The roads and railways that transverse the Swiss Alps are concentrated in narrow alpine valleys and passes, along which increasing amounts of goods are carried. North-south freight traffic between the major countries of the European Union, predominantly Germany and Italy, has contributed to existing infrastructure becoming overloaded, and worries about the environmental impact of the situation have been voiced for many years.

Under the unique Swiss democratic process, a referendum vote opened up Swiss government funding and provided fresh impetus to the concept of transferring freight traffic onto high-speed trains. The scheme is a combination of the Bahn 2000 project (to upgrade the Swiss railways in general) and the AlpTransit project to speed trans-Alpine routes. Of the later, the Gotthard and Lötschberg base tunnels are the major cost and time elements.

Of the two, Gotthard is the most ambitious, with 57km long twin tubes between Erstfeld in the north and Bodio in the south (figure 1). It will take up a third of the total investment in Swiss railways over the next 20 years, and on completion will provide a freight capacity of 40M t/y, carried on over 200 trains/day.

The geology along the Gotthard alignment (T&TI, November ’03) has posed some major headaches, particularly in the area of the Piora syncline and at the Tavetsch Massif, the latter occurring in the Sedrun construction section. It had been thought that the Piora Syncline deposit of sugar-like running sand with water under high-pressure would cut across the planned tunnel horizon. However the deposit was delineated in the early 1990s by a TBM-driven exploration tunnel, from which long probe-holes were drilled. Fortunately, the base of the syncline was found to be above, and clear of, the preferred main tunnel horizon, increasing confidence in the project and speeding up its enactment.

Sedrun developments

Having clarified the geological conditions on the most difficult section of the proposed alignment, construction work commenced in April 1996 on the 990m long access tunnel at Sedrun (figure 2). This is located towards the middle of the alignment, and provided access both to the planned position of a sub-vertical access shaft, and for development of the main foul air outlet between the proposed shaft and the Val Nalps.

Sinking work began on the 800m deep No 1 shaft in August 1998. The shaft was sunk conventionally by a JV of Shaft Sinkers, Murer, Locher, Marti, Zschokke and CSC, using a platform-mounted Atlas Copco multi-boom jumbo. The second (No 2) shaft was subsequently excavated by Thyssen Schachtbau in JV with Östu Stettin and Murray & Roberts RUC. Thyssen used a Wirth Vee-Mole shaft drill and slyped the hole to 7m diameter. Simax handled the installation of the lift in the main hoisting shaft.

Since completion of the No 2 shaft, which is equipped for the transportation of heavy equipment and improving ventilation, underground work has accelerated and the north and south faces have now advanced a distance of 1km from the shaft bottom.

Much of the excavation work on the multi-function station (emergency egress, maintenance access and ventilation) near the shaft, including crossovers and emergency stop station, has now been completed. Major temporary installations at the shaft bottom horizon, still 550m above sea level, are workshops, the main pumping facility, and ventilation plant.

In addition, a rock mechanics laboratory has been established, in which full circular steel arches can be tested for convergence in the difficult plastic rock of the Tavetsch intermediate massif of the Sedrun shaft location. This old formation is composed of gneiss, soft phyllites and slates in variable sequence of soft and hard rock strata in a nearly upright position.

Running tunnels

Once excavations for the major installations in the vicinity of the shafts are complete, the main running tunnels for the railway will be driven north towards Amsteg and south towards Faido, using drill and blast. These drives will meet the large Herrenknecht TBMs working towards Sedrun from both directions.

The 6.2km long Sedrun section and its connection with the Faido section to the south is on the critical path, so nothing can be left to chance. The running tunnels are being excavated at a standard diameter of 11m, 40m apart, with connecting galleries at 325m intervals.

To the south, the running tunnel drives have to negotiate the Urseren-Garvera zone to reach the Gotthard massif. This zone consists of metamorphic rock mixtures of varying strength from Mesozoic ocean sediments, which have been squeezed between the massifs by tectonic movement. These wedges are composed of rocks in different states of deformation, and rock quality cannot be taken for granted, particularly at this depth, Therefore investigation drilling ahead of the face is being undertaken (see below).

To the north of the Tavetsch intermediate massif is the Palaeozoic Aar massif, which is considered to be favourable for tunnelling. However, conditions along the way in the Sedrun section may be demanding.

