Christmas finish in sight for machines tunnelling beneath Sydney’s Darling Harbour

3 hours ago 3

Worming her way under motorway foundations and building basements, a tunnel boring machine is set to smash through into a giant cavern dug under central Sydney for the country’s largest rail project by Christmas.

At 130 metres in length, the boring machine named Ruby has passed the deepest point – 35 metres beneath the water surface – that twin tunnels will reach under Darling Harbour for the final section of the 24-kilometre Metro West rail line between the CBD and Westmead.

A boring machine tunnels under Darling Harbour for the Metro West project.

Ruby and another boring machine called Jessie have been operating 24 hours a day to burrow under Darling Harbour, which is the third and final under-harbour crossing for the mega project. Jesse is about 200 metres behind Ruby as they march eastwards, leaving concrete-lined tunnels in their wake.

In a sign of the challenges for the construction teams, just two metres of rock sits above the top of the tunnels beneath Darling Harbour, while above that is about 12 metres of sediment.

Construction director Scott Connor said the alignment of the tunnels under Darling Harbour ensured they were as far as possible from the highest risk area, which was the part that had the least amount of rock cover.

“It’s about reducing the risk. We have been working 24 hours a day … because we don’t want to stop in a high-risk area,” he said.

Connor said the construction team was hoping that Ruby would break into the cavern for the Hunter Street station by Christmas.

On the final 1125-metre stretch, Ruby has tunnelled within about seven metres of piles for a Maritime Museum jetty, and will pass 21 metres beneath Wynyard station before reaching her final destination beneath Hunter Street in the CBD.

Eastern tunnelling project director Bob Nowotny said vibrations from the boring machines had caused some methane gas to be released from pockets in sediment between Rozelle and Pyrmont, resulting in minor bubbles on the water surface, but not in the final part of the tunnelling beneath Darling Harbour.

“You get pockets of gas in [the marine sediment], which do get relieved as you’re going through sometimes,” he said.

The two boring machines have been slicing through rock at one-third of the pace of other boring machines working on Metro West because of the combination of hard and soft rock under the harbour, which is one of the most complex parts of the project.

The gradient of the tunnels is up to about 5 per cent, which means that for every 100 metres it rises or falls about five metres. That will be the operational limit of the driverless metro trains.

The Herald recently reported that the cost of Metro West is set to blow out by at least $2 billion to $27.3 billion, due to the skyrocketing price of building underground train stations and delays to awarding major contracts.

Questioned on Monday about the project’s cost, Premier Chris Minns said he was not anticipating it to be above $25.3 billion because the government was “very tough on costs”.

“There’s some room to move in that envelope. If it does change, we’ll update the public,” he said.

The underground line is the fourth and largest stage of Sydney’s metro network, which is one of the world’s biggest rail projects, costing about $67 billion. Metro West is due to open to passengers in 2032.