow with a different geotextile-based technology in two months.
VIDEO: Rivierenland Water Board animation of piping phenomenon and filter design
GROWING CONCERNS OVER PIPING
Ten years ago, piping undermined the concrete I-walls along New Orleans’ drainage channels when Hurricane Katrina hit the city hard. The event put the issue of piping high on the agenda of Dutch water authorities. Subsequent levee inspections revealed how many sections were vulnerable to this phenomenon.
The Rivierenland Water Board has many clay levees along large rivers, such as the Rhine and Meuse. These levees rest on sand layers. Thus, they are vulnerable to piping.
The traditional strategy to prevent piping is to widen the toe of the levee or by constructing a very deep sheet pile wall.
The Water Board studied cheaper and less space-consuming alternatives. It experimented with buried geotextile screens to stop sand migration but allow for seepage flow.
INVITATION TO CIVIL ENGINEERING SECTOR
Convinced of its potential, the Water Board invited Dutch civil engineering companies to develop a geotextile technology for anti-piping. As a result, two companies have now been invited to place their screens over a length of 500 meters. Boskalis/Cofra has been the first to place such a screen.
The water board will monitor the performance of the screens with glass fiber cables that have been attached to the geotextile. The cable measures the temperature of the seepage water; in the case of a free flow, the water temperature will be low. In the case of clogging of the geotextile by sand, the water flow will stop and the temperature will rise.
MODULE SYSTEM GEOTEXTILE SCREENS
Boskalis and Cofra developed a modular system of geotextile plates that can be assembled into one screen. First, a trencher digs a very small trench in the toe of the levee. One by one the geotextile plates are installed and locked together in underground.
This technology has the advantage that the screen can be placed with precision. This space-saving solution disturbs less land, making it both more sustainable for construction practices and practical for use in densely populated or limited access areas, such as installing the screens near houses and roads.
GEOTEXTILE FROM THE ROLL
Van den Herik, which will install its technology in two months, has developed a technology with a special designed mobile construction that houses both the trencher and the large geotextile rolls.
This machine can put the screen in place continuously, making it much faster.
Another advantage of this technology is that it uses geotextile on a roll that can be supplied by any producer.
ADDITIONAL RESOURCES
Boskalis Nederland
http://nederland.boskalis.com
Cofra B.V.
www.cofra.com
Dutch Water Sector
www.dutchwatersector.com
Van den Herik
www.herik.nl
Water Board Rivierenland
www.waterschaprivierenland.nl
**
A version of this article first appeared on www.dutchwatersector.com. Thank you, Bianca Dijkshoorn, for helping arrange this special publication.
