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Turning the Tide

Construction of Venice’s gigantic flood defence scheme is racing ahead. Adrian Greeman reports on the fabrication of the massive concrete caissons that will house the steel floodgates.

Eight years into construction, Venice’s £4bn mobile barrier system MOSE flood scheme is becoming highly visible.

The main founding blocks of the project are huge concrete caissons that will be inserted flush with the seabed in the three channels connecting Venice Lagoon with the sea outside. The three channels are the Lido channel to the north, the central Malamocco channel and the Chioggia channel to the south.

Sitting within cavities in these giant cellular concrete blocks will be the steel flap gates that will rise from the bottom during storms and tidal surges to protect the canal city and its unique architecture, history and irreplaceable artistic heritage.

Gate housings

The gate housing blocks are enormous, the largest rising to a height of 11.5m with a 60m length and 48m width - the size of eight tennis courts. Even the smallest are 36m wide and the same length and height.

The differing widths will accommodate different gate sizes, according to the depth of the various channels. Four lines of gates are needed in total, one each for the two southern inlets and two in the northern Lido inlet, the closest entrance to the cluster of islands making up the old city.

The designer, consultant Technital from Verona, decided early on to divide the Lido channel in two by the formation of an artificial island, now fully dredged and with construction already under way. Conveniently, the island could also be used as the location for all the control and monitoring equipment and for the air compressors and other machinery needed for the gates once they are in operation.

Lido’s Treporti channel, passing north of the new island, is the shallowest with a 6m depth of water.
It will have 21 gates in a row. Each caisson will seat three of the 20m-wide gates, which means seven will be needed. At either end there will be two higher and narrower caissons to form the abutments and give access to the underwater units.

Seven caissons

The other Lido channel, San Nicolò, has another seven caissons plus two abutments. It is slightly narrower and only 20 gates are required, so one of the caissons will be smaller, holding just two gates. But the water is deeper here at 11.5m, to allow the big tourist cruise ships through to the city, and so the gates are longer and the units must be longer too.

In the far south the lagoon’s Chioggia channel will need a 12m depth to allow fishing vessels through to their nearby port of Chioggia. Here, 18 gates are needed, housed in six caissons with again two additional abutment units.

Largest of all the works are for the central Malamocco entrance. This is the most important for shipping with a deep water channel continuing right across the lagoon to the modern industrial port of Marghera. It will have 19 gates in seven caissons with another two abutment units.

“The equipment for the ship lift is literally Rolls Royce standard”

Enrico Pellegrini, GLF site manager

 

All these huge units are now in construction. For the smaller Treporto channel in the Lido inlet, contractor Mantovani is taking advantage of a small 500m long harbour space built as part of the new channel. The harbour is a storm refuge and also a waiting space for a small side lock to allow boats to pass through even during gate closures.

Harbour workspace

“By strengthening the walls and sealing the entrance with a cofferdam, the harbour makes a useful workspace for caisson construction,” says a spokesman for client Consorzio Venezia Nuova. The 9m deep dock will be flooded eventually and the units floated out. A similar temporary dry dock arrangement prevails at Chioggia where a larger lock and harbour refuge will allow the fishing crews to pass. Contractor Condotte d’Acqua is working on these.

For contractor Grandi Lavori Fincosit (GLF), which is carrying out the work in the Malamocco and southern Lido channels “another solution was required”, says GLF site manager Enrico Pelligrini. That was to build the caissons on land and move them, on completion, to the sea, where they will be lowered into the water using a giant ship lift.

“The equipment for that is literally Rolls-Royce standard,” says Pellegrini. It has been designed and assembled by US company Syncrolift, part of Rolls-Royce’s marine equipment division since the 1980s. “But at Malamocco, there was no land suitable for a giant work yard,” says Pellegrini. “It had to be large, because the caissons need a year of construction work. With some 18 to build and install by 2013 they all needed to run concurrently in a single space.”

Reclaimed land

Land had to be made. Conveniently there was a space close to the channel where a new breakwater had been built out to sea as part of the scheme to slow the fastest tides. It could be joined to land with a sheet pile quay wall extension to protect a shoreside space which could be filled.

Casting is complicated as each unit is a highly complex structure in itself with multiple cells and walls to form, all linked together with piping and ducts. These will allow water to be pumped to and fro in the float out stage, explains Pellegrini, to help balance the buoyancy of the irregularly shaped elements.

Once immersed the units will also be filled with water to weigh them down and with concrete in the cells around air-filled tunnels, which are needed for inspection and maintenance.

Reinforcement for the big box-like units is extensive, with a density of up to 350kg/m3 and bars of 22mm to 26mm thickness everywhere. For air/water boundary walls, highly expensive stainless steel bar is used.

So congested is the steel in the cell walls, which are often only 250mm thick and a maximum of 500mm, that ordinary vibrator compaction is ruled out. Instead, a self-compacting concrete mix has been designed.

“It is a difficult mix with tight tolerances,” says Pellegrini, and mainly for that reason the contractor has set up his own batching plant on site even though there is a local ready mix firm nearby. “That also makes delivery very quick,” he adds.

