Rosebery Tailings Storage Facility

1/ Transforming 2/5 Dam

We initially advised the client that they could raise the Bobadil TSF embankment as far as the Stage 9 raise, which was completed in 2015. At this point, the Bobadil TSF was approaching its design capacity with a Life of Mine of 5 years, so MMG subsequently requested us to investigate a new tailings storage option on the site. We turned our attention to the 2/5 Dam TSF, which is a historical TSF that was built in 1939 and is made up of three tailings storages: 1 dam, 2 dam, and 5 dam.

The dam presented some challenges, including acid mine drainage and seepage. The historical embankment was also constructed with acid-forming rock. We designed an integrated TSF that encompassed all three historical dams, addressing the acid drainage issues and delivering additional tailings storage capacity for the mine. We completed pre-feasibility and feasibility studies, followed by detailed designs for a two-stage raise. Construction of Stage 1 was completed in 2018 and Stage 2 in 2024, providing the site with an additional 5 years of tailings storage.

The geology of the sub-surface conditions below the 2/5 Dam Northern Embankment Buttress is represented by sediments and landforms created by late Pleistocene to Recent climatic changes, particularly associated with ice cap development, glacial incursion and retreat over the last two glacial/interglacial cycles (>130,000 years).

Cambrian felsic volcanics constitute the basement rock, forming a shallow basin like paleo-valley derived from glacial erosion. The basin was subsequently infilled with relatively flat layers of inorganic glacial soft silty sediments derived from fine suspended rock flour in ice melt water. After glacial retreat, during the Pieman Interglacial, the Stitt River valley was revegetated with the basin infilling with a peat layer, followed by organic silty clays during the Margaret Glacial. A recent erosional unconformity is represented by a layer of unconsolidated fluvial outwash gravel, sand and silt sedimentation.

Due to the presence of soft sediments within the foundation, the buttress embankment was constructed in stages to allow excess pore pressure dissipation, consolidation, and strength gain. An extensive array of instrumentation, including piezometers, inclinometers, and sacrificial settlement plates, was installed to monitor pore pressure and movement daily, ensuring safe embankment raises without overloading the foundation. A Trigger Action Response Plan (TARP) was prepared, and instrument readings were reviewed against the TARP daily to guide decision-making and maintain safety.

2/ Innovative solutions for Bobadil TSF

In 2020, the client asked us to explore options for further storage at Bobadil TSF, as production was set to continue beyond the Stage 9 capacity. The Bobadil TSF consists of upstream raised embankments, where each new embankment raise sits on top of the tailings placed during the previous stage. This approach presents challenges because tailings are susceptible to liquefaction and can lose strength or flow under certain conditions. When this occurs, the embankment can also lose strength as it relies on the tailings underneath to support it.

Through a series of investigations, we proposed an innovative “step-in” embankment design, which would allow additional embankment raises and provide the additional storage that the client required.

Designing the Bobadil TSF upstream raises from Stage 9 – 12 was highly complex and required sophisticated and highly skilled investigation and analysis. Analysis was conducted using critical state soil mechanics, stress path lab testing, estimation of state parameters, assessment of contractive and dilative behaviour of the materials, and liquefied shear strength analysis. We used detailed deformation analysis and quantified the deformations to validate the safety of the embankment raise, which was a critical element of the embankment design.

Our laboratories in Melbourne and Newcastle carried out the majority of the testing, partnering with the University of Sydney and NSW for additional testing where necessary.

In 2025, we completed construction of the Stage 10 Bobadil TSF embankment raise, which is now nearing capacity. We also completed the design documentation and necessary documentation for EPA approvals for a Stage 11 raise, for which construction was also completed in late 2025. We are currently in the process of designing a Stage 12 raise, which is likely to be constructed between late 2026 and early 2027.

Looking further ahead, we are currently examining Stage 13 and 14 raises for the Bobadil TSF, which would provide the client with critical storage capacity until a new TSF is completed in 2030. For this, we are completing more sophisticated investigation work, analysis, and assessments to determine whether future raises are possible. We are currently conducting advanced numerical analysis to assess possible static and seismic liquefaction and the expected deformations, while meeting the requirements of the ANCOLD guidelines and the Global Industry Standard on Tailings Management (GISTM). The findings of these analyses will provide confidence in the viability of additional raises.

Longterm partnership with MMG

In addition to supporting MMG with the design and construction of TSF embankment raises, we provide monthly facility audits and ongoing reporting, offering guidance and collaboration on day-to-day TSF operations. We also conduct comprehensive annual audits of the facilities and are the Engineer of Record for both the 2/5 Dam and Bobadil TSF.

Continuous communication, collaboration and on-site presence have been key to our success at Rosebery. As well as having a stable team, led by project manager and EOR Arun Muhunthan, who has been involved in this project since 2008, we have built strong relationships with MMG ‘s senior management, site staff, and operations teams.

Our long-term involvement has granted us a deep understanding of the site which is advantageous in fulfilling our EOR role and building towards compliance with the GISTM guidelines.

With close to twenty years on site, we have exceptional knowledge of the site, our client and their operations. We are dedicated to ensuring that the existing TSFs continue to meet operational needs through to 2030.