Volume 1 — From Chambers's Journal. THE MONT CENIS TUNNEL.

The tunnel through the Alps at present being pierced to connect the railway system of France and Italy, has acquired the title of the "Mont Cenis Tunnel;" but its real position and direction have very little in common with that well-known Alpine pass. On examining a chart of the district which has been selected for this important undertaking, we shall observe that the main chain of the Cottian Alps extends in a direction very nearly East and West, and that this portion of it is bounded on either side by two roughly parallel valleys. On the North we have the valley of the Arc, and on the South the valley of the Dora Ripari, or, more strictly speaking, the valley of Rochemolles, a branch of the Dora. The Arc, flowing from East to West, descends from Lanslebourg to Modane, and from thence, after joining the Isere, empties itself into the Rhone above Valence. The torrent Rochemolles, on the other hand, flowing from West to East, unites itself with the Dora Ripari at Oulx, descends through a narrow and winding valley to Susa, and thence along the plain to Turin. The postal road, leaving St. Michel, mounts the valley of the Arc as far as Lanslebourg, then turns suddenly to the South, passes the heights of the Mont Cenis, and reaches Susa by a very steep descent. On mounting the valley of the Arc, and stopping about eighteen miles West of Mont Cenis, and a mile and a half below the Alpine village of Modane, we arrive at a place called Fourneaux. Here, at about three hundred feet above the level of the main road, is the Northern entrance of the tunnel; the Southern entrance is at the picturesque village of Bardonnêche, situated at about twenty miles West of Susa, in the valley of Rochemolles.

The considerations which decided the Italian engineers upon selecting this position for the contemplated tunnel, were principally the following: first, it was the shortest route that could be found; secondly, the difference of level between the two extremities was not too great; and, thirdly, the construction of the connecting lines of railway—on the North, from St. Michel to Fourneaux, and on the South, from Susa to Bardonnêche were, as mountain railways go, practicable, if not easy. The idea of a tunnel through the Alps had long occupied the minds of engineers and of statesmen both in France and Italy; but it is to the latter country that we must give the credit of having worked the idea into a practical shape, and of having inaugurated one of the most stupendous works ever undertaken by any people. To pierce a tunnel seven and a half English miles long, by ordinary means, through a hard rock, in a position where vertical shafts were impossible, would be an exceedingly difficult, if not, in a practical point of view, an impossible undertaking, not only on account of the difficulties of ventilation, but also on account of the immense time and consequent expense which it would entail. It was evident, {61} then, that if the project of a tunnel through the Alps was ever to be realized, some extraordinary and completely new system of mining must be adopted, by means of which not only a rapid and perfect system of ventilation could be insured, enabling the miners to resume, without danger, their labors immediately after an explosion, but which would treble, or at least double, the amount of work usually performed in any given time by the system hitherto adopted in tunnelling through hard rock. To three Piedmontese engineers, Messrs. Grandis, Grattoni, and Sommeiller, is due the merit of having solved this most difficult problem; for whether the opening of the Alpine tunnel take place in ten or twenty years, its ultimate success is now completely assured.

A short review of the history of this undertaking, and a summary of the progress made, together with a description of the works as they are conducted at the present time, derived from personal observation, cannot fail to be interesting to English readers.

Early in 1857, at St. Pier d'Arena, near Genoa, a series of experiments was undertaken before a select government commission, to examine into the practicability of a project for a mechanical perforating-engine, proposed by Messrs. Grandis, Grattoni, and Sommeiller, for the more rapid tunnelling through hard rock, and with a view to its employment in driving the proposed shaft through the Alps. This machine was to be worked by means of air, highly compressed by hydraulic or other economical means; which compressed air, after performing its work in the perforating or boring machines, would be an available and powerful source of ventilation in the tunnel. These experiments placed so completely beyond any doubt the practicability of the proposed system, that, so soon as August of the same year, the law permitting the construction of the tunnel was promulgated.

