Mechanism and control of strong ground pressure in shallow buried ultra-large mining height working face with directly overlying double key strata

Shallow-buried working faces with an ultra-large mining height and two directly overlying key strata are prone to intense periodic roof weighting. Taking the 122,104 working face of the Caojiatan Coal Mine as a case study, field monitoring, physical similarity simulation, theoretical analysis, UDEC modeling, and engineering verification were combined to investigate overburden migration, support loading, and roof-control measures. The results show that the lower key stratum forms a long cantilever after initial fracture and a stepped rock-beam structure during periodic fracture, controlling overburden movement within 22 m and inducing minor periodic weighting. Fracture, rotation, and subsidence of the upper key stratum activate a wider overburden range and may promote premature fracture of the lower key stratum, resulting in major periodic weighting. A stage-dependent support-resistance model was established. The calculated support requirements are approximately 11,000 kN during initial mining, 15,000 kN during minor periodic weighting, and 30,000 kN during major periodic weighting, indicating that the rated resistance of the ZY29000/45/100D support is slightly insufficient under the most unfavorable condition. Physical tests further show that hard-roof pre-splitting reduces the initial and periodic fracture intervals of the lower key stratum by 57.98% and 52.97%, respectively, and those of the upper key stratum by 56.60% and 60.78%. The maximum stress-concentration coefficient decreases by 19.86%. Field monitoring confirms that hydraulic-fracturing weakening shortens the weighting duration and improves the stability of ground-pressure behavior.