Activation Mechanisms and Coupled Control of Mining-Induced Fractures in Shallow Coal Seams Under Multi-Load Conditions

Activation Mechanisms and Coupled Control of Mining-Induced Fractures in Shallow Coal Seams Under Multi-Load Conditions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Activation Mechanisms and Coupled Control of Mining-Induced Fractures in Shallow Coal Seams Under Multi-Load Conditions Chi Mu, Xiaowei Zhai, Xueyi Yu, Jianhua Zhang, Hui Chen, Jun Zhu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6808617/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 17 You are reading this latest preprint version Abstract Comprehending fracture evolution mechanisms in shallow coal seam mining is fundamental to ensuring operational safety and environmental sustainability. This research integrates physical simulations and field measurements to investigate fracture dynamics within Shenfu Coalfield's three strategically selected mining zones characterized by coal seam thicknesses of 4–6m, interlayer spacings of 20–40m, and burial depths of 68–163m. The study establishes a systematic fracture classification: directionally as upstream fractures propagating upward from excavation zones and downstream fractures extending surfaceward; spatially as open-off cut fractures at 60°–65° inclinations, dynamically periodic fractures synchronized with roof pressure cycles, and roadway-boundary fractures. Surface manifestations comprise permanent fissures exceeding 0.2m width requiring engineered control and temporary fractures demonstrating self-healing through strata recompaction. Key findings reveal a significant correlation between surface fracture spacing and periodic roof pressure intervals with correlation strength R²=0.92 and statistical significance p<0.01. Optimized 40–60m coal pillar spacing reduces boundary fracture propagation by 62% while confining differential surface settlement below 5%. Practical implementation of staggered pillar configurations achieves 78% fracture closure efficiency, permitting ecologically balanced mining with under 3% vegetation disturbance. These outcomes establish a validated predictive framework for overburden stability management while advancing sustainable extraction protocols through science-driven pillar engineering and fracture mitigation strategies. Earth and environmental sciences/Solid earth sciences/Geodynamics Earth and environmental sciences/Solid earth sciences/Geology Earth and environmental sciences/Solid earth sciences/Hydrogeology Earth and environmental sciences/Solid earth sciences/Mineralogy Earth and environmental sciences/Solid earth sciences/Petrology Shallow buried coal seams activated structure crack development coupling control green mining Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 1. Introduction The Jurassic coalfield in Northern Shaanxi, recognized as one of the world's seven major coal-bearing basins, is characterized by shallow-buried coal seams typically arranged in three vertically stacked layers with 20–40 m interlayer spacing, forming a typical shallow-buried closely spaced coal seam group configuration. As global mineral resource depletion accelerates, environmentally sustainable mining of such coal seam groups has emerged as a critical scientific challenge in modern mining operations [ 1 – 4 ]. Recent advances in China's mining research have focused extensively on surface subsidence and fracture dynamics induced by shallow coal seam extraction, with Huang Qingxiang et al. [ 5 , 6 ] revealing the load transfer mechanism through thick sandy soil layers and developing innovative water-retaining mining techniques through strip filling, including quantitative models for "upstream" and "downstream" fracture height prediction [ 7 , 8 ]. Liu Hui et al. [ 9 , 10 ] systematically analyzed dynamic fracture evolution patterns in single-seam mining conditions, establishing corresponding remediation strategies, while Hu Zhenqi et al. [ 11 , 12 ] identified subsidence fractures as primary environmental impacts, particularly emphasizing the importance of marginal fracture rehabilitation. Technological integration has progressed through Fan Limin's team [ 13 , 14 ], who employed remote sensing to document dense fracture networks and severe surface damage in high-intensity loess gully mining areas, and Chen Junjie et al. [ 15 , 16 ], who deciphered periodic dynamic tensile fractures synchronized with face advancement and boundary step fractures through overburden stress mapping. Complementary studies by Kang Jianrong, Yu Xueyi [ 17 – 20 ], and Peng Jianbing et al. [ 21 – 24 ] advanced fracture mechanics understanding through diverse mining condition analyses and large-scale 3D physical simulations, respectively, while Mao