Geotechnical parameters for pit slopes in weak volcanic rocks on Simberi Island, Papua New Guinea

Authors: Baczynski, NRP; Moncrieff, JN; Woodward, KA Download Paper

DOI https://doi.org/10.36487/ACG_repo/2135_19

Cite As:
Baczynski, NRP, Moncrieff, JN & Woodward, KA 2021, 'Geotechnical parameters for pit slopes in weak volcanic rocks on Simberi Island, Papua New Guinea', in PM Dight (ed.), SSIM 2021: Second International Slope Stability in Mining, Australian Centre for Geomechanics, Perth, pp. 297-314, https://doi.org/10.36487/ACG_repo/2135_19



Abstract:
Simberi gold mining operations occur on a 6 × 6 km island at the easternmost edge of Papua New Guinea. Rainfall is 3–5 m per year. Groundwater is variable; perched watertables establish after major rainfall events and artesian pressures exist locally. Earthquake magnitudes of 5–6 (Richter scale) occur at nearby tectonic plate boundaries; mine specific seismic risks were assessed. Lithology and structure are complex and spatially variable; rock mass is hydrothermally altered, deeply weathered and intensely brecciated. Intact rock unconfined compressive strength (UCS) is often between 1–10 MPa and variable. High strength porphyritic rocks are locally 50–140 MPa but low strength rocks often persist with depth. Simberi is traversed by numerous closely spaced sets of faults related to volcanic intrusions and plate tectonics. Initial geotechnical drilling was done in the mid-1980s and gold mining started in early 2000s. Current open pits are 120–150 m deep and extract oxide ore. Future pits will be 250–300 m deep (i.e. to 50 m below sea level) and will mine sulphide ore. Due to the shallow depth and good stability of the early pit slopes, pre-2016 external geotechnical auditing was completed on an as-required basis. However, from 2016, regular annual reviews were undertaken. The geotechnical model for the sulphide pits is based on slope mapping, orientated core drilling and laboratory testing. Major investigations were completed in 2020; a challenging task due to COVID19 travel restrictions. Wide scatter exists in the data but UCS and rock quality designation averaged over 25 m relative level depth intervals improve linearly with depth. In general, occurrence of clayey-breccia zones abates with depth. Hoek–Brown and Step-Path rock mass strength models were considered. Investigation results, data trends, assessed parameters and pit slope models are presented and discussed.

Keywords: RQD, UCS, weathered hydrothermally altered rocks, low strength, 300 m deep pits, Hoek–Brown, Step-Path

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