At 16.25 Italian time on Christmas Day 1870 a compressed air drill working on the Bardonecchia or Italian side holed through the last 3.8m of rock separating the headings from France and from Italy of the Mont Cenis Railway Tunnel. After 13 years, the concluding stages of the construction of the first trans-Alpine railway tunnel had been reached.

A special train, carrying important personages from the Contractor and Engineer, left Turin early the following morning to be present at the breakthrough.

Meanwhile, a number of boreholes were drilled in the intervening plug of rock. These were charged and fired in the afternoon and the headings were connected at 17.20 on 26 December. The error of alignment was less than 0.45m and the difference in level was 0.3m.

Signor Coppelo, surveyor of the tunnel and the engineer-in-charge of construction on the French side, having entered the tunnel at Modane, was the first man to walk through the whole length of the tunnel, then the longest in the world, when he arrived at Bardonecchia. He would also have been pleased with his survey carried out 12 years before. The errors of alignment and levels in the tunnel had been very small, although the actual length of the tunnel was 12,233.55m compared with 12,220m derived from the survey. Indeed, there had been some anxiety for a few days before breakthrough that the headings might be overlapping each other. It required only three more months to complete the tunnel and open it to traffic, although the official opening did not take place until 17 September 1871.

Preliminary stages

The Mont Cenis Railway Tunnel had taken a long time from conception to realisation and it was only the prospect of developing a workable compressed air drill that had made it a practicable proposition.

It all began in 1838 when Guisseppe Mendial, a native of Bardonneche (now Bardonecchia) who frequently crossed the Alps, submitted a plan to King Carlo Alberto of Sardinia for a railway tunnel piercing the narrowest part of the western chain of the Alps to connect Modane and Bardonneche (see Figure 1). Nothing came of Mendail’s bid.

In 1841, he submitted his plan to the Chamber of Commerce of Chambrey, but again without success and he died without seeing even the first steps being taken in the project he proposed.

In 1845, King Carlo Alberto commissioned a railway from Genoa to Turin, retaining Henri-Joseph Maus, a Belgian engineer, as consultant and asking him to investigate a suitable route across the Alps to extend the railway into Savoy, which at that time was also part of the kingdom of Sardinia. The Sardinian government also commissioned Professor Angelo Sisimunda to carry out a geological survey between Modane and Bardonecche.

Sisimunda found that the mountain consisted mainly of schistose limestone with carbonaceous schist, quartzite and compact limestone at the northern end of the tunnel (Figure 2).

Although Maus and Sisimunda and engineers accepted the desirability of the project, its sheer magnitude was overwhelming. It involved driving a tunnel of more than 12km with access at the ends only, without intermediate shafts, at depths of 1,200m. Opinion was that temperatures of 70ÞC would be encountered in the tunnel. There was also the problem of supplying air to the tunnellers working at distances of up to 6km from the portals.

The scheme intrigued fertile minds everywhere, and a plethora of patents were filed. For example Maus developed the first hard-rock tunnelling machine with funds provided by the Sardinian government. The device was constructed and tested at the Valdocco Arms Factory but the project was allowed to lapse.

The ventilation problem was solved in 1852 by Professor Jean-Daniel Colladon, a physicist from Geneva, who proposed the use of compressed air to drive the prospective machines to be used to construct the tunnel, with the spent air being used to sustain the tunnellers; mountain streams would provide power to produce the compressed air.

In 1854, Thomas Bartlett a British engineer designed a small portable machine for boring holes in rock using steam power. Some machines were taken to Savoy and used in 1855 for excavating cuttings on the railway between Chambery and St Jean-de-Maurienne. The machine was also demonstrated to Count Cavour the Prime Minster, Paleocapa the Minister of Public Works, the engineers who were to drive the Mont Cenis tunnel and Sommeiller (see box).

In 1956, trials were conducted on two Bartlett machines using compressed air instead of steam; penetration rates up to 59 times those achieved by hand drilling were achieved in a range of rocks. However, the machine was thought complicated and Sommeiller devised a drill, which was partly original and a partly improved Bartlett drill.

With the technical aspects of the tunnel out of the way only financial and political problems remained. Cavour recognised the importance of the Mont Cenis Tunnel as a symbol of Italian nationalism and was supported by Paleocapa and Sella Minister of Finance. On 29th June 1857 an Act was passed under which the government agreed to meet half of the estimated 40M lira (then about US$7.8M) cost of the tunnel*. The other half was to be paid by the Victor Emmanuel railway, a state railway, so in reality the cost was met by the Sardinian government.

(* Between 1860-70 the rates of exchange were approximately £1 = $4.87 = 25 francs = 25 lira)

Tunnelling begins

Work on site started on 18 August 1857 at the Modane Portal in the presence of King Victor Emmanuel 11 and Prince Napoleon. At the southern end tunnelling began on 4 November. The estimated completion time was 20 years.

Until 1861, all tunnelling was carried out by manual drilling (see Table 1). Holes were drilled to a depth of 0.5m to 0.9m by double-handed drilling, with one man holding and turning the drill steel which had a chisel tip and the second man striking it with a sledgehammer. Holes were charged with black powder, which was ignited by fuse.

