The Eiffel Tower, whose progress the whole world has followed with such curiosity and interest, is in a way the grandiose vestibule of the Exhibition.
Here, in its fullest details, is the summarised history of this unique construction, a striking and powerful manifestation of our national genius.
- The first work began on 28 January 1887. An army of diggers undertook the great excavations at the bottom of which the four legs of the immense tower were to be supported. The site, on the banks of the Seine, was occupied by a square, and it was necessary to start by uprooting the trees, removing the topsoil, diverting and rebuilding the sewer that crossed the piers on the river side, and finally setting up the site.
This was a considerable undertaking since the Tower occupies an area of more than 16,000 square metres.
The Tower forms a square of 129 metres 22 cm. on each side.
The Tower therefore occupies more than one hectare of surface area.
The axis of the Tower is placed in the axis of the Champ de Mars, and as this is inclined at 45° on the meridian, it follows that the four piers supporting the Tower are placed exactly at the four cardinal points.
The two piers in front on the Seine side (piers n° 1 and 4) are the north and west piers; those behind (n0 2 and 3) are the east and south piers.
-M. Eiffel, by means of the soundings he had made, had an exact knowledge of the composition of the ground. He knew that the lower foundation was formed by a layer of plastic clay about 16 metres thick resting on the chalk of the Paris basin. This clay, sufficiently compact to support foundations, is slightly sloping from the Ecole Militaire to the Seine, and is topped by a compact sand and gravel bank. Up to the beginning of the Champ de Mars itself, this layer of sand and gravel has a more or less constant height of 6 to 7 metres; beyond this point, we enter the old bed of the Seine and the action of the water has reduced the thickness of this layer to such an extent that it becomes almost non-existent when we reach the current bed. The solid layer of sand and gravel is itself topped by a variable thickness of fine sand, muddy sand, and fill of all kinds incapable of supporting foundations.
Special precautions had to be taken and two different systems of foundations had to be used.
For the piers furthest from the Seine, numbers 2 and 3 (pier 3, on the Grenelle side), the necessary layer of sand and gravel, 6 metres thick, was in fact found from the +27 level, which is the normal level of the Seine.
It was therefore very easy to obtain a perfect foundation for these two piers, the lower part of which consists of a 2-metre layer of cement concrete poured in the open air.
The two front piers, which bear numbers 1 and 4 (number 4 towards Passy), were founded differently. The layer of sand and gravel only reaches elevation +22, i.e. 5 metres below the water, and to get there, one crosses muddy and marly soil from recent alluvium. It was therefore necessary to use compressed air foundations with the help of sheet metal caissons, with strong iron reinforcements, measuring 15 metres in length and 6 metres in width. These caissons, in which, under a pressure of air forced by machines, about thirty workers worked for two months, clearing the muddy earth from under their feet, were sunk 5 metres under water, then filled with concrete, and from there the masonry in which the anchoring bolts of the Tower were fixed was built.
There are four caissons for each pier, and therefore, for each pier, four pyramids of masonry.
These masonries, which work at a maximum coefficient of 4 to 5 kilograms per square centimetre, are crowned by two courses of Château-Landon ashlar, whose resistance to crushing is 1,235 kilograms on average per square centimetre. The pressure under the cast iron shoes is only 30 kilograms per square centimetre, so the stone is only working at one fortieth of its strength.
Consequently, safety conditions are absolute.
- However, Mr. Eiffel foresaw, from the beginning, the possibility of having to maintain the legs of the Tower on a perfectly horizontal plane one day. For this purpose, in each of the shoes, a hydraulic press (or jack) with a force of 800 tons was installed, allowing the displacement of each of the edges at any moment and raising it by the necessary amount.
These hydraulic presses, a sort of gigantic screw, were never used during the construction, as it was carried out so safely and precisely.
In pile 4, a cellar was reserved for the housing of the machines and their generators for the service of the lifts.
As for the flow of atmospheric electricity into the ground, it was, from the first day, provided for each pile by two cast iron pipes of 0.50m in diameter, immersed below the level of the water table, over a length of 18 metres. These pipes turn over vertically at their ends until they reach ground level, where they are placed in direct communication with the metal part of the Tower. It can be seen that everything had been wonderfully planned and combined.
This earthwork, foundation and masonry work took five months and three days.
On June 30, 1887, the metal assembly work began.
The main difficulty lay in the start of the crossbeams at the base: they had to be directed into the space in an inclined position, the so-called "cantilever". The work was all the more delicate because the great builder and his valiant helpers had never done anything like this before, yet they have executed the most daring constructions in all parts of the world. Here is how they practised their first attempts:
They built, in wood, a model of one of the piers as the machinists of theatres do when they have to combine some great new scenery, and they studied in small ways the means of supporting this mass by offering it, in its rise, light points of support on goats or pylons of framework. Calculations showed that these first support points were only necessary at a height of 26 metres. Therefore, when the iron crossbeams reached the height of 26 metres, they found the support of the goats, which had been built without interrupting the work, close by. These goats were topped by sand boxes on which the iron chords were to be supported.
From 26 metres onwards, the centre of gravity of the already built structure began to project vertically outside the base square. But with the goats supporting them, it was possible to push beyond, in a new cantilevered position, until the crossbeams were able to take their upper support point and finally come to rest against the horizontal beams of the first floor, a sort of vertiginous bridge thrown at a height of 48 metres over a span of 42 metres.
Connection of the irons.
- The beams carrying this floor could not be launched into space because the support point was missing. So, in the place now occupied by Mr. Saint-Vidal's beautiful fountain, four large structural pylons 45 metres high were built, on which the large beams were established, connecting the four inclined faces and carrying out the most difficult part of the work.
In order to ensure the perfect coincidence of the support points of the crossbeams with the horizontal beams of the first floor, a certain amount of sand was drained from the boxes that contained it and that lifted the crossbeams: a general pivoting was thus provoked, and this overall movement gradually brought the mobile piers closer to the beams that remained fixed, and a complete and rigorous coincidence of the parts to be assembled was thus achieved. The operation was so successful that the holes of the large connecting gussets, about two hundred in number, were in absolute coincidence and did not require any reaming to make the riveting and to operate their definitive connection.
This was certainly one of the most imposing phases of the construction, due to the size of the masses in motion that obeyed the will of the engineers.
- The rest of the metal part was assembled by means of four special cranes which were fixed along the fittings, like so many teams of mechanical workers, and brought the large pieces of iron to the workmen perched in the frame like sailors in their ropes. It was as curious as it was interesting to see these four lifting devices climbing, as it were, after the iron crossbeams, turning to the right, to the left, inwards, outwards, drawing from the bottom the large pieces of metal which vanished into space to go and fix each one in the exact place it was to occupy.
From 150 metres upwards, the surface of the site being very small, the materials were assembled by the ordinary procedures by means of a locomobile.
The speed of all these manoeuvres was such that the second platform of the Tower was reached in twelve months and that, on 14 July 1888, the fireworks of the National Day were fired, as promised by Mr. Eiffel, at a height of 115 metres.
© Guide Bleu du Figaro et du Petit Journal 1889