The Hawkesbury River Railway Bridge near Dangar Island north of Sydney, NSW, was one Australia's major engineering feats of the 19th century. The river is almost a kilometre wide at the point of crossing which begs the question why this site was chosen in preference to a number of far easier ones upstream.
The width of the passage necessitated a large bridge and for this reason an American Whipple style truss design was chosen. The bridge had seven spans each weighting 1,000 tonnes and extending to a length of 1,265 metres. Built by the Union Bridge Co. of Pennsylvania, USA, the steel was fabricated by Arrol Bros. at the Dalmarnock Iron Works in Glasgow, Scotland, who were engaged in similar work on the Forth Bridge in Scotland. The base of the piers were of trachyte quarried at Mt Gibraltar at Bowral, NSW, and topped with locally quarried sandstone.
The bridge had seven truss spans, each 120 metres centre to centre of the bearings. Eleven bay trusses were modified by the use of counterbalances in the five central bays.
The reported ground conditions for the piers consisted of 'a bed of mud extending to a depth varying from 60 to 170 feet below high water mark, and overlying the sand'. The maximum water depth was 77 feet and the tidal range 7 feet. The bridge was designed with six piers founded on caissons, sunk through the 'mud' to the underlying sand. The caissons, 48ft x 20ft in plan, were constructed with hollow cellular walls and three internal compartments. Sunk to a maximum depth of 162 feet below high water, they were the deepest bridge foundations of their time in the world.
Commenced in April 1887 and completed 2 years later, a workforce of 800 men housed in a tent village on Dangar Island was needed to complete the task. As well as steelwork, 10 million bricks, 10,000 bags of cement, 110 tonnes of blasting powder and 10 tonnes of dynamite were used to construct the bridge and its approaches. This was almost as expensive and difficult a task as building the bridge itself. It involved cutting numerous tunnels, the one under Mt Wondabyne, at 1,925 metres, was Australia's longest.
Difficulty was experienced with the sinking of the first caisson at No. 5 (the river piers were numbered 1 to 6 from south to north). The caisson founded at a depth of 43.9 metres below low tide. It immediately began sinking towards the east. It had to be modified 1.5 metres from its original position. An attempt was made to sink and add another caisson but this collapsed under lateral pressure at 9 metres from the bottom. The solution was to corbel the masonry out from the foundations to support the pier.
The problem was overcome in the other caissons by a change in the cutting edge. Pier 6 also gave trouble as it was built in an area where the riverbed sloped downward to the north, finishing up 1.3 metres north of its original position. As a result, span 6 was modified to be 1.3 metres longer than the intended 125 metres.
The bridge was built in a similar manner to Como Bridge in that the spans were erected on Dangar Island on pontoons and floated into position. It was a dangerous exercise as the builders had strong tides and shark-infested waters to contend with. The third and sixth spans were almost lost when both broke free during the manoeuvring process and came perilously close to smashing into the abutments. The bridge was tested on 24th April 1889 and the line was opened to traffic by Lord Carrington on 1st May 1889. The final cost of the bridge was ?327,000.
By the 1930s, the piers had decayed to such a degree that trains using the bridge were reduced to a crawl as a safety measure, therefore it was decided to build a replacement bridge alongside it. The new bridge, which was of a similar design, came into service in 1946 at which time the older bridge was dismantled. The piers of the original bridge remain alongside the replacement bridge.