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Coastal connection

talian contractor Bonatti's redesign of a complex new motorway interchange in Savona on the Ligurian coast has achieved a 30% saving in project costs

A road atlas would suggest that links on the coastal motorways between France and Italy as far as Turin are well established. But a downside of privately operated toll motorways is that they do not always connect well, especially when different operating companies are involved.

A major bottleneck lies at Savona, a large industrial port and modern resort town 30km west of Genoa, where the coast-hugging east-west autostrada and the north-south Savona to Turin toll link converge.

With three different concession companies, no proper interchange for the approaches was constructed when the roads were built in the 1950s. While through traffic to Genoa and France flows without disruption, on other routes drivers have to leave the toll road, pass through toll booths, traverse a short section on local roads and rejoin the next concession through a second toll gate. At peak times this creates traffic gridlock into the local town.

Computerisation of toll collection and accurate calculation and breakdown of traffic flows mean it is now possible for the three operators to agree the appropriate distribution of toll revenues - without having to divert traffic through the toll booths.

This has given the green light to construction of a fully connected interchange that will allow unimpeded access in all directions.

Italy's Bonatti is contractor for the scheme which involves two major viaducts plus extensive earthworks and cuttings. Client Autostrada Genoa-Savona, and its design and supervising body Spea, originally costed the project at Lire 20,000M (US$10.5M). Bonatti, by redesigning the project with consultant Lombardi Italia reduced the cost to Lire 14,000M. This almost 30% cost saving was achieved 'without of course, comprising quality', adds Bonatti's technical supervisor or geometra Gianni Fornasier. Given the contract includes a 10 year maintenance liability, it is clearly in Bonatti's financial interest that quality is ensured.

One of the biggest construction issues is space, which in places is extremely limited. The work has not only to fit in with major changes in level and awkward road alignments, but it is also being squeezed in, around and above the municipal sewage treatment works.

As a result construction, which is currently two years into the two and a half year programme, has involved a challenging mix of earthworks, retaining structures, foundations and superstructure.

On top of this are the logistics of minimising disruption to traffic, which has meant 15 different temporary road layouts, some involving construction of substantial temporary roads.

The most dramatic elements are two 340m long viaducts on either side of the existing through section.

The viaducts cut across existing sliproads and ramps, which are to be demolished at a later stage.

The two 10m wide carriageways are supported on Y-shaped piers, up to 15m in height, and formed in 3m lifts.

Modular precast beams and decking panels span the piers. They provide the foundation for the poured concrete deck and the porous asphalt surface.

Steel and teflon plates and bearings accommodate the complex thermal expansion movements between the piers and the deck and its adjoining curved access ramps.

Some of the piers are positioned alongside the filtration tanks of the city's sewage treatment works. They are founded on 200mm rotary bored minipiles, which vary in length between 8m and 28m depending on the variable clay and silt conditions.

Piers have either a 5m by 8m footprint supported on 40 micropiles or a larger 7m by 10m base supported by 70 piles.

Parma-based foundation contractor CCP installed minipiles with a design working load of 600kN.

During static load testing, carried out by Genoa-based Edil Control, individual piles were typically settling just 10mm at one and a half times working load, with half the settlement due to recoverable elastic settlement in the pile itself.

Bonatti's redesign included a greater element of earthworks than the client's original scheme. In particular efficient use of cut and fill, minimised the volume of offsite disposal and reduced the need for expensive concrete structures.

Essentially traffic on the Turin and France-bound arms will run on earth embankments formed at slope angles of two vertical on three horizontal.

Cuttings, where space permits, are excavated at an optimistic one on one slope.

Access to the Genoa section runs in cut up to 8m depth. Retaining walls are constructed with 140mm diameter steel tube minipiles, spaced at 200mm centres. The cutting is then excavated. Three rows of ground anchors, at 3.6m horizontal spacing provide additional support.

Cast in place concrete panels provide the final finish.

MARINE MIX: This highly corroded pier is on one of the original access ramps and will be demolished to make way for one of the new viaducts. Concrete spalling on this and the adjacent pier is more significant than on the other 1950's built sections of the original structure. Both piers are orientated obliquely to the carriageway they support, which is on a tight curve and incline. Heavy traffic creates significant live torsional loads in the piers, which may explain the damage in these areas.

Nevertheless the project is just a few hundreds of metres inland and the harsh marine environment means special measures have been used to ensure concrete quality on the new structure.

All structural reinforcing steel has a minimum 50mm cover and the use of additives produces a low water content.

Concrete mix was designed by testing house Edil Control, which also carries out cube testing, providing onsite quality checks. To work on public works in Italy materials testing companies need government authorisation.

An extremely smooth finish is achieved by using veneered shuttering.

This minimises the likelihood of superficial fine surface cracking.

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