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Volume 118
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Top-coal flow behavior and caving process optimization in extra-thick coal seam LTCC based on 3D discrete element method
Xiangyi Zhu a b *, Haojie Song a b, Hongwei Li a
a School of Hydraulic and Ocean Engineering, Changsha University of Science & Technology, Changsha, 410114, China
b Key Laboratory of Water-Sediment Sciences and Water Disaster Prevention of Hunan Province, Changsha, 410114, China
10.1016/j.partic.2026.08.029
Volume 118, November 2026, Pages 246-259
Received 8 July 2026, Revised 24 August 2026, Accepted 29 August 2026, Available online 31 August 2026, Version of Record 17 September 2026.
E-mail: zhuxiangyi1990@163.com
Highlights

• A 3D discrete element model for top-coal drawing is developed and validated against field data from an extra-thick coal seam.

• Pressure arch height exhibits a logarithmic growth with cumulative draw volume, revealing dynamic load transfer mechanism.

• Velocity attenuation coefficient is identified as a key indicator linking caving step distance to recovery efficiency.

• Interval drawing outperforms sequential drawing in improving top-coal recovery by promoting more favorable flow conditions.

• Whole-layer caving achieves a significantly higher stable recovery ratio than slice caving, offering greater efficiency.


Abstract

To investigate top-coal flow behavior and optimize caving parameters in extra-thick coal seam longwall top coal caving (LTCC), this study establishes a three-dimensional discrete element model (PFC3D) based on the Yima Coal Mine conditions and validates it against field data. The flow behaviors under slice caving and whole-layer caving are compared, and the effects of caving step distance and drawing sequence on the recovery ratio are analyzed. Results show that the pressure arch within the coal mass undergoes a cyclic “formation–failure–upward migration” evolution during top-coal drawing, and the pressure arch height exhibits a logarithmic growth with the cumulative drawing volume. The flow velocity field of coal-gangue particles presents an ellipsoidal distribution with an exponential decay along the vertical direction. Interval drawing increases the total drawn volume by 11.0% compared with sequential drawing. The recovery ratio decreases with increasing step distance, stabilizing at approximately 75% for one-cut-one-draw, 65% for two-cuts-one-draw, and below 60% for three-cuts-one-draw. Whole-layer caving achieves a stable recovery ratio of 80%, significantly higher than the 60% from slice caving. Considering both top-coal recovery efficiency and mining productivity, the whole-layer caving with one-cut-one-draw and interval drawing is recommended for similar geological conditions. The findings provide theoretical guidance for improving extraction efficiency and reducing residual coal in LTCC operations.

Graphical abstract
Keywords
Extra-thick coal seam; Longwall top coal caving; 3D discrete element method; Top-coal flow behavior; Caving process optimization
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