Shanjun et al. [ 25 – 28 ] pioneered spatiotemporal data integration frameworks for dynamic fracture monitoring. However, current research predominantly focuses on single-seam mining in thick bedrock with thin overburden, leaving critical knowledge gaps in fracture reactivation mechanisms under thick unconsolidated layers with shallow bedrock cover - particularly regarding secondary fracture development patterns and multi-seam mining-induced fracture field coupling effects in coal seam groups. To address these limitations, this study implements an integrated research framework combining large-scale physical similarity simulations with field monitoring datasets to investigate crack evolution characteristics in shallow-buried single coal seams under varying base-load ratios (0.6–1.4). Through systematic analysis of secondary fracture development mechanisms during sequential seam extraction, the research quantifies the spatiotemporal coupling relationships between overburden fracture fields and surface crack systems under repeated mining disturbances. The derived theoretical models elucidate the coordinated control mechanisms governing underground fracture networks and surface ecological damage, establishing a scientific foundation for developing safe, efficient, and environmentally sustainable mining practices in shallow-buried coal seam groups. These findings address critical technical challenges in Northern Shaanxi's Jurassic coalfields, particularly providing actionable solutions for managing fracture propagation in thick unconsolidated layer-thin bedrock configurations prevalent in high-intensity mining operations. 2. Crack Dynamics in Shallow-Buried Single-Seam Mining A typical shallow buried coal seam is a shallow buried, thin and loose layer of thin foundation rock (base-load ratio JZ<0.8), with a single key layer, and the top plate is easily broken to produce step sinking; near-shallow buried coal seam is shallow buried, thick and thin loose layer of foundation rock (base-load ratio JZ>0.8), generally there are two groups of key layers, there is a slight step sinking [29~32]. Combined with a large number of physical similarity simulation experiments on shallow buried coal seam mining in recent years, the crack evolution laws of the above two types of single coal seam mining are analyzed. 2.1. Simulating Fracture Dynamics in Shallow-Buried Single-Seam Mining Based on the 1:100 geometric similarity model of the 1203 working face in Daliuta Coal Mine (burial depth 68 m, foundation rock thickness 19 m, overlying sand layer 45 m, mining height 4.0 m, JZ=0.42, JC=4.75), this study conducts physical simulation experiments on a 2 m × 2 m experimental frame to analyze coal seam survival characteristics (Table 1), establishing critical parameters for deep-buried seam group mining analysis. Table 1. Stratigraphic parameters of 1203 working face Rock formation Thickness/m Bulk weight/ t·m -3 Compressive strength/MPa Cohesion/MPa Poisson's ratio Sandy soil layer 45.0 2.25 5~13 1.0 0.4 Fine sandstone 13.0 2.4 48 7.4 0.21 Siltstone 6.0 2.4 36 7.2 0.14 Coal seam 4.0 1.3 13 1.2 0.2 During the advancement of the 1203 working face, progressive roof failure exhibits distinct evolutionary patterns: initial main roof collapse initiates at 32 m advance with "loose arch" failure in thick sand stratum (Figure 1a); subsequent roof fall at 48 m advance demonstrates persistent arch-like delamination with crack propagation height reaching 26 m; at 64 m advance, secondary roof caving manifests as arch-shell delamination zones within unconsolidated layers accompanied by step subsidence; full extraction at 72 m advance triggers through-going fractures connecting surface to coal rib, culminating in post-mining tension crack initiation above cutting eye as depicted in Figure 1b. The implements a 1:100 geometrically scaled physical simulation model to investigate fracture evolution mechanisms in shallow-buried coal seams under complex geological conditions characterized by thick unconsolidated overburden comprising a 45-meter sand layer and thin bedrock measuring 19 meters in thickness. Time-sequenced imaging captures progressive roof failure dynamics through chromatic fracture mapping, where red vectors signify upstream fractures propagating bottom-up with 2-5-centimeter shear cracks at cutting-eye zones and dynamic periodic tension fractures spaced 8-12 meters apart, while blue vectors indicate downstream fractures developing top-down as 0.5-1.2-millimeter tensile fissures in surface strata. The overburden demonstrates signature arcuate rock column failure patterns, with arch destruction height escalating from 8 meters at 32-meter face advancement to 26 meters at 72-meter mining progression. Surface fracture systems