THE MONT CENIS TUNNEL.

Lyttelton Times, Volume XXI, Issue 1230, 3 May 1864, Page 3

THE MONT CENIS TUNNEL.

The following paper upon the actual state of the works on the Mont Cenis tunnel, and description of the machinery employed, was read at the institution of Civil Engineers on the 16th of February : — This tunnel would form the completing

link of the Victor Emmanuel Railway, and be the means of putting France and Italy in direct railway communication. The railway on the French side was already opened to St. Michel, in Savoy, and on the Italian side to Susa, in Piedmont. When the whole line was completed, the mails and traffic with India might perhaps be advantageously transferred from Marseilles to some Italian port, as the Mediterranean seatransitwould. thus be materially shortened. During the last twenty years many routes had been surveyed and recommended for crossing the great barrier of the Alps. Of these, that by the Mont Cenis was generally considered the most feasible; and it was only a question, whether the mountain should be crossed by a series of inclines, or whether a tunnel should be made. In 1857, Messrs. Soraraeiller, Grrandis and Grattoni, brought beiore public notice a new system of boring by machinery, instead of by hand labour. A Government commission was appointed to examine and report upon it, and to see if it could be applied to the boring of the tunnel under Mont Cenis. Their report was favourable, and M. Sommeiller and his partners were shortly afterwards charged with the execution of the work. The ends only were available for attack, it being impossible, as was known from the first, to sink shafts. It was feared that the ventilation would seriously retard, or altogether prevent, the completion of the tunnel; but this fear was uncalled for, as the artificial ventilation in collieries overcame greater natural difficulties, and the ventilating current passed through a longer distance, than could possibly be required in this tunnel. M. Sommeiller also proposed to use compressed air for driving -the machinery, and calculated that on its escape, a volume of fresh air would be supplied, adequate to the requirements of the workmen. The tunnel at the Modane, or French side, was of the following dimensions: —25 feet inches wide at the base, 26 feet 2f inches wide at the broadest part, and 24 feet 7 inches in height; the arch being a semicircle nearly. At Bardonneche, the height was increased 111 inches. The exact length between the ends was 7*5932 miles. The present ends

would not be the permanent entrances, as it was intended that a curved gallery should leave the tunnel at the north side, 4*15 yards from the end, and at the south side, 277

yards. At Modane, the tunnel was built entirely with stone; at Bardonneche, for the greater part, the side walls only were of stone, and the remainder of brick. The Bardonneche end was 434 feet higher than that at Modane. For one-half the length of the tunnel, therefore, from Modane to the middle, the gradient would be 1 in 45§ ; the other side being driven with only sufficient fall, 1 in 2,000, to allow of the water escaping. When the tunnel was complete, it was expected that there would be a constant current of air from the north to the south ; the latter was not only the higher end, but the air was more rarified and exposed to the heat of the sun, whilst the entrance at Modane was under the shade of the mountain.

The establishment consisted at each end of machinery for compressing the air, workshops for making and repairing machinery, offices, storehouses, residences for the engineers, and barracks for the workmen. At Modane, the entrance of the tunnel was 328 feet above the bottom of the valley, where the workshops were placed, with which there was ft communication by means of an inclined plane, worked by a water balance. Different systems of tunnelling by machinery have been tried in England; amongst others one by Captain Penrice, R.E., in which it was intended to drive a gallery about 4-i feet diameter, and by means of repeated blows from a heavy frame loaded with knives, to reduce the whole of the excavated materials to dust. It seemed, however, to the author, that any system of tunnelling must be deficient which did not make so | cheap and readily applicable a power as gunpowder available; and that by the trituration of the rock to such small particles, as in Captain Penrice's system, a great amount of work was unnecessarily performed. In M. Sommeiller's system, whilst machinery was employed for accelerating the progress usually made by hand labor, gunpowder was also available. He had succeeded in producing a compact machine, not weighI ing more than 6 cwt., which could pierce, a

common borehole, about li inch in diameter and 3 feet deep, into a rock in twenty minutes, where two miners would havo required two hours. Further, he had arranged a moveable support capable ot carrying eleven such machines, any one ot which could be worked at almost any angle, aud ot allowing the free action of each, in a gallery 10 feet square. This support could be removed wbon it was necessary to explode the' holes bored by the machines. The machine was of very ingenious construction. It consisted of two parts: —one, a cylinder for propelling the borer against the rock; the second, a rotary engine for working the valve of the striking cylinder, turning the borer on its axis at each successive stroke, and advancing, or retiring, the striking cylinder as occasion required. It gave 250 blows per minute. The effective pressure on the piston in striking was 216 lbs.; the length of the stroke was from 2 inches to 7| inches. Although simplified as much as possible, the nature of the work the machines performed was so severe that they were liable to frequent derangement, and a large stock was kept on hand. The cost of each machine was about £80. The compressed air was used at a pressure of five atmospheres above atmospheric pressure, and was conveyed to the ' fore-head' of the advanced gallery by a pipe 71 inches in diameter. The advanced gallery was the only place where the machines were used; the enlarging of the tunnel to the full size, walling, &c., were performed by manual labor. The system of working was to bore eighty holes in the fore-head of the advanced gallery. The frame and machines were then withdrawn, and a set of men charged and fired the holes; afterwards replaced by another set to remove the deblais. The division of time amongst the different classes of labor was very variable. It might, however, be averaged as

