Tunnels are always a fascination for engineers, be they road, rail or canal. Perhaps it is the idea of disappearing into a dark chasm that entrances the human brain. Writes Clive Kessell

Whatever the reason, it is a fact that many more tunnels are being built today than ever before. Much of this is because of increasing environmental concerns over the impact of new surface railways. Also, with the need to build new urban transport systems, there is often no other way except to go underground.

The more famous rail tunnels of past and present are known to pretty much everyone – Channel Tunnel and Crossrail being well publicised in the UK – but what is happening elsewhere? A recent visit to Sweden and Denmark revealed some surprising tunnel projects that are virtually unheard of in Britain. Yet they represent some real engineering challenges that may set new standards for construction in due course. Three specific tunnel projects are described; one that is complete – one that is being built and one that is only at the design stage.

Malmö tunnel project

Sweden’s third city took on a new business and tourist perspective with the opening of the Øresund Link to Copenhagen back in 1999. Linking the two cities by a bridge / tunnel combination and incorporating both a motorway and rail connection, it made travel between the countries much easier and faster. This has resulted in a massive upsurge in traffic and, for the rail network, the creation of congestion at both ends.

In Malmö, the main station was once only a terminus with all passenger trains having to reverse if their destination was beyond the city. Mixing all these movements with increasing volumes of freight traffic was causing a real problem and public complaints became a dominant issue. The Swedish authorities therefore took the brave step of planning the provision of through platforms at the station and to provide two additional stations (Triangeln and Hyltie) in the city centre area so as to enhance local journey opportunities at the same time. The only way to achieve that has been to build a new line in tunnel, some 6km in length with two single bores and periodic cross passages.

Tunnelling technology has progressed considerably since sub-surface railways were first built. Equally, risk factors and safety requirements are better understood with associated technology to ensure near- perfect reliability and availability plus safe evacuation should anything serious occur. Boring the tunnels is almost the easy bit and the excavating machines soon got to work to create the rail tunnels and station sites. At Malmö, the new underground platforms occupy an extensive area of the station so this construction has used cut and cover techniques similar in style to the new St Pancras Thameslink station in London.

Equipping the tunnels with all track, electrification, signalling, and other ancillary systems was to become a virtual project in its own right. The Authorities insisted that safety standards should be no different from those on a surface railway. Getting approval for an operational railway meant satisfying the authorities that all the engineering content could deliver the required safety edict. It could be argued that the onerous requirements placed upon the project were over the top and that this railway would take little cognisance of other underground lines that have been operating safely for decades. However the world moves on and safety provision can become ever more demanding.

An important element has been the provision of emergency access points and five of these have been required including ones at Malmö Central and Triangeln stations. The prime concern is a train fire and 24 hour fire brigade cover is insisted upon. This is provided by a central team but contracts are in place with the city fire brigades to assist if an incident develops that overwhelms the dedicated team.

Other systems needed to support the safety regime are fire alarms, de-watering, fire ventilation, normal ventilation, lifts and escalators, tunnel lighting, pressurised water and CCTV plus, of course, resilient power supplies and a SCADA network to link it together. The new line has been in operation since 2010 and has achieved its objectives. Not only is the link to Copenhagen more reliable but important new suburban traffic has been generated.

Hallandsås Tunnel

North-westwards from Malmö is the important main line to Gothenburg and Oslo. Opened in 1885, it is known as the West Coast Line (why is it that West Coast lines always seem to achieve notoriety?). It is mainly double track but has a single track section with steep inclines through the Hallandsås ridge, the land mass being 10km wide and 40 km long. This has long been a constraint to operations and a means of increasing capacity

Photo: Francis Vigouroux.

Photo: Francis Vigouroux.

is required so as to achieve 24 trains per hour and a doubling of freight train weights.

Various options were considered but as is so often the case, the land around the ridge is sensitive and the only real solution was to build a tunnel. The ridge is mainly hard rock combined with seams of soft rock and clay and with a very high natural water pressure. The decision has been to build two single track tunnels each 8.7km long and nine metres in diameter. Work began in 1992 but was stopped in 1997 until the problem of intrusive water could be resolved.

A revised electric boring machine of some 2430 metres long, 3200 tonnes in weight and with a cutting head of 10.6 metres diameter was procured. Known as Åsa, it could cope with the geological conditions including high water pressure that were being encountered and work recommenced in 2003. The machine came from Herrenknecht from Germany and cost €50M.

