OF CAISSONS.
With regard to laying foundations by means of cais sons, this mode was first practised in England by that able engineer Mr Labelye, at Westminster bridge. In the sequel of a brief report relative to this work, (edi tion 1751,) he announced an intention of publishing a larger report, with plans and designs, calculations and details. This, however, never appeared ; but most of the essential materials have been preserved by Mr Tho mas Gayfere, a venerable old gentleman, aged 90, now living in Abingdon-street, Westminster. Ile was senior apprentice to Mr Jelf, the mason first employed on Westminster bridge, and was specially appointed to make working plans and models, and to be attendant on Mr Labelye's own directions ; and when Mr Tufnel succeeded Mr Jelf, Gayfere was continued in the same capacity till the bridge was finished. From his manuscript narrative and drawings, which he has allow ed to be in part copied for this work, will be seen the injustice to which engineers subject themselves by not publishing their designs,—most of Labelye's inventions having been copied, without acknowledgment, by others, who have since constructed large bridges, and to whom they have rendered similar operations a comparatively easy task.
By this valuable manuscript we are informed, that at the situation fixed for the bridge, in the ebb channel on the Surrey side, there was 6 feet at low water ; in the flood channel on the Westminster side, 5 feet 9 inches, and on a shoal between them, 4 feet ; also that the rise of tides at extra springs, with land frcshcs, was 15 feet ; neaps 7 feet ; and at a mean tide 10 or 11 feet ;—that the velocity of the surface was something greater with the tide of flood than that of ebb ;—that with the former it was about 3 feet per second, with the latter 21 feet ; and that the piers were laid about live feet below the bed of the river. About the middle, and at five feet under the bed of the river, the gravel is 14 feet deep: this continued from the Westminster side -1 the width of the river. The other fourth part, next the Surrey shore, is a loose sand and mud.
They commenced the operation by driving fender piles to keep off the barges. These were 34 feet long, 13 or 14 inches square, and driven 13 or 14 feet into the bed of the river. They were placed 30 feet from the
pier, and 7 feet asunder, and were connected by floating beams. For the first large pier there were 34 of these piles, for the others 26. They were driven by an engine constructed by M. Vauloue, a watchmaker. The weight of the ram was 1700Ibs. After the engine had worked for some time, it was found, that by three horses walking at an ordinary pace, when the ram was raised only 8 or 10 feet, 5 strokes were made in two minutes.
The foundation was formed by scooping the mud in the usual way of raising ballast in the Thames at that time. When they came to the gravel, it was raked level. The guage for proving the foundations, was a stone 15 inches square, and three inches thick, fixed to a graduated rod 18 feet in length. The rod was painted red, and the figures white.
In order to prevent the current from washing the sand into the pit, short grooved piles were driven before the two ends and part of the sides, not above 4 feet higher than low water mark, and about 15 feet distant from the caisson. Between these piles, rows of boards were let down into the grooves to the bed of the river.
The length of the caisson from point to point (the shape corresponding with that of the pier,) was 80 feet, the breadth 30 feet, the height, including the bottom, 18 feet. The sides were formed by fir beams laid hori zontally upon one another, and pinned with oak trenails. They were framed, and had oaken knees bolted and screwed at all the angles, except the salient angles, where they were secured by iron work, which being unscrewed, permitted the caissons, if necessary, to part in two halves. The sides were planked across the tim bers, both inside and outside, with three inch planks; the thickness of the sides was 18 inches at the bottom, and 15 inches at the top. The bottom was also formed with beams, with three inch planking across their under sides, and timbers nine inches thick across their upper side, making the whole two feet in thickness. Upon this bottom, around the outer edge, a strong curbing was fixed, to prevent the first course of stone from press ing against the sides. All the joints, both of the sides and bottom, were caulked and pitched.