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Ausgrid recently celebrated an early milestone on its City East Cable Tunnel (CECT) project in Sydney’s CBD.

The CECT tunnel is a critical component in Ausgrid’s project to future-proof Sydney’s electricity supply network.

After having launched the tunnel boring machine (TBM) at the Riley Street (Surry Hills) Substation in January last year, Thiess’ CECT project team achieved breakthrough at the City North Substation on 29 October 2013, 40 calendar days ahead of schedule.

At the time, Thiess project manager Tim Burns said the TBM excavation performance gave the project a good chance of finishing well ahead of its May 2015 completion target.

“This breakthrough is a huge moment, the culmination of four years of work.”

Mr Burns said the team had set up on-site 23 months prior, and had been excavating for 10 of those months.

Background

Ausgrid awarded Thiess the $141 million contract after its successful completion of Ausgrid’s City West Cable Tunnel in 2010. The project began in December 2011.

The CECT project involves the construction of a 3.2-kilometre, segmentally lined tunnel – with an internal diameter of 3.5 metres – beneath the CBD to carry 132-kilovolt cables.

The tunnel connects the City North Substation in Sussex Street to the new Riley Street Substation.

The project also includes the construction of two concrete-lined connectors, an extension to the City South Cable Tunnel, and installation and commissioning of all tunnel mechanical and electrical services.

For the excavation and lining of the cable, Thiess had to obtain a bespoke TBM. “Much of the equipment came from overseas,” Mr Burns said.

He said it was “a logistical exercise” to get all the equipment on-site and working in the required safe and productive manner.

Thiess also constructed a high-flow water cooler to recycle water used in the TBM’s cooling system, which managed to save an average of 26 megalitres per month.

Groundwater has been treated for reuse in the grout batching operations, reducing the need to use potable water.

Tunnelling success

Tunnelling underneath the vast and complex foundations and services of an established city demanded specific expertise.

On any shift, approximately 20 people were directly involved in the tunnelling operation, with nine people deployed on the machine and the balance engaged in tunnel logistics and maintenance.

Thiess emphasised safety as the number-one priority on the project. This was recognised when the CECT project won the Site Safety – Civil Construction award at the 2013 NSW Master Builders Association Excellence in Construction Awards.

The judges said that the project team had invested time and resources to significantly reduce risks for tunnel workers.

The construction of such a major project in a dense urban setting, with many residents nearby, presented specific challenges and required a solid understanding of how to work with the community and the environment.

Mr Burns said that with such a project, “You need to have good community and environmental support, which we’ve been blessed with from the Thiess side and the client side.”

Towards completion

Once civil works are complete, the final stage of the project  involves the mechanical and electrical fit-out of the tunnel.

Completion of the CECT will complete Ausgrid’s ‘ring main’ for the Sydney CBD, linking the City South Cable Tunnel and the City West Cable Tunnel.

Mr Burns said that the Thiess team has had an “excellent relationship with our client Ausgrid … based on open communication and trust. We’ve done a good job, and Ausgrid has supported us the whole way.”

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