If the ground proved firm and water-tight, this pro cess, although laborious, was regular and plain ; but as the French engineers inclosed very large spaces around their masonry, great inconvenience frequently arose from imperfections in so extended a coffcrdam,an s.,:ne, times from springs supplied fi om higher grounds within the inclosed space. From circumstances of thii nature, in one of the abutments at Orleans, frequent in terruptions took place ; they were forced to sadivide the enciosed space ; and, at last, after having discovered the precise situation of the springs, to raise them by tubes, so as to discharge the water made by them above the level of the water of the river. It is, therefore, advisable, in order to avoid cxpenee in the construction and after risk, to enclose the smallest space possible. Round the edge of the platform, they some times drove a row of pile planking, or laid a bed of dry rubble stones, about six feet wide, and about two to three feet deep. For Neuilly cofferdam, see Plate XCV.
The foundations of many bridges in England have been laid by means of cofferdams, so nearly similar to those which have just been described, that noticing them here would lead us into repetition: We shall therefore pro ceed to the second mode.
2. In constructing a bridge upon the river Severn, at Bewdley, in Worcestershire, designed by Mr Telford, and executed under his direction, Mr John Simpson of Shrewsbury, (who built the bridge by contract) con structed a cofferdam, which answered the purpose per fectly well. Here the low water was five feet in depth, for 7feet more there was gravel and rubbish, and under that a sand stone rock. The floods rose about 10 feet. In Plate XCVI., Figs. 1, 2, 3, the plan, section, and side view, will chew distinctly the manner in which the cof ferdam was constructed. The chief peculiarity is, that the sides and ends were formed on the shore, by laying the planking horizontally along upright piles, and dow elling them together ; they were then floated off, fixed against main piles previously driven, and there secured by cross braces. About five feet without these a similar inclosure was made, and the two secured together by iron bars in the middle of the height, and wooden bra ces at the top. The space between them was filled with clay in the usual way. Before the space for the founda tion of the masonry could be wholly cleared clown to the rock, it was found necessary to put down some small piles along the edge of the gravel, next to the outer edge of the stone work.
In the same Plate, Figs. 4, 5, represent the plan and section of a cofferdam, used in under building one of the piers, and also the eastern abutment of Pulteney bridge, in the city of Bath. The operations here were rendered peculiarly difficult, by several previous ineffectual at tempts ; they were at last successfully accomplished by Messrs Simpson and Cargill, employed for that purpose by Mr Telford. This work much exceeded in difficul
ty those in Languedoc, so minutely described by M. Gautier, the French engineer, as very extraordinary ef forts of art.
A still simpler mode of constructing cofferdams has been practised in Britain : that is, by driving guaging piles, about 12 inches square, at the distance of about 10 feet from each other, and about five feet from the edge of the intended platform. They are driven from 5 to 7 feet into the bed of the river, so as to be about 3 or 4 feet below the level of the platform, and rise about 3 feet above low water. They are grooved to receive the pile planks, and a leading beam, 6 inches by 9 in ches, is fixed about a foot below their top on the inside. These beams are notched on the guaging piles, so that their outer edge is in a line with the inside of the groove for the pile planks. The whole of the pile planks, which fill a space between the guaging piles, are entered to gether, and the middle pile has a wedge-form narrow est at the bottom. The pile planks receive a few strokes alternately, so as all to be driven nearly together, while the middle one wedges the whole, and makes the joints water tight. They are driven about three feet below the platform, and the whole properly braced. The wa ter being pumped out, and the ground cleared, piles are driven about two feet and a half distant from centre to centre, over the whole foundation ; those near the mid dle being driven 7 feet 6 inches below the platform, and 8 inches diameter ; those next the outside 9 feet long, and 10 inches diameter. Immediately under the outer edge of the platform, grooved guaging piles are driven about nine feet distant from centre to centre, and nine feet in depth. Nearly in the same range with these, common round piles are also driven. Upon the latter, a leading beam, 12 by 8 inches, is fixed, which also em braces the guaging piles, and guides the pile planks which were driven around the platform. The bearing piles may be considered as sufficiently driven, when by 30 blows of a proper ram they are not driven one inch, and 40 blows for driving the sheeting piles the same distance. See Plate XCVII. Fig. I. Along the top of each second row of piles, under the body of the pier, a beam, 12 by 6 inches, is laid, reaching quite across, and dovetailed into the beams which surround the platform. The spaces between the beams are filled up with ma sonry. Upon this platform the regular courses of ma sonry are laid.
In whatever manner the cofferdams are constructed, there should be means provided to discharge the water near the level of low water of the river, instead of rais ing it always over the top of the cofferdam ; and there should also be a sluice near the level of the bed of the river, to fill the cofferdam in case of any defect appear ing in the dam, or any risk of a flood rushing in too hastily.