Quality improvement of dredged sand on board a trailing suction hopper dredger.
Type:
Presented during:
CEDA Dredging Days 2005 - Dredging: The Extremes, Rotterdam
Authors:
Vlasblom W.J., Hoff J. van 't, and Rhee C. van
Abstract: As part of a feasibility study to produce concrete and mortar sand out of sea-sand by the sub- working group “PIA-sand” a research was initiated to the possibilities of pre-separation of the coarse fraction of a suitable sand within the sedimentation process onboard of a trailing suction hopper dredger (TSHD) dredging in the North Sea.
The hopper of the TSHD was divided in a short and long compartment. Sedimentation of the coarse material could take place in the short compartment, while via an overflow the finer fraction could settle in the long compartment.
At the North Sea the geological formation of Kreftenheye consists of medium and coarse sand and is overlain by a Holocene overburden of fine sand. It was assumed that this overburden was removed beforehand. On the basis of a number of characteristic particle size distributions the sedimentation process was calculated as well as the mean particle size distribution in both compartments.
Two scenarios were investigated, one for a large TSHD and one for a medium TSHD. It appeared that for the production of concrete sand out of sea-sand, the fraction 2-8 mm was the limiting factor. The condition that 80 to 90 % of the separated sand must fulfill the condition for concrete sand could only be met when selected coarse layers were dredged and a second pre-separation on shore was used to extract coarse fractions. The latter was necessary to avoid needless processing of fine material in a separation installation onshore. Calculation showed that concrete sand produced out of sea-sand via a pre-separation on board of a hopper dredger was about 2 times more expensive than the market price, without pre-separation onboard is was about 2.5 times more expensive.
Although these calculations are based on recent sedimentation theories it should be kept in mind that these processes are outside the present experience.
Although onboard separation of sand during the sedimentation process for producing concrete sand is expensive it might be interesting for upgrading sand for civil applications where coarse and fine material both are necessary.
Keywords: quality improvement, dredged sand, trailing suction hopper dredger, PIA sand