524 



BRIDGE. 



Practice, driven at one time j these frames were placed along, 

 and embraced three of the main or guide piles, and 

 were composed of two uprights, of the same thick- 

 ness as the pile planks, each sharpened at one end ; 

 these uprights were fastened together by two hori- 

 zontal pieces one below, and the other above, and 

 separated by the thickness of the uprights ; these 

 frames being fixed, served to guide the pile planks ; 

 the grooves were two to three inches wide, and two 

 inches deep ; the shape of the groove was sometimes 

 rectangular, and sometimes triangular ; and some- 

 times a groove was made in each plank, and a slip 

 or tongue driven down. The distance between these 

 two rows of piles, to receive the clay, &c. they made 

 in still water to be equal to the depth of the water 

 to be sustained ; in running water, once and a half 

 that height is recommended : The two rows were kept 

 together by two tiers of ties, six inches square. 



At Orleans, the two rows were driven ten feet 

 apart from centre to centre ; the piles were from 18 

 to 24 feet long, and nine to ten inches diameter 

 without the bark ; they were shod with iron, each 

 shoe weighed 20lbs. : The pile planks were 18 to 21 

 feet long, nine to ten inches broad, and four inches 

 thick ; each shoe weighed 81bs. The rams for driv- 

 ing the piles were about 1200lbs. ; those for the pile 

 planks 500 to 600lbs. At Neuilly, the piles were 

 22 to 24 feet long, nine to ten inches diameter, in- 

 cluding the bark ; they were driven six feet into the 

 earth. The pile planks were 21 feet long, and four 

 inches thick, driven six feet, by rams of from 600 to 

 900lbs. 



When these two rows of piles and pile planks were 

 driven and secured by ties six inches square, the 

 space between them was cleared of stones and gra- 

 vel down to the firm ground, by rakes and spoons, 

 and then filled up with clay or earth, which excluded 

 water. This being accomplished, hydraulic machines 

 were established to lift the water from the inside of 

 the cofferdam ; these were pumps worked by men or 

 horses. At Neuilly, water-wheels upon the Seine 

 worked bucket-wheels, which threw up the water 

 from 10 to 12 feet. After the water was pumped 

 out, and the stones and gravel removed, Gautier, af- 

 ter having levelled the ground, laid down a grating, 

 consisting of timbers, of 10 to 15 inches diameter, 

 with openings of two feet, to two feet and a half 

 square, and drove two piles in each opening. Hu- 

 peau and Perronet, after clearing the space within 

 the cofferdam, drove piles at the distance of three 

 to four feet from centre to centre ; these piles are 

 from 12 to 20 feet in length, and 12 inches mean 

 diameter without the bark ; their shoes weighed, in- 

 cluding branches and nails, from 13 to 25lbs. They 

 \vere driven with rams of 1000, 1181, 1384, and 

 198 libs, until they did not sink two lines by the 

 last sixteen strokes, after a continuation of thirty 

 strokes ; or until they did not sink two lines with 

 twenty, five strokes, and received ten strokes after- 

 wards. They began by driving the piles next the 

 centre of the space, and finished with the outer rows. 

 After the piles were driven, and, with the exception 

 of the tenons, cut off nearly level with the ground, 

 caps were morticed upon them, and upon these, 

 along each row of piles, beams, 12 inches by 8, were 



I'ractici 



xcv. 



fixed by pins or bolts ; the spaces in this grating 

 were cleared out for 18 inches in depth, and filled 

 up with rubble masonry, laid in lime mortar, which 

 grew hard under water ; this was brought up level 

 with the top of the grating ; thin planking, four 

 inches in thickness, was laid over the whole space, 

 and pinned or spiked down to the grating; this plat- 

 form extended seven feet beyond the face of the 

 masonry of the shaft of the piers and face of the 

 abutments. Upon this platform, the first course of 

 the masonry was laid. 



If the ground proved firm and water-tight, this 

 process, 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 

 cofferdam, aad sometimes from springs supplied from 

 higher grounds rising within the enclosed space. 

 From circumstances of this nature, in one of the 

 abutments at Orleans, frequent interruptions took 

 place ; they were forced to subdivide the enclosed 

 space ; and, at last, after having discovered the pre- 

 cise 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, ad- 

 visable, in order to avoid expence in the construc- 

 tion and after risk, to enclose the smallest space pos- 

 sible. 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. PtATI 



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 noti- 

 cing them here would lead us into repetition : We 

 shall therefore proceed to the second mode. 



2. In constructing a bridge upon the river Severn, 

 at Bewdley, in Worcestershire, designed by Mr Tel- 

 ford, and executed under his direction, Mr John 

 Simpson of Shrewsbury, (who built the bridge by 

 contract,) constructed a cofferdam, which answered 

 the purpose perfectly well. Here the low water was 

 five feet in depth, for 7 feet more there was gravel 

 and rubbish, and under that a sand stone rock. The 

 floods rose about 10 feet. In Plate XCVI., Figs. PLATI 

 1, 2, 3, the plan, section, and side vigw, will shew ^,^ 

 distinctly the manner in which the cofferdam was 3 ***' 

 constructed. The chief peculiarity is, that the sides J ' 

 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 secu- 

 red together by iron bars in the middle of the height, 

 and wooden braces at the top. The space between 

 them was filled with clay in the usual way. Before 

 the space for the foundation of the masonry could be 

 wholly cleared down to the rock, it was found neces- 

 sary 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 F'gs. 

 and section of a cofferdam, used in under building 

 one of the piers, and also the eastern abutment of Pul- 