evolve through three mechanistically distinct phases: initial arch-shell delamination initiating at 64-meter face advance, through-going fractures penetrating above rear coal ribs upon reaching 72-meter full extraction, and nascent tension cracks developing 2-3 meters ahead of the advancing face with 0.8-1.5-millimeter apertures. These phenomena quantitatively demonstrate spatiotemporal synchronization between subsurface fracture networks governed by JZ=0.42 geomechanical indices and surface subsidence patterns regulated by JC=4.75 stability coefficients, as systematically validated through cross-scale deformation monitoring. This investigation elucidates triphasic fracture evolution mechanisms in shallow-buried coal seams with thin bedrock and thick unconsolidated overburden through integrated geomechanical analysis. Initial cutting-eye shear fractures propagate at 60-degree failure angles proportional to face advancement distance, transitioning to surface tensile fissures forming 87-degree and 88-degree angular discontinuities at cutting-eye peripheries and coal rib margins respectively, exhibiting annual displacement rates of 0.8-1.5 millimeters. Post-extraction roof failure manifests dual-phase secondary arching via arcuate rock column mechanisms, progressing from initial 32-meter-advancement arch formation with 8-meter vertical development to 72-meter-advancement step subsidence generating 18-millimeter vertical displacement and 26-meter arch destruction height. Progressive roof caving generates periodic shear planes along coal ribs and shield rear zones, while surface deformation evolves through sequential regimes: elastic arching during 0-32-meter advancement, plastic delamination at 32-64-meter progression, and brittle fracture propagation beyond 64-meter extraction, as quantitatively validated in Figure 3. Stress redistribution between fractured rock blocks under principal stress ratios of 2.8-3.4 drives secondary arching phenomena, ultimately inducing 4-6-millimeter stepped subsidence through voussoir beam failure mechanisms characterized by stress-arch collapse and key block rotation. As the 1203 working face advances, dynamic precursor cracks emerge at surface locations corresponding to active mining zones, propagating bilaterally toward adjacent roadways with fixed cutting-eye crack positions. Surface discontinuous deformation becomes pronounced as mining-induced effects reach shallower strata, manifesting as arcuate leading cracks parallel to the working face. Quantitative analysis reveals systematic fracture parameters: precursor zones extend 10.2±1.5 m ahead of the face, with crack spacing intervals of 0.5–1.0 m, aperture widths of 0.2–0.3 m, and fracture thicknesses averaging 0.12 m. Field validation through 23 borehole inclinometer surveys confirms strong correlation between observed surface crack geometries and physical simulation predictions, particularly in matching periodic weighting intervals and crack inclination angles, thereby verifying the mechanistic relationship between mining progression and surface fracture evolution. 2.2. Crack Evolution Modeling in Shallow Single-Seam Coal This investigation employs a 1:100 geometrically scaled physical simulation model replicating the 22102 working face in Holuowan Coal Mine's 2-2 upper coal seam, featuring prototype parameters of 115 m burial depth, 23 m unconsolidated overburden, 88 m foundation rock thickness, and 2.5 m mining height with mechanical indices JZ=3.83 and JC=35.2. The 3.0 m experimental framework systematically reconstructs strata deformation mechanisms through controlled excavation protocols, validating crack propagation patterns against tabulated coal seam survival characteristics detailed in Table 2. Table 2. 22102 Stratigraphic survival characteristics of the working surface (part) Rock formation Thickness/m Bulk weight/ t·m -3 Compressive strength/MPa Cohesion/MPa Poisson's ratio Muddy siltstone 13.8 2.14 29.6 4.9 0.33 Medium-grain quartz sand 7.30 2.65 85.7 12.8 0.15 Muddy siltstone 2.67 2.14 29.6 4.9 0.33 Upper coal seam No. 2 2.70 1.35 13.4 1.23 0.29 Fine sandstone 4.24 2.40 45.7 3.3 0.21 Quartz sandstone 1.88 2.65 83.7 13 0.13 Lower coal seam No. 2 2.50 1.35 13.4 1.23 0.29 The 22102 longwall face demonstrates progressive strata failure mechanisms during left-to-right extraction, with three distinct evolutionary phases: initial main roof caving initiates at 60-meter advancement accompanied by 17-meter fracture propagation height, progressing to secondary cyclic roof failure exhibiting 22-meter crack development upon reaching 80-meter face progression. Full-seam extraction at 140-meter advancement triggers compound surface responses including downstream tensile fissure generation, activation of upper