Multi‑strata hydraulic fracturing via surface vertical wells for rockburst mitigation in an extra‑thick coal seam under a protective layer: a case study

In extra-thick coal seams under a protective layer, the advanced coal mass (ETUPAC) may experience stress recovery and even abnormal concentration after a prolonged mining cessation, leading to rockburst occurrences under strong mining-induced tremors. In this study, a 'combined cantilever + voussoir beam' overburden structure model was established, and a surface vertical well multi‑strata hydraulic fracturing (SVMSHF) method was proposed to regulate the overburden structure for rockburst control. The proposed method was validated through engineering practice at the Haishiwan coal mine. The results show that under the combined effects of the rotation of the 'combined cantilever' and the pressure arch formed by the high-position key strata, the goaf above the ETUPAC was progressively compacted. The vertical stress in the ETUPAC increased from 4.73 MPa to 29.18 MPa, which is 25.94% higher than the initial stress level. After SVMSHF treatment, fractures propagated along the direction of the maximum principal stress (NE154°), with a single-wing fracture length exceeding 100 m and a vertical influence range of approximately 54.8 m. Breakage of both the rock strata and the optical fiber cables were observed at a depth of -438 m. Within the ETUPAC area, the maximum microseismic event frequency density decreased from 130 events/m² to 54 events/m², representing a reduction of 41.5%. The spatial distribution of microseismic events became more uniform, stress concentration was significantly alleviated, and dynamic manifestations in the corresponding roadways were notably weakened. The 'combined cantilever + voussoir beam' overburden structure formed during extra‑thick coal seam mining under a protective layer was established, and its loading mechanism on the advanced coal mass of the working face was elucidated. The rockburst prevention principle of surface vertical well hydraulic fracturing for regulating multi‑strata overburden structures was revealed, clarifying its dual role in static stress release and dynamic load control. Field engineering demonstrated the significant control effect of surface vertical well hydraulic fracturing on the rockburst hazard in the advanced coal mass area of the working face.