The Sydney Harbour Bridge viewed from Milsons Point railway station.
The Sydney Harbour Bridge viewed from Milsons Point railway station. Contributed

Sydney Harbour rail project underway

WORK on the $10 billion second Sydney Harbour rail crossing began today with Premier Mike Baird wasting no time to get the project started.

Mr Baird said geotechnical drilling would begin up to 70m below the harbour in the coming weeks to work out the best location for the rapid transit railway tunnels.

"This new rail crossing of Sydney Harbour is the key to increasing capacity on our rail network by 60% - allowing us to move an extra 100,000 people every hour right across Sydney," he said.

Minister for Transport and Infrastructure Andrew Constance said about 300 bore-holes would be drilled during the initial works - half beneath the harbour and the rest on land either side of the route.

"From the end of the North West Rail Link at Chatswood, Sydney's new rapid transit network will continue under Sydney Harbour, through new railway stations in the CBD and west to Bankstown with capacity for trains to run every two minutes."

On the harbour, a barge will be towed into position before its four legs are lowered to the seabed up to 25m below the water surface, then pushed to firm ground on the harbour floor.

The barge will be raised clear of the water to provide a platform unaffected by tides, currents or other water movements.

Boring will then start with a diamond-tipped drill that takes core samples from depths up to 70m below the seabed.

To monitor the barge's interaction with the harbour floor, divers will be used as an extra visual check, with a preparation dive already completed on Sydney Harbour.

When the drilling work under the harbour is finished, the holes will be filled with concrete to create a permanent seal.

Mostly Sydney sandstone and shale is expected to be found along the route.

Geotechnical drilling will be done at Sydenham; in the CBD; North Sydney; Crows Nest and Artarmon.