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Coastal connection

Highways; Interchanges; Redesign of a complex new motorway interchange in Savona on Italy's Ligurian coast has achieved a 30% saving in project costs. Paul Wheeler reports.

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 (£6.8M). Bonatti, by redesigning the project with consultant Lombardi Italia, reduced the cost to £4.7M. This almost 30% saving was achieved 'without of course, comprising quality', adds Bonatti's technical supervisor or geometra Gianni Fornasier. Given that 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 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 managing and 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 also cut across 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 only 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. Where cuttings are required and space permits, these are excavated at an optimistic one on one slope.

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

Cast in place concrete panels provide the final finish.


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