Mr. H. LAW, through the Secretary, described a cofferdam, or shield of timber, which had been designed by him and employed in constructing the entrance of a graving dock at Rio de Janeiro. The shore being bare granite rock, an ordinary cofferdam of piles would have been impossible. The site of the proposed entrance was enclosed by a shield of timber, the bottom of the shield being cut to the shape of the rock against which it was to be placed. This shape was measured by constructing a temporary stage of the same form as the intended shield; the vertical depth was then taken with a rod at distances of 6 inches apart; and thus a series of ordinates were obtained, to correspond with which the bottom of the shield was cut. The shield (Fig. 1) was built upon the same FIG. 1. Line of Shore temporary stage in a horizontal position, the first or inside layer of planks being placed close together upon templates formed to the requisite curve, just in the same manner as the lagging of an ordinary centering. In the next layer the planks were bent round the circumference at right angles to the former, being secured at each intersection by a 5-inch spike; then succeeded another layer of planks parallel to the first, and similarly secured; then another parallel to the second, and so on, until a total thickness was attained of 18 inches, consisting of six planks of the ordinary dimensions, 3 inches thick by 9 inches wide, by 14 feet in length. The number of thicknesses of plank was increased according to the depth of water, the upper 5 feet consisting of only two thicknesses, and an additional plank being added at each additional 5 feet of depth. The shield was constructed in three weeks by half a dozen ordinary carpenters, and was caulked in two days; when completed it was lowered without difficulty into place, being loaded with sufficient ballast to counteract buoyancy. It simply bore against the rock at either side, there being no struts or bracing of any sort; a few pine wedges were inserted on the inner side wherever the shield was not in actual contact with the rock. Mr. R. G. Cunningham, who superintended the erection, wrote, "It proved a most brilliant success; I must confess that I never anticipated it would have been so strong and tight; we built in the sill of the dock and completed all the channel with perfect facility as if we were working on dry land. When we removed the ballast from the shield it floated, I towed it away, and the dock was ready to receive the steamer the 'Incansavel.' I may add that I recovered nearly all the timber used in its construction, and employed it in flooring coal lighters and the two large pontoons I have here." Mr. STONEY observed, through the Secretary, that his specification of the Dublin cofferdam provided that the dam should be made with double sheet piling and puddling clay between, so as to be watertight, and high enough to exclude tides rising 3 feet above standard high water; and that both sides of the foundation pit should be closely sheet-piled to a depth of 25 feet below low water, so as to prevent the water from the bottom of the river coming in or the road sinking, with whole-balk wales and wholebalk struts across the pit, and that the excavated material should be thrown outside the dam, so as to form a foreshore to stiffen and support the dam. The minimum width of the foundation pit also was specified. It would be observed that the cofferdam1 had the 1 The term "cofferdam" here includes the puddled dam, front and back piling of foundation pit, wales, cross struts, and other timber work forming the coffer or enclosure within which the quay wall was built. Sometimes the term is restricted to the dam alone.-B.B.S. river on one side and a public quay bounded with houses on the other; it was essential that the quay and houses should be kept from sinking or moving in any way, and as the pressure of the earth beneath the quay, and which was retained by the back piling of the foundation pit, was communicated by the cross struts to the dam on the river side, the simplest and most efficient way of providing the necessary resistance was to throw the excavated material out on the foreshore of the river so as to form a bank outside the dam capable of resisting the thrust of the earth communicated through the cross struts in the foundation pit. The total length of the cofferdam, measured along the exterior of the puddled dam, was 2,437 feet, and the price paid the contractor was £36,500. This included the puddled dam, the front and back piling of the foundation pit, cross dams, struts, and everything connected with the construction, maintenance and removal of the cofferdam. The cost per lineal foot of cofferdam was therefore £15, and its cost per lineal foot of quay wall was £15 16s. These prices contrasted favourably