Modelling pit wall erosion using photogrammetry, implications for water quality, and acid and metalliferous drainage assessment

Authors: Wright, T; Grant, S; Hertrijana, J Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2515_17

Cite As:
Wright, T, Grant, S & Hertrijana, J 2025, 'Modelling pit wall erosion using photogrammetry, implications for water quality, and acid and metalliferous drainage assessment', 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. 1-14, https://doi.org/10.36487/ACG_repo/2515_17



Abstract:
Exposed pit walls are prone to affecting water quality due to erosion caused by weathering. This work presents an updated case study from the high-sulphide Martabe gold mine in Indonesia, where a novel approach has been taken to assess spatial and temporal variations in pit wall erosion rates which influence debris generation and pit lake water quality. Understanding erosion rates of different alteration lithologies and waste classes enables forward planning and enhanced control over levels of acid and metalliferous drainage (AMD). Measurement of several test zones was undertaken using drone-mounted photogrammetric scanning targeting key lithotypes. The resulting 3D point clouds were used to generate digitised high-resolution surface meshes. Surfaces were remeasured at regular intervals and compared to the initial control surface to generate a 4D difference model. Observed differences show where material has been lost through dissolution and collapse or gained through the formation of scree piles. A key aspect of the study was the addition of manual debris and run-off collection in a series of small test pits to calibrate the findings of digital monitoring and provide important data relating to water quality and material characterisation. The net volumetric difference and calibrated mass of material removed was used to estimate the total amount of mass lost from each test area and extrapolated over the entire exposed pit surface before being forward modelled over periods of decades and centuries using a rate of erosion per unit area per unit time for each of the key lithotypes and waste classes encountered at the Martabe site. The mass and character of material removed plus run-off water quality were used to predict potential long-term AMD release incorporating 3D grade control block models developed during active mining.

Keywords: erosion, pit wall, AMD, photogrammetry, water quality

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