An Innovative bridge design for a flood prone Northern territory River has given the Australian Defence force all weather access to a remote warzone training area.
Designed by Consultants Sinclair Knight Merz and built by Steelcon Constructions (nt) Pty Ltd, The Bradshaw bridge across the Victoria River took out the Northern An Territory construction Association’s award for the best civil division project of 2002, and was also the outright winner of the associations outstanding project of the year award.
Completed in September 2002, the 8.9m bridge gives access for the Australian Army’s 1st Brigade and other Australian defence force units to move onto the new and previously almost inaccessible Bradshaw training area (BFTA).
The BFTA is a former cattle property known as Bradshaw Station, Which was purchased by the Australian Department of Defence in1996 for use as live fire combat training facility.
The area is located about 600km South West of Darwin and covers more than 8700sq km. It is bounded to the south by the Victoria River which separates it from the Victoria Highway; the only sealed all weather access road in the vicinity.
The department of Defence commissioned the construction Division of the Northern Territory’s Department of Infrastructure, planning and environment (DIPE) to carry out the procurement, Project management and construction supervision of the Bradshaw
Bridge and its associated access road.
In response of the DIPE’s call for tenders , Darwin based Steelcon Constructions engaged Sinclair Knight Merz (SKM) to develop an alternative to the Bridge design originally Proposed in the tender documents.
The submission of alternative designs is a standard feature of the NT Governments tendering process.
SKM Project design manager, Ross Dunkley says that the new design developed by his structural team was innovative yet simple; Robust enough to handle the challenged posed by the river and aimed at delivering cost savings to the DIPE and department of defence.
“At the height of the wet season, the Victoria River’s main flow has a peak velocity of
up to six metres per second and carries a large quantity of logs and debris, all of which put significant lateral forces onto a bridges structure.”
“this meant that we had to devise a substructure that would ensure the bridge is strong enough to with stand the rivers forces, yet be relatively quick and inexpensive to build” Dunkley Says.
The design resulted in the construction of a 270m long submersible bridge comprising 12 x 22.5m Spans supported on bipod piers, Capped with a pre- stressed concrete plank deck that sits 13m above the riverbed.
The upstream piles are set 3.6m into the rock to provide sufficient tension and anchorage against the forces of the river, and the downstream piles are set one Metre into the rock, providing a solid foundation anchorage.
Consultants from the Darwin Office of Douglas Partners provided geotechnical and rock mechanics expertise to assisting SKM in designing the rock sockets.
“Because we specified the tops of the steel socket liners to be at a higher level than the dry season river flow, the construction crew was able to continue working through the early part of the wet season as the river rose,” Dunkley Says.
“The Simplicity and strength of the design also enabled us to reduce the original number of bridge spans. Together, These factors produced cost savings of around 10% .”
The deck of the bridge is a cast insitu concrete 125 thick overlay acting compositely with the prestressed beams and is 7.4m wide with a 2m Cantilever walkway off the downstream side.
The all concrete submersible bridge has been designed for both vertical loads from army tanks – driven and transported to the off-road HLP 320, as well as with standing the sever lateral loads from the Victoria river in flood.
The design also incorporated construction logistics. Because it is in one of the most remote areas of the Northern Territory and Australia, the concretor set up a batch plant onsite with full QA.
The Beams were cast in Steelcon’s Yard in Darwin, each weighing 18T. Special permits were sought from the department of transport to use a road train made up of two extendable trailers to transport the beams. “Getting around corners was interesting” Dunkley remarks.
Other logistics built into the bridge design was the use of a 100T crawler crane to Launch the beams from the already constructed deck. This load case was built into the structure. Originally the beams were to be launched from below using the floating piling plant on the river, but this would have meant reconfiguring the plant to take the crane, with the cost and the time involved too great, Dunkley Says.
SKM Was engaged through out construction to assist the Steelcon with logistics and determine the length of rock sockets required and other quality issues such as techniques of concreting under water.
The BFTA work also involved the design and construction
of an access road of approximately 4km up an escarpment. Because of the relatively steep gradient, some blasting was involved, with material for the rock base acquired onsite. The road is a standard Rural road with a natural gravel compacted base and single bitumen seal.
The 2001/2001 Monsoon season meant everything was demobilised and the site abandoned until work could be resumed.
Prior to the construction of the Bradshaw Bridge and its access road, the only way into BFTA was either by barge cross the Victoria river from the south, by four wheel Drive across the river at low tide or Via a land track from the Northeast – Both only traversable during the dry season
or by air.
Now the Australian Defence force has all-weather road access to the training field during all but a few days of each year at the height of the monsoon wet season, when the bridge may become completely submerged.