with those of other cofferdams, having regard to the fact that the foundation pit extended in depth from 43 to 46 feet below the surface of the quay, and from 38 to 41 feet below high water of equinoctial spring tides with 13 feet range of tide. The contractor, however, was fortunate in having retentive clay both in the bed of the river and inside the cofferdam, so that the quantity of water to be contended with was comparatively small. Mr. Stoney had constructed departmentally, on the north side of the river in Dublin, a quay wall somewhat similar to that described in the Paper. It was 784 feet long and extended to 33 feet below high water. This wall was finished in 1869, and as the cofferdam was probably the lightest on record for such work, it might be interesting to have it described in the proceedings of the Institution. In general form and arrangement it closely resembled that adopted subsequently at the south quay, as described in the Paper; but the characteristic feature consisted in the slightness of the sheet piling, which was formed of 3-inch American spruce planks, requiring therefore only one-half or one-fourth the quantity of sheeting, and that of a cheaper class of timber than if it had been half or whole-balk piling. The ground below low water level was gravel, full of boulders and porous as a sieve. Wholebalk piles refused to penetrate it more than a few feet, and when taken up their points were generally broken off or broomed up, and the only way the sheeting could be got down was by excavating a few inches below each pile, and then tapping the pile with a small ringing engine with a 4 cwt. ram worked by hand. The quantity of water, which chiefly came through the bottom of the foundation pit, was very great, and required the use of several powerful steam pumps, which raised collectively 14,000 gallons per minute. The wall was faced and coped with granite ashlar. The total cost, including masonry, machinery, cofferdam, pumping, and every expense of construction, was £28,000, or £35 14s. per lineal foot of wall. If, however, the old plant remaining after the completion of the work was valued at only £2,000, the cost would be £33 38. per lineal foot. The cofferdam was represented in isometric perspective in Plate 3. The actual cost unfortunately had not been kept distinct from that of the rest of the work; though unlikely to have exceeded £10 per lineal foot. At the present time the Author of the Paper was constructing under Mr. Stoney's specification abutments for a new bridge in Dublin, behind a dam formed of a single row of whole-balk sheet piles with sides sawn parallel and driven in close contact and having the joints caulked with oakum like the planking of a ship. This dam had up to the present given great satisfaction. It was watertight, and, as it required only one row of piles and no puddle, it was likely to prove a valuable type of dam. The depth of the foundation pit was 17 feet below high water and 25 feet below the surface of the street. ANNUAL GENERAL MEETING. December 18, 1877. GEORGE ROBERT STEPHENSON, President, THE notice convening the meeting having been read, Messrs. J. D. Baldry, C. Frewer, C. E. Hollingsworth, H. Law, R. C. May, T. M. Smith, F. Stevenson, J. Thomson, and A. Williams were requested to act as Scrutineers of the Ballot, for the election of the President, Vice-Presidents, and other Members and Associates of Council for the ensuing year; and it was resolved that the Ballot Papers should be sent for examination every quarter of an hour that the Ballot remained open. The Ballot having been declared open, the Annual Report of the Council, on the Proceedings of the Institution during the past year, was read. (Vide page 163.) Resolved, That the Report of the Council be received and approved, that it be referred to the Council to be arranged for printing, and that it be circulated with the Minutes of Proceedings in the usual manner. Resolved, That the incoming Council be requested to prepare, for the consideration of a Special General Meeting of Members and Associates, such alterations in the Bye-laws as the suggestions contained in the Report with reference to classification may require. The Telford and Watt Medals, the Telford and Manby Premiums, the Miller Scholarship, and the Miller Prizes, which had been awarded, were presented. (Vide pages 176 and 177.) Resolved, That the thanks of the Institution are justly due and are presented, to the Vice-Presidents and other members of the Council, for their co-operation with the President, their constant attendance at the Meetings, and their zeal on behalf of the Institution. Mr. Bateman, Vice-President, returned thanks. Resolved unanimously,-That the cordial thanks of the Meeting be given to Mr. Stephenson, President, for his persevering en[1877-78. N.S.] M |