Study on roof partition fracture based on dominant primary fracture

As the length of the open-off cut increases, the roof pressure in the advancing direction of the working face exhibits distinct zoning characteristics, with prolonged pressure duration and challenging predictability. Based on the influence of primary fractures on roof fracture morphology, the concepts of dominant incidence and dominant position of primary fractures are introduced. Dominant primary fractures are considered the main factor leading to zonal roof breakage, while non-dominant ones act as weakening factors. Under the effect of dominant primary fractures with specific occurrence conditions, zonal fracture patterns and criteria for the first roof weighting are established. The study reveals: (1) Multiple zonal fracture models involving primary fractures, such as "double O-X", "X-Y", "X-Y-Y", "Y-X-Y", "OX-OX", and "three O-X", are derived, along with their fracture propagation processes. (2) Using the "double O-X" model as an example, the influence of the roof's length-span ratio and crack length on fracture toughness and stress intensity factor is analyzed, yielding influence coefficients for internal and external forces during roof bending. Criteria for crack initiation, propagation, and instability under dominant primary fractures are established based on bound theorems and fracture mechanics. (3) PFC3D simulations and three-dimensional physical modeling validate the theoretical fracture patterns, showing good agreement with numerical and experimental results. These findings help explain roof zonal fracture mechanisms and provide a basis for ground control and disaster prediction in super-long working faces.