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Dutch firm invents 'keyhole' dredging system


A NEW 'keyhole' dredging system allowing removal of clean sand from beneath contaminated ground was launched last month.

Dutch dredging firm Boskalis said its BeauDredge system could be used to deepen channels and lower ground levels in areas previously off limits because of cost and environmental concerns.

The system is crane- or excavatormounted and can be used on land or overwater. The dredging tool is pushed vertically into the ground until it reaches clean material.

It is then rotated and a horizontal high pressure water jetting head breaks up the sand, which is extracted back up the drill rods. Ingoing and outgoing fl ws are balanced, to avoid blow-out or collapse.

Boskalis said ground above the dredged zone, together with any fl ora and fauna, is lowered but left 'largely undisturbed'. Dredged sand can be used to either cap any contaminated soils or elsewhere as a building material.

Development of the technique resulted from Boskalis' decision to revisit some of the fundamental problems arising from dredging works in The Netherlands - where even the disposal of uncontaminated soft material is a major concern.

The study also took into account the need for future large-scale dredging to confront problems such as river flooding.

Boskalis said BeauDredge could be used to lower ground levels and create large water retention basins behind river flood defences.

These basins would store surface water during the winter months, for controlled release during the summer. Dredged material could be used to cap contaminated material and to reinforce defences.

Pilot trials of the BeauDredge system began in 2003 on a site at Vlijmen in the south of the country, to demonstrate its ground lowering capabilities. A second series of trials was carried out in spring 2004, when a water retention basin was built near the Herfterwetering River, now a wetland conservation site. The surface was lowered by an average of 0.7m, with the upper layer of agricultural soils left in place.

A third trial programme - essentially a marine operation - took place at the Ketelmeer inland sea in the centre of The Netherlands in the second half of 2004. A 3m thick clay layer was lowered by 2m over a 10 week period. Turbidity was minimal, with no detectable impact on water quality, Boskalis said.

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