Underground construction | Digging deep at Kidston


Various lessons have been unearthed during excavation of the underground shafts at the Kidston pumped storage project in Australia


THE 250MW KIDSTON PUMPED storage project is currently under construction and will be the first pumped hydro project in Australia for over 40 years. It will also be the first to be developed by the private sector and the third largest electricity storage device in the country. Forming part of the Kidston Clean Energy Hub in North Queensland, it also comprises a solar and wind power project. Genex Power, the company which owns the project,


Above: Aerial view February 2024


recently discussed the lessons it has learnt with respect to the excavation of the project’s shafts. Four shafts are required to be excavated: Ventilation Shaft – 4.1m diameter (excavated), 243m deep.


Cable Shaft – 4.1m diameter (excavated); 252m deep. Intake Shafts x 2 – 5m diameter (excavated); 244m deep.


Raisebore techniques were used to sink the shafts, with the shafts excavated from the bottom up. This was made possible due to the availability of underground caverns prior to excavation commencing. The general methodology followed was: Establishment and survey. Installation of pilot hole. Attachment of cutterhead and ream the shaft towards the surface. Removal of cutterhead and dis-establishment.


Genex discussed various key challenges and lessons learned relating to the raisebore operations and focused on ground conditions, the pilot hole, and reamer operations and removal.


Access road Canal Intake structure Powerstation cavern


Ground Conditions During raisebore operations, the excavated shaft


is an unsupported circular profile, and support can only be given once the reaming operations are complete. Raisebore shaft construction techniques are normally adopted where ground conditions are favourable, access is provided to the shaft bottom, and the project shaft diameter and length are deemed as being suitable for the equipment available. Where the ground conditions are less stable and at risk of collapse/ravelling, then structural ground support treatment is required. Following a comprehensive geotechnical investigation, Genex identified that all four shafts would encounter competent rock, with the exception of the top 20m of ground for the ventilation and cable shaft locations where extremely weathered rock was found. This could result in potential ground squeeze, ravelling and stability issues if left unsupported. As such, the project commenced the installation of a secant piled ring in the upper zone of both the ventilation and cable shafts as an advanced pre-sink activity to the raisebore operations. This activity was not included as part of the original scope of works as the geotechnical information needed to be confirmed. Further geotechnical information obtained during detailed design confirmed the need for pre-sink piles. Genex says the lesson learnt here highlights the importance of good geotechnical investigation and analysis to make informed decisions on ground treatments and supports. This was a known risk at the Kidston Project and was confirmed and addressed prior to raisebore operations commencing.


Pilot hole The pilot hole links the top and bottom of the shaft and


New permanent access road


Access tunnel


Eldridge Pit (Lower reservoir)


ultimately sets the shaft alignment. Ensuring verticality when drilling the pilot hole is of utmost importance. At Kidston, the pilot hole for each shaft was drilled using the rotary vertical drilling system. The pilot hole within the cable shaft encountered a


Wises Pit Dam crest elevation 552.70


Wises Pit (Upper reservoir)


rogue geotechnical monitoring instrument at depth, which twisted around the drill head and affected the verticality of the hole, and in turn the shaft. This was remedied by removing the pilot hole drill rods, clearing the drill bit, and recommencing pilot hole drilling. The lesson learnt relates to ensuring that the utilisation of state-of-the-art positioning and drilling equipment will provide the greatest chance of shaft verticality. Genex says that other methods are available but for the Kidston Project, verticality was paramount and would not have been achieved without the use of the rotary vertical drilling system.


Reaming operations and removal Once the cutter head is in action it is critical to ensure


that for safe operation the cuttings are free to fall to the 34 | November 2024 | www.waterpowermagazine.com


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