Alp base tunnels – concept
Two new railway axes are to be constructed through the Alps in Switzerland to connect the high-speed rail networks in Germany and Italy. These are known as the Alp base tunnels or the Alp Transit scheme. The project, solidly backed by the Swiss cantons and central government, has overwhelming support from the Swiss people, who voted in favour of the finance to build it in a 1998 national referendum. Environmental improvements will accrue from HGV transportation by train rather than using the Swiss roads. Furthermore passenger journey times will be reduced. It is a massive undertaking and several British contractors are chasing work in multi-national consortia and joint ventures. On 21 September, the British Tunnelling Society heard about the scheme from the Swiss point of view. Felix Amberg, president of Amberg Consulting Engineers from Zürich talked about the background to the scheme and illustrated the challenges for the future by more detailed reference to the Gotthard base tunnel and the Lötschberg Tunnel.
The old Gotthard Tunnel is 118 years old and the Lötschberg is 87. Both are too slow for the speeds required of present railway systems. The new tunnels, are to be of large cross-sectional area to allow for all types of goods transportation and will have generous radii to allow high speeds. Phase one comprises the Gotthard and the Lötschberg tunnels. Phase two, envisaged to start around 2005, comprises the Zimmerberg and Ceneri Tunnels.
Financing
The Swiss central government decided to finance the project with a levy on HGV traffic passing through Switzerland. An additional ‘permille’ of the VAT would be allocated to financing the project and a proportion of debt would be arranged. The total cost of the two routes was estimated at SFr15bn ($8.5bn) in 1991. Because of the high cost, the scheme was restricted to these four main tunnels. The 1995 cost estimate was SFr13bn ($7.3bn). A specially created company, Alp Transit, which is a subsidiary of Swiss Railway Company SBB, will supervise the construction of the tunnels. On completion, three tunnels will be handed over to SBB. The Lötschberg Tunnel will not, as it belongs to the BLS network, a separate company.
Lötschberg Tunnel
The 37.6km long Lötschberg tunnel has two tracks and a maximum overburden of 2200m. The north section, Frutigen to Mitholz, has one tunnel excavation to carry the tracks as the existing site investigation gallery acts as a service tunnel. In the remaining sections, Mitholz, Ferden, Raron, both tunnels will be excavated. One will carry the rail tracks; the other will act as a service tunnel with the two joined by a cross-passage every 330m. There are portals at Raron, Amsteg and Frutigen with intermediate attacks at Ferden and Mitholz. The northern section of the work has been awarded to the SATCO joint venture, comprising Ilbau, Dumez, Skanska, Rothpletz, Lienhard & Co and Walo Bertschinger. The other two sections were won by the Matrans joint venture comprising Marti, Züblin, Dywidag, Walter Bau, Heilit & Wörner Bau, Unternehmergruppe Porr and Balfour Beatty. Tenders have not yet been returned on the Ferden section.
Gotthard base tunnel
Some of the main features are:
The programme requires intermediate construction points to allow more points of attack. These intermediate shafts will serve as ventilation shafts when the construction is complete. The siting of the intermediate points of attack was chosen to divide the tunnel into roughly equal lengths and to allow difficult geological conditions to be dealt with at an early stage of the proceedings, as follows:
These are considered to be favourable for tunnelling. Most of the zones with significant overburden have an almost vertical dip and will be excavated perpendicularly to the dip. The technically difficult sections lie in the Tavetscher Intermediate Massif and in two younger sedimentary zones, the Urseren Garvera Zone and the Piora Basin. These tunnels through the Alps have, of course, extremely large overburden over most of their length. Some 35km have overburden of more than 1000m, 20km more than 1500m and 5km reach more than 2000m. The maximum overburden is 2300m. Throws into perspective the overburden experienced on the Channel Tunnel with that to be expected in the Alps.
Site investigations in difficult ground
gives a general geological seSitection along the Gotthard Tunnels. The Piora Basin and the Tavetscher Massif were regarded as the two most difficult zones for tunnelling. It was decided to carry out thorough investigations of these areas at an early stage of the planning. The investigation of the Piora Basin was done by an investigation tunnel 5.5km long, 350m above tunnel level. The structure and thickness of the basin was explored by boreholes from the tunnel. The boreholes revealed a complex structure consisting of dolomites, Rauhwacken, dolomite-gypsum-anhydrite with differing proportions of sugar-grain dolomite and high water pressure up to 130 bar. The Piora basin at that level covers 230m of the tunnel alignment. Based on these findings, a second stage of investigation was undertaken with further boreholes correlated by seismic measurements to quantify the size of the basin, research to find the optimum method for crossing this ground in the tunnel and the sinking of a 350m deep shaft to base tunnel level. The investigation revealed that at base tunnel level, the Piora Basin consists of stable dolomite, marble and dolomite-anhydrite. No sign of high water pressure was found in the boreholes at base tunnel level.
The geological model derived from the findings showed the Piora Basin to be solid Trias limestones/ dolerites with occasional lenses of anhydrite or gypsum. Rock fissures are filled with gypsum, preventing major inflow of water. Between the exploration level and the base tunnel level, geologists have interpreted the existence of a gypsum cap formed through the transformation of anhydrite to gypsum by water percolating from nearby valleys.
