The underground world of hydropower

06 May 2020



Sure our dams are impressive. But while you stand in awe of what's before you, you miss the underworld of hydropower.

 

Scattered throughout Tasmania’s mountain ranges, far from our cities and busy towns, some 80 kilometres of tunnels and pipelines channel water across our rugged terrain. Drawing it from our rivers and lakes, through our power stations and to the ocean.

 

Thanks to the people who built the hydroelectric power schemes, and those who continue to maintain them today, Tasmania's underground world of hydropower has stood the test of time.

Carl Von Savageri, civil assets portfolio manager at Hydro Tasmania, began his work as a specialist engineer for underground assets. He has been drilling and designing underground systems around the world for years. So why Tasmania? Because according to Carl, this little island is one of the top two or three territories in the world as a whole for renewable energy generation.  

 

Although Carl has designed underground systems around the globe, he tells us it’s not as straightforward as it sounds to make a tunnel. “We're interacting in a very hostile natural environment using good engineering knowledge, skills and experience, and cutting edge technology, which is what I find interesting.”

 

The west coast schemes were mostly built underground and teams faced many challenges during the construction phase in the late 1980s. At one point, a soft spot full of sand, clay and water collapsed. Unable to push forward, they backed up 5-600 metres and tunnelled around the collapsed section. Carl explains the difficulty they would have faced tunnelling in Tassie's rugged mountains. “The tunnel goes through very geologically diverse conditions. We start with the Mount Roland conglomerates, which have a lot of minerals in them. The majority of the mines on the west coast around Queenstown are mining this particular type of ground. It has a lot of copper and gold and a whole range of other things associated with it. The rock is made up of kilometres upon kilometres of fallen volcanic ash over millions of years that was squeezed and squashed and turned sideways. Now we have these layers of volcanic rock and little bits of stones that weathered over the years. That turns into granite, a very high quality, high strength rock." 

 

The Tribute Power Station, part of the Anthony Power Development Scheme in the upper Pieman River in western Tasmania was built underground; tunnels, power station and all.

Looking at the design of the Tribute tunnel system


 

 

In terms of underground works, Tribute is probably the most complex of our underground schemes. The headrace tunnel that leads to the power station carries water from Lake Plimsoll to the turbine, this one section alone is seven kilometres long. The water then leads away from the turbine via a tailrace tunnel which is 800 metres long to Lake Murchison. The final tunnel in the system is used to access the headrace and tailrace tunnels and is over a kilometre long. Access tunnels allow people to get to the turbine or sections of the headrace and tailrace tunnels.

 

Finding the right time to switch off a power station for at least a week so tunnels can be drained for inspection is very difficult. When the tunnels are finally drained, the specialist inspectors go in. Norm Cribbin is a technical specialist in the civil assets team. He has a helicopter view over assets like canals, flumes, pipelines, penstocks, intakes and other structures. He also plays a role in dam safety, engaging with civil engineers.

Lake Plimsoll feeds into Tribute Power Station


Norm describes the tunnels we use for hydropower as being the same as having a fuel line in your car. "If you look at our lakes as the fuel tank, and our power stations as the engine, the tunnel gets you under a mountain or through an obstruction. It's the easiest route in terms of distance. Rather than taking the long windy route to dig a canal, you go in a straight line through the ground to get water from one place to another place, pretty simple."

 

Before Norm goes in, he’s checking everything. The asset life cycle of the station, the history of the tunnel, the construction era, the geology, and civil construction reports. Once he has a full understanding of the asset, he enters for inspection.

Norm Cribbin conducting an inspection in a tunnel


"In some tunnels the water might be up to your waist in a few spots but generally speaking when they're drained it's usually only ankle depth. The nicest tunnels to walk through are those where we've taken the time to put a concrete invert in. It's more or less like walking on a nice concrete path all the way through the middle."

 

“You find some interesting creatures, like whole schools of fish that for some weird and wonderful reason that happen to be living in a tunnel. We inspected the John Butters headrace tunnel and there were thousands of fish. Not big ones, just little ones, up to about 100 millimetres. They would have come from the lake and just decided this is a much better place to live.”

Norm inspecting mussels in a tunnel

Sometimes when water is being transferred through tunnels, tiny spat (oyster larvae) get washed through and settling into back eddies where they breed. “At Trevallyn, we found a few mussels out of water and put them back in. There was a back eddy that made a sandy beach and they were caught in that area. They were still alive, so we just left them there.”

 

Carl and Norm were working on the Mossy Marsh tunnel inspection together when they found something rather unexpected. Norm recalls “We got to one section which had a back eddy (often created by a larger than expected blast) and there was this unusual pile of sticks, weed and grass. Next thing you know, this platypus comes out and says - what are you doing here, this is my spot! It became really aggressive, right there in the middle of the tunnel! We were about a kilometre in, so I guess he'd set up home in there. The Mossy Marsh tunnel has a dry crown and doesn't completely fill with water, so he could swim in and out and be protected. They're just some of the funny things you find.”

 

In recent months, Carl and his team of project managers, civil engineers, specialists and contractors has been undergoing routine maintenance at Tribute Power Station. Part of the work involves shotcreting the access tunnel walls. 

Shotcreting happening in the tunnel


Shotcrete is a projected concrete with fibers inside which adds to add strength and flexibility. The fibers add extra strength and limit the formation of cracks in the concrete. Like thousands of tiny pieces of reinforced steel tying the concrete together. The concrete is sprayed under high pressure with compressed air, and when it impacts the rock face, it forms a tight seal between the rock and the shotcrete. “The primary use of shotcrete is to stop small bits of rock falling. We install bolts that we tension up and push against the rock. That's what really holds the rock in place. The shotcrete provides the shell that stops small bits of rock falling out of the roof. It locks everything in place." says Carl.

 

“We’re also taking a lot of samples, taking cubes and cylinders, which then go to the lab to be tested to ensure the quality of that work.” Once maintenance inside the tunnel is complete, it is refilled with water and won’t be entered for another 10 years or so.

Tunnel

Carl describes the environment as visceral. “People ask me how do you design a tunnel? It's not like constructing a building where you can put things in place to build something. We're taking something away, designing in negative space if you like. It's a different way of looking at things. I suppose the underground is in my bloody. It's an environment I feel comfortable in. My job is to keep that environment safe for everyone else to work in.”

 

Carl believes we are the custodians of an amazing landscape. “We have this gift from past generations that we need to cherish and to look after. What we have here, not just for ourselves but for our children and grandchildren, is unique. Maintaining these assets to be functional as well as pleasant places to visit is a key part of Hydro Tasmania's part in the broader community.”

 

The photographs taken for this project were pre-COVID-19 social distancing requirements.

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