Experimental and numerical study on mechanical behavior of rock beam fracture under unloading with different thicknesses and spans in deep mining working face

Abstract

The stability of the roof in coal mining is crucial for ensuring safe extraction. Studying the mechanical behavior of rock beams under various conditions is essential for improving coal mining safety. However, research on the dynamic response of rock beams under sudden unloading remains limited. This study utilized a self-developed bidirectional loading and unilateral unloading test system to simulate how sudden lower strata subsidence induces the fracture of upper hard rock beams. Bottom unloading experiments were performed on rock beams with varying thicknesses and spans. The experiments recorded surface crack development and internal damage evolution using high-speed photography and acoustic emission monitoring. The results show that rock beams experience multiple stress reductions after unloading, with the largest reduction occurring in the first stage. Flexural deformation was observed, becoming more pronounced as the thickness-span ratio decreased. Greater thickness increased shear cracks and crack expansion angles, while larger spans promoted tensile cracks, arched crack formation, and notable rock spalling. Acoustic emission analysis showed that signal count and energy increased with thickness and span. Finally, discrete element numerical simulations revealed the critical controlling role of harder rock strata in rock beam failure: when the harder strata are at the top, cracks are sharp, and shear failure is more likely; when they are at the bottom, the overall failure range expands, and cracks tend to form arches. These findings improve the understanding of dynamic rock beam fracture under sudden unloading and offer theoretical guidance for roof stability control in deep mining.

Keywords

roof rock beam / bottom unloading / thickness and span / tensile crack / composite rock beam / harder rock strata

References

[1] RanjithP G, ZhaoJ, JuM-h, et al.. Opportunities and challenges in deep mining: A brief review [J]. Engineering, 2017, 3(4): 546-551

[2] HeM-c, WangQ. Rock dynamics in deep mining [J]. International Journal of Mining Science and Technology, 2023, 33(9): 1065-1082

[3] KangH-p, GaoF-q, XuG, et al.. Mechanical behaviors of coal measures and ground control technologies for China’s deep coal mines—A review [J]. Journal of Rock Mechanics and Geotechnical Engineering, 2023, 15(1): 37-65

[4] YanH, HeF-l, YangT, et al.. The mechanism of bedding separation in roof strata overlying a roadway within a thick coal seam: A case study from the Pingshuo Coalfield, China [J]. Engineering Failure Analysis, 2016, 62: 75-92

[5] TanY-l, MaQ, LiuX-l, et al.. Study on the disaster caused by the linkage failure of the residual coal pillar and rock stratum during multiple coal seam mining: Mechanism of progressive and dynamic failure [J]. International Journal of Coal Science & Technology, 2023, 10145

[6] NingJ-g, WangJ, JiangL-s, et al.. Fracture analysis of double-layer hard and thick roof and the controlling effect on strata behavior: A case study [J]. Engineering Failure Analysis, 2017, 81: 117-134

[7] FanK-f, HeJ-h, LiW-p, et al.. Dynamic evolution and identification of bed separation in overburden during coal mining [J]. Rock Mechanics and Rock Engineering, 2022, 55(7): 4015-4030

[8] JuJ-f, XuJ-L. Surface stepped subsidence related to top-coal caving longwall mining of extremely thick coal seam under shallow cover [J]. International Journal of Rock Mechanics and Mining Sciences, 2015, 78: 27-35

[9] YuB, ZhaoJ, XiaoH-T. Case study on overburden fracturing during longwall top coal caving using microseismic monitoring [J]. Rock Mechanics and Rock Engineering, 2017, 50(2): 507-511

[10] SeedsmanR W. Geotechnical sedimentology: Its use in underground coal mining [J]. International Journal of Coal Geology, 2001, 45(2–3): 147-153

[11] PoulsenB A. Coal pillar load calculation by pressure arch theory and near field extraction ratio [J]. International Journal of Rock Mechanics and Mining Sciences, 2010, 47(7): 1158-1165

[12] ZhuG L, SousaR L, HeM C, et al.. Stability analysis of a non-pillar-mining approach using a combination of discrete fracture network and discrete-element method modeling [J]. Rock Mechanics and Rock Engineering, 2020, 53(1): 269-289

