Test and application of hydraulic expansion bolts in a roadway under goaf with ultra-close separation

The roof of a roadway under goaf with ultra-close separation consists of thin rock strata and rocks caving in upper goaf. Influenced by the mining of the upper coal seam, the roof is loose and broken, and its integrity is poor. Resin anchored bolts cannot provide an effective anchoring force in such roof conditions. By conducting free expansion tests and field pull-out tests on a hydraulic expansion bolt, this study has analyzed the influencing factors and laws of radial expansion and anchoring force changes in the rod body. This has revealed the anchoring mechanism of such bolts, and has obtained reasonable water injection pressures and suitable drilling diameters (which are 20–25 MPa and 32–35 mm respectively) for the hydraulic expansion bolt (φ28 mm) used in these tests. Based on pull-out tests at different interlayer spacing, the applicability of hydraulic expansion bolts had been verified for controlling the roof of roadways under goaf with ultra-close distance. Combined with the deformation and failure characteristics of the test roadway roof, this paper proposes a united roof-control technology based on the use of hydraulic expansion bolts and advancing intubation for the roof. Engineering practice indicated that the roof of the test roadway did not generate leaking and caving phenomenon, and the amount of roof deformation was controlled to within 150 mm. Maintenance of the roadway roof has been improved significantly, which ensures safe mining in coal seams with ultra-close separation.