The Phu Kham Copper-Gold Operation (Phu Kham Operation) is located in Lao People’s Democratic Republic in mountainous jungle terrain and is exposed to a four-month annual monsoon season. An emergency spillway was initially developed at the eastern abutment of the main tailings storage facility (TSF) embankment dam.
Increased risks due to the potential for spillway flows to impact embankment dam integrity resulted in the project owner, PanAust Limited (PanAust), requiring the relocation of the spillway to reduce risks to the TSF. Knight Piésold designed a spillway exceeding current design standards, while providing PanAust with a cost-effective risk and water management strategy. The spillway—designed to safely divert the probable maximum flood (PMF) away from the TSF—ensures safe TSF operation during extreme flood events.
Knight Piésold proposed the relocated spillway to be developed in five stages throughout the remaining life of mine to allow for the spillway invert to be raised annually as the tailings dam is raised. Stage 1 construction required a 30-m-deep excavation through the western ridgeline of the TSF to achieve the spillway invert. Stages 2 through 4 require the construction of a zoned earthfill embankment to support a reinforced concrete chute spillway with an energy dissipation structure. Stage 5 requires the embankment and spillway complex to be converted to a saddle dam at mine closure.
Executing a design such as this required an innovative approach where the final arrangement (Stage 5) needed to be designed first to ensure a saddle dam could be feasibly executed. This design also required that all intermediate stages of spillway development could be feasibly executed with the understanding that the staging proposed at this initial design stage would undoubtedly be modified with the changing requirement for tailings and water storage. A modular reinforced concrete chute spillway was chosen as the preferred spillway alternative to allow for modifications to the chute during the raises.
Stage 2 of the spillway construction was completed in June 2018, ahead of schedule and under budget.