Efficient construction delivers lengthy M-Lock® Bridge at lowest cost

Wentworth Shire Council in NSW has completed one of the longest precast concrete Rocla M-Lock® bridge in Australia over the Great Darling Anabranch in the far southwest corner of the State.

Comprising nine spans of 12 metres each, the 108-metre Wentworth Bridge is built from precast reinforced concrete deck components manufactured by Rocla at its Adelaide precast facility and transported by road to site.

In addition, Rocla supplied hollow spun concrete piling manufactured at its Wodonga plant. By using the Rocla M-Lock® precast components, it was possible to construct the bridge economically using “span-over-span” construction, which allows the structure to be built over water, craning the components off the deck of the previously constructed span. This was the first post-tensioned M-Lock® bridge built using the span-over-span construction method. The new bridge replaced an 1860s timber bridge that was owned and maintained by the Roads & Traffic Authority of NSW (RTA). Most of the timber had been replaced at least once and maintenance costs were very high.

The RTA approached Wentworth Shire Council with a request that they take over ownership and maintenance of the bridge. Council agreed on the basis that RTA provided funding for a replacement concrete bridge. The project took six years from initial concept preparation by Council in 1998 to the opening date in December 2003. However, construction of the bridge was completed over a 5-month period. Construction involved the use of a “jack-up” barge in the river to pre-bore the pile holes and a 40-tonne crane on the deck of the bridge.

Lifting limits of the crane working off the deck meant the 20-metre Rocla® spun piles were supplied in two 10-metre sections and joined using the Rocla New Millennium™ joint.

Headstocks were also placed by the crane working from the deck. Again, the crane loading limit at full reach meant the 15-tonne headstocks had to be supplied in two 7-tonne sections, which were joined on site with a small cast in situ connection.

Finally, the 13-tonne bridge planks were placed from the deck. Although the mass of these plank components was twice that of the headstock sections, the crane reach was only half the distance (6.5m).

The completed structure is 8.4 metres wide between the crash barriers, with nine 12-metre spans between headstocks. The bridge alignment is on a 15-degree skew, which required all components to be cast with angled end sections to ensure the rows of piles remained parallel to the river.

Two piled abutments with Rocla® precast approach slabs and wing walls form the substructure on the banks. The eight intermediate groups of piers comprise four Rocla circular hollow-spun concrete piles of 585mm diameter. Decking for the superstructure comprises 7 precast and reinforced (but not pre-stressed) beams on each of the 9 spans.

The deck planks and barriers were post-tensioned transversely at 5 locations per span. Precast crash barriers were fitted to the outer edges of the deck. The bridge was designed to cope with existing traffic volumes and vehicle loadings as well as future AustRoads heavy vehicle loadings (SM 1600) and unlimited traffic volumes.

Thanks to the economy and efficiency of the construction method, Wentworth Shire was delivered a cost-effective bridge replacement, while the community now has a maintenance-free bridge designed for 100 years.