Construction access shaft for the Semmering-Basistunnel railway tunnel, sunk to 400 metres depth in the Fröschnitzgraben valley south of Steinhaus am Semmering.
Schacht Fröschnitz 2 is one of two construction access shafts making up the Zwischenangriff Fröschnitzgraben, the central intermediate attack point of the 27.3-kilometre Semmering-Basistunnel railway tunnel being built by ÖBB-Infrastruktur AG between Gloggnitz in Lower Austria and Mürzzuschlag in Styria. The shaft is located in the Fröschnitzgraben valley south of Steinhaus am Semmering. Preliminary site preparation works were carried out between 2012 and 2014.
Sinking of the two shafts — Schacht Fröschnitz 1 and Schacht Fröschnitz 2 — began formally on 23 July 2015. Both shafts were sunk by blasting to a depth of approximately 400 metres and have a diameter of approximately 10 to 11 metres. At their foot a large underground cavern was excavated to serve as the assembly and launch chamber for two tunnel boring machines and as the future site of an underground emergency stop station.
Tunnel boring machine driving in the direction of Gloggnitz began in July 2018; conventional driving towards Mürzzuschlag was also carried out from this point. The shafts served throughout construction for materials supply, spoil removal, personnel access, and ventilation. Final breakthrough of the Semmering-Basistunnel was achieved on 29 November 2024.
The tunnel is scheduled for opening at the December 2029 timetable change.
Schacht Fröschnitz 2 is one of the pair of construction access shafts constituting the Zwischenangriff Fröschnitzgraben — the intermediate attack point forming the central section of the Semmering-Basistunnel. The Semmering-Basistunnel is a 27.3-kilometre twin-tube railway tunnel connecting Gloggnitz in Lower Austria with Mürzzuschlag in Styria, crossing beneath the northern Alpine chain near the Semmering Pass. It forms part of the Baltic-Adriatic Corridor, a trans-European rail route from Gdańsk to Ravenna, and is intended to relieve the historic Semmeringbahn mountain railway while dramatically reducing journey times between Vienna and Graz.
The Fröschnitzgraben section (Baulos SBT2.1) is the longest and central of the three construction lots, with approximately 13 kilometres of tunnel driven in two directions from the Zwischenangriff. The shaft site is located south of Steinhaus am Semmering in the Fröschnitzgraben valley, within the municipality of Spital am Semmering. ÖBB-Infrastruktur AG awarded the contract for Baulos SBT2.1 in January 2014 to a joint venture (ARGE) of Implenia (Switzerland) and Swietelsky Tunnelbau GmbH, with Implenia holding a 50 per cent share and serving as technical lead; the contract value was approximately 623 million euros. Preliminary site preparation works ran from spring 2012 to 2014, including new drinking water infrastructure for the municipality of Spital am Semmering, road construction and improvement, a temporary motorway access point at the Semmering Schnellstraße, a bypass road at Steinhaus am Semmering, and preparation of the Longsgraben waste disposal site for spoil from the tunnel drives.
Sinking of Schacht Fröschnitz 1 and Schacht Fröschnitz 2 began with a ceremonial start on 23 July 2015, attended by the Federal Minister for Transport, the Styrian State Governor, and the CEO of ÖBB-Holding AG. Both shafts were sunk by blasting to a depth of approximately 400 metres — equivalent, as noted at the ceremony, to a skyscraper of over 130 floors — with a diameter of approximately 10 to 11 metres. At the foot of the shafts, a large underground cavern was excavated to serve as the operational base from which tunnel drives were launched in both directions. This cavern was subsequently fitted out as the site for an underground emergency stop station for future rail traffic through the tunnel.
The two tunnel boring machines deployed for the drive towards Gloggnitz — named "Carl" and "Ghega" — were lowered in parts through the shafts, assembled underground, and launched in July 2018. Each machine is approximately 120 metres long, weighs around 2,500 tonnes, and has a boring diameter of about ten metres. By late 2021, both machines had completed the approximately 8,100 to 8,300 metre drive eastward through the mountain. The conventional (drill-and-blast) drive towards Mürzzuschlag was carried out in parallel. Excavated material throughout the construction period was transported up through the shafts and deposited at the Longsgraben spoil depot nearby; a total of approximately 4.25 million cubic metres of material was placed there over the construction period.
The first breakthrough between the Fröschnitzgraben and Göstritz sections was achieved in June 2022. The final tunnel breakthrough of the entire Semmering-Basistunnel was completed on 29 November 2024. Fitout of the shafts that are no longer needed for operational use is being concreted over during the finishing phase. The tunnel, including the underground emergency stop station at the Fröschnitzgraben cavern, is scheduled to open with the December 2029 timetable change. Up to 12,000 people were involved in the overall construction; approximately 30,000 blasts were carried out across the project.