OF COFFERDAMS.
We shall first describe the measures pursued both in France and Britain, in working by means of batterdeaux or cofferdams.
The forms of these are varied according to local cir -cumstances, and the ingenuity of the persons who have the direction of the work.
1. By driving two rows of vertical piles and plank piles, and filling the space between them with clay.
2. By driving main piles, and working with strong planking, laid in a horizontal position.
3. By driving one row of guaging piles, and filling the spaces between them with pile planks driven vertically.
The most eminent French bridge engineers, viz. Sicur Gabriel, Gautier, Hupeau, and Perronet, in constructing their greatest bridges with cofferdams, directed piles, from nine to ten inches diameter, to be placed from three to four feet from centre to centre, and driven from three to six feet into the bed of the river, (if composed of mud, clay, or gravel,) and to rise (in their rivers) six feet above low water mark. Their pile planks were nine to twelve inches broad, and four inches thick ; one frame r•ontained 16 of these pile planks, which were driven at one time ; these frames were placed along, and embraced three of the main or guide piles, and were composed of two uprights, of the same thickness as the pile planks, each sharpened at one end ; these uprights were fastened together by two horizontal pieces, one be low, 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 tri angular ; and sometimes a groove was made in eaeil 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 he 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 20Ibs. : The pile planks were 18 to 21 feet long, time to ten inches broad, and four inches thick ; each shoe weighed Bibs. The rams for driving the piles were about 1200Ibs.; those for the pile planks 500 to 600lbs. At
Neuilly, the piles were 22 to 24 feet long, nine to ten inches diameter, including the bark ; they were driven six feet into the earth. '['he pile planks were 21 feet long, and ['our inches thick, driven six feet, by rants of from 600 to 900Ibs.
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 gravel down to the firm ground, by 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 re moved, Gautier, after having levelled the ground, laid Clown 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 hark ; their shoes weighed, including branches and nails, from 13 to 25Ibs. They were driven with rams of 1000, 1181, 1384, and 1981/bs. 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 afterwards. 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 fixed by pins or bolts ; the spaces in this grating were cleared out for IS 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 platform 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.