Fractures Evolution Characteristics of Pressure Relief Boreholes in Coal Mine Working Face under Stress Loading
The hidden danger of coal spontaneous combustion induced by air leakage in pressure relief boreholes of coal mining faces threatens the safety production of mines.Revealing the evolution characteristics of fractures in the pressure relief boreholes of coal mining faces or reserved coal pillars under stress loading is of great significance for effectively guiding the prevention of coal spontaneous combustion.Adopting a research method combining theoretical analysis and experimental testing to study the stress-strain curves of three types of specimens with the holes during a uniaxial stress loading.The failure process of the specimens is analyzed and divided,thereby the distribution pattern of fracture development and the number of fractures around the holes are determined under different stress loading rates.Resulting shows that compared with intact coal specimens,the peak stress intensity of porous coal specimens decreased by an average of 55.49%under various stress loads,and T-shaped fractures appeared around the coal specimens with peak stress exceeding 6 MPa.With the external stress increases,the fractures around the hole expand and extend until the specimen undergoes deformation and failure.Thus,countless of fracture networks appear around the hole,the radial and circumferential displacements of the specimen along the hole during the external stress loading process are both greater than 0.5 mm.This indicates that under the action of load stress,deformation and damage occur in the depressurization boreholes,resulting in a large scale of millimeter scale fracture networks.Without timely fracture sealing measures taken in the loose circle around the boreholes,it is easy to cause long-term air leakage and oxygen supply,leading to the hidden danger of coal spontaneous combustion.
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