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Grouting: Soft ground solution for Austrian railway

While the rock tunnel on the new Koralm Railway in Austria may be record- breaking in its length, several shorter soft ground tunnels on the route are also setting new standards. 

When the new 130km Koralm railway line between Graz and Klagenfurt opens in 2023, it will cut the journey time between the two cities from three hours to just 60 minutes. Central to the €5.2bn scheme is the 32.9km Koralm Tunnel which has been under construction using tunnel boring machines since 2012 but there some shorter tunnels that also pose some considerable challenges.

Koralm Railway

The St Kanzian tunnels are being built by a joint venture of Kostmann and Baresel with ground engineering support from Keller

Around 20km to the west of the headline-grabbing Koralm Tunnel are the soft ground tunnels at St Kanzian which will be bored through some tricky ground conditions. These structures are currently being delivered by a joint venture of Kostmann and Baresel with ground engineering support from Keller.

The tunnels – the 620m long Srejach cut and cover tunnel and the 664m long mined Untersammelsdorf tunnel – are to be driven through lacustrine clay deposits, which is renowned in Austria with wet cohesive matter that is highly variable.

“The site at St Kanzian lies within one of three known hot spots in Austria for the worst ground conditions in the country,” says Keller project manager Clemens Kumerer. “This is the first time a tunnel has been driven through these ground conditions in Austria.”

Project client Austrian Railways (OBB) knew that these clays could present a considerable challenge to the new rail route and commissioned full scale field trials to simulate the work in 2009.

Kumerer estimates that this work cost OBB in the region of several million Euros but provided essential information for the tender documents. “The information that OBB gave at the tender stage allowed us to evaluate the material and develop our own approach to deal with the problem,” he says.

The solution for the two tunnels that are separated by a 100m long at grade section involves a mix of techniques. Keller’s approach uses a combination of bored piles, soilcrete jet grouted columns, shotcrete and ground anchors.

Koralm Railway

Kumerer describes the €31.2M solution as the most challenging grouting job in Austria and believes it is one of the largest schemes being delivered by Keller in Europe at the moment.

One of the key challenges of the work is not actually the technical design and delivery of the work itself, it is the management of the spoil arising from the work.

Keller moved onto site in March and Kostmann and Baresel joint venture has also been working on constructing the work platforms for the piling and grouting work. For Keller, much of the work since moving onto site has been focused on trialling the planned techniques, as well as looking at how to deal with the high water content spoil. “We have been testing various plants to see what performs best,” says Kumerer. “Systems under consideration include filter presses, centrifuge based equipment and geotextile containers. Whatever approach we take, dealing with the spoil is a major logistical challenge on this project.”

The first part of the main works is expected to get underway in June or July this year. Kummerer said that piling work on the cut and cover tunnel will take 10 months and the grouting will take another 12 months. Keller’s work will then move onto the mined tunnel which is more complex because of the depth of ground cover, so piling work there will take 12 months and the grouting 15 months.

Keller has put some figures against the planned work and expects to install almost 58,000 linear metres of bored piles and another 24,000 linear metres of jet grouted soilcrete columns to create a seal between the piles. A further 50,000 linear metres of jet grouting is anticipated for the slab on the cut and cover tunnel and crown stabilisation on the mined tunnel. The work will also call for 3,100m2 of shotcrete and 3,300m of ground anchors.

According to Kumerer, once Keller has completed its work it will take the joint venture contractors another two years to complete the excavation and tunnel lining work so the tunnels are scheduled for completion in 2018.

Kumerer has said that work on site has gone well so far and his team is gaining confidence that the required 3.35MPa uniaxial strength from the soilcrete jet grouted columns can be achieved and the other challenges can be successfully overcome.


The 32.9km Koralm rock tunnel is currently being constructed by Strabag.

The 9.93m diameter twin bore tunnels were launched in 2012 through a 60m deep shaft and are being driven westwards through complex geology with an overburden of up to 1.25km. Strabag is using two Aker Wirth telescopic shield tunnel boring machines.

The tunnel alignment takes the construction through partially weathered slate formed mostly from gneissschist and unweathered shales of mica gneiss with local fault zones.

The tunnel will also pass through the 4km long Lavanttaler fault system.

The tunnel is expected to open for rail traffic in 2023. 


On the Srejach cut and cover tunnel, Keller will install 1,200mm diameter cased bored piles to depths of up to 21m. The spaces between the piles will then be sealed using 800mm diameter jet grouted soilcrete columns installed to depths of 14.5m on a triangular grid. Jet grouting will then be used to form the base slab of the tunnel before the site is handed over for the excavation phase.


The Untersammelsdorf mined tunnel will be constructed by installing 1,200mm cased bored piles to depths of up to 30m with the bottom 16.6m section forming the pile. Keller will then use 10m long, 1,000mm diameter jet grouted soilcrete columns to create a seal between the piles. Jet grouting will then be used to create a 2m thick “umbrella” over the crown of the tunnel to protect the miners during the excavation phase. 

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