Two of 24 concrete spans erected for the "Skytrain" section of the new rail line to Sydney's north-west suburbs may need to be pulled down after cracking.
The skytrain spans, which have an average length of almost 40 metres across the project, are made up of pre-cast concrete segments locked together with steel cords.
But Transport for NSW, which confirmed the cracking after enquiries from Fairfax Media, says there should be no delay in the overall construction of the $8.3 billion Sydney Metro NorthWest project, due to open in early 2019.
On Thursday, Premier Mike Baird and Transport Minister Andrew Constance announced the early and under budget completion of tunnelling for the rail line – a mammoth undertaking that has involved drilling 15-kilometre twin tunnels.
But to the west of the tunnelled section, progress has been patchier on the eight kilometres of elevated track that will form the Skytrain and viaduct parts of the rail line.
These sections, running from Bella Vista to Cudgegong Road past Rouse Hill, are being built by Italian consortium Salini Impregilo.
Fairfax Media revealed last year the consortium had already claimed a $50 million contract variation, following design disputes and the discovery of asbestos at a construction site.
The latest issue involves cracking in the pre-cast concrete used to form the elevated rail track. The Skytrain is being built using two gantry cranes to assemble about 1200 pre-cast segments up to the level of 130 concrete piers.
These segments then form "spans" between the piers, and are partly held in place by steel cords that run through the segments. Across the whole of the project, these spans have an average length of about 40 kilometres.
In an emailed statement, Sydney Metro acting program director Tom Gellibrand said none of the 24 spans erected so far had been disassembled but, "following close inspections, there are two spans where some cracking has occurred".
"The rectification of the cracking may result in these two spans being replaced," he said. "This is a straightforward construction process.
"It is not uncommon for cracking to occur within re-enforced concrete and there is a comprehensive inspection program in place for the Skytrain construction, with any required repairs undertaken in accordance with quality assurance procedures and processes."
Mr Gellibrand, who said safety was the top issue for the project, said it would be finished in line with the program. That would mean finishing the section in 2017 to hand over to the next major contractor, in charge of both running the trains and fitting the line with track and signalling systems.
The construction timing will be crucial, partly because the overall project involves shutting down the existing Epping to Chatswood rail line for about seven months from late 2018, to link it in with the extension to Rouse Hill.
The Greens transport spokeswoman, Mehreen Faruqi, who has a doctorate in engineering, said problems in the project were "hallmarks of an infrastructure program where the public sector has been hollowed out of engineering expertise, and is unable to effectively oversee and supervise project delivery".
One source who has worked on the Skytrain section project said morale was low following the latest issue. "Most of the people that are there are completely disillusioned," he said.