Stability Assessment of Underground Structures under Overlying Goaf and Barrier Pillars during Mechanised Depillaring: A Field and Numerical Modelling Study

Abstract

Underground extraction of multiple coal seams presents substantial strata control difficulties, particularly when the lower seam is mined using caving methods beneath weak or laminated interbur-den and panels developed in a staggered configuration. Interactions between adjacent seams can intensify ground pressure and trigger several geotechnical problems, such as roof and rib deterioration, coal bumps, floor heave, and pillar failure. This paper investigates a case of lower-seam ex-traction in an Indian underground coal mine, where pronounced instability of fenders, pillars, and the immediate roof developed near the advancing extraction line during continuous miner-based mechanised depillaring. The No. 5 Seam, being extracted, is located approximately 42 m beneath the previously worked No. 3 Seam, which was mined about two decades ago and is separated from the lower seam by a weak interburden. During depillaring of Panel CM-1 in the No. 5 Seam, considerable ground deterioration occurred beneath the overlying barrier pillars. The observed conditions included side spalling extending up to 1.5 m on either side of the fenders, roof fractures with apertures of 3–30 mm, and premature roof collapses at multiple locations. Strata monitoring further recorded induced vertical stress reaching 26.57 MPa over a fender, indicating the severity of the loading conditions encountered during extraction. To investigate the underlying mechanisms, numerical simulations were integrated with empirical stress assessment and field monitoring data. The results demonstrated that the overlying barrier pillars substantially redistribute and concentrate stresses within the underlying workings, with the effects becoming particularly pronounced during pillar splitting. The numerical model estimated a peak stress of 27.33 MPa, closely matching the field observation and yielding a prediction error of approximately 2.86%. In response to the identified hazards, modifications to the pillar extraction sequence and the implementation of additional artificial roof support were recommended to improve ground stability and facilitate safer coal recovery in subsequent panels. The findings demonstrate that significant interactions between previously extracted upper seams and active lower-seam workings can persist even with a moderately thick interburden, underscoring the importance of site-specific extraction planning and ground support design for bord-and-pillar operations influenced by overlying barrier pillars.