Luying Shao

Due to the supporting effect of the coal pillar on the side of the roadway, there is often a problem of the top coal and roof in the working face end not collapsing in time. The single directional hydraulic fracturing method cannot directionally cut off the top coal of the working face end. To overcome the above shortcomings, this article first proposes a method of controlling top coal and roof in the working face end by pulse and directional hydraulic fracturing. The method of using pulse hydraulic fracturing to weaken the top coal and directional hydraulic fracturing to cut off the roof to induce rock pressure to break the coal is used to control the timely collapse of the top coal and roof in the working face end. When the vertical stress on the top coal is greater than the strength of the weakened top coal, the top coal and roof begin to collapse. The vertical displacement of the weakened top coal and the cut roof gradually decreases from low to high, and the caving top coal and roof finally fill the goaf. The maximum reduction in stress concentration of coal pillar is about 75%. Then, the principles for determining the parameters of this method were provided. Finally, the industrial experiment was conducted in the coal mine. The construction water pressure of the roof is between 30 and 40 MPa. The construction water pressure of top coal ranges from 8 to 16 MPa, with an amplitude of approximately 4 MPa. Directional crack is formed within an area of 3.3 m in the direction of the adjacent roof borehole. A crack network is formed within a 4 m area near the top coal borehole. After hydraulic fracturing, the top coal and roof in the working face end collapse with mining.

The stress concentration caused by the presence of the remaining coal pillar in the overlying goaf can easily cause large deformation of the roadway in the lower seam and other problems. To solve this problem, the reasons for stress concentration caused by the remaining coal pillar in the overlying goaf were analyzed. A hydraulic fracturing method for relieving stress concentration has been proposed. By using hydraulic fracturing technology to remove the hanging roof on both sides of the coal pillar and weaken the coal pillar system, high stress is transferred to the goaf. The degree of stress concentration in coal pillars and surrounding rock of roadway can be reduced by 47% and 26%, respectively. This method can also solve the stress concentration problem of the remaining ore pillar in the overlying goaf of the metal ore. This method has been applied in the coal mine. The deformation of the surrounding rock of the test roadway after hydraulic fracturing has been reduced by more than 60%.

The behavior of hydraulic fractures initiated along the radial and axial directions of borehole has been investigated by the author in previous hydraulic fracturing experimental and engineering applications. Based on this, and considering the stress fields at the tips of fractures, a mechanical model of the progressive initiation and propagation of radial and axial hydraulic fractures in the borehole is established in this paper. Subsequently, based on the mechanical model, criteria for the progressive initiation and propagation of radial and axial hydraulic fractures are postulated. According to the criteria, the axial fracture begins at the midpoint of the radial fracture. When the second and third principal stresses near the axial fracture tip are equal, the axial fracture begins to turn. Finally, through an actual triaxial fracturing experiment, the injection pressure curve and internal fracture shape of a test block were obtained, which are the discrimination factors of the criteria. The fracture initiation pressure of the radial and axial fractures of the test block satisfied the fracture initiation condition of the criteria. The accuracy of the criteria in predicting the turning position of the axial fracture was 89.6% in comparison to the experimental data. Radial and axial fractures can propagate horizontally and longitudinally on the hard roof to fully break it, which provides a new idea and essential theoretical basis for the control of the end hanging roof of coal mining face.