Mechanical mechanism of pore fluid on coal dynamic disasters under cyclic loading

To investigate the mechanical role of the pore fluid played in the gestation stage of coal dynamic disaster,a series of drained triaxial cyclic loading tests under different pore pressures (1 MPa,3 MPa and 5 MPa) and stress levels (50% and 80% of the triaxle axial strength of saturated coal) were carried out on saturated coal specimens.When the maximum cyclic loading stress is at the 50% of the triaxle axial strength of the coal,the axial and radial stress-strain curves of coal have no obvious change under 1 MPa and 3 MPa pore pressure,indicating that there are no substantial cracks produced in coal.When the pore pressure increases to 5 MPa,the axial and radial strains of coal change significantly with the proceeding of cyclic loading.Therefore,the difference of stress-strain curves above is attributed to the pore fluid.It is also found that the residual radial strain gradually decreases during the cyclic loading.This is caused by the pore and crack compaction in coal.As a result,the pore fluid is discharged during the cyclic loading.When the maximum cyclic loading pressure increases to 80% of the triaxial strength of the coal,the coal specimen fail,and this coal failure rate is positively correlated with the pore pressure.When the pore pressure increases to 5 MPa,the coal is severely damaged with large going-through cracks.Also,prominent small coal debris and powder are produced among the macro cracks.Microstructure analysis shows that the porosity of coal increases significantly after cyclic loading,and the greater the pore pressure,the greater the increase in porosity.The above results show that the coal far away from the working face undergoes low amplitude cyclic loading,and causes the coal shrinkage occurs along the horizontal direction due to the discharge of pore fluid.On one hand,such coal shrinkage reduces the permeability of the coal seam regionally.On the other hand,it reduces the horizontal stress,thereby reducing the triaxial compressive strength of the coal,favorable for coal weakening.Meanwhile,the discharged pore fluid migrates and accumulates regionally in the coal seam.Once the coal of the fluid accumulation zone undergoes a high-level stress in the later mining stage,high pore pressure would be easily developed,which in turn changes the stress state of coal and speeds up the gestation of dynamic disasters.