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The granite blocks are widely distributed on granite residual soil slopes, particularly in the circumstances of humid granitic terrain. These blocks may be partially buried in the soil slope with their long axis upright, which results in complex toppling mechanisms. The objective of this study is to investigate the mechanical stability of the blocks against toppling failure, considering extremely heavy rainfall. Based on the analysis of limit equilibrium, an analytical formulation taking the influence of the block overhang length and the slope inclination into account is proposed to calculate the factor of safety (FOS) of the blocks partially buried in the soil slope. The results of the model experiments verified the performance of this formulation by showing a consistent behavior regarding the stability of the block with the formulation. The FOS obtained from this analytical formulation first shows a trend of increase and then a decrease according to the overhang length of the blocks. Furthermore, the critical overhang length and the critical slope inclination were found related to the toppling failure of potential blocks. The analytical formula obtained in this study can be used to determine the factor of safety against toppling of the single blocks on granite residual soil slopes, which provide a quantitative method for risk evaluation of toppling disaster in granitic regions. • Single blocks on granite residual soil slopes are partially buried with long axis upright. • A calculating formulation for the factor of safety of single blocks is newly present. • Overhang length of blocks and slope inclination significantly affect the toppling instability. [ABSTRACT FROM AUTHOR]