The Hawkesbury Bridge, New South Wales.” by CHARLESORMSBY BURGE,M
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2 BURGE ON THE HAWKESBURY BRIDGE, N.S.W. minutes Of Associate iMernbers-continued. HERBERTSPON~ HAWKINS. HAROLDWILLIAM PAULING, Stud. Inst. JADlES ALLANHENDERSON. C.E. PATRICKNICHOLAS HILL JOXES, Stud. ALFRED CECIL PEREIRA,Stud. Inst. Inst. C.E. C.E. FREDERICKJORGEN~EN. CHARLESROBERTS, F.C.H. JOHSKNIGHT. ARTHUR SAXUELFRANCIS ROBINSOS. TOMGALLON LUMB. CHARLESTALLENT SPENCER, Stud. Inst. WILLIAWMCKELVIE. C.E. HENRY ALLANNoss. ROBERTANTHONP SYNGE, B.E.. JOSEPHMOYNAN, B.C.E. ALEXANDERTHOPSON WALKER. JOHNJAMES MYRES. HORACEREGINALD WATERS, B.E. ADOLPHEERNEST ORR. (Paper No. 2421.) The Hawkesbury Bridge, New South Wales.” By CHARLESORMSBY BURGE,M. Inst. C.E. THErailway systemof New South Wales until recentlyconsisted of two separate divisions,the one starting from Sydney, and branching in a westerly, southerly, and south-westerly direction, while the otheroriginated at Newcastleon the sea-coast, about 100 miles north of Sydney, and communicated with the northern part of the Colony, and with Queensland. The Hawkesbury Bridge is situatedupon the connecting link of railway which was designed to unite these separate divisions. At the site of the bridge, about 7 miles from the sea, the estuary of theHawkesbury has a totalwidth of about 6,600 feet, and is dividedinto two channels by Long Island (Plate 1, Fig. l). A solid embankmentcarries the railway across thecomparatively shallow southern channel, on leaving which the line is tunnelled through a high promontory forming the eastern end of Long Island (Plate l, Fig. 2), andthen crosses the mainchannel to Mullet Point by the bridge which is the subjectof this Paper. The Colonial Government, after a considerable amount of dis- cussion, decided to invite notonly tendersfor the construction of the work, but also designs for the same, and took the necessary steps t,o make the competition world-wide. A committee, consisting of Xr. W. H. Barlow, PastPresident Inst. O.E., &h.G. Berkley, Vice PresidentInst. C.E., andCaptain DouglasGalton, R.E., .lssoc. Inst. C.E., was appointed to adjlldicate on the tenders; and when the report of this committee hadbeen supplemented by others from Sir John Fowler, Past President Inst. C.E., and Mr. John Downloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] BURGE ON THE HAWKESBURYBRIDGE, N.S.W. 3 Whitton, M. Inst. C.E., Engineer-in-Chief for Railways, New South Wales, it was finally decided to accept the tender of the Union Bridge Company, of New York, who undertook to complete the bridge within two-and-a-half years for the sum of S327,OOO. The accepted design consisted of seven spans of 416 feet each, from centre to centre of the piers, the foundations for the latter being of concrete encased in steel caissons, while the upper portions of the piers and the whole of the abutments, are of masonry. The girders are formed of built steel compression-members, and solid steel eye-bar tension-rods, all the connections being made by steel pins. The cross-girders and rail-bearers are of riveted steel plate. The two main girders of each span are 410 feet 03 inch long, from end pin to end pin, and 58 feet deep at centre, and are placed 28 feet apart, from centre to centre, the bridge carrying two lines of railway. The general elevation of the bridge is shown in Plate l, Fig. 3. LOCATION. The line and spans of the bridge were located by a system of triangulation carried out by Mr. F. Small, and continued by the Author who succeeded him as representing the Government, and by Mr. 0. Schulze on thepart of the contractors. The results obtainedfrom the firstbase-line on the shore wereafterwards supplemented and checked byusing a portion of thestructure itself, when fixed, as a secondary base-line. FOUNDATIONS. The borings showed a bed of mud extending to a depth varying from 60 to 170 feet below high-water mark,and overlying the sand ; the greatest depth of water being 77 feet, and the range of tide 7 feet. Thegreatest depth of the foundations occurs at pier No. 6, which is carried down 162 feet below water, this being, as far as the Author isaware, the deepest bridge-foundation yet sunk. The caisson foreach pier isrectangular, with rounded ends, and its construction is shown inPlate l, Figs. 4 and 4a. It is 48 feet long transversely tothe bridge, and 20 feet wide,splaying out in the lowest 20 feet,, so as to form a tapered shoe which is 2 feet wider all round at the bottom. In the centre line on plan, parallel with its length, are three wrought-iron dredging-tubes, 8 feet in diameter, and 14 feet apart from centre to centre. These are connected to the outerskin and to each other by steel strutting of tees and angles. B2 Downloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/09/16]. Copyright © ICE Publishing, all rights reserved. 