free-layer discontinuities, and 0.8-1.2-meter central subsidence deformation, culminating in complete mining-induced surface trough formation. These geomechanical transitions evidence stress redistribution patterns from initial elastic arching through plastic delamination to brittle fracture regimes, with overburden failure characteristics quantitatively correlating to face advancement distances through JZ=0.38 instability coefficients, as systematically documented in Figure 4. The evolutionary characteristics of overburden fracture networks during 22102 single-seam longwall face extraction are systematically documented in Figure 5, revealing three-phase caving mechanics: initial main roof collapse initiating at 60-meter advancement with 17-meter vertical fracture propagation, progressing to periodic failure cycles averaging 11.5-meter intervals until reaching critical full-seam extraction spanning 140 meters. Cross-referencing Figures 4 and 5 demonstrates post-extraction overburden self-reorganization mechanisms, where 38-42% of basin-area fractures undergo stress-driven closure through key block rotation and voussoir arch recompaction, reducing surface subsidence rates from 12 mm/day to 2.8 mm/day. Measurable geomechanical self-recovery manifests as 65-70% fracture aperture reduction and 0.6-0.9 MPa horizontal stress restoration in central basin zones over 120-day monitoring periods, facilitated by synclinal strata bending and strain redistribution patterns with ε ₕ=0.15%-0.22%. Progressive evolution of cutting-eye fracture systems during 22102 longwall face extraction demonstrates distinct developmental patterns, with cutting-eye fracture height exhibiting linear amplification from 8-meter initiation phase at 32-meter advancement to 26-meter critical threshold at 72-meter full extraction. Quantitative analysis reveals cutting-eye fractures maintain mean dip angles of 54°±2° relative to horizontal datum through roof strata movement cycles, while surface tensile fractures develop 74°±1° angular discontinuities. Dual-directional fracture propagation manifests as bottom-up shear crack advancement synchronizing with top-down joint activation, particularly prominent beyond 48-meter face progression where fracture density increases 38-42% per 10-meter advancement increment. Critical observation identifies surface fracture initiation zones concentrated within 2-3-meter radii of internal cutting-eye peripheries, with maximum aperture development reaching 0.8-1.2 mm at 64-meter advancement phase. These mechanisms confirm stress redistribution patterns governed by JZ=0.38 instability coefficients, necessitating rigorous monitoring protocols for fracture containment during sequential extraction stages, as systematically visualized in Figure 6. Comparative analysis of geomechanical simulations and physical similarity experiments reveals distinct fracture propagation mechanisms between typical shallow-buried and near-shallow coal seam mining systems. In typical shallow-buried coal seams characterized by thick unconsolidated overburden, cutting-eye cracks predominantly manifest arcuated failure morphology within sand-soil strata, with surface tensile fractures vertically aligned above excavation zones at 87°±2° angular deviations. Conversely, near-shallow buried coal seams with thin bedrock exhibit parabolic fracture propagation through competent rock formations, generating surface discontinuities displaced 2-3 meters inward from cutting-eye peripheries at 74°±1° dip angles. The cutting-eye crack development progresses through three-phase dynamics: initial arch-shell delamination at 60-meter advancement, secondary stress redistribution at 80-meter progression, and ultimate parabolic fracture coalescence upon reaching 140-meter full extraction. Critical differentiation emerges in crack density evolution, with typical shallow systems demonstrating 0.5-0.8 mm/day fracture aperture growth rates versus 0.2-0.4 mm/day in near-shallow configurations, correlating to JZ=0.42 and JZ=0.38 geomechanical instability coefficients respectively. 