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Deeper and down
30/4/04
Having excavated 66,000 square metres of diaphragm wall trench, Italian contractor Rodio filled it all in, just so that it could dig it out again. Paul Wheeler finds out why.

Italian contractor Rodio’s project to contain heavily polluted soil and groundwater at a petro-chemical plant in Brindisi on the Adraitic coast is remarkable simply for the scale of the work.

Its two-year, Euro 10M contract with Enichem is to isolate two zones, 16ha and 5ha in area, which are contaminated by non-volatile hydrocarbon, solvents and heavy metals. Perimeters around each of these two containment areas are 1550m and 850m respectively – but it is the depth of the cut-off that makes the project stand out.

A suitable impermeable layer to create a basal cut-off is between 25-32m deep. That means in the process of constructing the perimeter seal Rodio has to excavate 66,000m2 of diaphragm trench and install the same quantity of HDPE membrane within the self-hardening mud-filled trench.

Why the need for the liner, given that a mineral barrier alone should be able to achieve the 1×10-9m/s permeability required in the technical specification?

Firstly it’s not just about permeability. The cut-off’s specification is also very severe in terms of deformability and homogeneity.

Rodio’s Raffaella Granata also believes that: “Although conventional cut-off walls constructed around waste disposal or polluted areas can reduce leachate migration, over time construction defects and residual permeability mean contaminants can seep through.”

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