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SETTLINGDIFFERENCES

FLOOD PROTECTION - Legend tells that English monarch King Canute attempted to hold back the tide to show sycophantic courtiers the limits of his power. Damon Schünmann went to Venice to witness Italian efforts to protect the city from the sea.

The troubled city of Venice faces two major problems. Everybody knows it is sinking, and most believe global warming is causing rising sea levels.

The Adriatic city is fighting a war on two fronts. Put together they have caused a 230mm loss in land level over the past 100 years and worsening high waters now reach the Piazza San Marco about 250 times per year.

So far the Venetians and the government have responded by raising street levels, but the historic architecture that draws in millions of visitors now hampers protection efforts.

'We can't raise the pavements too high as it would cover the bases of the columns and facades, ' says protection consortium Technital public relations officer Monica Ambrosini.

So with Venice going down and tides going up, a more far reaching solution is needed. Work is now centring on the three inlets to the lagoon within which the city sits, as part of a grand protection plan.

The inlets comprise Chioggia in the south, centrally located Malamocco and Lido in the north. Because Lido has two navigable channels - separated by an artificial rock island - this inlet is further divided into Treporti at the north end with San Nicolò below.

In each of these areas, Technital is building buoyant fl ood gates as part of a flood defence and lagoon restoration programme. Placed side by side on the sea bed, they will be raised by pumped air during extreme high tides to control lagoon water levels, an occurrence predicted to happen three to five times a year based on current sea levels. A hinge at one end will keep them anchored to their housing. The defence is designed to cope with a 2m water level difference between the lagoon and the sea.

Technital has planned work on the inlets in three phases, with the first - under way since May 2003 - now almost complete. This involved enabling work, trial areas for seabed consolidation, underwater archaeological surveys and checks for Second World War ordnance.

Phase two includes building gate abutments, ground improvement work and lock construction (for when the gates are raised). The final phase will see the building and installation of the gates as well as the caissons on which the gates will sit.

Work on the locks was well under way at the time of GE's visit and when complete, those at Lido and Chioggia will allow small craft to pass, while the Malamocco inlet will allow large ship transfer.

The chemical works on the mainland is the destination of the majority of the 5,000 ships using the Malamocco passage every year - equivalent to 10,000 crossings. Ambrosini says negotiations with the port authority resulted in agreement for this channel to be raised from 16m depth to 14m. This will still allow large ships to pass, but the shallower cross section will help reduce tidal speed at this point.

The large Malamocco lock will have internal dimensions 380m long by 50m wide to accommodate commercial vessels up to 280m long and 39m wide with a 12m draught.

Contractors are also building breakwaters from riprap and acropods at all three inlets measuring 520m, 1280m and 1000m respectively from south to north.

Before the gates can be built, the soft strata must be prepared for its future loading.

Technital president Alberto Scotti says: 'There are finely distributed layers down to 100m including sand, silty sand, clayey silt and silt. But the behaviour of the soil is more or less uniform so we consider it all as one soft layer of silty clay.

'This is important because the settlement is fast - a matter of months.

So with this type of soil we decided not to do any real foundations. However the caissons supporting the gates are big, 60m long by 45m wide and 11m high, and are rigid structures.

While we can accept some settlement it must not be excessive, otherwise the gates will hit each other.' Although the caissons will have compensated foundations where the weight of soil dredged is greater than that of the caissons and gates, this will not ensure protection from deformation. So the consortium has had to plan ground improvement for the areas beneath the caissons.

Contractors are now deciding which of three techniques will be used, depending on the various design loads. Being considered are cylindrical steel piles, precast concrete piles and jet grouting.

An advantage of the closed bottom end steel piles is that being driven inside casings, they can be installed before site workers dredge the 12m deep channels to accommodate the caissons and gate housings.

Both the steel and concrete piles will be 400mm diameter at the top but the latter will be conical, reducing in width to 280mm by the end of their 29m length.

The downside was Technital's concerns over the potential for punching shear piercing the bottom of the caissons and the possibility of cracking the concrete piles during driving, particularly at Chioggia where there are firmer sand layers.

But the design team is now confi dent that neither will hinder the project, with testing proving the cracking fear to be unfounded and any punching shear catered for with a load-distributing gravel blanket between the piles and the caissons.

Technital intends to drive the steel piles from their closed tips using a technique more commonly used for offshore platforms. After this process that Scotti calls 'submarine hammering', it will close the tops.

'The jet grouting had a question mark because it is normally a solution for granular materials, ' he says.

'On silty clay it could be a problem reaching diameters greater than 1m.

Cohesive materials make it hard to get beyond 600mm to 800mm unless there is very great pressure.

'We wanted to test continuity and how diffi cult it would be to recover the polluted residual materials that are normally present with such a solution; mud, cement and bentonite are all considered pollutants that we could not leave in the lagoon.' Although testing showed 1.4m diameter columns were achievable with good continuity, hydraulic waste removal to shore would still be difficult. So Technital made the decision to use jet grouting at only one site, Treporti, the northern of the two Lido channels. Here, the working depth of only 6m makes it easier for divers to operate compared to the other deeper inlets.

Whether all this work, scheduled to fi nish in 2011, will succeed in stemming the tide remains to be seen. But the Italian state has defined safeguarding Venice and its lagoon as a 'primary national interest' and the e4bn (£2.7bn) now earmarked for the project marks clear intent to preserve the historic city.

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