Abstract: The traditional block theory neglects the evolution of discontinuous structural planes to continuous ones induced by the effect of construction disturbance, such that the determination of key block is inaccurate. The criterion to distinguish the type of structural planes plays a significant role in the block stability analysis. Numerical simulation methodology is adopted to calculate the coalescence strength and peak strength of rock samples. A coalescence coefficient of structural planes is introduced as the ratio of the coalescence strength to the peak strength, which could potentially provide the indication of the formation of ligament. The correlation between the coalescence strength and various influencing factors such as dip angle of ligament and structural planes, friction coefficient of structural planes, lateral stress and continuity rate is analyzed quantitatively. Based on these observed relationships through analytical studies, the coalescence criterion for ligament is proposed to characterize two arbitrary discontinuous structural planes and to identify whether they can be formed as a single continuous structural plane. Incorporating coalescence criterion into block theory, the key block sliding triggered by the propagation of interior structural planes can be obtained accurately. This may provide better guidance for practical engineers to perform stability analysis.