Probabilistic Rockfall Analysis in 3D in the Peña Colorada Open Pit (Colima, Mexico)

ABSTRACT: Rockfall events are one of the most difficult geotechnical conditions to predict and control in a mining operation. This study is focused on the Peña Colorada open pit (Colima, Mexico), where a series of rockfalls occurred on the main slope during the period of Hurricane Lidia (October 2023). The proposed methodology is based on a probabilistic analysis and field inspections to identify potential rockfall sectors. The topographic survey was obtained with photogrammetry using a drone for identifying potential sectors. Then, the properties such as restitution coefficients and the friction angle has been calibrated based on field tests of the different terrains observed on the main slope (rock and spill). The results obtained in the Rfall 3D simulations are interpreted, compared, and calibrated with real-scale field tests performed on the main slope. Finally, solutions to mitigate rockfall risk are proposed, such as temporary or permanent closure of haul roads or implementation of barriers. 1. INTRODUCTION Open pit mining is a mining method for metallic and non-metallic minerals. While this approach has advantages in terms of accessibility and efficiency, it also presents significant challenges related to rockfall risks. Rockfall risk in open-pit mining represents a constant hazard for workers in mining operations. Rock falls can cause serious injury or even loss of life and its origin is related to earthquakes, blasting or even weather conditions, such as rain or hurricanes. The methodology proposed considered a field inspection, photogrammetry survey by drone, use of rockfall 3D software, calibration by field tests and geotechnical control implementation as barriers. 2. PEÑA COLORADA OPEN PIT 2.1. Open pit description The Peña Colorada Mine is a open pit mine located in Colima, México. It is owned by ArcelorMittal and produced an estimated 3.8 million tons per annum (Mtpa) of iron. As part of the business chain, Peña Colorada Mine is one of the key assets of Ternium and ArcelorMittal in México. Recommended interramp slope angles for Phase 4A are based on bench geometries that resulted from the backbreak analysis using an 80% reliability of achieving the required 12.7 m catch bench width for a double bench configuration. Resulting interramp slope angles for 28 m double benching range from 45 to 49°. The recommended slope design angles require average bench face angles of at least 72° (see Fig. 1).