c. 2020 pitt & sherry

Cethana Power Station Geotechnical Investigation and Ground Support Remediation

Cethana, Tasmania

The Cethana Power Station within the Mersey-Forth Scheme is one of four underground hydro-electric power stations in Tasmania, Australia. It was constructed in the latter half of the 1960s, with excavation being substantially complete by 1969, and the cavern roof having been established and supported by December 1967. The asset was commissioned in 1971.

As part of an ongoing asset management program, the owner and operator, Hydro Tasmania, scheduled a major outage for 2015 to upgrade the power-generation equipment. Two years prior to the upgrade pitt&sherry undertook an extensive geotechnical investigation of the underground excavations.

The investigation highlighted that much of the ground support and reinforcement system had degraded significantly and was rapidly approaching the end of its serviceable life. It was assessed that a substantial remediation program should be undertaken to upgrade and/or replace the ground support and reinforcement system throughout the underground excavations prior to the power generation upgrade.

Cethana Power Station was excavated before any of the rockmass classification schemes and empirical ground support and reinforcement design tools commonly used in the 21st century had been proposed. Nonetheless, pitt&sherry’s geotechnical investigation found that the extensive ground support and reinforcement system that had originally been installed closely matched that which would have been installed today.

The remediation program included the replacement of rockbolt reinforcement and shotcrete in the station access tunnel, replacement of rockbolt reinforcement and installation of corrosion protected mesh in the penstock access tunnel, replacement of the rockbolts securing the gantry crane beams to the walls of the power station chamber and replacing the rockbolt reinforcement in the roof of the power station chamber. Additional minor works were undertaken elsewhere.

It was a requirement of the remediation program that the upgraded ground support and reinforcement system have a design life exceeding 70 years.

That requirement called for the installation of double-corrosion–protected reinforcement elements.

Installation of the system was accompanied by an extensive QA/QC program including shotcrete testing, borehole gauging and photography, grout testing, rockbolt installation tests and pull-testing. The remediation program was scheduled for implementation over a 6-month period between May and October 2014. The work program was undertaken by The Mancala Group who also provided many innovated solutions to challenges we met along the way.

Despite the extent and intrusive nature of the work undertaken, the remediation program was completed successfully, on time and incident free, and without material interruption to power generation throughout the course of the work.

It was also necessary for the program to be undertaken in parallel with work needed to prepare for the power generation outage in 2015. pitt&sherry is unaware of such a remediation program being undertaken in these circumstances anywhere else.


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