[13] HeJ-h, LiW-p, FanK-f, et al.. A method for predicting the water-flowing fractured zone height based on an improved key stratum theory [J]. International Journal of Mining Science and Technology, 2023, 33(1): 61-71

[14] SunY-j, ZuoJ-p, KarakusM, et al.. Investigation of movement and damage of integral overburden during shallow coal seam mining [J]. International Journal of Rock Mechanics and Mining Sciences, 2019, 117: 63-75

[15] ZhangJ-h, RuiQ, YangY-s, et al.. Roof movement and instability fracture characteristics in shallow-buried thin coal seam conventional mining faces [J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2024, 10127

[16] BertonciniC A, HindersM K. Fuzzy classification of roof fall predictors in microseismic monitoring [J]. Measurement, 2010, 43(10): 1690-1701

[17] JiangL-s, WuQ-s, WuQ-l, et al.. Fracture failure analysis of hard and thick key layer and its dynamic response characteristics [J]. Engineering Failure Analysis, 2019, 98: 118-130

[18] SongC-h, LuC-p, LiuH-q, et al.. Moment tensor and stress field inversions of mining-induced seismicity in a thick-hard roof zone [J]. Rock Mechanics and Rock Engineering, 2024, 57(3): 2267-2287

[19] MondalD, RoyP N S, BeheraP K. Use of correlation fractal dimension signatures for understanding the overlying strata dynamics in longwall coal mines [J]. International Journal of Rock Mechanics and Mining Sciences, 2017, 91: 210-221

[20] DengG-d, XieH-p, GaoM-z, et al.. Fracture mechanisms of competent overburden under high stress conditions: A case study [J]. Rock Mechanics and Rock Engineering, 2023, 56(3): 1759-1777

[21] ChengG, ShiB, ZhuH-h, et al.. A field study on distributed fiber optic deformation monitoring of overlying strata during coal mining [J]. Journal of Civil Structural Health Monitoring, 2015, 5(5): 553-562

[22] LiH-t, QiQ-x, DuW-s, et al.. A criterion of rockburst in coal mines considering the influence of working face mining velocity [J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2022, 8137

[23] MajdiA, HassaniF P, NasiriM Y. Prediction of the height of destressed zone above the mined panel roof in longwall coal mining [J]. International Journal of Coal Geology, 2012, 98: 62-72

[24] JuY, WangY-l, SuC-s, et al.. Numerical analysis of the dynamic evolution of mining-induced stresses and fractures in multilayered rock strata using continuum-based discrete element methods [J]. International Journal of Rock Mechanics and Mining Sciences, 2019, 113: 191-210

[25] EreminM, EsterhuizenG, SmolinI. Numerical simulation of roof cavings in several Kuzbass mines using finite-difference continuum damage mechanics approach [J]. International Journal of Mining Science and Technology, 2020, 30(2): 157-166

[26] ZhouL, LiX-c, PengY, et al.. Material point method with a strain-softening model to simulate roof strata movement induced by progressive longwall mining [J]. International Journal of Rock Mechanics and Mining Sciences, 2023, 170105508

[27] GaoF-q, KangH-p, LouJ-f, et al.. Evolution of local mine stiffness with mining process: Insight from physical and numerical modeling [J]. Rock Mechanics and Rock Engineering, 2019, 52(10): 3947-3958

[28] SunX-m, ZhaoC-w, ZhangY, et al.. Physical model test and numerical simulation on the failure mechanism of the roadway in layered soft rocks [J]. International Journal of Mining Science and Technology, 2021, 31(2): 291-302

[29] MaQ, LiuX-l, QianR-p, et al.. Progressive failure processes and mechanisms of disasters caused by interrelated failure of residual coal pillars and rock strata [J]. Science of the Total Environment, 2024, 954176181

[30] LuJ, JiangC-b, JinZ, et al.. Three-dimensional physical model experiment of mining-induced deformation and failure characteristics of roof and floor in deep underground coal seams [J]. Process Safety and Environmental Protection, 2021, 150: 400-415

