Investigation of Chemical Weathering Effects on Rock Mass Classification at Open-Cast Mineral Sites in Imo State, South East Nigeria

Chemical weathering of rock masses in the humid tropical climate of Imo State, South East Nigeria, significantly influences geotechnical properties and slope behaviour at open-cast mineral extraction sites. This study investigates the effects of chemical weathering on rock mass classification at sand and laterite quarry sites in Owerri and Okigwe Local Government Areas of Imo State. Field rock mass characterisation was conducted using the Rock Mass Rating (RMR) system and Geological Strength Index (GSI), with weathering grades assigned in accordance with the International Society of Rock Mechanics classification scheme. Index tests including point load strength, Schmidt hammer rebound, and slake durability index were performed on samples representing different weathering grades. Results show a systematic decrease in RMR and GSI values with increasing weathering grade, from a mean RMR of 52 for slightly weathered rock to 19 for completely weathered material. The slake durability index declines sharply from grade III weathering onwards, indicating susceptibility to rapid deterioration on exposure. Slope failures at two of the study sites are shown to occur exclusively within weathered zones. The study recommends weathering grade mapping as a mandatory input to slope design at all new open-cast sites in Imo State and proposes conservative slope angle adjustments for different weathering classes. Keywords: chemical weathering, rock mass classification, open-cast mining, Imo State, slope stability.