At this time, absolutely nothing had been prepared, with the exception of a very general project presented by the proposers, and the model of the machinery with which the experiments had been made before the government commission; we cannot, therefore, be much surprised on finding that some considerable time elapsed before the new machinery came into successful operation, the more particularly when we consider the entire novelty of the system, and the unusual difficulties naturally attending the first starting of such large works, in districts so wild and uncongenial as those of Fourneaux and Bardonnêche. Fourneaux was but a collection of mountain-huts, containing about four hundred inhabitants, entirely deprived of every means of supporting the wants of any increase of population, and where outside-work could not be carried on for more than six months in the year, owing to its ungenial climate. Nor was the case very different at Bardonnêche, a small Alpine village, situated at more than thirteen hundred metres (4,225 feet) above the level of the sea, and populated by about one thousand inhabitants, who lived upon the produce of their small patches of earth, and the rearing of sheep and goats, and with their only road of communication with the outer world in a most wretched and deplorable condition. Under these circumstances, we can imagine that the task of bringing together large numbers of workmen, and their competent directing staff, must have been by no means easy; and that the first work of the direction, although of a nature really most arduous and tedious (requiring, above all, time and patience), was also of a nature that could scarcely render its effects very apparent to the world at large for some considerable time. Again, it was necessary in this time to make the detailed studies not only of the tunnel itself, but of the compressing and perforating machinery on the large scale proposed to be used. This machinery had to be made and transported through a country abounding in difficulties. Then, as might be {62} expected, actual trials showed serious defects in the new machines for the compression of air; and, in perfecting the mechanical perforators, unexpected difficulties were encountered, which often threatened to prove insurmountable. The total inexperience and unskilfulness of the workmen, and the necessity of giving to them the most tedious instruction; accidents of most disheartening and discouraging kinds—all tended to delay the successful application of the new system.

The first important work to be undertaken was the tracing or setting out of the centre line of the proposed tunnel. It was necessary first to fix on the summit of the mountain a number of points, in a direct line, which should pass through the two points chosen, or rather necessitated by the conditions of the locality, for the two ends of the tunnel in the respective valleys of the Arc and of Rochemolles; secondly, to determine the exact distance between these two ends; and thirdly, to know the precise difference of level between the same points. These operations commenced toward the end of August, 1857. Starting from the Northern entrance at Fourneaux, a line was set out roughly in the direction of Bardonnêche, which line was found to cut the valley of Rochemolles at a point considerably above the proposed Southern entrance of the tunnel. On measuring this distance, however, a second and corrected line could be traced, which was found to be very nearly correct. Correcting this second line in the same manner, always departing from the North end, a third line was found to pass exactly through the two proposed and given points. The highest point of this line was found to be very nearly at an equal distance from each end of the tunnel, and at but a short distance below the true summit of the mountain-point, called the "Grand Vallon." The line thus approximately determined, it was necessary to fix definitely and exactly three principal stations or observatories—one on the highest or culminating point of the mountain, perpendicularly over the axis of the tunnel; and the other two in a line with each entrance, in such a manner that, from the centre observatory, both the others could be observed. At the Southern end, owing to the convenient conformation of the mountain, the observatory could be established at a point not very far from the mouth of the tunnel; but toward the North, several projecting points or counterforts on the mountain necessitated the carrying of the Northern observatory to a very considerable distance beyond the entrance of the gallery—not, however, so far as not to be discerned clearly and distinctly, and without oscillation, by the very powerful and excellent instrument employed. These three points permanently established, remain as a check for those intervening, and serve as the base of the operations for the periodical testing of the accuracy of the line of excavation.

