Evolution of the Bige Stockpile Closure Design for the Ok Tedi Mine

The Lower Ok Tedi Dredge Project, which commenced operation in 1998, is located at Bige in the Western Province of Papua New Guinea (PNG). The project is an important component of Ok Tedi Mining Limited’s (OTML) environmental management program to reduce the mine’s impact on downstream riverbed aggradation, flooding, and vegetation dieback. Dredged river sediments (mostly sand and silt particles) are stored in on-land stockpiles on the east and west banks of the Ok Tedi River (note the term “Ok” translates to “River” in the local region language hence the term Ok Tedi used throughout this paper). Klohn Crippen Berger Limited (KCB) (formerly Klohn Crippen Consultants Limited) undertook the original site selection, geotechnical, and civil design for the project, including surface water management and landform design of the stockpiles in the mid-1990s. The original design intent of the Bige project was to store sand in a “semi-permanent” facility on the Ok Tedi flood plain, where erosion of the stockpiles by migration of the Ok Tedi was seen as a possibility. The expectation at the time was that after mine closure and recovery of the river system, gradual erosion of the stockpiled sand would not significantly impact the river system. In the early 2000s, following commencement of operations at Bige, further studies indicated that the dredged material stored in the stockpiles could be potentially acid forming (PAF) and could lead to acid rock drainage (ARD). Therefore, modifications to the landform design were undertaken to mitigate this risk.

The initial landform design of the stockpiles generally incorporated side slopes of 7.5H:1V. To facilitate surface water management from this surface, a series of hard engineered structures including gabion baskets, Reno mattresses, benches, graded bench drains and inter-bench spillways. This water management system has operated successfully over the past 15 years on rehabilitated sections of the stockpile. Developments in material availability, scheduling issues and construction practicalities associated with placing a proposed non-acid forming (NAF) closure cover necessitated a change in design philosophy for the surface water management system. In 2013–2014, OTML initiated studies to investigate the use of soft engineered water management features including flatter side slopes, vegetated drainage swales, and ephemeral surface wetlands. The updated closure design approach is intended to provide a landform that minimises long term care and maintenance requirements, is sympathetic to the natural environment, and includes a stable and resilient long-term closure cover.

Brand, K., M.J. Ind, M. Ridd, M. Thompson, and L. Murray. 2015. “Evolution of the Bige Stockpile Closure Design for the Ok Tedi Mine,” in Proceedings of the 10th International Conference on Mine Closure, 1-3 June 2015. Vancouver, B.C.: InfoMine.

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