It was just one small slab of concrete, but it represents a big milestone for the Buckle St tunnel.
The first 300-tonne concrete roof slab was poured in the cool early hours of this morning.
Heritage projects manager for the Ministry of Culture and Heritage, Brodie Stubbs, said the first roof slab was a big step in the construction of the tunnel.
"It's a milestone moment. We have moved from having a big trench to the start of a tunnel," he said."It's also the beginning of the creation of the park. It's very exciting to see that happening. It makes it a lot more real, because you can get a sense of the finished project."
Construction of the tunnel, opposite the national war memorial, near the Basin Reserve, began in October 2012.
The roof of the tunnel will form the base of the new Memorial Park, which will open in time for the Gallipoli centenary commemorations on Anzac Day next year.
The pouring of the tunnel's concrete floor began in December and walls began in February. The walls are 60 per cent completed and 130-metres of the 300m-long floor has been poured.
Memorial Park Alliance community relations manager Brian Aspin said pouring the slabs on site made the tunnel stronger.
"A benefit of pouring the roof slab, rather than placing a preformed slab across the gap, is that the strength of the underpass box can be increased," he said."The steel reinforcing in both the walls and the roof can be linked strongly to create a more resilient box able to resist a one in 2,500 year earthquake."
The underpass is due to be opened to cars this October.
Construction of a roof span:
-Timber scaffolding, a plywood deck and steel reinforcing for the concrete is prepared and all supported on steel legs.
- Concrete is poured through the steel reinforcing to form a strong concrete block.
- The steel legs, timber scaffolding and deck will be stripped off in about a week after the concrete has cured.
- The span poured today is 14.5m wide, 10m long and 0.8m deep: 116 cubic metres and 300 tonnes of concrete.
- 12 roof spans will be poured: 3600 tonnes of concrete.
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