Abstract
After mining the upper-goaf side, large coal pillars and part of hard roof exposed above the pillars remain. The hard roof can significantly deform the roadway by transferring high stress through coal pillars to the roadway. This paper reports the use of hydraulic fracturing technology to cut the hard roof on both sides (i.e. the broken roof slides to the goaf) to relieve the pressure. The position of the roof fracture is the key to controlling the pressure relief. The bearing characteristics of the large coal pillars and hard roof are analyzed to establish a mechanical model of the broken-roof sliding instability after directional fracturing and determine the width of the coal pillars that can collapse under maximum overburden load on coal pillars as a reasonable hydraulic fracturing position. The results show that the distance from the mine gateway to the fracture location increases with increasing hard-roof length, coal pillar depth, coal pillar thickness (mining height), and goaf width. In addition, the distance to the mine gateway decreases with increasing coal strength, support of the coal pillar by the anchor rod, cohesive force, and internal friction angle of the coal–rock interface. Engineering tests were applied in coal roadway 5107 of coal seam 5# of the Baidong Coal Mine of the Datong Coal Mine Group. Given the site conditions, a reasonable fracturing length of 8.8 m was obtained. Next, directional hydraulic fracturing was implemented. The comparison of the roof deformation before and after fracturing suggests that this method reduces the local stress concentration in coal pillars, which allows the surrounding rock deformation in roadway 5107 to be controlled.
Highlights
The mining of close coal seams with a hard roof is widespread in the Datong and Jincheng mining areas in China
The distance between the roof fracture line and the inward staggered gateway wall gradually decreases because the increase in cohesion at the coal–rock interface gradually increases the shear resistance of the coal–rock interface, increasing the clamping effect of the top and bottom plates on the coal seam and enhancing the ability of the coal pillar against the damage caused by the overburden rock layer
Upon increasing the coal strength, the distance between the roof fracture line and inward staggered gateway wall gradually decreases because the increase in the internal friction angle of the coal–rock interface gradually increases the shear resistance of the coal–rock interface, increasing the clamping effect of the top and bottom plates on the coal seam while enhancing the ability of the coal pillar to withstand the damage caused by overburden rock layer
Summary
The distance between the roof fracture line and inward staggered gateway wall gradually decreases because the increase in the length of the hard roof increases the load transmitted from the overburden rock in the goaf, increasing the width by which the fractured roof could crush the coal pillar below it. The distance between the roof fracture line and the inward staggered gateway wall gradually decreases because the increase in the depth of the coal pillar increases the load transmitted from the overburden rock in the goaf, increasing the width of the fractured roof that could crush the coal pillar below it.
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