Surface installations

The surface installations at Sedrun are located in Las Rueras, away from residential areas, and are designed for minimum environmental intrusion. Temporary dormitories are situated above the main site, which is on the valley floor. Man riding trains take the workers through the access adit to the shaft collar, where they are lowered in a cage to the working level. The Koepe winder takes just 80 seconds for the 800m trip.

A works railway is connected to the site by a dedicated spur from the famous narrow gauge Furka-Oberalp Railway for all-weather materials transport. Huge concrete batching plant storage silos are a dominant feature of the site.

Much of the rock being raised through the Sedrun shaft is crushed and recycled in a surface treatment plant, from which covered conveyors transport the waste fraction to prepared dumps further down the valley. A conveyor belt fire on the disposal facility in June has not delayed tunnelling progress.

Underground construction

Construction of the Sedrun section was awarded to Transco; a consortium of Batigroup, Frutiger, Bilfinger & Berger, and Pizzarotti, at a reported price of US$847M. Apart from its value as an intermediate attack point for the running tunnels north and south, the installations that will form the Sedrun Multifunction Station will also perform a major role in the safety and security for passengers travelling the Gotthard route.

The main shaft will provide fresh air ventilation for the tunnels, while the No 2 shaft will provide an independent exhaust to the Val Nalps. This system will allow fresh air circulation around the emergency evacuation installations in the vicinity of the shaft bottom, should this be needed. Passengers and crews of trains can then disembark at the emergency stop stations located on the west tube south of the shaft complex and on the east tube north of the shaft complex, in the event of an incident in the running tunnels. Rail crossovers between the main running tunnels will permit trains to enter either of the emergency stop stations.

The ground in the close proximity of the shaft is good for tunnelling, and most work has been undertaken using drill and blast with eight specially modified Tamrock Axera drill-rigs. However, in some sections of weak ground, umbrella drilling has been used to advance an arched profile, supported radially by grouted rockbolts. In places where rock conditions have worsened, the arch has been opened to full circle and sectional steel friction supports, installed to absorb the plastic deformation of the ground. In these places, the face area of excavation is increased to 13m diameter in the expectation that this will reduce to 11m over a period of 4-6 weeks, as the steel rings compress under the load.

Spoil removal is normally by a fleet of nine Toro loaders and haulers to the shaft bottom, with scaling by CAT excavators.

Drill and blast is not always possible, especially when the face has to be secured in advance using long steel rockbolts. In these situations, the 36t CAT325C CR tunnelling excavator, equipped with hydraulic hammer or scaling bucket, is used. This machine is purpose-built for tunnelling, with a short swing radius (1.9m), oversized undercarriage and short boom design.

The larger and more powerful custom-built 45t CAT 330C, with a two-piece boom and tiltable cab, is a real multi-purpose production excavator. There are now four CAT330C excavators on site, equipped with a 360? rotation device and a manipulator for precise installation of the steel arches and a rotary cutter, hydraulic hammer and ripper bucket for excavation. A specially developed cooling and filtration system also aids reliability and availability of the machines.

Special arch-erection platforms supplied by Rowa have been installed in anticipation of the poorer rock conditions of the Tavetsch intermediate massif and are being maintained at positions close to each face. These are carried on rails suspended from the roof, and are each equipped with two hydraulic arms with access baskets, from which loose ground around the crown of the drive can be excavated. A hydraulic manipulator can be used to install the upper sections of the steel arch support.

Considering the ground conditions, tunnelling on the running tunnels is reported to be making good progress, with daily advance rates of 9m/day towards the south on both drives. At the first week of October, 2,193m had been excavated in the east drive and 1,642m on the west side. Further tunnelling will be delayed by core drilling to investigate the ground in the Urseren-Garvera zone ahead of the faces.

To the north the transition to the ‘squeezing rock’ zone of the Tavetsch intermediate massif has been reached and work on special supports continues. It is believed to be the first time that deformable steel insert rings have been used in such large dimensions, although the principle has been used in German coal mines. These will be installed with the suspended Rowa platforms to allow work beneath. Each 13m diameter ring is composed of eight segments with yielding connectors to form two concentric rings. Testing to failure of rings installed in the underground laboratory so far has confirmed earlier small-scale results and that it would be a suitable method of tunnelling through the Tavetsch intermediate massif.

Related Files
Fig 1 – Map showing the lengths of sections on the Gotthard Base Tunnel
Fig 2 – Schematic diagram of the Sedrun section (not to scale)