For the canal between the port of Marseilles and the River Rhone a tunnel four and a half miles long was driven beneath a range of rocky heights. Now canal barges pass beneath these hills from the sea to the inland waterways of France
THE ROVE TUNNEL is a little over four and a half miles long. The tunnel is 72 ft. 2 in. wide and 50 ft. 6 in. high from invert to crown. Where the tunnel lining needed extra thickness the towpaths were built on the top of masonry piers spaced 7 feet or 8 feet apart. Altogether the tunnel involved the removal of 8,830,000,000 cubic feet of rock.
FOR size and boldness in conception and design, it is doubtful whether anything of the kind can compare with the great Rove Tunnel through which runs the Marseilles-Rhone Canal. The tunnel is part of an ambitious scheme for linking the port of Marseilles with the old city of Arles in the Rhone Valley, and making barge navigation between the two towns independent of the sea and of the varying moods of the River Rhone.
Through flat country formed partly of deposit brought down by the river in the course of many thousands of years, the main stream of the Rhone flows down into the Gulf of Lions, its mouth being about twenty-five miles west-by-north of Marseilles and debouching on to the bay called the Golfe de Fos. A secondary channel, the Little Rhone, breaks off from the main stream
at Arles and pursues a crooked course in a generally south-westerly direction to the sea, forming the natural boundary between the French departments of Bouches du Rhone and Gard.
The country is generally flattish, low rocky plateau alternating with boulder-strewn plain and dismal salt marsh and lagoon. It is therefore by no means unsuited for canals. In natural conditions, without the aid of canals, the passage of large seagoing barges from Marseilles to Arles is not easy. The Rhone, though undoubtedly a great river, is not ideal for inland navigation. It cannot be compared with the Rhine, for instance, and exhibits some of the fickleness of the Mississippi and the Missouri, though it is not muddy, as they are. A famous French engineer once said that the Rhone consists of two rivers, one of water and the other a flow of small stones, and that the two flow down from the mountains together, sharing the same river bed. The Mississippi in America brings down enormous quantities of soft silt, which gives it its coffee-coloured appearance, but the Rhone brings down masses of light gravel and shingle.
In parts the Rhone is almost a large mountain torrent that has wandered far from its native mountains — to such an extent does it abound in shingly shallows, pools and rapids. In places the gradient varies from 1 in 2,500 to as steep as 1 in 500, yet from. Arles to the sea it eases off to approximately 1 in 25,000. It might be thought, therefore, that navigation from Arles southwards would be relatively easy. This, however, is not so, for the lower part abounds more than ever in deposits of the solid material brought down from the swifter upper reaches. In relatively recent times, it is true, the regulation of the river has improved, matters, but it is still no boatman’s paradise.
THE MARSEILLES-RHONE CANAL was designed to link the Mediterranean port of Marseilles with Arles and the River Rhone and to make navigation between the two towns independent of the sea and the difficult river. Near La Lave, the Rove Tunnel takes the canal under the Nerthe Hills to a brackish lagoon known as Etang de Berre. Separated from the lagoon by a dike, the canal skirts the southern shore as far as Martigues, thence through the Etang de Caronte to Port de Bouc, opposite the main mouth of the River Rhone. A smaller canal runs between Port de Bouc and Arles, but the new scheme involves the cutting of a ship canal between those two places.
Were it not for the natural disadvantages of the Rhone as a navigable stream, it would form a vital link in a waterway right across France, extending through the Saone, the Burgundy Canal and the Yonne to the Seine, and thence down the Seine to the English Channel. Such a waterway would allow for the passage of vessels of moderate tonnage from Northern to Southern Europe without the necessity for rounding Ushant, Cape Finisterre and Gibraltar.
Towing has been possible on the River Rhone since 1878. Ordinary steam tugs have been used without accident on the stiller reaches. But where the current was too strong, a chain tug had to be introduced. This hauls itself up the river by a chain laid along the bottom. The vessel drags in the chain forward and pays it out aft as she progresses. So much does the river-bed shift about that every chain tug has to make a return trip up and down her reach every day, whether there is traffic or not, to keep the chain clear from shingle.
Conditions such as these have been responsible for a scheme destined to link Marseilles and Arles with a large canal. About seventy years ago a certain amount of improvement was effected, at any rate for smaller craft, by the building of a canal parallel to the Rhone between Arles and Port de Bouc, on the Golfe de Fos. This old canal cannot compare with the great works of the new Marseilles-Rhone Canal, but its existence explains why the Marseilles-Port de Bouc section, including the tunnel, was built before the second length of ship canal.
A Coastwise Waterway
Small craft such as were able to make the passage of the old canal were not always safe on the open sea passage from Port de Bouc to Marseilles. The building of the Rove Tunnel has made it possible for all canal craft to travel inland between Port de Bouc and Marseilles.
