By Phil Manning, Senior Curator, Cultures & Histories
Needed to cross the river. Designed to survive the river. Built using the river. The rail bridge over the Fitzroy River reveals how the forces of nature were used to overcome the challenges of the environment itself.
The Alexandra Bridge was part of the Rockhampton Junction Railway. George Willcocks was the contractor for its construction.
Designed for floods
Since its official opening on 6 November 1899 the Alexandra Bridge has withstood the forces of seven major flood events. This is a tribute to the design of HC Stanley, Chief Engineer for Queensland Railways. The design for the Fitzroy River rail bridge was based on a design he had previously introduced for the Albert Bridge, Brisbane, in 1895, after the first Albert Bridge had been washed away in floodwaters in February 1893.
Stanley sought to avoid the obstruction to flood waters posed by multiple piers, which had been the downfall of the first Albert Bridge.
The author deemed it advisable that in re-building the structure an entirely new design should be adopted providing for only two spans and one central pier…The foundation of this pier may be regarded as the most important work connected with the bridge.
Building concrete foundations in a river
Flooding in March 1898 delayed the construction of the Alexandra Bridge over the Fitzroy River. Once begun it was expected to take eighteen months to complete.
Just like the design, the construction methods employed for the Alexandra Bridge in 1898 was the same as those used for the Albert Bridge in 1895. The piers were made using prefabricated caissons, and floated into position. Divers worked inside them for weeks to excavate the riverbed and sink each caisson to the required depth.
The caissons for the Alexandra Bridge were launched on their sides, rather than the bottom entering the water first. It was the belief of Mr HJ Beatty, manager for the contractor George Willcocks, that the weight of the caisson would right itself as soon as it was in deep water – which it did.
The launching of a caisson is always an undertaking which is attended with considerable risk… This time there was rather more anxiety than ordinarily for a new plan of launching was to be tried – a plan that had never previously been attempted in the colony.
Unfortunately the water was not deep enough when the first caisson was launched. It touched the river bed before it was in position and came to rest at a 45 degree angle. With the aid of a donkey engine on a punt it was hauled into position. The same process was repeated for the second caisson a month later. Positioning the second caisson took 50 minutes, compared to the two hours it took to move the first one into position.
Floating the spans into position
As with the Albert Bridge, no falsework was used in the construction of the Fitzroy River bridge due to the risk of floods. This meant that the two long, central spans needed to be floated into position with the use of a hulk. This method had also been successfully used in constructing the Albert Bridge in 1895, and even the same hulk, Jennie Parker, was used for both bridges.
To shift such a mass is a work of no small magnitude – not one attended with such great risks as might be imagined, but one requiring the very strict attention to details. First, the time had to be carefully selected. A day had to be chosen when there would be enough water to enable the girder to be shifted out until the southern end was right over the top of the bed rocks and yet only a few inches above them. Then care had to be taken that the hulk should be so moored and held that it would go straight across without deviating either to the right or left and land its freight just in the exact position prepared for it. Then the time that it would take to do the journey had to be carefully calculated so that the girder should not get to the end of its journey a minute too late.
The Jennie Parker was moored in position at 5am, when the tide was low. As the tide rose the staging on its deck began to take the weight of the span at about 2pm. Once the weight was off the leading trollies the span could continue to be moved the remaining 200 feet (61m) to the midstream pier.
The move, planned for high tide, began around 3.15pm and took 50 minutes. Winches were used to drag the hulk straight across the river.
The height of the tide had been accurately gauged during construction. When the hulk was in position the outer end of the girder was about 10 inches (25.5cm) above the midstream pier. As the tide dropped the girder was precisely placed in position.
Success in design and construction
The height of the bridge factored in the highest recorded flood level – 20 feet (6m) above the high-water mark. At its lowest point, the girders were four feet two inches (1.7m) above the flood level, with the tracks a further four feet (1.2m) higher.
The bridge faced its sternest test in 1918. That was the year the Fitzroy River experienced its’ highest recorded flood. The bridge remained above the water and the piers retained their structural integrity.
The bridge was officially opened on 6 November 1899 and named Alexandra Bridge in honour of the Princess of Wales. To this day, the Alexandra Bridge remains integral to Australia’s east coast rail network from Cairns to Melbourne.
 Stanley, HC, ‘Re-erection of the Albert Bridge’, Proceedings of the Institute of Civil Engineers Vol CXXXII, 1898
 Morning Bulletin, 31/8/1898  TheCapricornian, 17/6/1899