We learn from the work of L. A. Cessart, that the mode of laying foundations in deep water, by means of caissons, was not practised in France till the year 1757, when M. de Voglio and himself (encouraged by Per ronet) introduced it with success at the bridge of Sau mur,after the manner of Labelye.
Having detailed the different modes which have been practised by eminent engineers upon large rivers, it may be useful to young artists to mention, that in smaller rivers, where the foundations are to be laid on firm gra vel, mud, or quicksand, it is frequently necessary to drive a row of dovetailed three-inch pile-planks around the pier or abutment, from five to ten feet in length, ac cording to the nature of the ground, and lay some clay round the outside of them ; then to excavate the founda tion about two feet below the level of the bed of the ri ver, and lay down sills nine inches by six longitudinally, at the distance of about four feet from each other, filling up the spaces between them with rubble masonry ; upon this to lay two thicknesses of three-inch planking crossing each other, and pinned with oak trenails ; around the outer edge of these, and immediately with out the first course of masonry, to fix a curbing six in ches by four, to which, after the masonry has been built up to the level of low water, and the clay removed from them, the pile-planking should be fixed by spike nails, and cut down to nearly that level. the ground is ve ry soft and loose, it is also necessary to drive piles un der the platforms. In some instances, besides the plat forms under the piers and abutments, gratings filled with masonry, and covered with planking, have been carried quite across the bed of the river. M. Blondel performed this in the bridge of Xaintes upon the Cha rante. In a bridge over the Lilly, in Dublin, Mr George Semple carried a solid mass of masonry across the bed of the river, between the piers. Inverted arches have also been made between the piers, and more especially between the abutments, in the case of having one arch only across the stream.
Excavations for the foundations and gratings in deep rivers can now be much more expeditiously performed by the improved dredging machines, worked either by horses or steam engines.
Having completed the cofferdams and caissons, it is, in large works, an important consideration to have a perfect command of the water which may rise within them. The modes formerly employed were hand
buckets, or pumps worked by men or horses, also wa ter-wheels ; but of late, in England, recourse has been had to steam-engines. These can now be suited to any exertion required, and are the most powerful and certain means yet devised. The gangways and scaffolding must be regulated by local circumstances, no general rules being applicable. We shall, therefore, only no tice, that the labour in removing large stones, may fre quently be much abridged by laying down cast-iron railways; and that much aid is to be derived from the use of properly constructed cranes. The form and pro perties of these will be explained under their respec tive heads.
To lay the foundations of piers in deep water, upon rock, and in the tideway, being a difficult operation, it becomes of serious importance to explain how it has been performed upon a large scale. We do not recol lect of this having been done in a satisfactory manner in any former publication, and therefore trust the follow ing narrative will prove interesting to those who may be connected with similar works. It affords a minute detail of every operation in the construction of a coffer dam upon rock, in deep water, and in the tide-way. It has been practised with perfect success at the western termination of the Caledonian canal, for the tide-lock built there by Messrs Simpson and Wilson, and is equal ly applicable to the abutments or piers of bridges.
The entrance from the canal into the tide-way having been fixed on the north-west side of a rock which pro jects into the sea about 100 yards from high-water mark, and which was covered at three-quarters flood, it be came necessary to discover in what direction, and at what depth, it lay below the silt or mud which compos ed the beach at that place. By boring with augers, it was found, at the distance of 21 yards from low-water mark at the lowest spring ebbs, that the rock, in the middle of the entrance, was 21 feet below high-water, neap-tides, which affording the desired depth, was fixed upon as the extremity of the space to be enclosed by a cofferdam for the sea-lock.