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Steep geosynthetic-reinforced slopes stabilize access routes for construction of 27-km tunnel

Case Studies | June 1, 2014 | By:

Semmering Base Tunnel project in Austria


Following two years of preparatory work, main construction began on Austria’s 27-km Semmering Base Tunnel in January 2014.

The tunnel between Mürzzuschlag and Gloggnitz on the Vienna–Villach/Graz main line is divided into three contract sections. The 13km central section is currently under construction, and two tunnel boring machines (TBMs) have been launched in a pit at Fröschnitzgraben to excavate the parallel single-track bores.

The central section is being built by a joint venture of Swietelsky Tunnelbau and Implenia under a €623m ($829m) contract. The second contract, which covers the 7km eastern (Gloggnitz) section of the tunnel is currently being tendered and construction will start in 2015. Construction is expected to start on the remaining 7km western section in 2016.

Once completed (scheduled 2024), the new Semmering Base Tunnel in Austria will eliminate one of the main infrastructure bottlenecks along the Baltic–Adriatic Axis (BAA). The uninterrupted route for rail traffic between Gdansk, Poland, and Ravenna, Italy, is expected to generate significant economic momentum for the adjoining regions.

As one of Europe’s most important north-south rail corridors, the BAA links the burgeoning economic regions of the three European Union member states—Poland, Czech Republic, and Slovakia—with the business centers of Austria and northern Italy. Once completed, the new Semmering Base Tunnel between Mürzzuschlag and Gloggnitz will eliminate one of the most critical infrastructure bottlenecks along the route between Gdansk and the Upper Adriatic. A key aim of the BAA is to stimulate economic development in the regions adjoining this corridor.

Semmering Base Tunnel

The Semmering Base Tunnel project required extensive enabling works, including the construction of steep geosynthetic-reinforced slopes to stabilize the access and haul roads near the Longsgraben spoil tip (the municipality of Spital am Semmering).

Given the difficult terrain, the need to reuse local tunnel spoil, and the complete relocation of a stream bed over a length of 2km, this work package posed complex geotechnical challenges. The extensive enabling works, which were split into several packages, have been in progress since 2012. The tunneling work proper started at Fröschnitzgraben in 2014.

The large quantities of spoil produced by the 27-km tunnel drive will be deposited at a specially-created tip in the Longsgraben dell. The provision of access to the site’s setup area at Fröschnitzgraben and the Longsgraben spoil tip necessitate the construction of haul roads and bypasses in difficult terrain.

The work for this phase was carried out between February 2013 and January 2014. The streambed at the Longsgraben site was first raised over a length of more than 1km to a level some 50m higher than the previous bed. Extensive reinforcement works were installed to stabilize the steep slopes adjoining the new streambed. Further stabilization measures were implemented for the site traffic routes and to provide access to the Longsgraben spoil tip from Pfaffensattelstraße (L117).

The incorporation of geosynthetic reinforcement products made it possible to reuse the variable-quality tunnel spoil for construction of the steep slopes. The steep slopes were built with geogrids using the wraparound method, with the geogrid layers installed at uniform intervals of 55cm. Overall, more than 120,000m² of geogrids were incorporated.

This flexible construction is particularly suitable in case of inhomogeneous subsoil conditions. An additional geotextile was inserted at the wrap-back positions to protect against erosion and the washing-out of the face soil. The efficiency of site operations was vastly improved through the incorporation of pre-formed steel-mesh angles as formwork, which also ensured the achievement of a smooth front face.

Compared to an unreinforced solution, the construction of up to 25m slopes with a 75° gradient and continuous asphalt surfacing along the top served to reduce both land take and bulk materials transportation.

The reinforced slopes were designed as a temporary measure (for 15 years). Eventually, when the areas along the slopes at the Longsgraben site have been filled with tunnel spoil, they will no longer need to fulfill a structural function. This limited service period allowed an economical sizing of the geosynthetic reinforcement.

To seal the artificial streambed at the top of the steep slopes, the client opted for the use of a geosynthetic clay liner (GCL).

Despite the tight construction window, the works for package SBT2.3 were completed on schedule. The prompt delivery of the necessary geosynthetic materials and the workmanship of the contractor ensured implementation of the cost-effective solution.

Sören Schmidt, Dipl.-Ing., is an engineer in the Department Geo DACH Anwendungstechnik for Huesker Synthetic GmbH based in Gescher, Germany.

Geosynthetics editor Ron Bygness also contributed to this article.

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