W. J. DOHERTY DELT 10 10 Stay pile, every 8ft Extra Struts 14"x 14" 430" Extra Struts 14"x 14" Mud RIVER Extra Wale Extra Wale O.D. S. AN Row of American Elm Piles Extra Wale AW DOCK, HULL. Minutes of Proceedings of The Institution of Civil Engineers. Vol. LI. Session 1877-78. Part 1. December 11, 1877. GEORGE ROBERT STEPHENSON, President, No. 1,523." Description of Cofferdams used at Dublin, Birkenhead and Hull." By WILLIAM JAMES DOHERTY, Assoc. Inst. C.E. In the summer of 1869 a tender from the Author was accepted by the Dublin Port and Docks Board, for the removal of the river wall at Sir John Rogerson's Quay, in the harbour of Dublin, and the erection of a new wall at a greater depth, to be carried out in accordance with the plans and specifications of Mr. Bindon B. Stoney, M.A., M. Inst. C.E., Engineer-in-Chief to the Board. The old wall was the second which had been built at this part of the harbour, and had been constructed about the year 1820, replacing a still older wall, built at a remote period. Owing to its foundations having been laid at a depth of only from 3 to 4 feet below low water, it was practically useless for the requirements of modern shipping, as dredging could not be carried on close to it without endangering its safety. Hence the necessity for constructing a new wall, founded at a sufficient depth to allow of the river being dredged, so as to accommodate the largest class of vessels frequenting the port. As no design had been furnished with the contract plans for the construction of the cofferdam, inside of which the work was specified to be built, it devolved upon the Contractor to design one which should meet with the approval of the Engineer, and which, by its cheapness and stability, should secure the economical and successful carrying out of the works. From the Author's experience, gained during the construction of some of the largest cofferdams in England, whilst on the engineering staff of Mr. William McCormick-a —as for instance that across the entrance to the deep Low-water Basin at Birkenhead Docks, and those at the Victoria Dock Extension and new West Dock at Hull, all of which had to resist great heads of water-he was enabled to form an estimate of what would be required in this present instance. The Author is of opinion that the cofferdam about to be described (Plate 1) was one of the lightest on record, considering the work it had to do. The design consisted in having main piles, of Memel timber, 32 feet in length by 12 inches square, driven as space piles, on the outer row 12 feet apart from centre to centre, with corresponding main piles, 40 to 42 feet long, by 12 inches square, driven as an inner row directly opposite, leaving a space of 4 feet 6 inches between, for the puddle. The space between the main piles of both the outer and inner rows was filled in with sheet piles, 12 inches wide by 6 inches thick; those on the outer row were driven 12 feet into the ground, and, reaching up to the level of high water, were 28 feet long. Those on the inner row being of a similar length, were driven down to the level of the foundation of the new wall, 24 feet below low water, their heads reaching 4 feet above low-water level. The sheet piles on the inside of the dam were driven with their faces in a line with the main piles, so as to offer no obstruction to the settlement of the puddle; and the lower edges of the horizontal planking, 3 inches thick, with which the dam was sheeted on the inside, from the level of 3 feet above low water to 3 feet above high water, were bevelled off with the same object. Two 12-inch tie-rods, of wrought iron, were passed through two half timber wales on the outside and the inside of the main piles of the dam. They were provided with cast-iron washers on each end, 8 inches square and 2 inches in thickness, screwed up with hexagonal nuts. The tie-rods were placed at 1 foot below high-water level, and the other at 2 feet above low water; these tie-rods prevented the dam from spreading while the puddle was being tipped into place. The cofferdam piling, for a length of 150 feet, had to be driven during the winter, when clay could not be readily procured by dredging in the bay, the only place where clay suitable for puddling was obtainable in the vicinity of Dublin. The Author therefore resolved to substitute peat-moss, which was available within easy reach of Dublin by canal. It was expected that the peat, from its spongy nature, would not offer sufficient resistance to the pressure, and might allow the dam to collapse. To prevent this, triangular struts, 6 inches to each side, were placed in two tiers within the dam, against each of the main piles. These answered the purpose for which they were intended; but, for want of density in the peat, a hollow appeared underneath each of the struts large enough to allow water to percolate through the dam, from the joints of the piles, on the outer row, at every place where a triangular strut had been put in. The collected leakage from this cause, when the dam was closed, rendered necessary the employment of two 12-inch steam pumps to displace the water. The struts had therefore to be removed; when the peat answered admirably, becoming improved by pressure and |