The tunnel was advanced by first driving a heading 3.4m high and 2.4m wide along the invert. For the five and three years at the Modane and Bardonecchia ends of the tunnel respectively, annual progress of the advance heading averaged 201m, or about 0.56m/day.

At this rate the tunnel would have taken just over 30 years to complete. Some 25m or so behind the face of the advance heading, work started to open the tunnel to its full height and break out the crown to the shoulders with temporary support as required. When the lining had been placed, the benches at either side were then removed with the arching in the crown being propped while the lining below was completed to the floor of the tunnel.

With tunnelling under way, Sommellier and his engineers concentrated on solving the problems of introducing pneumatic rock drills. The main difficulty concerned the hydraulic compressors which proved unreliable and had to be scrapped. They were replaced by reciprocating compressors having 564mm dia cylinders and a 1.5m stroke with a flywheel. A pair of such machines was driven by a waterwheel 6m in diameter and 4.9m wide. This machinery worked reliably for many years.

Introduction of pneumatic drills

Compressed air drills were first used at the Bardonecchia and Modane ends of the tunnel at the beginning of 1861 and 1863 respectively. The introduction of compressed air drills resulted in a dramatic increase in progress. Thus in 1870, the final year of tunnelling, progress was almost 3.6 times the best year using manual drilling (1858).

Each new drill cost about 2000 francs and weighed about 300kg, and up to about 11 of these would be installed on a rail-mounted iron framework provided with a compressed engine. Including the drills this set-up weighed about 15t. Behind this frame was another carriage with cylindrical wrought-iron tanks filled with water maintained with a pressure of about 5 bar by compressed air for water jets to cool the drills and flush out the borings.

In a 3m2 advanced heading, some 70 to 80 holes between 0.8m and 0.9m deep were drilled into the face for each round. Three or four large holes about 90mm in diameter were bored at the middle of the face and left uncharged, the remaining holes being 32mm in diameter. The holes around the larger ones were first charged and fired to create a cavity in the middle of the face into which the remaining holes were fired in turn, working outwards.

In 1863, the time taken to complete a round consisted of six to eight hours drilling, one and a half to two hours charging and firing and three to five hours removing the debris. In the last two or three years of the tunnelling, however, work had speeded up with three rounds often completed in 24 hours. The greatest monthly progress made in the advance heading was in May 1865 when 121.8m were driven; the least progress using pneumatic drills was in April 1866 in the quartzites with an advance of just 10.7m.

In all, 127 workmen were employed in the Modane heading, with 44 on each of the two drilling shifts and 30 on charging, firing and removing debris. Despite the success of the compressed air drilling in the advance heading the enlargement of the tunnel was still carried out manually until 1868.

Manual enlargement work was carried out simultaneously at up to six points at either end of the tunnel which seriously impeded the removal of rock debris, particularly from the advance heading. The introduction of pneumatic drills, although increasing the cost of enlargement, enabled work to be concentrated in a single location; this also reduced the effect of the atmosphere in the tunnel on the exposed rock surfaces.

The Figures (3, 4 and 5) illustrate some of the sequence of operations in the later stages of driving at the Bardonecchia end of the tunnel. The 2.9m high by 2.6m wide advance heading was driven using seven pneumatic drills and enlarged to 3.7m2 using five drills. The second enlargement to a height of 5.5m was carried with two drills (Fig 3). The third enlargement from crown to shoulders was done manually and secured by brick arching (Fig 4).

The lower part of the tunnel was then enlarged to the full size required – a length of a metre or less being extended at a time, with temporary propping to support the arch. Finally, masonry side walls completed the tunnel (Fig 5). On the Modane side the lining was almost entirely in masonry, with the exception of the key which was brick. About 300m of tunnel through the quartzite was unlined.

The completed tunnel

The two-track tunnel rises from the north portal at an elevation of 1,202.6m above sea level at a gradient of about 1:45 to a summit at mid-point of 1,338.4m and then descends at 1:2000 to an elevation of 1,335.4m at the south portal. The cost of construction was US$14.6M, almost double the original estimate. It is thought the Italian government was able to recoup some US$6.2m of this from the French government and a further US$2.5M from the Victor Emmanuel Railway Company.

The exhaust air from the pneumatic drills ventilated the advance heading adequately, but conditions at the enlargement were poor. Here temperatures rose to 30°C or a little more, but never to the predicted 70°C, and a ventilation plant had to be installed to remedy this situation.

Today the tunnel continues in service carrying railway traffic through the Alps. After the Second World War it was also used to shuttle cars and trucks. Frejus II, a road tunnel (see box), was constructed during the 1970s to provide much needed additional capacity on this important route.

Related Files
Figure 1: Plan and location of the Mont Cenis Railway Tunnel
Figure 5: Completed tunnel with masonary lining below brick arching
Figure 2: Longitudinal and predicted geological section of the tunnel
Figure 3: The second enlargement to a height of 5.5m
Figure 4: Third enlargement followed by brick arching