HomeInfrastructureThe Semmering Base Tunnel: overcoming the Alps

The Semmering Base Tunnel: overcoming the Alps

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One of the highlights of the Institution of Mechanical Engineers Eastern European Rail Tour in May/June 2024 was a journey from Austria’s Vienna Hauptbahnhof over the Semmering pass to Mürzzuschlag Bahnhof for a visit to the offices and southern portal of the Semmering base tunnel which is under construction.

When European railways were developed in the 19th century, civil engineers had to overcome the barrier of the Alps to deliver north-south links in Austria as well as in other Alpine countries. These early lines were characterised by long, and often heavily curved climbs up to passes or comparatively short tunnels though mountains. Ruling speed limits were typically between 50km/h and 70km/h. All of this was challenging for early locomotives and the descents challenged early braking systems.

Today’s trains are more capable but, as both passenger and freight train speeds have increased, these routes have imposed time and capacity constraints. Tunnelling techniques have improved immeasurably since then and many railway administrations have built or are building so-called base tunnels allowing much higher speeds with smaller climbs and many fewer curves, although the tunnels are much longer. The Swiss Lötschberg base tunnel was the first, built over 20 years ago. However, the high cost resulted in a 20-year gap between the completion of tunnelling and its fitting out for double track operation throughout.

Reducing journey times

The Semmering base tunnel, an ÖBB-Infrastruktur AG (Austrian state railway infrastructure company) project, is being built on the ‘The Southern Line’ between Vienna, Graz, and Klangenfurt as part of an EU transport plan to improve north-south rail services though the Austrian Alps. It is expected to reduce the journey time between Vienna and Graz by 30 minutes and will benefit the approximately 3.5 million people who live around this route.

The tunnel will be 27.3km long running between Gloggnitz and Mürzzuschlag and cutting off the lengthy, steep, and curved route previously above. It is not taking a straight line partly to provide a reasonable 0.8% gradient to cover the 280-metre height difference between the two stations and partly to avoid the worst rock formations and fault lines on straighter routes.

Several options were explored and around 280 core drillings with a combined depth of about 41km were made around the Semmering area to explore rock composition. The most northern option was discarded quite early as it would have influenced water flow into Vienna’s water supply. The project brochure suggested that all this exploration allowed the ideal route to be chosen, but the tour group were left in no doubt that this amounted to the least worst option. The various rock types are shown in the diagram below which highlights fault lines and rock types. There is more information on rock types in the panel.

Anyone used to tunnels in the UK, built by setting off a tunnel boring machine (TBM) from one end of a tunnel to the other, would be amazed at what has been involved. There were many problems when mining through faults in the rock, and what might look like a straight section blasted through the rock could soon become misshapen as the blasting released locked in stresses, leading to rework.

Assembly of TBM in Fröschnitzgraben central cavern.

Construction

The tunnel has been constructed from five sites/access points. Alongside reconfiguring the track layout and constructing portals at Gloggnitz and Mürzzuschlag, three other sites at Göstritz (northern end), Fröschnitzgraben (approximately the middle), and Grautschenhof  (southern end) were set up. Each end of the tunnel has seen extensive works to remodel the tracks and stations together with constructing the portals and the initial tunnels, mostly using rock mining methods.

The main construction was let in three lots:

Construction section SBT1.1 Gloggnitz Tunnel. This section of two 4.6km long tunnel bores with 16 cross passages started construction by conventional mining in 2015 and is due for completion in 2028. Intermediate access has been provided at Göstritz. This site required one kilometre of temporary tunnel including two 250-metre-deep vertical shafts with logistics caverns at the top and bottom of the shafts. Two tunnels, approximately 1.6km length are being excavated in each direction from the bottom caverns. This involved dealing with groundwater ingress of up to 100 litres/second and, until the area was sealed, the water had to be pumped out continuously.