The first rough tracing out of the line was completed before the winter of the year 1857, and it was considered sufficiently correct to permit the commencement of the tunnel at each end by the ordinary means—manual labor. In the autumn of 1858, the corrected line was traced, and the observatories definitely fixed, and all other necessary geodetic operations completed. Contemporaneously was undertaken a careful levelling between the two ends, taken along the narrow path of the Colle di Frejus, and bench-marks were established at intervals along the whole line. All the data necessary for an exact profile of the work were now obtained. The exact length of the future tunnel was found to be twelve thousand two hundred and twenty metres, or about seven and a half English miles; and the difference of level between the two mouths was ascertained to be two hundred and forty metres, or seven hundred and eighty feet, the Southern or Bardonnêche end being the highest. Under these circumstances, it would have been easy to have established a {63} single gradient from Bardonnêche down to Fourneaux of about two centimètres per mètre—that is, of about one in fifty. But a little reflection will show, that in working both ends of the gallery at once, in order to effect the proper drainage of the tunnel, it would be necessary to establish two gradients, each inclining toward the respective mouths, and meeting in some point in the middle. This, in fact, has been done, and the two hundred and forty metres' difference of level has been distributed in the following manner: From Bardonnêche, the gradient mounts at the rate of 0.50 per one thousand mètres—that is, one in two thousand as far as the middle of the gallery; here it descends toward Fourneaux with a gradient of 22.20 mètres per one thousand, or about one in forty-five. The highest point of the Grand Vallon perpendicularly over the axis of the tunnel is 1615.8 mètres, or 5251.31 feet.

The difficulties encountered in the carrying out of these various geodetic operations can scarcely be exaggerated. It is true that nothing is more easy than to picket out a straight line on the ground, or to measure an angle correctly with a theodolite; but if we consider the aspect of the locality in which these operations had to be conducted, repeated over and over again, and tested in every available manner with the most minute accuracy, we shall be quite ready to accord our share of praise and admiration to the perseverance which successfully carried out the undertaking. In these regions, the sun, fogs, snow, and terrific winds succeed each other with truly marvellous rapidity, the distant points become obscured by clouds, perhaps at the very moment when an important sight is to be taken, causing most vexatious delays, and often necessitating a recommencement of the whole operation. These delays may in some cases extend for days, and even weeks. To these inconveniences add the necessity of mounting and descending daily with delicate instruments from three thousand to four thousand feet over rocks and rugged mountain-paths, the time occupied in sending from one point to another, and the difficulty of planting pickets on elevated positions often almost inaccessible. All these inconveniences considered, and we must admit the unusual difficulties of a series of operations which, under other circumstances, would have offered nothing peculiarly remarkable.

As has already been pointed out, the excavation of the gallery at both ends had already been in operation, by ordinary means, since the latter part of the year 1857; this work continued without interruption until the machinery was ready; and the progress made in that time affords a valuable standard by which to measure the effect of the new machinery. In the interval between the end of 1857 and that to which we have now arrived, namely, the end of 1858, many important works had been pushed forward. At Bardonnêche, the communications had been opened, and bridges and roads constructed for facilitating the transport of the heavy machinery. Houses for the accommodation of the workmen had been rapidly springing up, together with the vast edifices for the various magazines and offices. The canal, more than a mile and a half in length, for conveying water to the air-compressing machines, was constructed, and the little Alpine village had become the centre of life and activity. At Fourneaux, works of a similar character had been put in motion; only here the transport of the water for the compressors was more costly and difficult, the water being at a low level. At first, a current derived from the Arc was used to raise water to the required height, but afterward it was found necessary to establish powerful forcing-pumps, new in their details, which are worked by huge water-wheels driven by the Arc itself. Early in the month of June, 1859, the first erection of the compressing machinery was commenced at Bardonnêche. The badness of the season, however, and {64} the Italian campaign of this year, delayed the rapid progress, and even caused a temporary suspension of this work. The results obtained by the experiments which had previously been made on a small scale at St. Pier d'Arena, failed completely in supplying the data necessary to insure a practical success to the first applications of the new system; numberless modifications, both in the compressing-engines and in the perforating-machines, were found necessary; and several months were consumed in experimenting with, modifying, and improving the huge machinery; so that it was not before the 10th of November, 1860, that five compressors were successfully and satisfactorily at work. On the 12th, however, two of the large conducting-pipes burst, and caused a considerable amount of damage, without causing, however, any loss of life. This accident revealed one or two very serious defects in the manner of working the valves of the engine; and in order to provide against the possibility of future accidents of the same nature, further most extensive modifications were undertaken.

