ON THE ERECTION OF ALEXANDER III. BRIDGE IN PARIS. 471. 
obviously adequate to the various conditions of the problem. The moments 
of flexure being almost independent of the temperature in such arches, 
and only becoming a little important near the articulations, it is possible 
to reduce the thickness of the section in the central part, more than in 
any other system, without any inconvenience. It is equally possible, by 
a judicious distribution of the metal, to prevent changes in the direction 
of the stresses, which are always well known in this system, and to obtain 
a sort of steel vault in which every part of the metal is compressed under 
all circumstances. The joints of the elements of the arches or voussoirs 
have not to be riveted, and may be made with screw-bolts. There is, 
moreover, no objection to the use of cast steel, so that the operations left 
to be carried out on the works for mounting the ribs are very few, 
provided sufficient preparatory care has been taken. 
It was easy, on account of the suppression of riveting, not only to 
erect each rib in a short time, but also to erect them successively with 
the help of a moving system so as to secure the fairway wanted for navi- 
gation during the whole time of erection. 
Fcundations.—On account of the flatness of the arches the thrust is 
very considerable, though it is not greater in proportion than in many 
arch bridges made of masonry. It amounts to 288 tons for each metre 
in length of the abutments—about 12,700 tons for each abutment. 
The subsoil of the valley of the Seine is not to be compared with the 
regular subsoil of the Thames in London, where engineers are sure to find 
an excellent water-tight, hard, uniform clay. The strata in the subsoil 
in the southern part of Paris have a general gradient towards the hills at 
Meudon, so that the bridges over the river have been built with all kinds 
of subsoil for foundations. Whilst, for example, the ‘Pont Neuf,’ the 
‘Pont de la Concorde,’ and other bridges of the centre of Paris have been 
erected on the solid calcareous bed-rocks belonging to the Lutetian 
strata, the ‘Pont des Invalides’ stands on sand, the ‘Pont de ]’Alma’ 
on soft clay, the ‘Pont de Jéna’ and the ‘ Passerelle de Passy ’ on hard 
clay, the ‘ Pont Mirabeau’ on chalk. 
When foundations are laid upon calcareous rock or on chalk, 
engineers agree on their being perfectly stable, but when the foundations 
are to rest upon the intermediate strata of sand or clay, precautions must 
be taken, because these strata lack regularity in depth and, in the case of 
the clay, in firmness, so that works resting on it run the risk of settlement. 
It was decided that the foundations should be built as if they were to 
be supported by the worst strata of Parisian subsoil. Therefore the 
pressure on the bottom ground was limited to three kg. per square 
centimetre (about 40 lb. per square inch), and the weight of each abut- 
ment was fixed at such an amount that it could resist the thrust by the 
friction of the ground alone, without depending in any way on the earth 
backing. 
It is easy to see that no result satisfactory for both conditions could 
be obtained without developing to a great extent the surface of the 
abutments, the back parts of which were carried to a distance of 
33°50 metres (110 feet) from the front part in the river bed. 
Each abutment consists, in consequence, of an enormous block of 
solid masonry in the form of a parallelogram 44 metres (144 feet) in 
length (along the river) and 33-50 metres in width, its surface area being 
1,474 square metres (15,850 square feet). 
The right abutment has been sunk to a depth of 8-25 metres (27 feet) 
