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A system of geogrids and gabions is providing robust and unusual support for a new high speed railway in Italy. Alexandra Wynne reports.

Italferr, the engineering company owned by Italian rail operator Ferrovie dello Stato, is responsible for upgrading and renewing the country's main railway network. As part of this agenda, Italferr is in the process of building a new high speed line linking the two cities of Milan and Bologna in the north of Italy.

The new 182km long railway had to be kept close to the country's A1 highway or existing railway line to minimise land take for the railway embankment and avoid, as far as possible, disrupting agricultural land of the Po Valley plain.

However, this created a potential problem because of the close proximity of the railway to the high volume of traffic using the highway. There was a concern that a truck could career off the road and into the railway embankment, causing a major accident.

As a result, the team, which included Rome-based engineer Lotti and geosynthetics firm Maccaferri, came up with a design for a safety embankment using gabions and geogrids for its clients – Italferr and the train operator's procurement arm Rete Ferroviaria Italiana.

Maccaferri project manager Paolo Di Pietro says the solution was different from anything the firm had been involved with previously: "Nobody knew how to do this before we started," he says. "In practice we were about to build something no one had built before and we were using soft organic soils."

These soft organic soils turned out to be one of the triumphs of the project – worth in the region of 12M (£9.6M) to Maccaferri – according to Pietro. The new railway for the most part crosses the floodplain of the River Po, which is the longest river in Italy and runs eastward across the north for about 650km.

The soils in this region comprise alluvial deposits – most of which were considered unsuitable to bear the load of the new railway. To reach stable and harder foundation ground for new railway embankments, site workers removed a large amount of organic soils with low shear strength – with excavations in many areas going 2m deep.

Disposal of these soils could have cost a substantial amount, but the team working on the crash barrier has been able to make use of the spoil as fill material.

The geosynthetic solution comprises a series of Maccaferri Duna barriers (so called because of their resemblance to dunes) designed to absorb high-energy impact. Each comes in the form of preassembled units that are made to measure.

The kit includes a range of layered materials including welded wire mesh on the outside face, twisted mesh, and a combination of geosynthetics to encourage vegetation. The Duna will eventually have an entirely green face on the highway side.

Site workers are installing these units to form embankments ranging in angle from 60˚ to 80˚ from the horizontal on the highway side. On the railway side the angle is between 60˚ and vertical.

Each is preformed in a flat pack so that when the crew is ready, a unit can be opened up, with the angled face on either end held in position by tie rods. The fill material is then compacted inside the unit.

This phase is repeated to build up the barriers, which have a maximum height of 4.5m.

Maccaferri began its work to install the system in September 2004 along an 8km stretch near Parma (the middle section between Milan and Bologna). The results of the first installation led to further contracts being awarded to the company for more barriers, which flank the highway and railway for a total of 50km.

The Duna system is interrupted in places to allow for infrastructure such as existing drainage systems. As a result, the ends of each block, or shoulders, are reinforced with geogrids that tie into further reinforcement in the form of stone-filled gabions.

This is to help with resistance to tearing forces if and when a vehicle crashes into the barrier. But Pietro says that because the Duna is installed in sections this also helps with maintenance – if one portion is damaged, it would not impact on other portions and can be replaced.

Maccaferri is expecting work on the barriers to be complete by December.

Click here for figure: Typical isometric view of Duna barrier

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