TD1/2 Stage 2 Capping Design and Support
Mangoola Coal Operations, located in the Hunter Valley, is a major open-cut mining operation committed to responsible resource management and progressive rehabilitation. As part of its closure planning, the site required a comprehensive capping solution for two decommissioned tailings dams: TD1, with a capacity of 3,400 ML, and TD2, with a capacity of 3,900 ML. The Stage 2 capping works covered an area of approximately 222,000 m2.
Mangoola Coal Operations (MCO) required a comprehensive capping solution for two decommissioned tailings dams (TD1 & TD2). The goal was to ensure long-term stability, environmental compliance, and safe access for future land use. The tailings presented multiple challenges, with variable shear strengths and some areas requiring reinforcement to safely support capping activities.
ATC Williams supported Mangoola Coal Operations with targeted engineering support to ensure the successful execution and certification of Stage 1 capping works for Tailings Dams 1 and 2. Over a five-month period, our team provided project management and geotechnical advisory services that began with a kick-off meeting. Fortnightly field investigations were carried out to validate the tailings shear strength, reducing the risk of failure. This was supported by site inspections and construction certification. All this gave the client confidence with clear documentation for internal and regulatory assurance. One of the key successes of this work was achieving time and cost savings for the client.
By scheduling investigations on non-construction days and efficiently using internal resources, we minimised site disruption and avoided unnecessary expenses.
Building on the success of stage 1, we progressed to stage 2, where we developed a detailed Stage 2 capping design for the site. Comprehensive geotechnical investigations, including shear vane testing (SVT) and cone penetration testing with pore pressure measurement (CPTu), were undertaken to delineate strength zones across the tailings surface. These investigations informed a series of stability analyses, covering local, global, punching, and bearing failure modes, to ensure the design would perform under all loading conditions.
The design is a layered capping system constructed from mine waste, tailored to both the strength of the underlying tailings and the operational demands of equipment working on the surface. Although high-strength woven geotextile (PET 200-50) was initially proposed and designed for the weakest areas, rigorous testing and stability assessment demonstrated that the capping could be delivered safely without it, and the geotextile was eliminated from the design.
Throughout construction, strict protocols will be adhered to, including setback distances, access limits, and monitoring regimes.
ATC Williams delivered the project on time and within scope, maintaining clear communication between MCO, contractors, and our own team throughout. We provided the client with comprehensive documentation, including technical specifications, construction drawings, and inspection checklists, ensuring transparency and consistency at every stage. Our team also offered ongoing technical advice and field validation during construction.
With Stage 1 complete and Stage 2 capping now ongoing, the works will place more than 1,032,000 m3 of fill across TD1 and TD2. Through rigorous testing and staged stability assessment, the design achieved minimum Factors of Safety across all potential failure modes without the geotextile reinforcement originally proposed, ensuring long-term stability and compliance.
This approach not only delivers a safe and cost-effective closure solution for legacy tailings infrastructure but also provides the client with confidence in the durability and sustainability of the final landform.
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