Construction & Technology

Dust, fuss, roaring in the half light. The Koralm Tunnel is being built. Two tubes of around ten metres in diameter are drilled into the mountain. Two sets of 33 kilometres - through a piece of the Lavant valley Alps. With 10,000 PS or a good 7,000 kilowatts, the machines fight their way through rock and debris. They dig and drill further and further inwards. They have been doing this almost constantly since the first tunnelling work in 2009. Since then two of these machines have struggled through the Koralm mountains, both of them on the Styrian side. The third has just started on the Kärnten side.

Tunnel drilling machines – Loud giants in the mountain

The tunnel diggers call these whining wonder works tunnel propulsion machines, or “Mauli 1” and “Mauli 2” (after the beloved character Mauli the mole). These are their Styrian nicknames. The Kärntners have lovingly christened their giant “Kora”. With a weight of 2,000 tonnes and around 200 metres long, these tunnel drilling machines work like underground factories. Leading the machine operation is the drilling head of the heavy cutting wheel, which rotates at the front. It has a diameter of approximately ten metres and it digs through several metres of stone every day. Not unlike glass cutting, the 30 to 40 centimetre large, sharp chisels on the cutting wheel break off rock chips from the mountain. Behind the drilling head, the follower rolls on the rails with vans and platforms with drill units, cement sacks, pumps, wires and power units.

Excavated material is partly reused

The tunnel drilling machines have to be supplied with power, compressed air and cooling water and conveyor belts and wagons take what they remove from the mountain away afterwards: gneiss and mica slate, marble, quartz and feldspar. In the tubes that have already been drilled out on the Styrian side, the conveyor belts are 16 and 17 kilometres long. Part of the rock formations can be used for building the routes. A better quality part can be used for the lining of the tunnel itself. Above ground, in a gravel plant, it is ground down and processed with water and cement to make concrete. This concrete is later reintegrated on the inside of the mountain in the form of concrete inner shells and segments.

160,000 ring segments made from reinforced concrete – the segments

Another service that the colossal machine provides besides the thunderous propulsion: it supports and lines the raw stellar vault as soon as it has been created. It drills and builds at the same time - with the help of segments, precast curved steel concrete parts, which provide a protective outer ring when hefted and jointed. Thousands of these ring segments are brought into the mountain with the tunnel railway. Just one segment weighs over seven tonnes. The remaining hollow space between the mountain and the segments is filled with special pea gravel.

Will water flow from the rock?

“You can never talk about day-to-day business,” say the tunnel construction experts. Despite refined logic, despite precise test drilling and preliminary geological explorations, every step forward is also a step into the unknown: the mountian has a life of its own. Will water flow from the rock? Will it seep from the ground? Has a machine gone stagnate? Do the chisels need to be replaced? Every task is its own challenge. The geologists keep a constant eye on the stone that is primarily changed in this way. One minute you could come across loose mountain rock and the next hard “Koralm crystal”.

“Continuous” and “cyclic” tunnelling

The tunnel drilling machines, which power the “continuous” tunnelling into the mountain cannot work alone. For the vastly changing rock formations and suspected problem areas of the mountain, another method is used. The tunnel diggers then talk about “cyclic” tunnelling. Austria has made a name for itself with the “New Austrian Tunnelling Method”, for which engineers and workers drill and blast the uncovered hollow space immediately with construction steel grids, secure them with sprayable concrete and anchors and finally remove the loosened stone with diggers. And they keep doing it, thousands of times, in cycles. While about a quarter of the Koralm tunnel is produced cyclically, around three quarters of the shorter, 27 kilometre long, Semmering base tunnel is dug this way.

Semmering: a complicated tube system

With its two tunnels, it crosses the dark, crystalline Greywacke zone, which also contains a lot of water. They are built in three large sections and from several sides at the same time. To build a railway tunnel, lots of small tunnels need to be built around it: vertical supply and air shafts; drainage so that the water can all flow away; cross cuts every 500 metres to connect the two main tunnels. You need escape routes and emergency stations, large rock caves, so-called caverns, where machines can be repaired and maintained. Constructing several work shafts or so-called intermediate points is a unique and huge task, which requires a lot of time. The material logistics for the middle section of the Semmering Base Tunnel, for example, has an approximately 400 metre deep supply shaft in Fröschnitzgraben in the middle of the Semmering countryside.

Coordinate peak performances

All of the processes have to be precisely coordinated with one another. The tunnel drilling machine is the master clock for the preparation of the tunnel breakout material, the segment production and the dam fillings on free stretches and it influences the project progress. In 2025, the company hopes that the Koralm tunnel, one of the longest railway tunnels in the world, will be complete. In 2027, the last touches will be put to the tunnel through Semmering. At the end of 2027, train passengers will then be able to race from Vienna to Graz in 1 hour and 50 minutes and beyond through the Koralm tunnel from Graz to Klagenfurt in 45 minutes. Freight transport will then be able to travel with high power on what is known as the flat track with a maximum slope of just eight per miles - in comparison to this, the existing mounting stretch to Semmering has a maximum slope of 25 per miles.