4 BURGE ON THE HAWKESBURP BRIDGE, N.S.W. [Minutes of At thebottom, the dredging-wells splay out in a trumpet mouth so as to meet the outer skin, andalso each other, in a strong cutting edge formed of heavy steel plates. The method of sinking the caissons was as follows :-The shoe, having been built on shore at Dangar Island, and provided with a timber false bottom, was floated out to position and sunk to the bottom of the river, by removing the temporary bottom, and par- tially loading the caisson with concrete. The caisson was then sunk through the mud by dredging the material from the bottom of the wells and by loading the space between the wells and the skin with concrete, more steel being built up as the caisson went down. As soon as the structure was firmly in the sand, the dredging- wells were filled with concrete, and the masonry was then begun at a level somewhat below low-water. The concrete was composed of l part of Portland cement, 3 parts of sand, and 6 parts of stone, broken to 2fr-inch gauge. The stone was what is locally known as Kiama blue stone, the material being mixed by Jamieson’s concrete-mixers, each passing through about 5 cubic yards per hour. The concrete in the shoe was made stronger by the addition of one-fifth of a cask of cement per cubic yard. The caisson for No. 5 pier was the first one started, and, having undergone greater vicissitudes in its downward progress than any other, was the last to be completed. The sinking was begun on the 9th of December, 1886, and the foundation was only ready for the masonry on the 9th of October, 1888. Shortly after it had well entered the mud, the caisson showed a tendencyto work eastwards, that is to say,transversely to the direction of the bridge. Efforts to recover its position were first made by endeavouring to cantit eastward at thetop, by excavating the eastern well in advance of the others, thus pointing the central vertical axis downward in the westward direction required; but this was not successful, as even when the eastern well excavation was 15 feet deeper than the western one, the cant was still west- wards. Dredging outside was then resorted to without effect, also dumpingthe excavatedmud outside on the easternend. When the caisson had reached 75 feet below the river bed, the divergence had amounted to 5 feet at thebottom and 3 feet at thetop, the cant still continuing westward, notwithstanding constant extra sinking of the eastern wells. The margin for lateral divergence allowed by the specification for the caissons was 2 feet. A recovery of about 18 inches at the bottom was affected about Downloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/09/16]. Copyright © ICE Publishing, all rights reserved. froceedings.1 BURGE ON THE HAWHESBURY BRIDGE, N.S.W. 5 this time, and when the west cutting edge first felt the indications of sand, the caisson, as had been expected, commenced a righting movement towards the vertical, but of course at the expense of still further lateraldivergence at the top. At this juncture, the 7-feet grab working in the ea,stern well, got caughtin thebottom splay, and as aconsiderable period elapsed before it was recovered, work was suspended at this caisson for a time. Later on, twograbs were similarly caught in the centre well, one of which was never recovered-diving operations, at the depth then attained, being attended with greatdifficulty. The contractors now began driving piles, at the eastend, to sustain a cribwork, which was loadedwith stone and was intended to forma buttressagainst the structure and prevent further movement ; and, subsequently, a similar cribwork was putin about 400 feet upstream or westwards, with the intention of anchoring the top of the caissonfirmly to it; and then, by digging per- sistently in the eastern well, it was hoped that the bottom would be brought back to its position. But as to all these proposals, it was always the Author’s opinion, that, as the purchase, in each case, was in the same unstable medium of mud, any horizontal movement in the caisson could not be corrected by either tying it to, or resting it against, any other structure not founded in the sand. This opinion was verified, as, on further sinking, the top, in righting itself, pushed over the eastern cribwork. Operations, as regards No. 5 caisson, were now suspended for some months, except for the recovery of the lost grabs, and the construction of the cribwork upstream, as mentioned above. This experiment having failed, a proposal was made to cease sinking, as the caisson was well in the sand at 144 feet below high-water of ordinary spring-tides, and then it was proposed to rectify matters by sinking an additionalcaisson at thewest end.