A new downgrading system for weak rock mass

Estimating the strength of weak rock mass is challenging in rock mechanics and rock engineering. This study introduces the development of a downgrading system for weak rock mass strength (GSI w ), a novel methodology designed to bridge the gap between soil and hard rocks. A conceptual model is proposed, which simplifies weak rock mass downgrading by considering the influence of the rock matrix, joint distribution, and surface conditions. This model provides a systematic framework for interpreting weak rock mass properties as a function of intact rock and discontinuity influences. Additionally, rock mass properties obtained from both Australian open-cut coal mines and literature are utilised to derive the best-fit surface for interpretation. The proposed GSI w system provides a reliable and applicable tool for estimating weak rock mass strength. To facilitate its practical application, a guideline is outlined, and two case studies are presented alongside the final GSI w chart.