By the beginning of January, 1861, the five compressors were again at work; and on the 12th of this month the boring-engine was introduced for the first time into the tunnel. Very little useful result was, however, obtained for a long and anxious period, beyond continually exposing defects and imperfections in the perforators. The pipes conducting the compressed air from the compressing-machines to the gallery gave at first continued trouble and annoyance; soon, however, a very perfect system of joints was established, and this source of difficulty was completely removed. After much labor and patience, and little by little, the perforating-machines became improved and perfected, as is always the case in any perfectly new mechanical contrivance having any great assemblage of parts. Actual practice forced into daylight those numberless little defects which theory only too easily overlooks; but there was no lack of perseverance and ingenuity on the part of the directing engineers; one by one the obstacles were met, encountered, and eventually overcome, and the machines at last arrived at the state of precision and perfection at which they may be seen to-day. About the month of May, 1861, the work was suspended for about a month, in consequence of a derangement in the canal supplying water to the compressors; and it was considered necessary to construct a large reservoir on the flank of the mountain, to act as a deposit for the impurities contained in the water, and which often caused serious inconvenience in the compressors. In the whole of the first year 1861, the number of working days was two hundred and nine, and the advance made was but one hundred and seventy metres (five hundred and fifty feet), or about eighteen inches per day of twenty-four hours, an amount less than might have been done by manual labor in the same time. In the year 1862, however, in the three hundred and twenty-five days of actual work, the advance made was raised to three hundred and eighty metres (one thousand two hundred and thirty-five feet), giving a mean advance of 1.17 metres, or about three feet nine inches per day. In the year 1863, the length done (always referring to the South or Bardonnêche side) was raised to above four hundred metres; and no doubt this year a still greater progress will have been made.

At the Fourneaux or Northern end of the tunnel—owing to increased difficulties peculiar to the locality—the perforation of the gallery was much delayed. A totally different system of mechanism for the compression of air was necessitated; and it was not before the 25th of January, 1863, that the boring-machine was in successful operation on this side, or two years later than at Bardonnêche. The experience, however, gained at this latter place, and the transfer of a few skilful workmen, soon raised the advance {65} made per day to an amount equivalent to that effected at the Southern entrance. Thus, on the South side (omitting the first year, 1861) since the beginning of 1862, and on the North side since the beginning of 1863, the new system of mechanical tunnelling may be said to have been in regular and successful operation.

In the beginning of September of this year were completed in all three thousand five hundred and seventy metres of gallery. From this we deduct sixteen hundred metres done by manual labor, leaving, for the work done by the machines, a length of nineteen hundred and seventy metres. From this we can make a further deduction of the one hundred and seventy metres executed in the first year of experiment and trial at Bardonnêche, so that we have eighteen hundred metres in length excavated by the machines in a time dating from the beginning of 1862 at the South end, and from the beginning of 1863 at the North end of the tunnel. Thus, up to the month of September, 1864, we have in all four years and six months; and eighteen hundred metres divided by 4.5 gives us four hundred metres as the rate of progress per year at each side, or in total, eight hundred metres per year. Basing our calculation, then, on this rate, we find that the eight thousand six hundred and fifty metres yet to be excavated will require about ten and a half more years; so that we may look forward to the opening of the Mont Cenis tunnel at about the year 1875. The directing engineers, who have given good proof of competency and skill, are, however, of opinion that this period may be considerably reduced, unless some totally unlooked-for obstacles are met with in the interior of the mountain. As has been indicated above, sixteen hundred metres in length of the tunnel was completed by manual labor before the introduction of the mechanical boring-engines, in a period of five years at the North and three years at the South side, equal to four years at each end; and eight hundred metres in four years gives us two hundred metres per year, or just one-half excavated by the machine in the same period.

In using the machines, up to the present time, a perfect ventilation of the tunnel has been secured by the compressed air escaping from the exhaust of the boring-engines; or by jets of air expressly impinged into the lower end of the gallery to clear out rapidly the smoke and vapor formed by the explosion of the mine. It should be remembered, moreover, that in working a gallery of this kind, where vertical shafts are impossible, by manual labor, a powerful and costly air-compressing apparatus would have been necessary for the ventilation of the tunnel alone, so that the economy of the system, as applied at the Mont Cenis over the general system of tunnelling in hard rock, is evident. I propose, in the second portion of this article, to give a short description of the machinery employed and the system of working adopted, both at the South and North ends of the Mont Cenis gallery.