[31] JuM-h, WangD-p, ShiJ-c, et al.. Physical and numerical investigations of bedding adhesion strength on stratified rock roof fracture with longwall coal mining [J]. Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2021, 7124

[32] FuZ-l, ZhengY-r, LiuY-X. Rock bending creep and disturbance effects [J]. Journal of Central South University of Technology, 2008, 15(1): 438-442

[33] ZuoJ-p, YuM-l, HuS-y, et al.. Experimental investigation on fracture mode of different thick rock strata [J]. Journal of Mining and Strata Control Engineering, 2019, 1(2): 88-96

[34] LiH-z, JiangZ-b, FanZ-y, et al.. Experimental study on dynamic propagation characteristics of fracturing crack across coal-rock interface [J]. Rock and Soil Mechanics, 2024, 45(3): 737-749(in Chinese)

[35] ZuoJ-p, YuM-l, LiC-y, et al.. Analysis of surface cracking and fracture behavior of a single thick main roof based on similar model experiments in western coal mine, China [J]. Natural Resources Research, 2021, 30(1): 657-680

[36] ZhaoT-b, ZhangP-f, GuoW-y, et al.. Controlling roof with potential rock burst risk through different pre-crack length: Mechanism and effect research [J]. Journal of Central South University, 2022, 29(11): 3706-3719

[37] YangS-l, YueH, LiQ, et al.. Study on failure behaviors of roofs with varying thicknesses in longwall coal mining working face [J]. Rock Mechanics and Rock Engineering, 2024, 57(8): 6259-6282

[38] YangS-l, YueH, LiQ, et al.. A novel numerical modeling method for studying the failure mechanism of the main roof with different thicknesses in longwall coal seam mining [J]. Computational Particle Mechanics, 2024, 11(3): 1487-1502

[39] WangF, XuJ-l, XieJ-L. Effects of arch structure in unconsolidated layers on fracture and failure of overlying strata [J]. International Journal of Rock Mechanics and Mining Sciences, 2019, 114: 141-152

[40] ZhaoY-h, WangS-r, ZouZ-s, et al.. Instability characteristics of the cracked roof rock beam under shallow mining conditions [J]. International Journal of Mining Science and Technology, 2018, 28(3): 437-444

[41] ZhengZ, DengB, LiS-j, et al.. Disturbance mechanical behaviors and anisotropic fracturing mechanisms of rock under novel three-stage true triaxial static-dynamic coupling loading [J]. Rock Mechanics and Rock Engineering, 2024, 57(4): 2445-2468

[42] ZhouX, ZhangD-m, NowamoozH, et al.. Failure and acoustic behavior of combinational flaws sandstone subjected to dynamic compression loads [J]. Theoretical and Applied Fracture Mechanics, 2024, 129104169

[43] LiJ-y, LiuD-q, HeM-c, et al.. Excess energy characteristics of true triaxial multi-faceted rapid unloading rockburst [J]. Journal of Central South University, 2024, 31(5): 1671-1686

[44] ZhangX-p, WongL N Y. Cracking processes in rock-like material containing a single flaw under uniaxial compression: A numerical study based on parallel bonded-particle model approach [J]. Rock Mechanics and Rock Engineering, 2012, 45(5): 711-737

[45] MaQ, LiuX-l, TanY-l, et al.. Experimental study of loading system stiffness effects on mechanical characteristics and kinetic energy calculation of coal specimens [J]. Rock Mechanics and Rock Engineering, 2024, 57(11): 9941-9957

[46] SongJ-f, LuC-p, ZhangX-f, et al.. Application of Bayesian method for mining-induced tremors: A case study of the Xinjulong coal mine in China [J]. International Journal of Rock Mechanics and Mining Sciences, 2024, 174105635

[47] HeM-c, WangQ, WuQ-Y. Innovation and future of mining rock mechanics [J]. Journal of Rock Mechanics and Geotechnical Engineering, 2021, 13(1): 1-21

[48] SunX-m, JiangM, MiaoC-y, et al.. Study on large deformation and failure mechanism of deep buried stratified slate tunnel and control strategy of high constant resistance anchor cable [J]. Engineering Failure Analysis, 2023, 144106953

RIGHTS & PERMISSIONS

Central South University