Villegas, F, Calderon, J & Vasquez, P 2025, 'Observational method applied to pit slopes informing the characterisation, 
design and monitoring process for an open pit mine: a case study', in JJ Potter & J Wesseloo (eds), SSIM 2025: Fourth International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, https://doi.org/10.36487/ACG_repo/2535_16
(https://papers.acg.uwa.edu.au/p/2535_16_Villegas/)
Abstract: This paper presents the application of the observational method (OM) to slope design in open pit mining, highlighting its effectiveness in managing geotechnical uncertainty and enhancing slope reliability. While widely applied in underground works, the OM remains underutilised in open pit environments. Through a structured cycle of predict–monitor–reconcile, the OM enables dynamic updates to design parameters based on actual slope behaviour. A case study at BHP's Escondida mine illustrates how the OM guided design adjustments following unexpected slope instability, particularly where rock mass properties deviated from initial assumptions. The methodology supported back-analysis, model reconciliation and outcomes such as reduction of inter-ramp angles, improving slope stability and increasing reliability in the mine plan. Such backanalysis and reconciliation are necessary considering the uncertainty in the critical geotechnical parameters that drive slope instabilities, impact the mine plan and may lead to a decrease or increase in inter-ramp angles if the actual slope performance is significantly worse or better that the expected one. Integrating reliability assessments and parameter uncertainty indices further weaker or strengthened the design process. This work advocates for the systematic implementation of the OM in open pit slope engineering as a best practice for managing geotechnical risk.

Keywords: observational method, pit slope design, design guidelines, geotechnical monitoring, failure mechanisms, slope design process