Theme: 08. Metals

Topic Area: 8b. Mining and Uranium Site Restoration

Formatted Title:

Ranger Uranium Mine: Pit 3 Wick Drain Installation and Injection Well Directional Drilling

Background/Objectives:

At closure of most mines, tailings from mining operations (i.e., material left over after milling material of value out of mined ore) are required to be contained on site as part of rehabilitation requirements. This is because they are often contaminated and need to be stored underground, however they lack the geotechnical substance (best described as a soupy mix/toothpaste) to effectively cap the tailings in line with rehabilitation requirements and timelines. In the case of Ranger Uranium Mine in Australia’s Kakadu National Park (a UNESCO World Heritage Site), ~32 Mt of uranium tailings are to be interned in Pit 3 – one of the largest pits used by the mine.

In 2022-2023 Energy Resources of Australia (ERA), with Ventia as its partner in the field, installed 39,768 prefabricated vertical drains (PVDs) / wick drains within the Pit 3 tailings layer to meet a consolidation design and their rehabilitation requirements and closure timeline. The purpose was to release pore pressure from the tailings, and drain water from the body, thus accelerating consolidation of the tailings from years to months and allowing the pit to be capped and returned to a state like the surrounding Kakadu National Park within the legislated rehabilitation deadline. The wick drains were installed on a 2.5 m grid to depths of 20 m to 40 m into tailings. One year has passed since completion of the installation, and results from bathymetric surveys and further cone penetration tests (CPTs) show consolidation is matching or exceeding design estimations.

The installation of wick drains at this scale into a large body of tailings was novel - to the best of our knowledge. This paper aims to detail the performance of some of the innovations used to successfully execute the work, and provide some lessons learned from this impressive undertaking.

As a further undertaking, ERA also selected Ventia as its partner to install several brine injection wells around Pit 3. Directional drilling was used to drill from the surface and down the outside of Pit 3’s walls to finally penetrate the walls at depth, entering a 30 Mt waste rock underfill/underdrain layer placed beneath the tailings layer. Brine is a product of the mine’s water treatment operations, and it also was required to be stored underground as part of the mine rehabilitation requirements.

Approach/Activities:

Within the Pit 3 tailings layer, a CPT campaign was undertaken to finalize detailed design of the wick drain field. To meet the design during field works, specialized wick drain installation plant and equipment was developed to reach the required target depths into tailings and successfully anchor into the soup mix, combat high excess pore pressures to succeed in installation, and protect the delicate PVD material from foreign objects during installation. Furthermore, as it was impossible to use the tailings layer as a working platform, a bespoke wicking barge was designed and built to float the operation above the soupy mix, all while achieving positional accuracy and station holding within 200 mm and 1% vertical.

Simultaneously, outside of Pit 3’s walls, directional drill rigs drilled to depths of approximately 200 m below ground level, turning to penetrate the pit walls and reach the underfill layer the base of the pit at the base of the pit, allowing for brine to be injected through the well in the future.

Results/Lessons Learned:

Within the Pit 3 tailings layer, our work successfully overcame a varying tailings condition, high excess pore pressures, and anchoring difficulties to deliver this world first. Performance of the wicks has been verified through ongoing bathymetric surveys and more recently visual confirmation as the pit gets drained by pumps.

The project required good preparation though also being adaptable and open minded throughout implementation. Lessons learned include how to use water to combat high pore pressures, what anchor plates work best, and how to adapt to the inevitable variety of conditions within the tailings body.

Outside of the pit, the directional wells were also a success, allowing for brine to be injected as required.

With more mines approaching closure and rehabilitation in Australia and the world, the lessons learned could be invaluable for future projects.

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Jayson Gaul