The Snowy 2.0 project client ‘Snowy Hydro’ has given an update as to the current status of TBM Florence, recently reported to having been forced to stop tunnelling following the creation of a surface depression that originally appeared in December 2022 at the project site in Tantangara.
“The project team is currently conducting works to stabilise encountered soft ground and enable TBM Florence to restart excavating the headrace tunnel.
Surface depression at Tantangara February 2023
TBM Florence has tunnelled about 150m and is currently located at a depth of approximately 30m below ground. A surface depression recently emerged in the ground above TBM Florence, which has temporarily delayed progress on this particular worksite. The depression is approximately nine (9) metres deep and continues to be monitored.
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.
Snowy 2.0 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.
Grout is injected in front of the excavation to improve any weak zones and limit any water flows.
Grouting is well underway from TBM Florence, with 20 grout holes drilled to date in the current location.
FGJV is currently installing a slurry plant for the TBM that will mean the machine can switch from open excavation to a closed, pressurised mode. This will deliver stable excavation and efficient progress through the softer ground conditions, while enabling the TBM to switch back to open mode when in harder rock.
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 2.0, a pumped-hydro expansion of the Snowy Scheme that will link two existing dams, Tantangara and Talbingo, through a 27km tunnel and a new underground power station is critically important, and will help underpin Australia’s transition to a renewable energy future.
Elsewhere construction of Snowy 2.0 continues to make progress across multiple worksites. At Lobs Hole, TBM Lady Eileen Hudson has completed excavation of the 2.7km main access tunnel and TBM Kirsten is almost finished excavating the adjacent 2.9km emergency, cable and ventilation tunnel (ECVT).
At Lobs Hole, in the main access tunnel, works are continuing on multiple drill and blast cross tunnels. These will link with the ECVT, where TBM Kirsten has tunnelled 2.8km and has reached the site of the underground power station cavern complex. This is a great milestone for Snowy Hydro as we now have 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 is setting a global standard in tunnel boring technology, just as the original Snowy Scheme developed rock bolting techniques that are still used throughout the world today. The machine 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.
According to Snowy Hydro, despite the ongoing challenges faced by all large infrastructure projects, Snowy 2.0 continues to achieve progress across a large number of remote work fronts.
It would have been faster to dig Florence out of the soft spot and relaunched her into the hard ground.
In my view you should grout firstly underneath the machine and then in front and above the machine because ground collapse is aring from the ground underneath the TBM machine not the ground in front and above the machine. First the foundation under the machine should be stabilized so that further settlement above and in front of machine can be safely stabilized
But I have read here – 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.
— One more point due to topography effect – the surrounding rocks of the inclined shaft (I think it is being driven by another TBM) going up with the slope of the valley will be overstressed which means weak rock or ground