Construction contractor Thiess has achieved a significant milestone in its work on a 420-kilometre tunnel which forms part of a multi-billion dollar gas project in Queensland.

In its latest announcement, Thiess said it had successfully tunnelled under ‘The Narrows’ of Gladstone in Queensland to connect Curtis Island to the mainland.

Part of the $US18.5 billion Santos GLNG Project, the 4.3-kilometre tunnel is set to provide a conduit for installation of part of a 420-kilometre gas pipeline which will transfer gas from fields around Roma and Fairview to a liquefaction plant currently under construction on Curtis Island.

Thiess was contracted to build the tunnel by head contractor Saipem Australia, which is overseeing the building of the overall pipeline.

Commencing in 2013, the project saw a 277-tonne tunnel boring machine measuring one hundred meters in length plough its way through the earth, lining the tunnel as it went by installing more than 21,750 pre-cast concrete segments. A cement-based grout was injected behind the segment lining to permanently seal the tunnel, which has a finished internal diameter of 3.4 metres.

The machine was named Rose-Ella by children at the local Rosella Part State School.

Not all was smooth sailing. At one stage, work had to stop for several weeks to allow the crew to repair the cutter-head – a complex task which required 2.4 bar of compressed air pressure to prevent soil and water ingress in order to allow access, meaning the repair crews had to go through compression and decompression processes similar to those of commercial scuba divers.

Thiess general manager of tunnelling Steve Wille praised the way his team handled a difficult situation.

“We got into a section of ground which we knew was going to be tough and we went and did some inspections on the machine before we entered it,” he said.

“Fifty metres later, we found we had done a significant amount of damage to the cutter-head of the machine and the guys spent effectively a month working in compressed air and in very tough conditions to repair that.”

“It can’t be understated how difficult the conditions were for those guys (who went in to fix the machine) and I’m extremely proud of the way they fixed the cutter-head and brought the machine home.”

The operation employed 75 people and took 420,000 man hours to complete. All up, around 55,000 cubic metres of earth was excavated, which is being used by the Gladstone Regional Council to rehabilitate an ash pond associated with a coal fired power station.

The tunnel is now being stripped of services and cleaned, after which it will be flooded to provide buoyancy for the pipeline and handed over to Saipem Australia to then connect up the pipeline.