If high water flows are expected, the rock in front of the machine is sealed with cement grout that is pumped into the borehole through apertures in the cutting head. A watertight tube is created as part of the machine design by the insertion of concrete segments (some 40,000 being needed for the complete two tunnels) after which any remaining space between the rock and the segments is filled with pea gravel and cement grout. Other parts of the ridge consist of poor quality rock that is inherently unstable when disturbed. To counter this, the rock is stabilised with a freezing solution in advance of the cutting head and becomes sufficiently stable for Åsa to then move forward. A gradual thawing out then occurs with the concrete segments being strong enough to hold the tunnel in position.

As is customary nowadays, safety has to feature prominently and 19 cross passages are built for evacuation purposes as well as enabling maintenance areas and resilient cable routing. Both tunnels are now complete with only the cross passages to finish off. The main tunnel contractor has been Skanska-Vinci HB.

Work now has to start with the equipping of the tunnels for rail traffic. This will follow current technologies for a design speed of 200kph. Some 28km of catenary, 465km of cable and 35km of cable route will be required.

Initially, the signalling system will be a proprietary Bombardier product which is capable of being upgraded to ERTMS Level 2. GSM-R will exist in both tunnels but with separate cables for transmit and receive channels to avoid interference risks. A multiple repeater system is being installed to additionally allow public cellular services in bands 2G/3G/4G ranging from 800MHz to 2.6Ghz plus the emergency services radio channels. Every other cross passage will house one or other of the required repeater equipment.

TBM Åsa ute i snö_TRV [online]

Tunnel boring machine Åsa at Hallandsås.

Rolling stock will be the existing train fleet which will require retro-fitting with radio equipment. Completion will be in late 2015 with initially 100 trains per day. Trains with dangerous goods will not be permitted through the tunnels so the existing line will be kept in service at least for a while. The emergency management system is not yet finally decided but it is not the intention to have a smoke control or forced ventilation facility, nor an elevated walkway. There will be lighting and CCTV coverage. Total cost is €1.3 billion (around 11 billion SEK) for the project.

Fehmarnbelt Tunnel

As part of a modernised transport corridor from Denmark to Germany, works will include the upgrading and electrification of the rail line from Ringsted to Rødby (to complement the new high speed line from Copenhagen to Ringsted) with en route a replacement Storstrømsbridge linking South Zealand to the Lolland-Falster islands as the existing bridge has cracks in it. There will also be a new tunnel linking Rødby to Puttgarden in Schleswig- Holstein. This is currently a ferry connection and the major constraint to improved journey times.

A treaty between Denmark and Germany in 2008 led to various options being considered for crossing this gap but the decision was taken in 2011 to adopt an immersed tunnel solution. This will be a major engineering challenge and the resultant design has come up with a 40 metres wide tunnel incorporating two road sections and two rail sections. In effect it will be a rectangular box of 18.1km in length which will be a world record. Total cost is expected to be €5.5Bn plus €1.2Bn and €1.0Bn for the Danish and German hinterlands respectively and €0.5Bn for the new bridge. A joint venture between Rambøll, Arup and TEC has been established (known as the RAT team!) for the main design work with the main tendering process started back in September 2013.

The method of construction will be to dig out the seabed so as to create a flat bottom, then float the tunnel sections out and sink them into position. The dredged material will be re-used as reclaimed land of some 330 hectares to include new beaches and wildlife habitats including a vertical cliff. The tunnel will not be weather dependent and will be well lit but nonetheless there is a disaster potential. The edict has gone out that it must be demonstrated to be as safe or safer than an open road or bridge.

The portal at Lolland

The portal at Lolland

An intelligent management system will be devised primarily for the safeguarding of road traffic and will include:

» No queuing in the tunnel section and unidirectional traffic;

» Robust ventilation including longitudinal ventilation from portal to portal;

» Over pressure system for smoke control;

» Fans to assist exhaust extraction;

» Fire fighting and fire suppression systems to cool hazardous substances;

» Pumped drainage;

» Automatic incident detection.

Construction work has yet to begin but the plan is to open the link for traffic in 2021. The world will be watching progress with interest.

Three, all different

So, three different tunnel projects, all built or being built for different reasons and using different construction methods. It is also apparent that safety regimes differ from country to country and the rules for tunnel construction can vary from project to project. Whilst striving for absolute safety may be desirable, a pragmatic approach should recognise the many hundreds of rail tunnels that have been there for decades, with only a tiny number of instances where safety has been a real concern.

All three tunnels have or will consume considerable public financial expenditure and getting value for money has to be an objective for any projects of this size.