Chap. VI.

FRANCE.

259

The first soundings indicated a bed of gravel from 13 to 16 feet thick, and the length ofthe piles was fixed at from 26 to 30 feet. The foundations of an abutment and the ad-joining pier were first laid, but the exhaustion of the water was so difficult, that only onepier could be finished in the first season, and the piles were cut off’ 4 feet 4 inches below thesurface. The abutment was finished the next season, and the piles cut off 4 feet 4 inchesbelow low water.

It being found impossible to establish cofferdams for the piers in the middle of theriver, a proposal was made to adopt the method indicated by Belidor , which consisted incutting off the piles under water, and sinking a platform loaded with masonry, by meansof several screws firmly fixed. jYI. de Cessart invented a saw to perform this operation,but more careful consideration induced the engineers to use the caissoons, which Labelychad just employed at Westminster Bridge, and which were merely placed on the groundcarefully levelled. The bottom of these caissoons was composed of pieces from 10 to 11inches thick, which could rest oil the piles throughout; the sides were moveable, and mightbe adapted to a new bottom after sinking the former one.

All the other piers and the second abutment were founded in this manner. The pilesof the second pier were cut off 7 feet 6 inches, and some 12 feet 9 inches below the surface.Care was taken to dredge the sand between the piles as much as possible, and to fill up theintermediate space with rubble, the upper surface of which was levelled G inches below theheads of the piles, so as not to affect the placing of the caissoons.

The piles were driven by a ram moved by a wheel adapted to a horizontal axle; thissaved one-half the expense and number of men. The saw for cutting off the heads, whichhas since been employed in several other places, was completely successful. The inventionof this machine may be considered as a very important era in the art of building underwater, and one of the most powerful methods at the disposal of constructors to overcomethe difficulties which nature opposes to them; it lias sometimes cut off twenty-two piles ina day.

Founding the piers at the bridge of Saumur, by means of a caissoon, by De Cessart in theyear 1757, was thus accomplished.

The bottom of the caissoon was 48 feet in length, and 20 feet in width, from outside tooutside.

The ends were in the form of an isosceles triangle, two sides of which were 13 feet3 inches.

The outside timber of the frame-work was 18 by 16 inches, scarfed in their length, in themanner termed traits de Jupiter , and rebated on the inside edge to a sufficient depth toreceive the timbers which crossed from side to side, and were dovetailed at every 3 feet, the

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Fig.265.

SAUMUR PIERS.

others between them being laid with square edges; they were secured by wooden pins aninch in thickness, not driven entirely through, in order that the bottom of the caissoon shouldpresent no inequality to the heads of the piles on which it was to rest. To fasten them,however, more effectually, a wedge was driven into the points of the pins previous totheir being inserted, which forced them upwards, and formed beneath a second head, ren-dering their withdrawal almost impossible.

After the first planks were laid and pinned down, another piece, 10 inches in width, and8 inches in height, was spiked over their ends and united to the main timber by iron bolts,15 lines in diameter, at every 3 feet; on this was laid another longitudinal timber, 12inches high and 8 inches in width, bolted through the planks, and laterally through themain timbers: the space between these outer timbers was then covered with planks,

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