Over a century ago proposals were made for the Port of Marseilles to be connected with the Rhone as far above its delta as possible by some sort of a ship canal. The first plans for such an undertaking were drawn up in 1820. There were no railways then, and canals were the only means of carrying heavy freight in bulk across country. A canal of this description would therefore have enjoyed a monopoly. Even so, the idea came to nothing.
Twenty years later, however, the scheme, in a modified form, was revived, and though this 1840 scheme also proved abortive, it is interesting for a particular reason. It involved the building of a big canal tunnel. Along the northern side of the Bay of Marseilles runs a range of rocky heights called the Nerthe Hills. It was through this range that the tunnel of the 1840 scheme was to be pierced, and it is through the same range that the present tunnel runs on its way between the sea and the inland salt-water lagoon called Etang de Berre, lying on the other side of the hills. The original plans for this tunnel made it rather shorter than the present one, the difference being made up by locks, taking the canal to a higher level for its passage through the boring.
This second scheme for a canal shared the fate of the first. But at least a tunnel was built in 1843, on the site of the proposed canal tunnel, to carry the Lyons-Marseilles line of the P.L.M. Railway. The canal scheme remained in abeyance until after the Franco-Prussian War had ended in 1871. After that people busied themselves with the idea again, and in 1873 engineers brought forward a plan for a coastwise canal passing right along the shore from Marseilles to Port de Bouc, and separated from the sea by a wall. This, however, was abandoned because of the excessive route mileage which it involved. Another plan was drawn up, in which the proposal for a tunnel was revived.
This second scheme of 1873 was to provide the following route. The canal was to run from the North Basin of Marseilles Harbour, along the coast past L’Estaque (just below the point where the 1843 railway tunnel emerges), until it reached the promontory called Point Lave, whence it was to plunge into the hills. The tunnel would carry the canal in a straight line through the range, and would then enter the Etang de Berre. Through this and a smaller salt-water lake it would reach the existing canal system linking up Martigues, Port de Bouc and the Rhone itself. Such a canal was estimated to cost about £3,200,000, which, in the light of later experience, seems optimistic, even allowing for the changes which have taken place in money values since then.
Yet another thirty years were fated to elapse before anything further was done in the matter. Then, in 1903, the plans for the Marseilles-Rhone Canal received public attention once again. The new project formed part of an ambitious though only partly realized scheme of inland navigation development, as a “work of public utility” under an Act of the French Chamber dated December 22 of that year. This caused, at long last, the building of the Marseilles-Rhone Canal, though a long time elapsed before its completion.
The canal, as it is to-day, runs along the coast to beyond L’Estaque, as was originally intended in the scheme of 1873. Then it pierces the Nerthe Hills and emerges to find its way into the Etang de Berre. It skirts the southern side of this brackish piece of water, from which it is separated by a mole or dike, eventually reaching Martigues.
Diversion of Railway Lines
The canal continues, by way of the smaller sheet of water called Etang de Caronte, until it reaches Port de Bouc, on the bay opposite to the main mouth of the Rhone. The second part of the scheme, which had to wait for the completion of the first, involved the building of a long canal section up the valley parallel to the left bank of the river to Arles.
The southernmost section is 9 ft. 10 in. deep to allow for the easy passage of seagoing barges plying between Marseilles and industrial wharves which have sprung up round the shores of the Etang de Berre. Elsewhere a depth of 6 ft. 6 in. was considered sufficient, with a width of 82 feet on the straight sections.
By far the most interesting feature of engineering is the Rove Tunnel. The original designers intended to give the tunnel a width of 59 feet at the springing of the arch, with a 4 ft. 11 in. towpath throughout its length. The towpath was to be supported on brackets built into the side of the tunnel. This narrow towpath, however, promised to be a fruitful source of trouble, and there was little clearance, even allowing for the bracket system of support.
The Marseilles Chamber of Commerce came to the rescue. With their arbitrary powers they laid down that the width of the tunnel at the springing of the arch was to be 72 ft. 2 in., with a 6 ft. 6 in. towpath on either side, the width of the channel thus amounting to 59 ft. 2 in. The largest barges used on the canal have a beam of 26 ft. 3 in., so there is a clearance of 6 ft. 8 in. altogether for large barges passing one another in the tunnel. The Chamber of Commerce undertook to shoulder the responsibility for the increased cost entailed by these modifications.
GIGNAC CUTTING carries the Marseilles-Rhone Canal from the northern entrance of the Rove Tunnel to the Etang de Berre (in the background). The cutting is about 1¼ miles long and 95 feet deep at the mouth of the tunnel. The excavation was carried out by two 70-tons steam navvies and a bucket dredger. Some 42,000,000 cubic feet of rock spoil were removed.