Preplanning

An extensive programme of pretesting and pour trials was carried out, as well as trying out the packed-in reinforcement assembly. Trials included pressure testing a variety of waterstop options for joints. In the end a stainless steel strip with bitumen was selected. “The sequencing of pours and timing to compensate for shrinkage is critical,” says Pellegrini. The yard has to follow a stringent routine.

“One unit would be difficult, but doing all of them on a production line basis makes programming especially complicated,” he adds.

About one third of the units have been completed so far.

Finished rails will be laid under each unit on which will run small hydraulic jack trolleys each capable of lifting 350t.

The trolleys are specially designed versions of Norwegian TTS mobile jacks used in shipbuilding and oil rig work.

About 80 units will be linked by computer to lift the caisson and move it to the Syncrolift.

This too is based on shipyard design with 24-strong rank of 500t winch units each side and a platform of beams between them. These are twice as wide as a normal ship lift and 5m deep to resist the deflections.

Load testing was under way when NCE was on site, but it will be a while before the lift is used for a complete caisson. That has to happen before the gates will be ready for installation in 2013.

But that is a way off yet and Pellegrini says even the precise method for the immersion stage has not yet been worked out. He has enough on his plate for now.

 

Preparing the ground for the caissons

Before caisson work could begin last year there was plenty more to do, GLF site manager Enrico Pellegrini. Each of the four channels has to be prepared and shaped, partly to receive the caissons and partly for tailoring the overall hydraulic regime.

“First work was strengthening and defending the lagoon islands,” he says. The sea edge of the lagoon is little more than sand spits stretching its 50km length and only a few hundred metres wide, narrowing at one point to around just 50m. It was here the sea broke through in 1966’s devastating floods.

New groynes and other sea defences have been made. New additional breakwaters and reinforcements to the existing ones, shelter the sea entrances.

Then each channel had to be dredged to a square profile, from the natural V-shape. At Malamocco dredging will fill in the 16m deep centre, while cutting back the edges to 14m.

3D images

Grab barges fitted with sonar did this work. “And it is amazing how precise you can be with the 3D images you get from modern multi-beam underwater survey equipment,” says Pellegrini.

To protect the new sea floor, giant mattresses were made from aggregate sandwiched in geofabric and steel mesh tied across with steel bar. These are like the seafloor protection used on the Osterschelde barrier on the North Sea coast in the Netherlands.

The 200m long blankets, each 10m across, were made up on land and then rolled onto large floating rollers. A specially designed and built vessel could hold the rollers in Y-shaped arms and then gradually unroll the mattresses across the channel width. A heavy weighted section formed the start point.

But this was easier said than done says Pellegrini. Accuracy in placing the rolls was vital, particularly as they had to overlap by about a metre. “That can be achieved with modern GPS positioning equipment controlling the balance between six anchor cables” he says.

Tide problem

But a major problem for all the channel works is the tides. The water from the lagoon wants to pour through the three narrow entrances and back twice a day, explains Pellegrini, “which can mean currents up to five knots, which is almost impossible”.

For the mattresses, only dead calm would suffice, which limited the work window to just over half an hour on the turn of the tide, just long enough for one mattress. “But you only get one chance,” says Pellegrini.

Above the mattresses a 2m thick layer of rockfill has been placed by grab barges, the material coming from limestone quarries in Croatia across the Adriatic.

This shaped layer extends 200m either side of the gates which have to sit flush in a trench in the middle. Forming and excavating this is the most crucial work.

Sheet piles

The first step was to make a heavy sheet pile wall either side of the future trench. This is being driven by a pontoon-mounted hydraulic pile hammer, driving 30m into the dense clays.

Accuracy is vital to form the trench wall and positioning crucial - even using GPS and four anchor cables it can sometimes take half a day to set up, says Pellegrini. The tide race means the pontoons have been fitted with extendible legs to fix their position once it is done.

Work is complicated by the need to keep the channel open for the big ships. Closure of one side is possible with constant liaison with the port authority, but switching from side to side has been needed.

The inside of the sheet pile walls was next dredged 15m down with grab barges and then comes a second stage of piling. This time concrete piles have to go in, to form a grid pattern across the bed of the trench, some 1,800 in all.

Another purpose-made and fairly large barge has been used for this work which was completed just before Christmas. It carries two track-mounted pile masts with hydraulic hammers, one each side. They could move up and down to cover the grid each time the barge was repositioned.

Pile extenders

Driving the piles 30m down requires pile extenders. The 500m diameter circular precast piles arrived from a local supplier and were delivered by a small barge and lifted out by crane to slide into the pile rig.

Another 2m thick layer of stone goes on to the trench floor and this time must be compacted carefully to form a flat base. The contractor has made a giant plate compactor for this, 3m square with vibrator units and kentledge on top. It will be suspended by barge cables.

Some of the trench piling continues at the edges, but attention is now on the casting yard where the huge caissons began construction last year.

Each is built on a concrete slab above which is a grid of 2.3m high pillars, keeping a cavity underneath to be used later to move the 24,000t units.

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