Environmental remediation of beaches and seabed contaminated sediments

Challenge

Located in the southeastern part of the Port of Naples, the work area at San Giovanni a Teduccio is within the perimeter of ‘Napoli Orientale’, a site of national interest. Preliminary investigation has shown the presence of hydrocarbons and heavy metal, which contaminate both the sediment from the beaches and from the seabed. The contaminated material has been qualified as Red (‘Hot Spor’), with concentrations in the sediment in excess of 90 percent of limit values; Orange/Yellow sediments with concentrations lower than 90 percent of the limit values but above the target values; Green sediments with concentrations lower than the target values.

With respect to the remediation of this coastal area, Sviluppo Italia Aree Produttive S.p.a. awarded this design and build project to SIDRA, a subsidiary of DEME. The contract was signed on behalf of Commissario di Governo Bonifiche e Tutela delle Acque nella Regione Campania, and all operations were approved by the ministry of the Environment. The contractor proposed various technical improvements, such as the construction of a cement-bentonite wall (called ‘Plastic Diaphragm’) instead of a sheetpiles wall for the design of the confining system, and the assignment of a Mobile Soil Washing Treatment Plant for re-using part of the highly contaminated sediments in the CDF-filling. 

Our solution

By way of excavation and placing of a self-hardening mix of cement and bentonite, a 750 m long wall, called ‘Plastic Diaphragm’, was constructed in the first place. This impermeable barrier along the water edge was needed to prevent groundwater contamination spreading in the marine environment. A casted armor concrete beam was constructed on the head of the diaphragm. To give the diaphragm wall a bearing function, in view of the future use of the area as a marina, armor concrete sheetpiles were in some parts put inside the cement-bentonite mix as structural elements. Behind the wall, a draining trench was constructed inside which a pierced plastic pipeline was inserted as drainage system.

A 190.000 m³ Confined Disposal Facility (CDF) was constructed for the final disposal of 90.000³ of Orange and Yellow-qualified sediments. The fill was executed  by a Cutter Suction Dredger, pumping directly the sand in the CDF. For the construction of the CDF, some 780 m of iron sheetpiles were driven in the sediment layers until tuff bedrock. A tie bar system, anchored to special contrasting structures, improved the stability of the CDF-walls.

Some 1.250 m³ of ‘Hot Spot’ Red sediments were highly selectively removed by environmental mechanical dredging. This was carried out with a cable crane with a 4 m³ Eco-Grab watertight hydraulic clam shell, mounted on a self-propelled pontoon. Precautions were taken to prevent turbidity and a silt screen was used. Since the high concentration of contaminants could not directly be pumped in the CDF, a Soil Washing Treatment Plant separated the contaminated fines from the clean sand, in order to reduce the quantity to dispose in the CDF. After mechanical dewatering and analysis, rest fractions were disposed in a non-hazardous waste landfill or re-used – depending on the results. All process water was treated. A thorough environmental monitoring was carried out before, during and after the environmental dredging and treatment.

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