van der Merwe, C, Stimpfl, M, Gale, J, Carter, G, Noonan, D & Pershke, D 2026, 'Pit Impact Risk Assessment Tool: using explicit uncertainty to drive progressive 
mine closure planning across a portfolio of open pit voids', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-12, https://doi.org/10.36487/ACG_repo/2615_138
(https://papers.acg.uwa.edu.au/p/2615_138_van_der_Merwe/)
Abstract: Mine closure planning is fundamentally a problem of time-bound knowledge applied to permanent landforms. A pit void that will persist in the landscape for millennia must be assessed and managed using information that is incomplete at the time decisions are made and that improves only as the mine advances through its life cycle. The conventional industry response has been to defer detailed closure risk assessment until knowledge is considered adequate. This paper argues that a more effective approach is to make the current state of knowledge explicit at every stage, couple it visibly to a risk and uncertainty rating, and use that transparency to drive knowledge acquisition rather than waiting for it.
This paper describes the development of the Pit Impact Risk Assessment Tool (PIRAT), commissioned by BHP’s Western Australia Iron Ore (WAIO) closure planning team to manage closure risks across its Pilbara open pit portfolio (Noonan et al. 2025). The tool applies a source-pathway-receptor scoring framework across 14 risk categories. Its distinctive contribution is a parallel knowledge base (KB) certainty assessment in which every risk score is accompanied by a certainty rating calibrated against the minimum acceptable knowledge standard for the pit’s current stage of life. Where certainty falls below the threshold, the tool automatically flags to the user, making knowledge gaps as visible and actionable as the risks themselves. The tool’s forward work program output translates these flags directly into a prioritised schedule of studies. PIRAT is a screening and prioritisation tool. It identifies which pits warrant detailed study and in what sequence, and does not replace the detailed hydrogeological, geochemical and ecological studies that follow. 
The paper presents the tool’s design rationale, risk architecture, scoring methodology and preliminary outputs from 2 pits in the test hub, representing the first test data entered into the tool. The results illustrate how pits with the same closure strategy and life cycle stage can carry meaningfully different risk profiles and how the certainty framework reveals that both are currently relying on an inadequate knowledge base for their most critical closure control. The project is ongoing; findings are presented as at March 2026.

Keywords: mine closure planning, closure risk assessment, knowledge uncertainty, source-pathway-receptor framework, progressive closure planning.