Hawkesbury River Railway Bridge

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Hawkesbury River Railway Bridge
The Hawkesbury River Railway Bridge spans the Hawkesbury River just north of the town of Brooklyn on the northern outskirts of Sydney, Australia. The railway bridge was to be the last link in a railway network that linked Adelaide, Melbourne, Sydney and Brisbane and was a major engineering feat at the time.

Before The bridge
On 7 April 1887, the single line section of railway track was opened between Hornsby and the Hawkesbury River. Passengers and goods heading north were unloaded at the River Wharf platform located on the eastern end of Long Island. Here they boarded a double decker, rear paddled-wheeled steamer named "General Gordon" that conveyed them out into Broken Bay and up Brisbane Water into Gosford where they could rejoin trains heading north. Once the 1.6-kilometre (0.99 mi) long Woy Woy tunnel was completed, the three hour trip was considerably shortened as the boat only had to cross the river and negotiate the lower reaches of Mullet Creek to reach Wondabyne railway station.

First bridge
The Union Bridge Company from New York was awarded the contract to construct the bridge in January 1886. However, subcontractors were also involved in the actual construction work. The piers consisted of concrete below water with sandstone masonry above. The spans were assembled on Dangar Island and floated 1,500 metres (4,921 ft) or so across to the bridge site on barges. The bridge had seven spans of 416 feet (127 m) each for a total length of 2,910 feet (890 m). Five of the piers were sunk to then record depths of between 150 to 160 feet (46 to 49 m) below high water. The bridge was officially opened on 1 May 1889.

Load testing
At the time, it was the longest structure to be load tested in Australia and so two distinct and separate methods were used. Firstly, optical measurements was taken of the deformation under load at the centre of each span, taken by two sets of observers positioned atop the stone piers. The second test used a water gauge to accurately measure the deformation and the amount of deflection or "set" for each span. Load testing was carried out on 24 April 1889 in the presence of various dignitaries including Henry Deane, the Assistant Engineer in Chief of Railways. During the morning, each span was tested separately by slowly running trains out onto the span, taking the required measurements, backing the train off again and retaking the measurements. This was repeated for each span but on the second span it was found that the optical readings did not match with the water gauge. The cause was found to be a slow leak in a connecting pipe of the water gauge equipment. There was only one water gauge available and as the leak would become worse as the equipment was moved from span to span for each reading it was decided to abandon this method and rely wholly on the optical readings. A speed test was undertaken during the afternoon by four locomotives, coupled in two pairs. They were started off by a flagman standing above the Long Island tunnel and the trains ran across the causeway from Hawkesbury River station through the Long Island tunnel and out across the bridge at maximum speed.

Problems arise
Although the track on either end of the bridge was single line, the bridge itself was constructed to double track width with an eye to the future duplication of the line. This led to the undesirable practice of using the bridge as a crossing point for trains operating on this section of line, thus regularly subjecting the structure to maximum stress loads. Perhaps it was this reason that resulted in the double track being replaced by a single track in June 1893. The bridge gave admirable service until 1927 when it required strengthening and then during the 1930s cracks developed in one of the piers and it became necessary to replace the entire structure. The depth of sediment had made it impossible to reach bedrock with the foundations on the southernmost pier and it seems that this was the cause of the structural faults. The amount of traffic being carried (up to 100 trains a day) during World War II made the replacement extremely urgent and prior to the new bridge being brought into operation the speed limit on the old bridge was restricted to 15 miles per hour (24 km/h) and finally down to 5 mph (8.0 km/h).

Second bridge
Work commenced on the new bridge in July 1940 and despite best efforts it was not completed until after the war finished, opening for traffic on 1 July 1946. The new bridge was positioned 60 metres (197 ft) to the west or upstream of the original bridge and consists of eight spans in three different lengths and piers sunk to depths of up to 183 feet (56 m). New tunnels were bored through Long Island to the south and Cogra Point on the northern approach. The spans for the new bridge were constructed adjacent to the bridge site on the northern side of Long Island, raised to the correct height, placed onto barges and floated out to the piers at high tide. Upon completion of the new bridge, the old bridge was removed, however the sandstone piers remain.

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