Three large tunnel boring machines (TBMs) have arrived in Australia to begin work on Snowy 2.0, Australia’s largest renewable energy project. Two engineers working on the project explain how the technology behind one of these machines will work.
Webuild leads a joint-venture of companies constructing the Snowy 2.0 project. Remo Grandori, manager of the Plant and Equipment Design division at Webuild, told create that it has developed a TBM that can excavate a tunnel at a sharp incline and decline. It will be able to work at a variable slope from -9 per cent downhill to +47 per cent uphill.
“Once we succeed in excavating this variable slope shaft, it will open a new field for the design of hydropower and general underground works,” he said.
“It’s [one of] the first machines in the world that can follow this wide range of variable slopes and bore a very steep slope.”
As for the other two TBMs, one will excavate the Headrace Tunnel from Tantangara Reservoir for about 15 kilometres with a variable slope from -10 per cent to -0.5 per cent, while the other will bore downhill two tunnels – the Main Access Tunnel then the Tailrace Tunnel for a total length of about 9 kilometres.
The third — and most innovative — TBM will first bore three kilometres of the Emergency Cable and Ventilation Tunnel (ECVT) with variable slopes from -9 per cent to flat. The ECVT will lead to the future cavern where the powerhouse with six reversible Francis pump-turbines reside nearly one kilometre underground, before boring uphill for 1.6 kilometres of the Inclined Pressure Shaft (IPS) with variable slopes of up to +47 per cent. Finally, it will bore a 2 kilometre section of the Headrace Tunnel uphill on a +0.5 per cent slope.
The overall length of this third TBM is 205 metres fully assembled, with an operational weight of 3,270 t.
“We had to develop several new technologies, not only the TBM, but the transport systems for materials and personnel suitable for variable and extreme slopes, as well as provisions to ensure we meet strict Australian safety standards,” Grandori said.
This third TBM will essentially do the work of two traditional machines. This will save a lot of time that would otherwise be spent changing the machinery midway through the excavation work.
“To make a similar scheme without the TBM technology that we have introduced would have required the use of two different TBMs or to stop for around six to eight months at the bottom and at the top of the shaft to excavate large caverns to complete the reconfiguration of the machine,” Grandori said.
“Basically, we save almost one year in terms of project duration.
“In terms of benefits, this will allow us to develop new design alignments for the new hydropower project, and reduce the time it takes to execute that,” he added.
Snowy 2.0 is a pumped-hydro expansion of the existing Snowy Mountains Hydro Scheme’s network of nine hydro power stations. Arriving by ship from Germany and China, the components of the three TBMs — some as heavy as 174 t — were lifted from the vessels and onto special trailers for transport to storage, and then to the project site.
Webuild’s engineers have also had to prepare for other difficult scenarios once the tunnelling begins.
“It’s a very long drive through variable rock conditions,” Grandori said.
“We have a complex system in place in case we find naturally occurring asbestos material. In that event, the TBM will switch to slurry mode, and the excavated muck will be treated in a special shed under negative pressure in order not to lose asbestos fibre.”
Putting it to work
Damon Miller, Engineering Manager for the project at Snowy Hydro, explained how the team will put the TBM into action.
“The TBM will first bore the emergency cable and ventilation tunnel (ECVT) to enter the power station 800 metres underground. It will bore the cable tunnel at a decline of 5 per cent. When it gets to the power station, it starts excavating the inclined pressure shaft,” he said.
“The transition at the shaft from -9 per cent to +47 per cent is very steep territory for a TBM, and it presents a unique challenge. The steepness of the inclined pressure shaft means that all of the equipment within the TBM needs to be able to switch to work on the incline, all the stairways and walkways will transition with the incline and pivot so these features remain horizontal.”
If everything goes to plan, Miller said the first stage of boring will take about six months. Modifications to the TBM will then be undertaken before working more slowly on the upwards incline.
“There’ll be around 25 people working on the machine, including TBM operators as well as geologists, geotechnical engineers, mechanical engineers and various trades,” he said.
“The machine needs to be supported with electricity, water, ventilation and a constant supply of segments for lining the tunnel.”
The three TBMs each have an 11-metre diameter, which Miller said is as large as you get for hydropower tunnels. As they bore through the rock, the next big task of the machine is to put the segment lining in place.
“There is a ring of nine individual segments made out of concrete and steel reinforcement, which weigh around seven tonnes each,” he said.
“They’re currently being made at a precast factory in Cooma, [New South Wales], which is employing around 150 people to make the segments. There will be 130,000 segments for the whole project.”
The Snowy Scheme already generates about a third of the renewable energy in Australia’s National Electricity Market (NEM). Snowy 2.0 will add another 2,000 MW of on-demand energy generation and 175 hours of large-scale storage to the NEM.
Webuild is part of Future Generation, a joint-venture created specifically to build Snowy 2.0 on behalf of Snowy Hydro Limited. Future Generation brings the combined engineering expertise of three companies: Australian-based Clough, Italy’s Webuild (formerly Salini Impregilo) and US-based Lane Construction. The teams pair local expertise with global experience.
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Engineering is about facts and analysis.
The public perception of engineers is not enhanced by referring to Snowy 2 as “Australia’s largest renewable energy project”.
Snowy 2 is a pumped hydro scheme that stores excess energy for consumption at peak times. It has a round trip efficiency of not greater than 67%, that is, requires 1.4 times as much pumping energy as it can generate.
There is no guarantee that all pumping energy will come from renewable sources.
I have been hoping that the Queensland Government will get a similar priority interest in an updated Bradfield Scheme in the Townsville area.
I would be pleased to know more of the geotechnical details, the range of rock types to be tunnelled, the cutting heads that do the job, whether the cutting head is changed or modified for different rock types, how it is powered, and details of performance such as expected rate of progress in hard granites of the locality.
As a surveyor who worked on Snowy T2 and T3 excavations, that is also my concern.
I read in the tunnelling of the Sydney road tunnels that the boring machine was diverted to the side of the tunnel and abandoned due to logistics of disassembly and recovery,
What happens to the TBM when its work is completed, does it get recovered or is it abandoned? Also how are the segments joined together, is there a slurry of mortar pumped into the joins with overlapping reinforcement or is it some kind of mechanical joint?
Interesting this article is headed “Transport”. Surely we could find a more appropriate heading. As engineers we are supposed to have brains and intellect. Would not “Energy”, Water Resources” or “hydropower” or “tunnelling” be more appropriate?
I’d be wary of hoping for a Bradfield Scheme coming to fruition. It was imagined in an era when economics was not considered and adverse environmental impacts would not have been understood because ‘all development is a good thing”.