Quite close to the southern end of the proposed tunnel was a little port called La Lave, adjacent to extensive stone quarries from which stone had been shipped for years. Between La Lave and the docks of Marseilles, the canal builders erected a long mole running parallel to the shore of the gulf, thus forming a protected waterway between the tunnel mouth and Marseilles Harbour, flanked by the mainland on one side and the open sea on the other.
The mole was intended eventually to take the form of a substantial masonry structure, but for the time being the dike was built of loose stone blocks, the tideless nature of the Mediterranean permitting this expedient. Along the southern shore of the Etang de Berre was built a similar rock dike. Altogether these two dikes absorbed 14,800,000 cubic feet of stone, the average weight of the material working out at 176 lb. a cubic foot.
Another important work consisted of the diversion of several railway lines in the immediate vicinity of the southern mouth of the tunnel. This was a necessary preliminary. The most important of these lines was one which left the main line just south of the Nerthe Tunnel and ran along the coast to Martigues, whence it eventually linked up again with the main line to Arles. This line ran right across the site of the tunnel entrance by La Lave. To divert it, the engineers had to carry a new line up inside the face of the cliff, traversing in one place an open-sided gallery which is a prominent feature in views showing the southern entrance to the Rove Tunnel. A little farther westwards the same line had to be carried across an intervening ravine on a stone viaduct of several arches. The levelling down of an old disused quarry close to the southern portal was another work that had to be carried out before a start could be made on the tunnel itself. The building of the moles between the canal and the sea, on the other hand, was carried out simultaneously with the boring of the tunnel.
Shaft 430 Feet Deep
Boring was accomplished with comparative ease, primarily from the southern end and from two shafts driven down through the rock of the Nerthe Hills. It was not, on the other hand, possible for the engineers to make an immediate start from the northern portal, as this was to be approached by a rather long and deep cutting which had to be excavated first. The sinking of the two vertical shafts downwards along the centre line of the proposed tunnel enabled the engineers to bore their headings from several points simultaneously. It also ensured a thorough ventilation of the galleries and a means for removing spoil as they proceeded. Each of these shafts had a diameter of 11 ft. 6 in. In their construction, the masons followed the miners, putting in a 9·8-in. cement lining.
One of these two shafts reached the level of the future tunnel after having been carried down through the rock for a distance of 230 feet. The other shaft, however, had to be sunk from a much higher level, as the line of the tunnel followed the rising contour of the hill ridge. In this instance the distance between the tunnel’s centre line and the top of the shaft was no less than 430 feet. One of the shafts was sunk at a distance of about 1½ miles from the site of the northern portal, and the other was 3½ miles from that point, the total length of the tunnel having been fixed at over four miles.
The first thing the engineers did in building the tunnel itself, apart from the sinking of the shaft, was to drive forward three headings, first one on the right side, on a level which was later to coincide with the springing of the arch, then one on the left side, at the same level, and finally a top heading, along the line to be taken by the crown of the arch. The three headings formed a triangle. As the work progressed, the working face of the right-hand heading was always in advance of that of the left-hand heading, the top heading coming last. The material to be penetrated consisted of hard rock. At intervals of 330 feet, as soon as they were able, the miners drove cross-galleries between the right-hand and the left-hand headings. These cross-galleries were bored partly to assist in ventilation of the workings and partly to check the distance maintained between the two headings and to ensure that this remained constant.
THE SOUTHERN OR SEAWARD ENTRANCE to the Rove Tunnel, which carries the Marseilles-Rhone Canal beneath the Nerthe Hills. Along rock dike running parallel to the shore forms a protected waterway between Marseilles Harbour and the entrance to the tunnel. The tunnel was completed in 1927 and on April 25 of that year the canal was opened by the late President Doumergue.
With the top heading the procedure was somewhat different. At intervals of 60 feet along each of the side headings, the miners bored chutes upwards from the heading roofs to the site of the top heading, on the boring of which they were then able to proceed. The rock spoil from this top heading, as it progressed, was tipped down the chutes into wagons running on tramway tracks, which had previously been laid down in the more advanced side headings.
The whole process resembled that of open cut mining with bench and “glory hole”, except that it was carried out deep down in the core of the hill ridge instead of on an open mountainside or quarry. As work advanced, the rock between the chutes was removed in its turn, so that in due course the three headings were merged into a single crescent-shaped boring. In the preliminary boring, the two bottom headings averaged 90 square feet in section, and the top heading about 45 square feet. As soon as the crescent formation had been attained, the tunnel began to assume its eventual shape. It remained for the central core of rock to be removed and the canal itself to be excavated. As soon as sufficient space had been cleared, the masons went to work on the lining of the tunnel.