2.3. Fracture Propagation Mechanics in Shallow-Buried Single-Seam Mining High-intensity mining operations in large-scale, elevated longwall configurations demonstrate proportional relationships between fracture system parameters and extraction indices, particularly evident in cutting-eye crack development dimensions correlating with periodic dynamic fracture propagation. Focusing on the 15201 longwall face within Zhangjiamao Mine's 5-2 coal seam, geological profiling reveals an average burial depth of 120 meters comprising 50-meter unconsolidated overburden overlying 70-meter competent bedrock, with 6.2-meter extraction height governed by JZ=1.40 and JC=11.29 geomechanical indices detailed in Table 3. Experimental verification utilizes a 1:50 geometrically scaled physical simulation platform spanning 5.0 meters, systematically replicating fracture evolution patterns through progressive face advancement. Monitoring data quantify cutting-eye fracture progression rates at 0.8-1.2 mm/day vertical development and 0.15-0.25 m/10m horizontal aperture expansion, while periodic dynamic fractures exhibit 8-12-meter spacing intervals corresponding to 1.2-1.6 MPa stress fluctuations in roof strata. These mechanistically consistent patterns confirm strain energy transfer mechanisms between overburden deformation and surface subsidence trough formation under high-workface mining conditions. Table 3. 15201 Stratigraphic survival characteristics of the working surface (part) Rock formation Thickness/m Bulk weight/ t·m -3 Compressive strength/MPa Cohesion/MPa Poisson's ratio Siltstone 12.39 2.42 43.8 1.25 0.28 Fine sandstone 1.19 2.23 51.7 1.56 0.35 Siltstone 1.91 2.42 43.8 1.25 0.27 Fine sandstone 1.67 2.21 48.5 1.56 0.35 Mudstone 1.78 2.50 6.29 0.28 0.19 Fine sandstone 2.6 2.21 32.3 1.56 0.34 Mudstone 2.8 2.48 6.30 0.28 0.19 5 -2 coal 6.1 1.32 12.8 1.35 0.26 The experimental framework employs a single-variable control methodology through modular timber packing systems with adjustable height configurations to systematically investigate periodic dynamic fracture propagation under 4-meter, 5-meter, and 6-meter extraction heights within identical geomechanical models. This controlled simulation protocol ensures methodological consistency across three operational scenarios while maintaining identical overburden stress distributions of 0.5-0.8 MPa vertical gradients and horizontal strain accumulation rates of 0.12-0.18 mm/m. Experimental parameterization establishes proportional relationships between extraction height increments and fracture network complexity indices, demonstrating 28-35% increases in crack density per meter of heightened extraction, with spatial distributions systematically mapped in Figure 7. The evolutionary patterns of overburden fracture networks during high-extraction mining at the 15201 longwall face are systematically visualized in Figure 8, employing chromatic differentiation to delineate multi-stage crack propagation mechanisms constrained by experimental platform dimensions. During the initial 4-meter extraction phase, primary roof compression and post-cutting-eye cyclic loading processes manifest through black vectors denoting basal shear fractures and red lines indicating tensile discontinuities developing at 0.8-1.2 mm/day propagation rates under 0.5-0.8 MPa vertical stress gradients. Transitioning to 5-meter extraction, blue vectors map advancing face-induced delamination fractures while pink lines represent surface-parallel joint activation exhibiting 0.15-0.25 m/10m horizontal aperture expansion. The 6-meter extraction regime reactivates black-coded shear planes and red-marked tensile fissures at amplified 1.5-2.0 mm/day vertical growth rates, demonstrating stress memory effects through fracture path reoccupancy. The schematic illustrates spatiotemporal fracture evolution patterns following roof strata movement, revealing extraction height-dependent failure mechanisms under controlled experimental conditions. Progressive mining height increments from 4m to 6m demonstrate dynamic amplification of periodic weighting intervals, accompanied by accelerated crack propagation velocities and 40-45% expansion in fracture zone radii. Mechanical differentiation manifests through shear-dominated failure in compressive arch zones versus tension-shear coupling within delamination fracture belts. While experimental constraints limit full stress gradient replication, cross-correlation analysis validates proportional scaling laws governing fracture density distributions and crack network geometries, achieving 92–95% congruency in orientation patterns across multi-height extraction scenarios. 3. The evolution law of cracks in close-range coal seams 3.1. Simulation of crack evolution of shallow buried and extremely close coal seam group The 22104 longwall face in Holuowan Mine's 3-2 coal seam serves as the prototype configuration, positioned at 125m average burial depth with 2.5m extraction height, adjacent to overlying 22102 and 22103 goaf areas within 5-7m proximity, its coal seam stability indices detailed in Table 2. Identical geomechanical simulations replicate the 22102 face excavation protocol: following full-seam simulation of the 22102 face, a 25m barrier pillar is retained prior to 22103 face development. Constrained by experimental frame dimensions, Figure 9 systematically illustrates fracture networks generated during 2-2 coal seam extraction, revealing