Opened in September 1871, the 13.7km long Frejus Rail Tunnel was the first rail tunnel to be constructed through the Alps and is one of the most historic tunnels built. Thought to have been conceived in 1845 by King Carlo Alberto of Sardinia; his son Vittorio Emmanuel II, later proclaimed the first king of Italy after its unification in 1870 (coincidentally a few months before the tunnel opened), was instrumental in its initiation in 1857. The original purpose of the tunnel was to link Savoie with Piemonte, the two parts of the Savoy Kingdom divided by the Alps; as it was not until 1860 that Italy swapped Savoy and Nice with the French for Lombardy from the Austrians.
Prior to construction of the tunnel, passage across the Alps was extremely difficult and could only be achieved via cols high above the valley floor, which were often impassable in winter. The Romans had established early roads through these cols, which were substantially further developed by Napoleon. By the mid-1800’s railways had been pushed up the valleys to the foot of the mountains on both sides, yet no one considered it conceivable that railways could haul freight up the steep inclines necessary to cross the Alps – a tunnel was therefore the only option.
The Frejus tunnel follows the most direct route through the Alps from Savoie to Piemonte, this is 1600m beneath Cima del Gran Vallon between the towns of Modane in France and Bardonecchia in Italy. Three Italian engineers, Sommellier, Grandis and Grattoni, were commissioned in 1850 by the Italian Government to investigate the feasibility of running a railway below the Alps in a tunnel.
The single bored tunnel was designed and set out for the most part in a straight line; it is approximately 7.9m wide, 6m-6.3m high and horseshoe in shape. A 600m long covered tunnel brings the railway, on a curve, into Bardonecchia from the original construction portal, still visible close to the Frejus road tunnel portal.
The construction of the tunnel was a feat of technological excellence and many innovative techniques were used in its construction (see box). Not least, was the fact that with construction taking place from opposite sides of the Alps they had to meet in the middle. The meeting below the Alps was exulted at quarter past four on Christmas Day in 1870 and later reported in Harper’s Magazine as being as proud a moment as that of Wellington “when he saw Napoleon’s Imperial Guard tumbling back in rout from its charge upon his solid square.”
Not to be confused with Frejus Road Tunnel that opened 108 years later in 1979, the Frejus Rail Tunnel now forms the vital means of connection between Italy and France carrying through trains on twin tracks from Paris and Lyon to Turin and thence onto Milan and Rome. The tunnel currently carries approximately 9Mt of rail freight per year.
Alpine Rail Motorway
Over the past twenty years there has been a dramatic switch from rail freight haulage to road haulage. Unfortunately the increase in traffic has taken its toll on the environmentally sensitive Alpine region and the nearby Frejus Road Tunnel is reaching capacity, carrying up to 8000 HGV’s per day.
Furthermore, recent incidents in the Mont Blanc, Tauern and St Gotthard tunnels have highlighted the consequence of increased road traffic. All Alpine countries are making huge efforts to improve trans-Alpine communications and move freight from road to rail. A new rail tunnel has been proposed to link Turin and Lyon, between St Jean de Maurienne and Bussoleno. At 52.7km long, this deep tunnel is unlikely to be completed, at the earliest, before 2020.
In a move to immediately redress the balance, a new Alpine Rail Motorway service has been introduced jointly by SNCF and Trenitalia (through the joint venture company, Autostrada Ferroviaria Alpina) in cooperation with the French and Italian Governments. The service takes HGV’s off the road and loads them onto wagons, which run between freight transfer hubs located in Orbassano in Turin, Italy, and Aiton, close to Chambery in France (figure 1). The service commenced in November 2003 and currently runs four times a day using specially modified wagons lower in height than the average rolling stock. After 2006, when the upgrade work to the Frejus Tunnel will have been finished, HGVs will be loaded onto standard wagons for the 175km transit between France and Italy. It is envisaged that 30,000 to 50,000 HGVs can be hauled this way per year.
So as to allow passage of HGV loaded freight wagons it is necessary to increase the size of the tunnels on this portion of the railway and to readjust the track layout. In the Frejus Tunnel the existing track needs to be lowered by about 600mm, to a level close to the base of the foundation of the existing masonry tunnel lining. To do so requires removal of the existing track, track bed and formation and excavation of the underlying rock forming the invert of the tunnel.
To safeguard the tunnel lining the masonry arch foundation has been improved and the masonry tunnel lining strengthened. New niches are also required to accommodate electrical and mechanical equipment and provide safety refuges. All this has to be accomplished whilst not interrupting international rail services using the tunnel.
Structural solution
The original tunnel was constructed by drill and blast and lined with masonry using a technique known as ‘ripiena a sacco’, which involves filling the annulus between the masonry lining and the rock using construction rubble. The lining springs from rock masonry foundations located approximately 1m below rail level.
