Tailings Dam No.2
Tailings Dam No. 2 (TD2) at the Mount Thorley Warkworth Mine was the primary storage facility for coal tailings from the North Coal Handling and Processing Plant between 1998 and 2010. It also received ash from the Redbank Power Station from 2003 until deposition ceased in 2014. Yancoal has since run a staged program to progressively cap and rehabilitate the facility.
ATC Williams has supported TD2 throughout almost its entire operational life, first becoming involved at Mount Thorley Warkworth in 1999. Each investigation and design builds on previous monitoring, geotechnical assessments and operational experience, so strategies can be refined as tailings conditions evolve.
In 2025, Yancoal engaged ATC Williams to run a detailed geotechnical investigation and design a Stage 2 capping extension, assessing tailings strength gain around the decant pond perimeter and identifying where conventional mine spoil capping could be safely advanced.
Yancoal rehabilitates TD2 in stages as the tailings dry and gain strength. Annual geotechnical investigations identify areas capable of supporting conventional mine spoil capping, advancing rehabilitation incrementally in the most economically efficient way.
Tailings strength around the remaining decant pond is highly variable. Progressive drying had improved conditions across much of the facility, but areas that historically stayed saturated remained weak.
The challenge was to maximise capping extent while keeping construction safe and the tailings deposit stable. That meant identifying where strength gain could support conventional earthmoving methods, and recognising which areas needed further drying first. The design also had to accommodate plant ranging from low-ground-pressure dozers to large mining trucks, with capping thicknesses, sequencing and operating controls tailored to varying ground conditions.
ATC Williams carried out shear vane testing at 30 locations around the decant pond perimeter to depths of 4.5 metres, reassessing crust development and measuring strength gains since the 2023 investigation.
Continued drying, solar desiccation and extensive vegetation growth had produced measurable strength improvements across several areas:
These results gave the geotechnical basis for advancing the next rehabilitation stage, identifying areas strong enough for conventional mine spoil capping using low-ground-pressure civil equipment combined with standard mining plant.
ATC Williams completed local and global stability assessments using the Morgenstern-Price limit equilibrium method in SLOPE/W, in accordance with ANCOLD guidelines for short-term loading conditions.
The assessment evaluated combinations of capping thickness, equipment and placement sequence across the rehabilitation zones surrounding the decant pond, establishing the maximum extent of conventional capping that could be safely advanced. The design covered:
The assessment identified eight rehabilitation zones (A3C, B, B2/B3, B3, C, C2, C3 and C4) capable of supporting further conventional capping.
ATC Williams delivered construction drawings, equipment operating limits, sequencing requirements and monitoring protocols, supporting safe placement of approximately 190,000 m³ of additional mine spoil capping using civil earthworks and conventional mining equipment.
More than 70% of the TD2 tailings surface has now been rehabilitated using the staged capping approach. ATC Williams’ long-term involvement has enabled Yancoal to advance rehabilitation with confidence, reducing technical uncertainty, optimising sequencing and supporting efficient closure of one of the Hunter Valley’s major legacy tailings storage facilities
Areas surrounding the northern decant pond perimeter remain too weak for conventional capping. These are recommended for continued drying and reassessment during future annual geotechnical investigations, letting rehabilitation advance safely as tailings conditions improve.
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