A parallel investigation was being carried out in the Tavetscher Massif. It concluded that the tunnel would traverse very poor quality rock of highly fractured phyllites, shales and metamorphic gneisses. This is the most demanding area of the tunnel works determining overall programme rate.
Tunnel climate control
The great length of the tunnel combined with high rock temperatures and the high frequency of trains have been factors which characterised the analysis of the tunnel climate and in turn ventilation and safety requirements, especially those related to fire. Considering the routine running of the railway, it is desirable to minimise the maintenance needs of equipment. Nevertheless inspections and maintenance is necessary on a rolling programme and to achieve this in the temperatures expected, mobile cooling units will be required and shorter shift. Trains will be air-conditioned so the effects of the tunnel climate will not impinge on passengers. The following criteria have been drawn up to define the optimum tunnel climate:
Modelling has shown that these temperatures will be achieved near the ends of the tunnels but that the humidity is expected to be around 30%. This climate modelling led to the following conclusions:
Under these conditions it is possible to achieve the criteria for minimising the corrosive effects from humidity and to attain an acceptable temperature of 35°C. However, the geological forecasts for water inflow exceed the requirements. Therefore the tunnels will be sealed by a membrane.
Concrete requirements
The high rock temperatures also have an effect on construction materials and in particular shotcrete and concrete which must stand up to the following criteria:
In essence, the aggregates are client-supplied materials. So the client decided to run pre-qualification trials for companies who wished to supply cement and additives. After preliminary submissions, companies who prequalified could take part in main trials. Due to the rock petrography and construction techniques, three different classes of trial were run. Trial mixes for prequalification consisted of two mixes for cast insitu concrete and two for sprayed concrete.
OB1 Cast insitu 40/30 mix with requirements for early strength and watertightness
OB2 Cast insitu 40/30 sulphate-resisting mix with requirements for early strength, watertightness
SB1 35/25 mix with requirements for early strength and water-tightness
SB2 35/25 sulphate-resisting mix with requirements for early strength and watertightness
Shotcretes are wet-mix. To ensure concrete durability, some restrictions on water/cement ratio and cement content were imposed. All mixtures had to remain pumpable for six to eight hours. Chambers mimicking tunnel conditions were set up for the trials at the Hagerbach test gallery. The 75% of the excavated material not used for aggregates has to be disposed of. Some can be accommodated near the project, but most must be transported to Bodio. Transportation can only be by train or conveyor. This places a priority on connecting the Bodio and Faido by tunnel as early as possible. A contingency tunnel 5m in diameter is offered to assist in the logistics.
Operational safety
Planning has taken account of safety from the outset. The safety of passengers and the event of accidents involving goods trains have been the main considerations. The system finally chosen comprises twin tubes without a service tunnel. At the third-points of the tunnel, emergency stops are planned which contain the required equipment and infrastructure to help passengers to safety and deal with fire and ventilation issues. The rail tunnels are connected by cross-passage every 312.5m. Additional safety measures include remote train-guidance systems, video support and communication systems. The emergency stops have emergency stations, side galleries, connection galleries and caverns. The galleries are divided longitudinally into two separate sections – a waste air section and a fresh air section. This ensures the safety of passengers in fresh air until they are able to get out of the tunnel.
Dimensioning and construction
The main tunnels will be driven by TBM with the exception of the critical zone in the Tavetscher Massif. However, the multi-functional stations will be constructed by drill and blast. The standard TBM cross-section is shown in Figure 5.
It is based on a double-shell lining system with an inner cast insitu concrete lining. The temporary rock support is by shotcrete with mesh reinforcement, lattice girders and rock bolts as conditions dictate. A waterproof membrane is to be incorporated between the temporary support and the cast insitu lining. This will contain drainage strips to conduct any eventual leakage to drains and out of the tunnel.
The TBM criteria for the work are as follows:
These requirements are derived from practical experience in the Vereina Tunnel where Amberg Consulting Engineers was the designer and site supervision team. More elaborate design of the rock-support system is required in the geologically difficult areas where unfavourable and occasional squeezing ground can be expected.
Present progress
To access the tunnel levels at the critical Sedrun area, an early start was made to the works in that area. An access tunnel has been constructed with an 800m deep shaft at the end. At the foot of the shaft, heading operations will commence to the north and south. The shaft has been equipped for muck handling, equipment handling and transport of personnel. The double-deck muck hoist equipment can carry two muck-cars with a capacity of 11m³ or 25t per deck at a speed of 16m/s (60km/h). Work started at Sedrun in 1996 with the shaft beginning in 1998. In 1999 the drill and blast work began at the intermediate attack points in Faido and Amsteg. In Bodio the transport gallery began in 1999 together with various other smaller packages of work. All tunnels giving access to the base tunnels are now under construction. Contracts will be signed for Faido and Bodio at the end of September 2001, for Amsteg in October 2001 and at the end of January 2002 for Sedrun.
Related Files
Tunnel cross-section.
Overburden Alps and Channel Tunnel
General layout.
