Failure characteristics of entry under ultra-close goaf and graded control technology using double I-beam support systems

Abstract Mining within ultra-close coal seams induces significant fracturing of interlayer strata and destabilizes the surrounding rock in underlying entries. Variations in interlayer spacing further complicate entry control, rendering uniform support schemes ineffective for maintaining global entry stability. This study focused on the 3210 headgate of the Dengmaotong Coal Mine in Linfen City, Shanxi Province, China, and integrated field observations, mechanical testing, numerical simulation, and engineering validation to investigate failure mechanisms and develop a graded double I-beam (DIB) support strategy. Field investigations revealed heterogeneous deformation characteristics across different interlayer spacings, with borehole integrity indices of 13%, 41%, and 68% at spacings of 1.5, 2.5, and 3.5 m, respectively. These findings indicate that conventional single I-beam (SIB) supports are insufficient for effective surrounding rock control. Laboratory results demonstrated that DIB supports enhance tensile, compressive, and flexural capacities by 87%, 79%, and 119%, respectively. A three-dimensional numerical model incorporating DIB structures was developed to evaluate surrounding rock control performance under varying support schemes (row spacings of 0.8, 0.9, and 1.0 m) and geological conditions. The simulations revealed average reductions in roof subsidence of 64%, 62%, and 59% for interlayer spacings of 1.5, 2.5, and 3.5 m, respectively. Field implementation of the graded DIB support system further reduced maximum roof subsidence by 75%, 57%, and 44% compared with conventional SIB support across the corresponding spacing conditions. The proposed spacing-adaptive design methodology provides critical guidance for entry stability in ultra-close seam mining and offers practical significance for similar underground engineering environments.