Construction section SBT 2.1 Fröschnitzgraben tunnel. The middle section through Fröschnitzgraben is a 13km section with approximately 4.3km of twin tunnel using mining (drill and blast). The emergency refuge areas are approximately 950 metres long with several cross passages and approximately 8.2km of twin tunnel being built with two tunnel boring machines (TBM). Work started in 2014 and is due for completion in 2027. Access to this section was provided through two 400-metre-deep shafts, 8 metres and 11 metres in diameter. At the bottom of the shafts, 950-metre-long emergency refuge caverns were constructed into which sections of TBM were lowered for assembly underground.

The TBMs were removed in sections at the end of construction. There was significant ingress of water in some parts of the mining tunnelling section. When completed, the cavern at the bottom of these deep shafts will have means for emergency intervention and will have fresh air pumped in through one shaft and foul air or possible smoke exhausted via the other. All this will be provided by what was modestly described as ‘a large fan system’. Spoil from construction was removed via the 400-metre shafts and deposited in a landfill at Longsgraben where the local population has been involved in landscaping the site.

Construction section SBT 3.1 Grautschenhof tunnel. Work started on this section in 2016 and is due for completion in 2026. It involves twin tunnels, both 7km long with 16 cross passages. From two temporary 100-metre-deep shafts, four tunnel drives are in progress using drill and blast excavation.

Fit out and system commissioning

Construction started in 2012 and is expected to be complete in time for traffic to start in 2029. The original cost was estimated at €3 billion but, after difficulties caused by Covid, the geology of the site, and global inflation, the cost has increased by approximately €1 billion.

Our tour group was unexpectedly given the privilege of visiting the Mürzzuschlag portal and of walking along the tunnel for approximately 100 metres where we observed teams installing waterproofing sheets and steel reinforcement mesh prior to the in-situ casting of a smooth concrete lining.

We were able to see the foundation of the track bed and the channels that will be used to direct water from the inevitable leaks. For your writer who is used to the London Underground, the tunnel seemed absolutely huge. This is necessary, of course, to accommodate European-gauge double deck coaches.

Work has continued since the visit, of course, and recently ÖBB-Infrastruktur announced that on 11 September the last break through was made in the first tube in the Gloggnitz construction section, meaning that Gloggnitz and Mürzzuschlag are now completely connected underground. The report added that work on the second tube is expected to be completed in the first quarter of 2025, when tunnelling will be completely finished.

Finally, in case anyone was in any doubt, ÖBB-Infrastruktur intends to retain the existing route, partly for tourism, and partly to cover for the twice-weekly, eight-hours periods that the Semmering Base Tunnel will be closed for maintenance. It will also be useful in the unlikely event of an incident, such as that on 10 August 2023 in the Swiss Gotthard base tunnel, which put at least one track out of action for several months.

With thanks to Roland Leitner who provided an excellent presentation in perfect English and all his colleagues who accompanied the tour group into the tunnel. Photos and illustrations courtesy of ÖBB-Infrastruktur unless indicated otherwise.

Malcolm Dobell BTech CEng FIMechE
Malcolm Dobell BTech CEng FIMechEhttp://therailengineer.com
SPECIALIST AREAS Rolling stock, depots, systems integration, fleet operations. Malcolm Dobell worked for the whole of his 45-year career with London Underground. He entered the Apprentice Training Centre in Acton Works in 1969 as an engineering trainee, taking a thin sandwich course at Brunel University, graduating with an honours degree in 1973. He then worked as part of the team supervising the designs of all the various items of auxiliary equipment for new trains, which gave him experience in a broad range of disciplines. Later, he became project manager for the Jubilee Line’s first fleet of new trains (displaced when the extension came along), and then helped set up the train refurbishment programme of the 90s, before being appointed Professional Head of Rolling stock in 1997. Malcolm retired as Head of Train Systems Engineering in 2014 following a career during which he had a role in the design of all the passenger trains currently in service - even the oldest - and, particularly, bringing the upgraded Victoria line (rolling stock and signalling) into service. He is a non-executive director of CPC Systems, a systems engineering company that helps train operators improve their performance. A former IMechE Railway Division Chairman and a current board member, he also helps to organise and judge the annual Railway Challenge and is the chair of trustees for a multi academy trust in Milton Keynes.

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