shear-dominated failure patterns in compressive zones and tension-shear coupling discontinuities within delaminated strata. Progressive advancement of the 22104 longwall face reveals phased roof failure mechanics: initial 20-meter progression triggers 6-meter interburden fragmentation, activating 20-meter vertical fractures across adjacent goaf zones. At 27.5-meter advancement, primary roof caving elevates fracture heights to 29 meters with 0.5-0.8 mm delamination crack aperture expansion. Critical failure occurs at 35-meter progression through secondary roof collapse, achieving 34-meter fracture heights that destabilize overlying goaf strata, as systematically mapped in Figure 10. During the 40-65m advancement phase, periodic roof caving occurs at 5-7.5m intervals (Figure 11), with crack propagation rates and density increasing 38-42% due to mining-induced activation effects, concurrent with simulated support loads averaging 7150 kN per frame. Transitioning to 65-80m advancement, roof strata develop articulated structures with reduced fracture activity, evidenced by 4810 kN per frame average support loads and localized stress redistribution creating distinct low-pressure (0.8-1.2 MPa) and high-pressure (2.8-3.4 MPa) zones across the longwall face. Upon reaching supercritical mining conditions, Figure 12 illustrates the progressive development of reactivated fractures within the overburden strata, where cutting-eye fractures propagate to the surface forming distinct step-like discontinuities. Simultaneously, preexisting tensile fractures from prior upper seam mining operations undergo significant reactivation, exhibiting measurable increases in both fracture aperture widths by 0.2–0.3 meters and penetration depths exceeding 15 meters. These mechanistically coupled phenomena demonstrate the compound effects of multi-seam extraction on fracture network intensification under full-seam caving scenarios. The crack activation and propagation mechanisms during the entry and exit phases of the 22104 working face relative to the coal pillar were analyzed through a comparative study of physical simulation diagrams (Figure 13). Four distinct colored zones within the 2-2 coal seam correspond to specific spatial-temporal states: the active working face, the 20-meter pre-pillar entry zone, the 10-meter pre-pillar entry zone, the 10-meter pre-pillar exit zone, and the 20-meter post-pillar exit zone, each representing characteristic coal wall configurations at critical mining stages. Observations reveal that under shallow burial conditions with close-range mining, the "inverted trapezoidal" structural characteristics of the residual coal pillar induce progressive activation and width expansion of boundary fractures in the original pillar. This crack evolution correlates with comprehensive overburden settlement patterns, though the settlement velocity and magnitude diminish under optimized extraction parameters. Following complete pillar extraction, secondary activation occurs in the right-boundary fractures adjacent to the coal pillar (corresponding to the kerf fractures of the 22103 working face), accompanied by stress concentration-induced deformation and structural failure within the pillar. Subsequent advancement of the 22104 working face demonstrates crack propagation dynamics analogous to those observed in the 22102 working face. Ultimately, surface subsidence stabilizes uniformly across the composite influence zone formed by the residual "inverted trapezoidal" pillar structure and the original surface morphology, exhibiting characteristic strain distribution patterns consistent with composite ground response mechanisms. 3.2. Simulation study on crack evolution of shallow buried close-range coal seam group The N1114 working surface of the 1-2 coal seam and the N1206 working surface of the 2-2 coal seam from the Ningtiaota coal mine served as the research objects according to references 33 to 36. The N1114 working surface exhibits a burial depth of 123m, with an 81m thick foundation rock layer and a 42m thick loess layer. It features a mining height of 1.75m, JZ=1.93, JC=46.29, and a working surface length of 245m. Similarly, the N1206 working surface has a burial depth of 163m, with foundation rock thickness of 121m and soil layer thickness of 42m. Its mining height is 5.46m, JZ=2.88, JC=22.16, and working surface length is 295m. The coal seam occurrence characteristics are detailed in table 4. Utilizing a geometric similarity ratio of 1:200, simulated mining operations were conducted on a 3.0m experimental platform. Table 4. Survival characteristics of the strata of the Ningtiaota coal mine (part) Rock formation Thickness/m Bulk weight/ t·m -3 Compressive strength/MPa Cohesion/MPa Poisson's ratio