Each of the bottom headings had a ditch or gutter running along one side, below the level of the tramway track carrying the spoil wagons. Luckily the quality of the rock remained constant, and no underground springs of any size were encountered. There was always a certain amount of underground water, but this was easily run off along the ditches at the sides of the bottom headings. Little pumping, however, was necessary in the headings themselves, and pumps installed at the shafts carried the surplus water to the surface where it could run off. The two lower headings had their floors on the same level as the future towpaths.
ENTRANCE TO THE TUNNEL at La Lave. On the hillside in the background is the open-sided gallery which carries a railway line across the face of the cliff. Vessels of up to 1,200 tons burden can use the Rove Tunnel.
Where boring was carried out apart from the two vertical shafts, and before these were reached, the ventilation of the workings was successfully accomplished by a 21-in. air pipe in connexion with a big fan driven by a 40 horse-power motor. The shaft having been reached, another fan was installed at the top of it, delivering 460 cubic feet of air a second to the underground workings.
The drilling and excavation of the rock were carried out by pneumatic drills and by blasting with dynamite. Two electric air compressors were installed, one at either end of the tunnel. These supplied air to the drills at a pressure of 142 lb. per sq. in. For haulage of wagons along the tramways in the headings and in the partly finished tunnel, compressed-air locomotives were used. These were similar to steam locomotives but, because of the constant loss of power associated with the container, which replaces the boiler in such an engine, the air was supplied to them at a pressure of about 1,420 lb. per sq. in. The locomotives were charged by the same compression plant as the pneumatic drills, and each exerted a tractive effort equal to 200 horse-power when fully charged. The exhaust from these locomotives and from the pneumatic drills provided an additional source of fresh air to the men at work in the headings. In the course of excavating the Rove Tunnel, no fewer then 1,300 tons of dynamite were used.
The building of the tunnel lining presented no great problems. It took the form of an ellipse above the tow-paths on either side of the canal, and varied in thickness from 27 in. to 48 in., according to the degree of thrust. Where this was low, the designers had the sides of the channel lined with masonry walls 23-in. thick. The tops of these walls were paved, and formed the two towpaths. Through those sections where the thickness of the tunnel wall had to be increased, the engineers carried the tow paths along on top of a series of masonry piers, placed at intervals varying from 7 feet to 8 feet. The materials used for the masonry lining of the arch originated partly in the several quarries of the neighbourhood and partly in the better quality rock from the excavations.
On the northern side of the ridge, where the tunnel was to emerge into daylight, the canal builders had to dig a deep cutting through the rising slope of the hill. This is known to-day as the Gignac Cutting. It is about 1| miles in length and as much as 95 feet deep at its southern end, where the tunnel mouth opens into it. Two 70-tons steam navvies and a big bucket dredger were used in its excavation.
World's Longest Canal Tunnel
Most of the spoil was dumped in a neighbouring lagoon called Etang de Bolmon, to form the site of a future system of docks. Each of the sloping sides of the Gignac Cutting, at its deepest part, contains two intermediate benches or terraces between, the towpath and the top. Between these, the sides are sloped to a gradient of 1 in 1·25.
The outbreak of war in 1914 caused considerable delay and it was not until 1927 that the great Rove Tunnel was at last completed. Altogether, the tunnel section had involved the removal of 8,830,000,000 cubic feet of rock. Part of this was mixed with the building materials for the tunnel lining, and part was incorporated in the dikes.
The tunnel is a little over four and a half miles in length from portal to portal, making it the world’s longest canal tunnel, apart from hydro-electric power tunnels and other non-navigable borings. It is also believed to have the greatest section of any of the world’s tunnels, of any type, with its width of 72 ft. 2 in. over the two towpaths and its height, from invert to crown, of 50 ft. 6 in. Its opening did not mark the completion of the whole of the canal works, for even after it had been finished vessels were still able to proceed only as far as Port de Bouc. Navigation thence to Arles had to be carried out on the Rhone itself, as before.
But the tunnel itself forms one of the most notable recent engineering works in the world, with its remarkable dimensions and capacity. It can take vessels of up to 1,200 tons burden continuously in either direction. The canal was formally opened by the late President Doumergue on April 25, 1927.
THE SLOPES OF THE GIGNAC CUTTING at its deepest part contain two intermediate benches, or terraces, between the towpath and the top of the cutting. Between the benches the slopes have a gradient of 1 in 1·25.
[From part 29 & part 30, published 14 & 21 September 1937]
Each of the bottom headings had a ditch or gutter running along one side, below the level of the tramway track carrying the spoil wagons. Luckily the quality of the rock remained constant, and no underground springs of any size were encountered. There was always a certain amount of underground water, but this was easily run off along the ditches at the sides of the bottom headings. Little pumping, however, was necessary in the headings themselves, and pumps installed at the shafts carried the surplus water to the surface where it could run off. The two lower headings had their floors on the same level as the future towpaths.