Gas Migration Mechanism and Source Identification in Roadways Under Water-Induced Floor Mudstone Damage in Deep Coal Mines

This study investigates abnormal floor gas emissions at the 151302 driving face of Yuecheng Coal Mine. Numerical simulation and carbon–hydrogen isotope source apportionment were first used to characterize floor damage-controlled gas migration and quantify methane contributions from different coal seams. An extended three-source gas emission method and field interception boreholes were subsequently used for flow-balance and engineering validation. The results demonstrate that roadway excavation induces significant stress redistribution in the surrounding rock, with compressive stress concentration zones developing along the roadway sides and tensile stress concentration zones forming in the roof and floor. As the water-saturated zone of the floor mudstone progressively extends into deeper strata, plastic damage gradually propagates from the immediate floor of the No. 15 coal seam toward the roof mudstone of the No. 15 lower and the No. 16 coal seams, and the maximum floor heave displacement reaches 0.84 m at 1440 h. The highly damaged zones within the floor develop a coupled gas migration pathway characterized by “deep connection–central upward migration–release through floor corners and the roadway sides”. The tensile-dominated high-damage zone in the central floor primarily governs the vertical migration of gas, whereas the compressive–shear high-damage zones at both floor corners and the roadway sides control lateral gas release. The numerical results reveal a three-peak preferential gas emission pattern, with gas preferentially released through the central floor region and the two floor corner–sidewall zones. Quantitative source apportionment based on hydrogen and carbon isotopes indicates that the average methane contributions from the No. 15, No. 15 lower, and the No. 16 coal seams are 69.70%, 12.14%, and 18.16%, respectively. The three-source prediction method estimates the floor-associated additional gas emissions in the belt intake roadway and auxiliary intake roadway as 0.87 m3/min and 0.85 m3/min, accounting for 30.3% and 31.1% of the measured total gas emissions, respectively. These results are generally consistent in magnitude with the contributions of the underlying adjacent seams identified by isotopic source apportionment. After the construction of floor interception boreholes and the resumption of roadway excavation, the maximum absolute gas emission rates in the two roadways decreased from 2.87 and 2.73 m3/min to 2.10 and 1.96 m3/min, respectively. Meanwhile, the maximum methane concentrations in the return airflows decreased from 0.41% and 0.39% to 0.30% and 0.27%, respectively. The results demonstrate that water-induced swelling and damage propagation of floor mudstone can provide preferential pathways for gas from underlying adjacent seams to contribute to roadway gas emissions. The consistency among source characteristics, gas flow balance, and field-scale mitigation responses provides a basis for gas source identification and targeted floor gas control in roadway excavation under similar geological and mining conditions.