Block theory is a commonly used method in stability analysis of geotechnical engineering

but there is no research on non-coalescence structural planes

which makes the key block searching neither accurate nor adequate. It is an essential issue for block theory studies that how to deal with the non-coalescence structural planes and whether they should be connected. Coalescence strength and peak strength of rock samples containing two coplanar structural planes in different rock bridge angles

frictional coefficient of structural planes

lateral press

and connectivity rate are calculated through numerical simulation method. Coalescence coefficient is introduced to quantitatively describe the relationship between coalescence strength and peak strength. The coalescence strength is a standard to estimate whether the rock bridge is coalescent or not. The function is established between coalescence strength and rock bridge angle

frictional coefficient of structural planes

lateral press

and connectivity rate

which is the rock bridge coalescence law of rock mass containing two coplanar structural planes. The law can accurately determine that whether the rock bridge should be connected for the rock mass containing two coplanar structural planes. Key blocks

which are slipped due to rock bridge coalescence

are searched by block theory based on rock bridge coalescence law.