Core blimey!

12 April 2019

The Tarraleah hydropower scheme is nestled amongst picturesque hills in central Tasmania and we can only imagine the efforts that were required to build this impressive piece of infrastructure some 80 years ago.



We’ve recently been taking a close look at the ground conditions that our early pioneers would have encountered; embarking on geotechnical investigations all along the scheme’s length. 



Geotechnical investigations is the technical term for poking around in the ground to find out more about the soil, sub-surface structure and the rock layers beneath. And it’s ground breaking stuff… literally!



Our teams love getting their hands dirty and love big things, so getting the opportunity to combine both at Tarraleah was a match made in heaven. And they don’t come much bigger than a 30-tonne truck mounted drill rig or a 20-tonne excavator.



The drilling machine is pretty cool – it’s essentially a big drill with a hollow core. The machine sends the drill rods down deep into the ground and it takes a ‘core’ of the earth – like when you core an apple but on a much bigger scale. 



That core sample can then be analysed to see what it’s made up of and whether it’s what we expected to find. The drill is impressive as it can penetrate down to vast depths – at Tarraleah, we hit close to 200 metres! That’s where the proposed power shaft might go. 



Quick techy fact – a power shaft (sometimes also called a pressure shaft) is an underground tunnel that directs the water down quickly at a steep angle to the power station. You can think of it like an underground penstock.



So let’s get to the core of the issue – why we’re doing this work



The Tarraleah hydropower scheme has been faithfully generating power for the state since 1938.  Do the maths and you’ll quickly realise that those assets are now over eight decades old. That’s old enough in human years, let alone when we’re talking about assets that we depend on to move huge volumes of water over long distances and then generate the power that we all rely on.  Time for a facelift! 



But we didn’t book the surgery straight away…



Enter the Battery of the Nation idea. What if we could reimagine the scheme so that it generated energy, more flexibly without using any more water? What if doing that was not only great for Tasmania but gave us the kudos of helping out the rest of the nation…



Now that was an idea that got a lot of people excited, including the Australian Renewable Energy Agency who gave us funding to prove the concept.



So back to the facelift. 



We’ve got two options - we keep the scheme that we’ve got and ‘pretty it up’ (the refurbishment option).  The other option requires a little more ‘surgery’ - essentially, we’re talking about replacing what’s there with brand new infrastructure (the redevelopment option).



If the redevelopment goes ahead, we’re talking construction of a new power station, many kilometres of new tunnels and hundreds of millions of dollars in construction cost. When it comes time to build, we don’t want any surprises. 



So before anything gets started, you need to know what you’re dealing with.



Enter the drilling team



Quick techy fact – did you know Tasmania has the biggest exposure of dolerite (a really really hard rock) in the world?



And now that you know this, imagine thinking that you’re excavating deep into the soil and you suddenly hit super hard rock – your project budget and timeline goes out the window. We want to know if we’re going to run into this rock before we get serious about digging, tunnelling and building!





Now you’re curious, right? What did we find? After 690 metres of drilling at sites from Lake King William right down to where a new power station might go, we ended up with loads of core samples! Everything from surface rock to basalt to sedimentary rock (like mudstone) through to the really hard stuff, dolerite.



A selection of these samples are now being thoroughly analysed in a lab and we’ll get a report on exactly what the geotechnical conditions are on site. This is all grist for the engineering mill as our clever engineers can the use that detail to refine their designs.



The good news is that the site conditions are pretty much what we expected based on our previous studies and analysis.



So what’s next?



A feasibility study essentially involves a progressive gathering of knowledge and increasing levels of detail. Here are a few things that need to come together as we work towards wrapping our studies at the end of the year…



  • Geotechnical, environmental and heritage assessments that feed information into the engineering design process. 
  • A construction assessment to determine how and in what order the redevelopment will be built, how long it will take and how much it might cost. 
  • System modelling to look at how a redeveloped scheme will interact with the rest of the hydropower system (things like storage levels and river flows). 
  • A commercial feasibility assessment to determine project risks and recommend the best strategy for the Tarraleah scheme.


So stay tuned...




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