258
HISTORY OF ENGINEERING.
Book I.
fo 19 feet 2 inches thick : the abutments are 23 feet 5 inches thick ; the middle arch is7 feet, and that which joins the abutments 5 feet 10 inches thick. The breadth of thebridge is 49 feet. The plan of the starlings is formed by two arcs, one-sixth of acircle up the stream, and a semicircle down it.
The foundations are established on piles carrying a framework and platform of carpentry :the soil is a bed of sand from 4 feet to 13 feet in thickness, which covers irregular layers ofmarl and tufa. A foundation of this kind being very permeable to water, the pumping wasattended with great difficulties. In the sides of the cofferdams, with which the abutmentand piers were successively surrounded, several springs worked, which it was impossible toexhaust. They were at length enclosed in cisterns, in which the water was allowed to riseand round which a cofferdam was constructed with planks and clay : the nature of the soilalso rendered pile-driving in some places very irregular; it often occurred that by the sideof a pile which would only drive 6 feet into the tufa, another would go 16 feet, accordingto the nature of the beds they traversed.
The arches were constructed on trussed centres, which being found too weak werestrengthened by adding some pieces in the upper part, but after their completion, a settle-ment was perceptible in the seventh pier from the town, and a load of rubble weighing1} tons was added to the two arches which it supported; the pier gradually sank 1 foot7 inches, and the weight remained on it for five months afterwards. The pier and hauncheswere then relieved by small vaults in the upper part, which do not appear on the outside,being concealed by the facings. The same precaution was taken for the fifth, sixth, andseventh piers. The two arches adjoining the seventh pier have not experienced any accidentand have only a slight irregular curvature, hardly perceptible. The only reason whichcan be assigned for the settlement is, that under the bed of tufa there was probably a soil,so light, that it only acquired sufficient consistence by the weight and compression to whichit was subjected.
The year after the construction of the bridge a sinking of the earth under the tufa tookplace under three of the arches and some of the starlings, to the depth of 2 feet 2 inches,and it was deemed necessary to drive two rows of piles close to each other, and 12 feet9 inches apart, the whole length of the bridge, and 6 feet 6 inches below the starlings; theywere cut off 3 feet 3 inches below the surface, and the space between was filled with rubble.All the other portions that had settled were treated in the same manner.
The cost of Hupeau’s design amounted to 2,084,000 livres, and the additional outlaywas 587,000 livres. It was opened in 1768 by Porronet, who has published the details ofthe construction.
Bridge of Saumur , on the Loire . The designs were made by INI. de Voglia, the engineer-in-chief, and presented to the Ponts et Chaussccs in 1753. The works were begun in 1756,and finished in 1764, under the superintendence of De Cessart.
This bridge consists of twelve elliptical arches of 64 feet span, rising a third. The piersare 12 feet 9 inches thick; some are 17 feet high from the springing.
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