Abstract: Blocks are one of potential failure modes appeared mainly in structurally-controlled rock mass. Factors of safety (FoS) for blocks identified in underground caverns of a huge hydropower station are obtained using the limit equilibrium method and strength reduction method. A simple graphical method for stability analysis based on block parameters is proposed by the calculated FoS of the block, and the reliability of results is validated by in-situ monitoring data. It is found that the FoS values obtained by these two methods are inconsistent even for the same block, which thus further influence the stability judgment. Results obtained by the strength reduction method are conservative since it is affected by software algorithm realization and mesh sizes. FoS of simple blocks (i.e. blocks consist of 3 joints and excavation boundary) can be easily obtained by the limit equilibrium method, while FoS of complex blocks (i.e. blocks consist of more than 3 joints and excavation boundary) are more conveniently calculated by the strength reduction method. Four block parameters, namely vertical in-situ stress, block volume, the maximum vertex depth, and equivalent joint shear strength, in combination with the stability partition chart, are applied to assist in the stability judgment. Once blocks are determined as unstable, the proper support and reinforcement measures should be adopted promptly. These findings can be used to other cases with similar unstable problems of blocks.

Key words: block, limit equilibrium method, strength reduction method, factor of safety, stability judgment