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Cutting contamination

CONTAMINATED LAND

Stability and protection for the cut-off wall is provided by two parallel reinforced concrete diaphragm walls.

These are 0.8m thick and vary in depth from 4m to 10m, penetrating the top surface of the marl by 1.5m. The walls act as protection for the plastic cut-off in-between, particularly in the alluvium.

In some sections the cut-off wall is protected by an anchored micropiled wall. This Berlin-wall consists of a micropiled curtain - reinforced with single steel pipes and varying in depth from 11m to 15m. It is supported with a row of inclined ground anchors that range in length from 14m to 24m.

Trevi is using a rotary drilling system, since the use of rotary percussion systems is strictly forbidden. As company director Gianluigi Trevisani explains: 'The use of compressed air could cause the dispersion of the contaminated water and dust in the air.'

The slurry wall is excavated deep into the marl using a hydrocutter. A trench is formed 1m across and varying in depth from 8m to 16m, giving penetration into the marl of up to 12m.

This is filled with a plastic slurry mix of water, concrete and bentonite into which the HDPE sheets are carefully installed in panels to create a continuous watertight curtain. The target permeability for the slurry wall is 2 x 10 -8 m3/s.

The drainage diaphragm wall is located immediately upstream - in terms of groundwater flow - of the cut-off wall and the supporting concrete diaphragms. The 1m wide, and up to 9m deep drainage trench intercepts contaminated groundwater and collects it for treatment - so in theory no contaminated water actually reaches the cut-off wall.

It will be excavated under the support of a biodegradable mud and then filled with crushed stone.

Control and monitoring is provided by a network of piezometers placed both up and downstream of the cut-off.

They will provide a check that protection measures are working.

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