Venice faces two major problems - it is sinking while at the same time global warming is causing sea levels to rise. The mean tide mark has risen by 230mm in the past century, and the Adriatic floods the city's most famous square, Piazza San Marco, about 250 times a year.
Venetians have fought the upwardly creeping waters by raising street levels, but this is now treading on the toes of the famous architecture that draws in millions of tourists and keeps the city's economy afloat.
'We can't raise the pavements too high as it would cover the bases of the columns and facades, ' says Monica Ambrosini, public relations officer for protection consortium Consorzio Venezia Nuova (see box).
Work on a more dramatic and far-reaching ó4bn ($4.67bn) protection plan is now gathering pace. Venice sits within a lagoon, partially cut off from the open sea by two long, thin islands (see photograph). Plans were approved in spring 2003 to install gates across the inlets to the lagoon, plugging the inward flow of water whenever higher tides than normal threaten.
From north to south, the inlets are Lido, Malamocco and Chioggia. Italian consultant Technital, which forms part of the Consorzio Venezia Nuova, has designed a system of buoyant, hinged flaps that will lie in concrete caisson housings on the sea bed most of the time. In anticipation of a flood, compressed air will be blown into the gates, bringing them bobbing to the surface. Although not completely watertight, the defensive line will be able to hold back a 2m height differential between the Adriatic and the lagoon.
Construction of the defence scheme is being carried out in three phases, with the first - under way since May 2003 - almost complete. This has involved enabling works, seabed consolidation trials, underwater archaeological surveys and checks for World War II ordnance.
Phase two includes building gate abutments, ground improvement work and construction of locks, which will permit shipping entry to and exit from the lagoon when the gates are raised. The final phase will see the building and placement of the caissons which will house the gates, and installation of the gates themselves.
Before the caissons can be built, the soft seabed must be strengthened. 'There are finely distributed layers down to 100m including sand, silty sand, clayey silt and silt, ' says Technital president Alberto Scotti. 'But the behaviour of the soil is more or less uniform so we consider it all as one soft layer.
'With this type of soil we decided not to do any real foundations, as settlement is fast, but complete within a matter of months. However, the caissons supporting the gates are big, 60m long by 45m wide and 11m high, rigid structures. We can accept some settlement but it must not be differential, other wise the gates will hit each other.' To reduce the amount of settlement that is likely to occur 'compensated foundations' are being used, says Scotti.
Basically, material will be dredged, allowing the caissons to be recessed into the sea bed.
But because this alone will not ensure against movement, the consortium plans to carry out ground improvement beneath the caissons.
Consorzio Venezia Nuova is now deciding which of three techniques to use where - cylindrical steel piles, precast concrete piles and jet grouting.
Tubular steel piles with closed bottom ends can be driven before dredging of the 12m deep rebates needed to accommodate the caisson gate housings.
They would be driven using a 'bottom of the hole' hammer installed into the pile and acting against the closed tip.
Alternatively, concrete piles tapered over their 29m length from 280mm diameter at the toe to 400mm at the head could be driven into the sea bed.
The taper would help the piles penetrate the sea bed before developing the right bearing capacity through skin friction.
But the downside with both solutions was the potential for punching shear piercing the bottom of the caissons, and with the concrete piled solution, the possibility that the piles would crack during driving, particularly at Chioggia where there are firmer sand layers. Trials have shown Technital's fears about cracking to be unfounded, while punching shear can be eliminated by placing a blanket of aggregate between the pile tops and the caissons.
Jet grouting was also considered. 'This had a question because it is normally a solution for granular materials, ' Scotti says. 'In 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 see how difficult it would be to recover the polluted materials arising with such a solution - mud, cement and bentonite are all considered pollutants that we could not leave in the lagoon.' Although testing showed that 1.4m diameter columns were achievable with good continuity, hydraulically removing waste to shore would still be difficult. So Technital made the decision to use jet grouting at only one site, to the north of the Lido channel.
Here, a working depth of only 6m makes it comparatively easy for divers to operate equipment.
Whether the work, scheduled to finish in 2011, will succeed in containing the tides remains to be seen.
Locks the key
Construction of the locks for shipping is well under way. When complete, those at Lido and Chioggia will allow small craft to pass, while the Malamocco inlet will accommodate an estimated 10,000 ships of up to 5,000t per annum, plying trade with the chemical works on the mainland. The Malamocco lock will measure 380m long by 50m wide.
Draft has been kept to a 14m minimum to limit the throughflow of water. Contractors are also building breakwaters using riprap and accropodes at all three inlets, measuring 520m, 1,280m and 1,000m long at Chioggia, Malamocco and Lido respectively.
In all there will be 78 gates, all 20m wide but ranging in height from 18.5m to 29.6m, and in depth from 3.6m to 5m. The gates are divided between four channels.
Lido-Treporti 21 Lido-San Nicolò 20 Malamocco 19 Chioggia 18
Members of Consorzio Venezia Nuova
Technital Impresa Costruzioni E Mantovani Spa Fip Industriale Mazzi Scarl Sociatà Italiana Condotte d'Acqua Astaldi Mantelli Estero Costruzioni Grandi Lavori Fincosit Grassetto Lavori Sacaim Intercantieri Vittadello Cantieri Costruzioni Cemento Consorzio Rialto Consorzio Lepanto Saipem Impregilo Consorzio Grandi Restauri Veneziani Barbato Impresa Costruzioni Errico Costruzioni e Restauri