Hawkesbury River Railway Bridge

About

The Hawkesbury River railway bridge is a heritage-listed railway bridge in New South Wales, Australia which carries the Main North railway line across the Hawkesbury River. The bridge crosses between Brooklyn on the northern outskirts of Sydney and Cogra Bay in the Central Coast region. The railway bridge was to be the last link in a railway network that linked the state capitals Adelaide, Melbourne, Sydney and Brisbane and was a major engineering feat at the time. The original railway bridge was built in 1889 and replaced by the current bridge in 1946. The 1946 bridge was added to the New South Wales State Heritage Register on 2 April 1999.

On 7 April 1887, the single track section of the Main North line was opened between Hornsby and the Hawkesbury River. Passengers and goods heading north now unloaded at the River Wharf platform on the eastern end of Long Island, transferring to the double decker, rear paddled-wheeled steamer General Gordon for a three-hour trip out to Broken Bay and up Brisbane Water to Gosford where the train service recommenced. Once the 1.7-kilometre-long (1.1 mi) Woy Woy Tunnel was opened on 15 August 1887, the boat trip was considerably shortened as the boat only had to cross the river and negotiate the lower reaches of Mullet Creek to reach Mullet Creek station.

The Union Bridge Company of New York was awarded the contract to construct the bridge in January 1886. The railways engineer-in-chief, John Whitton, who designed and built the railway was not invited to design the bridge due to fallout from a mid 1880s enquiry into railway bridges. 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 127 metres (416 ft) each for a total length of 887 metres (2,910 ft). Five of the piers were sunk to then record depths of between 46 and 49 metres (150 and 160 ft) below high water. The bridge officially opened on 1 May 1889.

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 were 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 the New South Wales Government 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 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.

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 the bridge being used as a crossing point for trains, thus regularly subjecting the structure to maximum stress loads.

The bridge showed signs of problems within 12 months and the contractors were called back to repair some faults with the piers. Through the 1920s and 1930s, many design faults and problems became evident. By 1925 the original loading of the bridge was considered inadequate and in 1925 it was decided to strengthen the deck. The work took nearly six years between 1926 and 1931 and numerous problems were experienced with the pin-jointed construction of the trusses, which while easy to assemble were difficult to maintain and strengthen.

In 1938 a severe crack in one of the piers was discovered 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 23 km/h (14 mph) and finally down to 6 km/h (4 mph) with tracks altered to gauntlet configuration to ensure trains could not pass.

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 were 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 the New South Wales Government 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 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.

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 the bridge being used as a crossing point for trains, thus regularly subjecting the structure to maximum stress loads.

The bridge showed signs of problems within 12 months and the contractors were called back to repair some faults with the piers. Through the 1920s and 1930s, many design faults and problems became evident. By 1925 the original loading of the bridge was considered inadequate and in 1925 it was decided to strengthen the deck. The work took nearly six years between 1926 and 1931 and numerous problems were experienced with the pin-jointed construction of the trusses, which while easy to assemble were difficult to maintain and strengthen.

In 1938 a severe crack in one of the piers was discovered 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 23 km/h (14 mph) and finally down to 6 km/h (4 mph) with tracks altered to gauntlet configuration to ensure trains could not pass.

Design and construction of a replacement bridge commenced in 1939, due to concerns that the original bridge would not hold up to extra loading and traffic caused by transport demands of World War II. 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 56 metres (183 ft). New tunnels were bored through Long Island to the south and Cogra Point on the northern approach.

The design, foundation work and fabrication of the new bridge were undertaken by the New South Wales Government Railways and over 500 men worked on the project, with six dying during construction. A plaque commemorating the lives lost is at the southern end of the bridge, at the northern portal of the 1946 tunnel through Long Island.

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 capped piers remain to this day. The construction docks remain in situ.

In 2016, a report revealed cracking in concrete pylons as well as "consistent defects" in the steel frame of the bridge. As a result, limits are planned to be imposed on the weight capacity of freight trains crossing the bridge, pending possible strengthening of sections of the bridge to allow heavier loads.