Fast track beneath the Alps

Engineers have spent almost 20 years burrowing the world’s longest and deepest rail tunnel under the snow-capped peaks of the Swiss Alps.

The idea of creating a short cut through the Alps to help improve transport links through Switzerland and boost trade has been tantalising engineers since the end of the Second World War.

Basel-based engineer Carl Eduard Gruner first voiced the idea of a rapid transit tunnel beneath the Saint Gotthard Massif as far back as 1947 when he proposed a low gradient bore through the Gotthard base.

But it took 40 more years for the idea to really start to gain traction with proposals for a route between Erstfeld in the north and Bodio to the south as part of a wider trans-Alpine railway.

This rail link, known as the Alp Transit, was intended to drag heavy goods traffic off the congested roads that wind through the region, not only slashing transportation time and costs but also reducing the impact of heavy traffic on the environment.

 

The Gotthard Base Tunnel was officially opened in June 2016

 

By 1995, the Swiss authorities had accepted the proposals from Swiss Federal Railways (SBB) to build two 57Km-long, single-track railway tunnels between Erstfeld and Bodio, connected by 180 cross passages and two multifunction stations at Sedrun and Faido. With a total length of more than 150Km, the tunnels have a rock overburden of almost 2.5Km at their deepest point.

The scheme is expected to carry freight traffic at speeds of 110Km/h and passenger trains at speeds of 250Km/h

The scheme is expected to carry freight traffic at speeds of 110Km/h and passenger trains at speeds of 250Km/h, with an anticipated traffic mix of five goods trains and two passenger trains each hour and in each direction.

 

Tunnel vision

Following a series of exploratory bores, Alp Transit Gotthard, a wholly owned subsidiary of SBB was formed in 1998 with the remit to oversee the construction of the Gotthard Tunnel.

Split into five sections for construction purposes, the twin tunnels pass beneath the snow-tipped mountains at depths where the geothermal heat can reach 45 degrees centigrade. This presented an issue for the engineers, not just in cooling the workforce and providing extra ventilation, but also that it exacerbated the problem of the high water pressure around the tunnels and the huge rock load above them.

These massive rock loads were so great that in locations along the tunnel, the bore would be subject to ‘squeeze’. Here the heavily faulted and cracked rock could have the tendency to push inward into the tunnel bore during excavation, squeezing and deforming the tunnel cross section.

 

Excavating under the Alps for the Gotthard Base Tunnel

 

Breaking new ground

In a bid to overcome this problem, the project team borrowed from mining technology and introduced a series of steel arch support sections featuring sliding friction joints. These can be adjusted to set exact levels of loading and are engineered to ‘give’ slightly as stresses build.

“Working through squeezing ground requires specialist equipment that offers ground support around the tunnel,” says Ivor Thomas, Vice Chairman of the British Tunnelling Society and tunnel manager at UK contractor BAM Nuttall. “Simple colliery arches will just buckle, so these steel sliding arch sections are engineered to ‘give’ but without affecting the integrity of the tunnel lining.”

With such technical challenges conquered, the construction team broke through the final section to complete the tunnel in October 2010, 11 years after the first blasts were taken and following the removal of 28.2 million tonnes of excavated rock.

 

A worker pours concrete during construction

 

High-performance rail

But the success of the Gotthard Base Tunnel does not depend solely on its successful construction. Without a comprehensive, high-quality rail fit-out of the tunnels, the scheme would fail to meet its target to carry six freight trains and two passenger trains each hour.

Part of this fit-out is the provision of the 290Km of heat-treated pearlitic steel rails and crossover points throughout the scheme.

The heat treatment of rails modifies the microstructure of the steel ensuring they become particularly resistant to wear, rolling contact fatigue and other defects, and is especially important on sections of track that are heavily trafficked at high speeds. It helps boost running safety and reduces maintenance throughout, as well as increasing the operational lifetime of a rail.

On a project where all of these are of primary importance, the job of providing the required 18,000 tonnes of high-strength rails and the 43 high-performance turnouts for switching trains between tracks fell to specialist steel producer voestalpine.

“The top speeds reached in the Gotthard Base Tunnel by both passenger and freight trains place particularly high demands on the quality of the track and turnout systems”

Its 120m standard rail length improves safety by reducing the number of potentially weaker weld points. Manufactured at voestalpine’s Leoben/Donawitz mill, the rails are heat-treated through a self-developed inline process that ensures the highest quality rails can be produced at the volumes required to feed projects such as the Gotthard Base Tunnel.

“The top speeds reached in the Gotthard Base Tunnel by both passenger and freight trains place particularly high demands on the quality of the track and turnout systems,” says Franz Kainersdorfer, head of voestalpine’s Metal Engineering Division. Those rails underwent stringent tests as the Gotthard Base Tunnel prepared for its opening to traffic in December 2016.

Already the project has spurred on the development of the next generation of heat-treated rail steel, with voestalpine unveiling its ultra-high carbon heat-treated steel specifically designed for increasing track loads in mixed and heavy load transport. It claims these can halve rail maintenance and double service life compared with standard heat-treated rails.

Another huge scheme like the Gotthard Base Tunnel could offer the chance to prove those claims.

 

Images: © AlpTransit Gotthard Ltd, Getty and iStock