Structural investigations carried out in 2000 on the Italian side of the tunnel (6.7km long), indicated the possible presence of voids in the crown of the tunnel and also identified sections of the tunnel lining that were in distress. In order to design the most suitable remedial measures it was necessary first to determine the geotechnical nature of the rock mass within which the tunnel was situated.
A series of geotechnical investigations were carried out including in-situ and laboratory investigations. The tunnel was originally excavated within calc-schist belonging to the Piemontese Series – a rock type that displays a wide range of fabric and tectonism varying from massive to foliated. Geotechnically, the rock mass characteristics as a consequence were found to vary significantly. Using parameters derived from a GSI classification of the rock mass it was possible to back analyse the behaviour of the tunnel lining for different states of lining condition. Flat jack tests were carried out in the masonry lining and these revealed that the loading conditions were highly asymmetric and that the in-situ horizontal stress was greater than unity. Long term compression tests also revealed the rock to exhibit considerable creep properties. Analyses have confirmed the tendency for the lining to converge and for high stresses to develop in the lining, particularly in the crown.
Consequently a structural solution was required to prevent continued convergence and distress of the tunnel lining, prevent settlement of the foundations and support sections of the tunnel where new niches were required (figure 2).
It was essential that structural continuity be restored between the tunnel lining and the rock mass and this was provided by grouting of voids within the annulus and by pinning the masonry using rock bolts. CT bolts 3m long were deployed systematically over the lower portions of the masonry lining. These bolts were favoured due to their durability and ability to be shortened into 1.5m lengths to allow them to be used within the tunnel during single track working, each section of the bolt was connected using a threaded connector.
The remaining portions of the tunnel arch adjacent to the niches are supported using 3m long Swellex Mn24 dowels, which have been sleeved where they pass through the lining so as to prevent expansion and loading of the masonry lining (figure 3). Contact grouting was continued through the remainder of the annulus. The foundations were improved by the injection of cementitious grout through inclined valved tubes installed through the base of the tunnel lining.
Where niches up to 4.4m deep are required then the opening within the masonry is to be supported using 6m long self-drilling rock bolts (divided into 1.5m lengths), which strengthened the rock mass immediately above the niche. The niches themselves are supported using steel sets and shotcrete. Having previously installed self-drilling anchors around the opening, the new niches will be formed by first cutting the masonry lining using circular diamond saws and then demolishing the masonry using a robot. Rock excavation is to be carried out in the same manner as explosives are not permitted on the live railway.
Following strengthening of the tunnel lining on both sides of the tunnel, excavation can then commence to lower the tunnel invert. Due to single track working it will only be possible to excavate half the invert at one time over a width of 3.9m. Excavation is to be carried out using a Voest Alpine AHM 105 roadheader. The excavation process will be carefully controlled in two stages so as to ensure that the cutting head is fully stabilised in the typically 30MPa to 60MPa strength rock mass. Following an initial cut 50mm to 60mm deep to form a step in the rock, the step will subsequently be removed.
Geotechnical monitoring
Extensive geotechnical monitoring has been installed to monitor the performance of the masonry tunnel lining and the surrounding rock mass. The monitoring includes convergence monitoring of cross-sections at 50m centres along the length of the tunnel. At ten sections the stress in the lining will be monitored using flat jacks installed in cuts made in the masonry and load cells on CT bolts. Vibrations will also be closely monitored during roadheader excavation. During niche construction the masonry lining will be monitored using electronic distance measurement techniques (lasers) and by crackmeters. The results from the instrumentation will be downloaded to a project web-hosted GIS for real time distribution.
Construction logistics
There are only two windows a day when it is possible to gain access to the entire tunnel: two hours in the early afternoon and three hours in the early morning. With such short time periods available for complete possession, the works are being undertaken during single-track possessions. All the construction work described above is therefore being carried out on a live railway carrying Eurostar, TGV and freight services, which are diverted onto the adjacent track. None of the construction methods chosen can jeopardise the safety of the tunnel or the passing trains and particular care has been taken in this regard. Indeed, all equipment has to fit within the half width of the tunnel (one working track), this includes the drilling jumbos, the lengths of the rock bolts and even the roadheader.
Current progress
The construction works described above were awarded to a joint venture comprising seven contractors (CLF, AR FER, GEFER, SALCEF, PASOLINI VCB, RTS and VALD TERRA) in November 2002 for US$62.1M. Geodata SpA carried out the construction design for the JV and are assisting during construction. Italferr SpA is the scheme designer and engineer for owner and promoter RFI (Rete Ferroviaria Italiana).
As T&TI went to press all the drilling and grouting works had been completed and the roadheader had been delivered to site. The entire project is due for completion in February 2006. Similar upgrading works in the French portion of the tunnel are being procured and managed separately.
Related Files
Map showing the location of the Frejus rail tunnel and the route of the Alpine Rail Motorway
Fig 3 – Typical cross section of the tunnel
Fig 2 – Cross section of the tunnel at a niche location
