Mahayasa, M, Johnson, M, Witler, M & Standridge, L 2025, 'Lower Wanagon 3200 update design as part of Grasberg surface mine closure', in S Knutsson, AB Fourie & M Tibbett (eds), Mine Closure 2025: Proceedings of the 18th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-15, https://doi.org/10.36487/ACG_repo/2515_76
(https://papers.acg.uwa.edu.au/p/2515_76_Mahayasa/)
Abstract: The Lower Wanagon Overburden Stockpile (LWOBS) is a nearly 1 km high overburden stockpile located in a steep valley near Grasberg open pit in Papua, Indonesia. To ensure long-term stability of the LWOBS for closure, PT Freeport Indonesia is currently re-sloping to an overall slope of approximately 2.8H:1V with the objective to meet stability requirement, implement erosion control structures and reduce long-term maintenance for the stockpile.
A critical key to successful closure is the design and construction of an engineered toe structure. Design of the LWOBS toe was difficult, as material properties and volume of rehandle-sediment material at the toe was unknown before 2022. Vehicle access to the toe was developed in September 2022, when a geotechnical and hydrology investigation was performed, consisting of test pits, ground penetrating radar (GPR) surveys, and core drilling. The GPR and core drilling helped identify the bedrock contact beneath the valley fill, while the test pits provided aquifer permeability measurements and particle size distribution. Nested vibrating wireline piezometers were also installed in these boreholes to monitor water levels. 
The results of both investigations furthered the conceptual design of a toe stabilisation and erosion control structure at the LWOBS toe. The design includes a shallower stockpile regrade slope at the toe followed by the LWOBS engineered toe structure. The engineered toe structure consists of tightly graded screened coarse material to control phreatic levels at the stockpile toe, as well as large riprap to mitigate against erosion as water discharges from the stockpile. 
This paper presents the result of the geotechnical and hydrology investigations, design, and planned construction sequence at the LWOBS toe to provide management of the slope stability, groundwater, surface water, and potential erosion mitigation in this area.

Keywords: Wanagon overburden, slope stability, erosion control, acid rock drainage, water management