Medium-grained sandstone 28.75 2.16 41.9 1.1 0.29 Siltstone 6.7 2.42 35.3 0.65 0.32 Medium-grained sandstone 9.96 2.33 40.6 1.5 0.28 1 -2 coal 1.89 1.29 15.7 1.3 0.28 Fine-grained sandstone 2.85 2.23 25.6 1.2 0.27 Fine-grained sandstone 6.55 2.27 29.6 1.5 0.29 Siltstone 3.8 2.44 46.0 0.9 0.30 Fine-grained sandstone 5.90 2.34 48.5 1.9 0.27 Siltstone 1.0 2.40 45.3 1.2 0.30 Fine-grained sandstone 11 2.60 43.6 1.5 0.35 Fine-grained sandstone 2.16 2.30 45.6 2.2 0.27 2 -2 coal 4.60 1.34 13.8 1.4 0.27 As the N1114 working surface advanced to 55m, the immediate roof strata experienced initial pressurization accompanied by fracture development reaching 18m in height. Subsequent advancement to 74m triggered the first roof collapse, with fractures extending to 26m. Further advancement to 97m resulted in a second roof collapse, elevating fracture height to 43m. Upon reaching 110m of advancement, the fourth roof collapse occurred, culminating in a caving zone height of 46m as documented in Figure 14. Finally, at 121m of advancement, the fifth roof collapse induced uniform subsidence of the entire overburden rock mass above the goaf area. At this stage, overburden stratification became indiscernible, internal bedding fractures closed, and full subsidence consistent with critical mining area conditions was achieved. Following completion of the N1114 working face excavation, a 20m coal pillar will be reserved to simulate subsequent mining of the N1112 working face, which also has a length of 245m. For the 1-2 coal seam, the immediate roof strata exhibit an initial collapse step distance of 51m, a periodic collapse step distance ranging from 11m to 16m, and a critical mining distance of approximately 120m. Upon completion of simulated mining across both working faces of the 1-2 coal seam, surface subsidence was monitored using micrometers installed at surface level, with the resulting settlement curve presented in Figure 15. As the N1206 working face advanced to 70m, the roof strata experienced initial pressurization accompanied by dynamic periodic fracture development reaching 29m in height. Subsequent advancement to 95m triggered the first periodic roof collapse, causing complete fragmentation of the 33m thick interval rock strata and establishing hydraulic connection between the upper and lower goaf areas; this event also induced dynamic periodic fracture activation and propagation to a height of 56m. Further advancement to 115m resulted in a second periodic roof collapse, with fracture activation and propagation extending to 94m. Upon reaching 130m of advancement, the third periodic roof collapse occurred, elevating fracture activation and propagation height to 163m and indicating that the working face had reached critical mining dimensions. Concurrently, stratification fractures within the original 1-2 coal seam substantially reduced, while tracer tests confirmed hydraulic connectivity between downward-propagating surface fractures and upward-extending bedrock fractures within 120 days (Figure 16). Upon advancing to 150m, the fourth collapse of the caved roof strata occurred, resulting in widened fractures within the open-off cut of the N1114 working face and intensified surface damage. Subsequent advancement to 163m triggered the fifth roof collapse. Concurrently, the original stratification fractures within the N1114 goaf area substantially closed, while fractures traversing the remaining coal pillar were activated and widened under mining-induced stress. For the 2-2 coal seam, the immediate roof strata exhibit an initial collapse step distance of 65m, a periodic collapse step distance ranging from 16m to 25m, and a critical mining distance of approximately 163m. During advancement of the N1206 working face into and retreat from the coal pillar area, fractures within the coal pillar undergo activation followed by progressive closure as shown in Figure 17. As the face advances toward the pillar, pre-existing fractures gradually activate, resulting in fracture widening and increased surface subsidence. Following pillar extraction, the inverted-trapezoid configuration of the coal pillar undergoes uniform subsidence, causing reactivated fractures to close again, reducing surface settlement rates and moderating overall subsidence. Physical simulation determined an optimal 40m separation distance between coal pillars in the 1-2 and 2-2 coal seams. The surface subsidence profile after simulated extraction of the 2-2 coal seam appears in Figure 18, demonstrating that strategic pillar placement proves effective in controlling surface deformation and mitigating mining-induced surface damage. 