Tunnel completed for Sydney Metro under Sydney Harbour

TBM Kathleen breaking through north of the harbour. Image courtesy of Sydney Metro.

Twin metro railway tunnels under Sydney Harbour are complete. All tunnelling is now finished on the Sydney Metro City and Southwest project.

Kathleen was the last of two tunnel boring machines to arrive north of the harbour.

The tunnel boring machine dug two 885 metre tunnels from Barangaroo to Blues Point.

The second harbour tunnel took two months to build, one month faster than the first. Transport for NSW puts this down to lessons learned such as modifying the giant cutter head and changing tunneling processes. This enabled the machines to better deal with the clay material at the bottom of the harbour.

Over the course of the project five tunnel boring machines dug through about 175,000 tonnes of sandstone, clay and marine sediments to build both harbour tunnels that are about 40 metres below sea level at the deepest point.

A crew of 50 ran tunnel boring machine Kathleen, almost a century after her namesake helped build the Sydney Harbour Bridge above them.

The specialised tunnel boring machine is named after Kathleen Butler, who played a vital role in the construction of the Sydney Harbour Bridge. She was the technical advisor to legendary engineer John JC Bradfield.