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Scheldte terminal builds on revolutionary system

LEADING EDGE ground improvement techniques have been used in preparation of a section of rail track being laid to Antwerp's new Scheldt North Sea terminal in Belgium. In its final approach to the new terminal, the track crosses over sludge lagoons, infilled with 8m of super soft and saturated silt, dredged from Antwerp's harbour.

Faced with expensive removal costs, Belgium based DEME Group called on the services of its operating companies Hydro Soil Services, Silt and Dredging International, to develop an alternative approach. While the resulting solution involved removal of nearly half of the silt sludge, it also required insitu consolidation of the material, which has not been attempted on such a scale before.

Technical complexities were aggravated by a demanding schedule, in which site preparation had to be completed within six months.

Variations in soil in particular the presence (or not) of a sand layer underlying the lagoons - led to the use of both vacuum drainage and stabilisation by stone columns. Vacuum drainage using vertical sand drains was used over 34000m2 - approximately two thirds - of the linear site, where the natural sand layer was present.

The stone column ground improvement was used in the remaining 13,000m2.

The consolidation project was developed in association with the University of Ghent. On site work began with placing a 1m thick granular layer to form a safe working platform for the necessary heavy plant. From this vertical sand drains, hermetically sealed at their tops, were installed at 2.75m spacing, reaching down to the natural underlying sand layer at 10-12m depth.

Pumping out water migrating through the drains to this sand layer created a pressure gradient, resulting in forced accelerated consolidation of the silt which reached its required density in just three months.

Where the deep sand was not present, 800mm diameter stone columns were installed on a 2.5m grid, with installation taking place from a 1.5m thick granular blanket which doubled up as a surcharge and working platform.

This network of stone columns, together with the overconsolidation created by their installation and the modest surcharging from the platform, created adequate bearing capacity in the soil. The target was to achieve a bearing capacity of 28kN/m2 and a minimum compressibility coefficient of the top layer of 17MPa, both of which were achieved on schedule.

Performance was assessed using a gigantic plate bearing test. This involved testing the treated ground using a huge 40t, 10m by 4m rectangular reinforced concrete raft.

At six locations across the site this was loaded up to 110t, which resulted in a maximum settlement of 25mm, half of that calculated to be acceptable.

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