Excavation work has resumed at Snowy 2.0 in New South Wales, Australia, with two tunnel boring machines (TBMs) back in action to complete key tailrace and headrace tunnels.

In late July, TBM Lady Eileen Hudson began work on the 6km tailrace tunnel to connect Talbingo Reservoir to the underground power station complex. TBM Florence will shortly begin excavating key a headrace tunnel of Snowy 2.0. Tailrace tunnels carry water already used to rotate turbine blades away from a hydroelectric plant. Headrace tunnels, a key element of reservoir powered hydropower systems, take water from the reservoir or intake structure to the powerhouse, where it drives turbines to produce electricity.

The Snowy 2.0 hydroelectricity complex is being built for project promoter Snowy Hydro by Future Generation (FGJV). FGJV is a joint venture between Webuild, Australian firm Clough and an American subsidiary of Webuild called Lane Construction. Once complete, the scheme will provide 2GW of fast-start dispatchable energy and provide 350GWh of storage for use at peak times.

Snowy 2.0 will connect the higher Tantangara Reservoir with the lower Talbingo Reservoir via tunnels through the Snowy Mountains. A hydroelectric power station 800m underground between the two reservoirs will generate green energy as the water passes turbines in the tunnels, sending it to the grid. The reversible turbines are designed to push water back upwards, meaning the same water will be pumped back and forth all the time, allowing for hydro-electricity on demand.

However, delivery of the scheme has encountered a series of setbacks, leading to confirmation in May of a 12 to 24-month delay its completion date.

TBM Lady Eileen began her work after teams spent July preparing to launch her into the mountain at the Talbingo adit - a horizontal underground passage. The objective of the works is to excavate the 6km tailrace tunnel to connect Talbingo Reservoir to the underground power station complex. Water used to generate 2,000MW of power from Snowy 2.0 will flow out of the tunnel.

At 137m in length and weighing 2,300t, the TBM is described by Snowy Hydro as “a moving factory with many parts working together in a complex operation.”

Snowy 2.0 Hydropower Project TBM tunnels construction manager Derek Whelan confirmed Lady Eileen’s components include a new 378t cutterhead made up of 70 cutting discs, each weighing 290kg. They will excavate the diverse geology of the 11m diameter tunnel. More than 27,000 concrete segments manufactured at a precast facility in nearby Polo Flat, Cooma will be used to line the tailrace tunnel. The segment erector, segment feeder, segment cranes and grouting system are all located onboard the TBM. The machine is also equipped with a guidance system that provides data to the machine’s pilot, to the nearest millimetre. This information is mirrored above ground and monitored by Snowy 2.0’s construction and engineering teams.

Excavation of the Talbingo tailrace tunnel follows Lady Eileen's completion of the 2.8km main access tunnel (MAT) in 2022. Teams have been working to disassemble and safely extract the TBM and reassemble her with new components before re-commissioning. Other key equipment has also been retrieved from the MAT portal to the Talbingo adit. These include a conveyor stacker, grout batch plant, and chiller plant.

Meanwhile work has been undertaken in preparation for TBM Florence to restart excavation of the Tantangara headrace tunnel. The 143m long TBM was forced to a halt last December, after encountering soft rock, causing a 9m hole to appear on the surface above it in Kosciuszko National Park. Work to remediate the problem has included ground stabilisation works and construction of a slurry treatment plant by Snowy 2.0 teams.

The onsite slurry plant is currently undergoing final commissioning tests. It will enable the dual mode TBM to switch from open excavation to a closed, pressurised mode. This will allow stable excavation and efficient progress through softer ground conditions when required. The slurry treatment plant will process and separate the materials used when the TBM is in closed excavation mode, including bentonite - an absorbent, swelling clay - which will be stored for re-use.

A testing regime has confirmed grouting and stabilisation of the loose material around TBM Florence has been effective. Additional stabilisation continues from the surface to solidify loose material above the machine and directly in front of the TBM cutterhead.

“We’re carrying out rotations of the cutterhead and minor advances of TBM Florence to ensure the machine is correctly positioned for final commissioning and to confirm the ground support activities are working,” a Snowy Hydro statement confirmed.

“Additionally, we are working with New South Wales departments on environmental approvals for remediation of the surface depression above TBM Florence and to provide assurance that this will not reoccur.”