Abstract:
With the development of coal mining depth to KM level and the research and application of large mining height and super high mining technology, the far-field hard roof problem in overlying large space during coal mining is very serious. It has significant impact on the strong mining pressure in working face and the deformation of the roadway on the free face side of working face. Ground fracturing is one of the control measures for the hard roof in overlying large space, however, it always forms fractures with single shape that controlled by in-situ stress and has a limited effect on weakening far-field hard roof. For this purpose, the experimental and theoretical researches on the fracture pattern of slot guided borehole fracturing were conducted. The competitive law of hydraulic fracture and slot guided fracture was revealed. The slot guided fracture initiation model and well borehole initiation model were built and fracture initiation pattern criteria were formed. The influence of different critical characteristic lengths of plastic zone, different radius of slot, different radius of well and different depth on fracture initiation pattern were explored. The results show that three hydraulic fracturing modes exist in hydraulic slotted ground fracturing (transversely type, longitudinally type, composite type). The characteristic lengths of slot plastic zone and borehole plastic zone show positive correlations with fluid pressure. The corresponding critical length of plastic zone that forming composite type fracture shows positive correlation with the radius of slot while negative correlation with the radius of borehole. The bury depth of coal seam has significant impact on fracture initiation pattern that transient from longitudinally type to transversely type and composite type as bury depth increases. Under specific buried depth, the composite type fracture can be formed by adjusting both characteristic lengths of borehole plastic zone and slot plastic zone equal to the critical length of plastic zone through an appropriate design of the radius of well borehole and slot.