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Getting in step

Access to position the piers was one of the most complex and time-consuming operations during construction of a viaduct on the Turin to Savona road as Paul Wheeler discovered

The Turin to Savona toll road runs through the stunning Alpine scenery of Piemonte in northern Italy. The route passes regularly from viaduct to tunnel and back to viaduct as it hugs the narrow, steep sided mountain valleys.

The terrain is so demanding that much of the 1950s constructed route, although a designated tolled autostrada, is only single lane in each direction.

A series of projects concentrated in the most mountainous sections, will upgrade the entire route to full motorway standard within the next few years.

This, together with the improved new interchange at the road's southern end in Savona (see below) will significantly improve Turin's road links with the Italy's northern ports and the French coast.

One of the most striking contracts is 30km north of Savona, where contractor Garboli-Conicos is currently constructing two high-level viaducts traversing two valleys, linked by a 68m long steel deck that crosses (essentially at ground level) the ridge dividing the two viaducts.

Largest of the two main structures along the 800m contract is the 493m long Viadotto Zemola Nord, which cuts obliquely across a narrow wooded valley, the floor of which is steeply inclined.

The viaduct is being constructed from eight slipformed concrete piers, many of which appear to rise from impossible locations mid-way up the valley sides.

Work started two years ago and is due for completion this summer. Until quite recently visible progress on site was slow. Conicos assistant engineer Gabriella Mellogno says the biggest problem was getting started. 'When we arrived we couldn't get access at all. We were faced with a wooded mountainside, no access tracks, nothing, ' she says. 'You only have to look across to the opposite side of the valley to get a feel for the task we faced', says Mellogno, pointing to the dense and impenetrable terrain.

Even before an access track could be established, there was much work to be done. The new viaduct is beside and downslope of the existing carriageway and a major retaining structure was needed to prevent the new works undermining the old carriageway. This would also establish access to the pier positions.

Specialist foundation contractor Consonda constructed a 168m long, and up to 14m high micropiled retaining wall. It used steel tubes, 180mm in diameter, placed in 220mm diameter rotary cored bores sunk to around 8m to 10m below excavation level. The annulus surrounding the steel tubes was then filled with a sand and grout mixture.

With the micropiles in place, excavation started in front of the wall. As the retained height was increased, rows of ground anchors were installed typically at 3m vertical intervals, and at a horizontal spacing of between 2m to 4m.

Four strand anchors were needed near the top of the wall, while lower down an additional strand increased anchor capacity.

Wall stability relies on good drainage to reduce water pressure at the rear.

This has been achieved through a series of shallow upwardly inclined drains drilled 9m to 12m into the face at around 10degrees. Each hole is filled with a geotextile filter protected plastic slotted pipe.

Construction and excavation in front of the wall finally gave access to the pier positions almost one year into the contract. Again because of access limitations, pier foundations use micropiles, since these can be installed using small, compact drilling rigs.

Depending on loads, between 60 and 110 micropiles have been installed at each pier position, generally to a depth of 19m.

At three locations, deeply dipping superficial deposits across the pier footprint required a different foundation solution. It is thought the alignment runs across the edge of an ancient infilled river channel.

The main concern here was lateral confinement of the piles in the ground.

Project designer Lombardi Italia came up with an alternative foundation. Sixty micropiles were installed in a 9m diameter ring, with each pile reinforced by a steel I-beam. Excavation inside the piles produced a heavily reinforced cofferdam that was then filled to create a huge mass concrete foundation to support the slip-formed piers.

With all piers complete, the balanced cantilevered construction road deck is beginning to take shape.

Sections are cantilevered out symmetrically from each pier position so that each pier is balanced in static equilibrium, with the centre of gravity remaining over the centre of the pier base throughout deck construction.

Massive post-tensioned cables running through the deck segments hold each 5m section in place and transfer the load back to the pier. Each segment is held by a new set of cables. This process extends to within 0.5m of the centre of the span on each side when a 1m key section is dropped in. The underside of each deck section is shaped to create a shallow arch profile in longitudinal section.

However there is still much to do to meet the summer project completion target - especially given the short and harsh winter days the construction team currently faces.

Work on the taller - although shorter in length - Viadotto Magnine, is marginally behind its bigger brother. Construction process is the same, and at the time of NCEI's visit, work had yet to begin on the deck.

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