Empirical methods for support system design in deep serpentinite excavations: A critical evaluation

Underground mining excavations in fragile serpentinite rock masses represent a significant challenge today. The complicated features of these rock formations, especially at great depths, make it difficult to classify them using commonly employed methods. This paper focuses on the application of empirical methods for the classification of serpentinite rock masses and the design of support systems in underground mining excavations. Specifically, it concerns an access tunnel excavated entirely in serpentinite at a depth of 460 m. Geomechanical classification systems indicate that the Tunnelling Quality Index (Q-system) and Rock Mass Rating (RMR) evaluate serpentinites as rock masses of exceptionally poor to very poor quality, with Q-system values ranging from 0.005 to 0.2 and RMR system values between 19 and 30 depending on the type of serpentinite. Furthermore, the uniaxial compressive strength, ranging from 5 MPa to 34 MPa, confirms these particularly unfavourable geotechnical stability conditions, necessitating reinforced support systems to limit deformation in deep underground mining excavations. The present research has pointed out the limitations of empirical classification systems in effectively dealing with such an exceptional serpentinite-rich environment as that exhibited at the Bou-Azzer mine. The findings of the study indicate that tunnel engineering indeed needs a more tailored approach and advanced methods like numerical modelling. This may complement empirical classifications for better stress distribution estimations and failure zone predictions in such challenging mining conditions.