The Koralm connection


Involving tunnelling through 33km of mountain, the Koralm railway tunnel is set to become the longest entirely within Austrian territory.

Due to open in December 2023, it will not only help to connect northern Italy to north-eastern Europe, but will also form part of the 2,400km-long Trans-European Transport Network, which will link the Baltic and Adriatic coasts and connecting 40 million people. It is due to stretch from Gdansk and Gdynia on the northern coast of Poland to Bologna in northern Italy, crossing through the Czech Republic and Slovakia along the way and providing a direct link between the capital cities of Warsaw and Vienna.

The Koralm tunnel is also part of the 130km-long Koralmbahn, being built for ÖBB-Infrastruktur, which will provide a direct rail link between the cities of Klagenfurt and Graz, the capitals of the states of Carinthia and Styria respectively, reducing the current travel time of three hours to less than one hour. It will also allow heavy freight trains of up to 2,000 tonnes to be pulled by only one locomotive. The line will carry up to 256 trains a day running at a maximum speed of 250kph.

“The line will serve as a missing link from north-east Europe to northern Italy,” says Dietmar Schubel, head of Koralm tunnel KAT 2 for ÖBB. “It also provides a new alpine crossing line and a connection between Styria, especially the Graz area, and Italy, as well as the Carinthian area and Hungary.”

This improved access is expected to bring economic benefits to the two Austrian regions, he adds.

Carinthia and Styria are separated by the Koralpe mountains so a key feature, and the largest infrastructure element, of the new railway is the 33km-long Koralm tunnel under this range. The tunnel comprises twin running tube – the north and south tubes – 40m apart and connected by crosspassages every 500m. There are 42 crosspassages in total and there will also be a 900m-long emergency station and refuge at the mid point.

Before the main excavation for the tunnel, 130 exploration wells and a 10km-long exploratory tunnel system were excavated to carry out in-depth analysis of the geological and hydrogeological conditions.


Each tunnel-boring machine (TBM) is making an average monthly advance of between 500 and 700m and so far the greatest daily progress has been 44m.

However, the mountains’ hard rock geology has meant the average daily gain is 12m rather than the predicted 17m. Despite this slower than expected progress, Schubel says the project will still be finished in time for test runs in early 2023 and for full service in December of that year.

An ongoing challenge is the blocky rock areas and the fractures and rock burst that damage the cutters and the cutterhead on the TBM. As a result, several cutterhead revisions have had to be made, says Goliasch.

In February 2015, a fire on the backup generating set at the rear of one TBM stopped work in the north tube for two weeks. Fortunately no-one was injured and Schubel was pleased with the way the emergency evacuation procedures were implemented.

“The execution of the rescue concept worked well,” he says. “There were 25 people on the machine when the fire started; 13 rescued themselves using the rescue train; five were able to reach the next cross heading by foot and the remaining seven men were evacuated by the rescue team.”

To ventilate the tunnel during construction JV ARGE Kat2 is using a system similar to that employed on the Gotthard base tunnel.

“Because of the length of the tunnel we use one tunnel, the south tube, for blowing in fresh air. The fresh air is distributed from the last finished crosspassage to the TBM north and south and the exhausted air diverted out via the north tunnel. Crucial for such a ventilation system is a complex control system for all fans, flaps and lock gates, to fulfil the guidelines of the emergency plan,” Goliasch explains.

According to the latest schedule the TBM in the north tunnel will finish in summer this year and in the south tunnel in spring 2017.

The 9.93m-diameter TBMs are installing a 350mm-thick segment lining. Along the NATM sections a double lining of shotcrete and a second concrete lining is installed.

Mobile Baustoffe, a subsidiary of Strabag, has supplied 1.2 million m3 of concrete required for the prefabricated elements of the first two sections and exploration galleries. Some 82,000 tonnes of additives and 350,000 tonnes of cement were delivered by rail from Lafarge, tailoring the chemical composition of the cement for the geology.

* This is a version of an article that first appeared at Tunnels & Tunnelling:

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