Stability control of narrow coal pillars in the fully-mechanized gob-side entry during sequenced top coal caving mining

In order to enhance the weakened lateral constraint of narrow coal pillar induced by support failing along the gob-entry side,a stability control method by grouting the caved rock and coal mass at the working-face end was proposed.Several boreholes were drilled throughout the coal pillar,and the caving forms of coal and rock mass at the end area of 6302 longwall top coal caving working face in Baodian coal mine were investigated by conducting borehole detecting tests.The collapse process was also observed.Based on these tests,the feasibility of grouting reinforcement method was analyzed.The double-sided confined (before grouting reinforcement) and three-sided confined (after grouting reinforcement) models of coal pillars were established.Based on these models,some experiments were conducted to reveal the recovery effect of the bearing capacity of coal pillar.The results show that ① the narrow coal pillar along the gob side is basically filled by the caved coal and rock mass from the end of the former working face and the top coal above the roadway.Therefore,it is feasible to employ caved coal and rock mass as aggregate for grouting.② The recovery effect of the bearing capacity of coal pillar could be simulated by conducting double-sided confined and three-sided confined tests.During the compression tests,it takes some time for the filling body and pillar to contact each other.Then,the overall bearing capacity of coal pillar gradually increases under the lateral constraint of the fill body.③ After grouting the caved rock mass,the coal pillar changes from the original man-shaped shear failure to the failure mode of expanding,which could enhance the stability of the pillar.④ The residual bearing capacity of coal pillar can be increased by about 30% under ideal conditions.The in-situ test shows that the narrow coal pillar that strengthened by the lateral grouting method is relatively intact and the stability is well controlled.Although some fractures in the coal pillar were developed,the phenomenon of gob spontaneous combustion has not been found due to the backfill effect of the filling body.