Building a world heritage tunnel in Switzerland

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Photo: Keith Fender.
Photo: Keith Fender.

It is not only the Swiss main line railways that are benefitting from tunnel investment. The Rhätische Bahn (or Rhaetian Railway) is an extensive metre-gauge network in the southeast of the country. The section from Tirano (in Italy) to Tiefencastel via Pontresina has been declared a UNESCO World Heritage site, and it is easy to see why.

With origins dating back to 1889, the line was initially built to promote the tourist trade and offered a spectacular route traversing the northern valleys in the canton of Grisons (Graubünden). With many spirals to gain height in this alpine region, especially in the Albula Bernina area, the highest point, at 1,800 metres above sea level, is crossed by the 5,864 metre long single-track Albula tunnel, linking the stations of Preda and Spinas, which opened in 1903.

Originally operated by steam traction, the line was later electrified with the standard Swiss 15kV, 16²⁄³Hz system.

Having given over a century of service, the condition of Albula tunnel was assessed in 2006 and it was deemed to need significant renovation as rock falls were becoming an ever-present threat. This would have meant considerable disruption for the 7.4 million passengers, including 2.3 million commuters, and significant volumes of freight that use the line every year.

As an alternative, building a brand new tunnel alongside the old was duly evaluated and, in 2010, was adopted as the best solution.

The area is typical of the Swiss Alps, with considerable snowfall in the winter months that severely limits access to the tunnel site between mid December and the end of February. As such, construction work is suspended during this period but proceeds with 24-hour shift working during the rest of the year.

The new tunnel cannot be a replica of the old as current standards now prevail and safety considerations tend to dominate. Thus the tunnel bore is much larger and has to accommodate the provision of walkways plus the latest clearances for the OLE fixtures.

Building the first tunnel circa 1900. Building the first tunnel circa 1900.

Constructing the tunnel

Construction work follows conventional practice for boring a tunnel through rock. Firstly, and most importantly, a construction site has to be established close to each of the portal areas. These are major undertakings in their own right, with machinery and conveyor systems capable of handling all the material needed for the boring activity plus the disposal of the excavated spoil as the tunnel progresses.

Whilst road access is available, the local infrastructure would not be suitable for numerous lorry movements, so rail sidings and engineering trains are needed to cater for the main material removal. The site is extensive and, with the number of workers involved, a strict safety regime is needed. This includes protecting the staff during engineering train movements, with radio-based Schweizer portable trackside warning systems being used for this purpose.

Having established the portal locations, ingress into the mountain is by the normal routine of drilling, blasting, spoil removal and securing the worksite ready for drilling the next section. It sounds straightforward, but these activities have to be accompanied by other engineering ancillaries to ensure safe progression.

Grisons (Graubünden) canton and the Rhätische Bahn network. Map: Alyssa James.

Grisons (Graubünden) canton and the Rhätische Bahn network. Map: Alyssa James.

Firstly, there needs to be fresh air. Air is pumped into a tube that takes it to the rock face where it creates a slightly higher pressure in the tunnel atmosphere. This then helps to blow out the dust and diesel fumes as the excavation takes place.

Secondly, and like most tunnels, water ingress is considerable and channels are needed for this to flow down to the portals. Thirdly, the sides and roof have to be stabilised, this being achieved by ‘shotcreting’, or the spraying of wet-mix concrete.

Conventional tunnel machinery is used, so no temporary rail tracks are provided, the profile of the tunnel being sufficiently large and the floor of the tunnel being flat enough for road vehicles to journey to and from the rock face.

Even with all this, it is a wet, dusty and unpleasant environment. Progress on an average day is 6.5 metres.

Whilst the geology of the tunnel is mainly ‘Albula granite’, a 110-metre section close to the Preda portal consists of three different types of rock, known as cellular dolomite. A 20-metre section consists of a fault zone where a soft porous version of the dolomite is encountered, the material being akin to silty fine sand. It was found impossible to extract any solid material and thus the risk of tunnel collapse became a real threat.

The solution has been to drill freeze holes into the surrounding ground to a depth of 60 metres. The freeze zone must be at least 2.5 metres thick outside of the excavation area, this being strong enough to absorb the ground and hydraulic pressures. Once freezing has occurred, excavation can take place in that cross section followed by a 120 cm thick reinforced shotcrete lining applied within seven days of excavation.

The result is a strong tunnel profile that will withstand the expected pressures. However, because of the geological complications on this section, the progress rate through the frozen ground is only about 0.7 metres per day.

Spoil removal

Even though the new tunnel will be slightly shorter than the original at 5,860 metres, a tunnel of this length creates a vast amount of waste material for disposal. In a sensitive environmental area with world heritage status, this presents a considerable challenge.

As the waste is brought out through the portal on a conveyor belt system, it is graded into varying categories. The solid granite is broken down into manageable lumps, the larger of which can be used for future track ballast. Medium and smaller lumps are useful for making concrete.

However, it is the slurry and loose shale that causes the problem as this has to be dumped. Whilst a suitable site has been found during the construction period, it is a controversial longer-term dilemma. Either the waste has to be transported elsewhere, which is an expensive exercise, or it has to be blended into the nearby landscape.

Looking forward

From detailed planning work that began in 2010, it will take 12 years for the project to be completed. Construction began in 2014 for a period of 8½ years with breakthrough expected to happen in October 2018. Some 244,000 cubic metres of solid rock will have been excavated.

When completed in 2022, 15,000 trains will traverse the tunnel each year at a maximum speed of 120km/h. Total cost is estimated at 244 million Swiss Francs, equivalent to £188 million sterling.

The old tunnel will not be abandoned, however. Twelve cross passages are being excavated from the new tunnel to the old at intervals of 450 metres, stopping short of break out into the old tunnel until it is closed for rail traffic. Once the new route is commissioned, the cross passages will be opened up to allow access to the new tunnel, both for maintenance purposes and to provide a safe egress route should a train in the new tunnel ever need to be evacuated. Repair work to the old tunnel will be carried out so as to stabilise the walls and roof.

The Albula tunnel represents an impressive commitment to the local Swiss economy, which is difficult to see being replicated in the UK or other European countries. Maybe the Ffestiniog Railway Moelwyn tunnel, built during the 1960s, is the nearest comparison to be made but the standards and finances for heritage and commercial railways are very different.

The new portal at Preda.

The new portal at Preda.


Read more: Tunnel management, Swiss style
 

This post first appeared on Rail Engineer