Coal Extraction Beneath Buildings: Mining Parameters and Surface Subsidence Control Using Three-Roadway Continuous Mining and Backfilling

With the gradual depletion of high-quality and readily mineable coal resources, recovering coal beneath buildings while controlling surface subsidence is important for sustainable resource development and protection of surface structures. Using the 11205 working face of Taitou Coal Mine as a case study, a three-roadway continuous mining and backfilling scheme combined with compact backfilling by grout injection into bags was proposed, and Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) simulations were performed to evaluate the effects of branch-roadway width and backfilling ratio. At a 97% backfilling ratio, increasing branch-roadway width from 4 to 6 m increased maximum roof subsidence from 40.41 to 65.21 mm, maximum horizontal displacement from 1.220 to 2.174 mm, and the stress concentration factor from 2.62 to 2.96. At a 5 m width, increasing the backfilling ratio from 83% to 89% reduced maximum roof subsidence by 14.4%, whereas increasing it from 90% to 97% reduced subsidence by only 0.35 mm, indicating a pronounced diminishing-return response with markedly reduced sensitivity around 89–90%. Considering both subsidence control and construction efficiency, a branch-roadway width of 5 m and a backfilling ratio of 95% were selected. Field monitoring showed maximum strike and dip subsidence of 16.60 and 17.93 mm within 30 days. These results provide a reference for subsidence-controlled coal recovery beneath buildings under similar conditions.