Investigations continue after Snowy 2.0 TBM stalled

In December TBM Florence, which is digging the headrace tunnel at Tantangara, was transitioning from soft material into harder rock conditions when a depression developed on the surface above the machine. The TBM has tunnelled about 150m and is at a depth of 30m. The depression is around 9m deep.

In a statement, Snowy Hydro said principal contractor Future Generation Joint Venture (FGJV) is drilling probe holes through the front of the excavation to understand the ground conditions immediately ahead of the TBM. So far 20 grout holes have been drilled in front of the excavation to improve any weak zones and limit any water flows.

FGJV is also installing a slurry plant for the TBM to allow the machine to switch from open excavation to a closed, pressurised mode. Snowy Hydro says this will enable stable excavation through the softer ground conditions, while enabling the TBM to switch back to open mode when in harder rock.

“Importantly, the integrity of the tunnel, which is supported by a concrete lining and steel ribs, has not been compromised, and nor has the capacity of the TBM. The project team has paused the machine until about 10-15m of weak material in front of it is stabilised, prior to striking hard rock, when we expect normal tunnelling to resume,” the statement says. 

Stage two of the works to stabilise the ground in front and above the machine will involve ground improvement from the surface. Once the area is grouted, the depression will be backfilled and revegetated.

Snowy Hydro reiterated that, despite the current problems, the project was making progress.

At Lobs Hole, in the main access tunnel, works are continuing on multiple drill and blast cross tunnels. These will link with the emergency, cable and ventilation tunnel, where TBM Kirsten has tunnelled 2.8km and has reached the site of the underground power station cavern complex. This provides access to the excavation site from both ends of the cavern, which will be one of the largest and deepest in the world.

TBM Kirsten will soon be modified to excavate the inclined pressure shaft on a steep uphill angle. Modifications include altering the levels of the TBM’s tanks and mechanical equipment so they are level when travelling up the 25-degree incline, and converting the walkways on the machine’s sides into steps and ladderways.

Meanwhile TBM Lady Eileen Hudson is being reassembled at the Talbingo adit with some new components, including a cutterhead and shields. It will be launched into the mountain later this year to excavate the 6km tailrace tunnel, which will feed into Talbingo Reservoir.

Excavation of intakes at Talbingo and Tantangara is advancing steadily, along with the giant surge shaft on the Marica plateau that is already excavated to a depth of more than 20m.

Snowy 2.0 is Australia’s largest renewable energy project. It will link Tantangara Reservoir (top storage) with Talbingo Reservoir (bottom storage) through 27km of tunnels and a power station with pumping capabilities. This will enable water to be released for energy generation at times of peak demand and pumped back to the top storage when there is excess renewable energy in the system, ready to generate again.