From 6 to 8 hours for the machinists, From H to 2 hours for charging and firing, and From 3 to 5 hours for removing the deblais. Thus there were almost two complete shifts every twenty-four hours. An alignment was made once in three months, from an observatory at each end. As yet no error had been detected. Three or four large holes, each about 4 inches in diameter, were bored near the centre of the fore-head. These were not charged and exploded, their purpose being to weaken the surrounding rock. The remainder were charged, those adjoining the centre being first fired, and the result of these explosions was a cavity. The remaining holes were then exploded from this cavity outwards. The workmen were industrious, under circumstances which required more than ordinary perseverance. A premium on their wages Avas given for more than a certain advancement per day. At the time ot the author's visit, one metre per day was the standard. For a progress of 1 l-ioth metre per day 1 l-10th day's wages was paid 1 l-sth „ „ 1 l-sth „ „

1 3-10 th „ „ 13-10 th „ „ 1 4-10 th ~ „ » » This scale was subject to adjustment every fortnight. The deblais resulting from the explosion of the eighty holes was removed in small wagons. Its removal was well organised, arid considering the circumstances, quickly effected. It would be much accelerated,Jif it was possible to construct an iron frame, strong enough to be placed close to the forehead 'at the time of the explosion, and receive without injury the products of explosion, which could be removed en masse, or nearly so.

Two descriptions of machines for compressing air were in use, —one on the hydraulic ram principle, the other resembling a pump. In the first, the water was admitted, with a, pressure of 85£ feet, in a column, or vessel, containing air, about 14 feet high and 2 feet in diameter. The water by its momentum rushed up the column, compressed the volume of air, and forced it through a valve into a reservoir. The pressure valve being closed, the exhaust valve was opened, and the water fell in the column, at the same time its place was taken by air, and the machine became ready for another stroke. This machine made strokes per minute, and was capable of supplying about 20 cubic feet of air, compressed to five atmospheres, per minute. The other machine consisted of a horizontal pump, and two vertical branches. The piston was surrounded by water, which rose and fell alternately in the two columns : when it rose, compressing the air, and forcing it through the outlet valve; and when it fell, creating a vacuum, which was filled by air at amospheric pressure. The tunnel, on the 30th June, 1863, had been driven (including the advanced gallery) at Modane 1092-25 metres, and •at Bardonneche 1450 00 metres. The advancement in June last, at Modane, was v at the rate of 4 719 feet per day. At this rate of progress at both ends, the tunnel would be finished in 9 years 2-J- months from that time. It was not, however, too much to expect a progress of 2 metres per day at each end, seeing that machines had only been in use at Bardonneche about two years and a half, and at Modane half a year. A greater part of that time had been taken up in experiments, and the men were not thoroughly habituated, as yet, to the manipulation of the machines. The machines were alsobeing much improved. With an average rate of 2 metres per day from June 30th, 1863, 6 years 7 months would be required for the completion of the tunnel, as compared with 26 years 3 months by hand-labour, at 1.655 feet per day at each end, the average rate of progress previous to the introduction of the machinery. The machinery in use at Mont Cenis was made, for the greater part, at Seraing, near Liege. M. Sommeiller confidently expected an advancement of 3 metres per day at each end. If that were the average lrom June 30th, 1863, the work would be completed in 4 years months from that time. In the advanced gallery at Modane, the number of workmen employed during the twenty-four hours was as follows: —

88 machinists, in two sets of 44 each. 9 chargers in one set. 30 laborers for removing the dehlaia, in one set. 127 344 men were also engaged in enlarging and walling, giving a total employed underground of— 471, and including blacksmiths, stonedressers, and other labourers at the surface, there were employed at the tunnel 700 Mechanics, brakesmen, &c., in the workshops, machinery, &c. ... ... 240 Occasional laborers ... ... ••• 200 Or n total at Modane of ... ... 1140 At Bardonneche the number was greater; 1200 to 1400 being generally employed, giving a total of 2540 on the works. The result of a rough comparison was to show that, in the present development of the Sommeiller system, an advancement three

times quicker than by hand-labour might be effected, but at about two and a-half times the cost; judging rather of places where it might be generally applied, than by the Mont Ceniß only. The proportion of two and a half to one increase ot cost referred only to what was known as mining charges in the advanced gallery, i.e., wages, tools, candles, and gunpowder. This proportion was notably diminished in the case of a railway tunnel, where enlarging, timbering, walling, laying of rails, &c., were charges common to both systems. In the case of a tunnel through rock, costing, when completed, £30 per yard, the two systems might compare as follows:—an increased advancement in favour of machinery of 3 to 1, at an increased cost of 4 to 3.

The ventilation was good in the advanced gallery, the exhaust air from the machines affording an ample supply. During the time of exploding the holes, a jet of air was left open. Further back, where the men weire employed in enlarging the tunnel, the ventilation was insufficient. The tunnel was therefore being divided with a horizontal brattice, —the upper section being in communication with a chimney on the mountain side. The air was intended to pass along the under side of this division, and then return by the upper part, which had an area of 7 square metres. The works were now performed at the charge of the Italian Government. On their completion the French Government was to pay £760,000 for that portion of the tunnel situated in its territory—one-half the entire length —together with a premium of £20,000 for each year, by which a term of 25 years, counting from January Ist, 1862, was reduced. This premium would be increased to £24,000 for each year, by which a term of 15 years was reduced, counting from the same time. In addition, the French Government would pay interest at 5 per cent, per annum on such portion of the tunnel as was finished. If, however, the Italian Government did not complete the work within twenty-five years from the time of making the agreement, or if they renounced the works before that time, the French Government was absolved from further payment. If the works were finished, as there was every reason to suppose they would be, in ten years from June 30th, 1863, the French Government would pay £1,287,000 for the construction of one-half of the tunnel, or at the rate of £210 per metre.