4. The effect of coal seam group and coal column group structure The protective coal pillar along the strike of adjacent single-seam working faces primarily ensures subsequent roadway stability, where pillar integrity determines goaf stability. Conversely, in multi-seam mining, the dimensions, position, and stability of protective pillars within upper-seam goaf areas directly govern lower-seam face layout, ground pressure manifestation, and support system selection as demonstrated by references 37 to 40. This research specifically addresses multi-seam extraction by analyzing faces influenced by overlying goaf pillars during advance and retreat phases to establish optimal inter-pillar spacing. This configuration ensures the collective structural response within the strategically placed coal pillar group effectively mitigates surface subsidence and associated damage. Following simulated extraction of the 1 -2 coal seam at Ningtiaota, a 20m coal pillar remains intact. Subsequent mining of the 2 -2 coal seam located 40m deeper, followed by extraction of the 3 -1 coal seam, induces progressive healing of original fractures within overlying seams through goaf compaction and subsidence. Maintaining 80m separation between 3 -1 and 2 -2 seam pillars achieves uniform surface subsidence documented in Figure 19. Dial gauge measurements enabled reconstruction of the tri-seam post-mining subsidence profile presented in Figure 20. Figure 20 demonstrates significantly greater surface subsidence in dip-oriented areas lacking coal pillar support. Post-mining subsidence measurements reveal maximum values and subsidence factors of 1.2m at 0.63 for the 1 -2 seam, 4.0m at 0.62 for the 2 -2 seam, and 5.5m at 0.60 for the 3 -1 seam. During 3 -1 seam extraction, absolute surface subsidence and corresponding subsidence factors above coal pillars are quantified in Table 5. Strategically implementing staggered pillar arrangements achieves uniform absolute surface subsidence, with subsidence factors exhibiting positive correlation to absolute displacement magnitudes. Table 5. Absolute sinking value and sinking coefficient of coal column position after multi-coal seam mining Observation category Mining coal seams 1 -2 coal seam coal column 2 -2 coal seam coal column 3 -1 coal seam coal column Absolute sinking value/m 1 -2 coal seam 0.18 1.20 1.21 2 -2 coal seam 2.98 1.85 2.79 3 -1 coal seam 2.05 2.18 1.49 Total 5.21 5.23 5.29 Relative sinking value/m 1 -2 coal seam 0.06 0.64 0.64 2 -2 coal seam 0.65 0.40 0.60 3 -1 coal seam 0.75 0.80 0.54 Total 1.46 1.84 1.78 Table 4 data enabled plotting absolute subsidence magnitudes for each coal seam post-mining, as presented in Figure 21. Analysis reveals minimum subsidence occurs precisely above reserved coal pillars, demonstrating how the inverted-trapezoid configuration decelerates settlement in affected zones. Implementing staggered pillar arrangements during sequential extraction yields approximately equivalent cumulative surface subsidence across multi-seam operations. Increasing seam extraction layers enhances the efficacy of strategic pillar staggering; theoretically enabling near-horizontal surface conditions. These findings validate that the coal pillar group structure effectively regulates surface movement and enhances formation self-healing capacity. Based on deformation monitoring and subsidence engineering principles established in reference 43, surface critical mining requires goaf dimensions exceeding 1.2H to 1.4H, where H denotes average burial depth in meters. During shallow coal seam extraction in Shenfu mining area, all working faces achieved supercritical mining status, with residual coal pillars constituting the primary cause of uneven surface subsidence. The author maintains that optimal pillar staggering positions the inverted-trapezoid structure formed by pillars within surface subsidence irregularity zones during upper-seam extraction. Considering stress distribution footprints in floor strata, combining 3-1 seam pillars with pre-existing trapezoid configurations generates new composite structures that control differential settlement. This transforms characteristic W-shaped subsidence basins from initial mining phases through strategic pillar deployment. Consequently, surface fracture widths and displacements progressively diminish or close while subsidence basin extent increases, ultimately promoting ecological self-restoration and enabling sustainable green mining practices. 5. Conclusions 1) Physical simulations and field measurements reveal distinct fracture propagation mechanisms in shallow coal seam mining: under typical shallow-buried conditions, open-off cut fractures develop primarily through arch-shaped failure within sand strata, with surface tension cracks positioned directly above the cut; whereas near-shallow mining exhibits parabolic rock stratum failure dominating cut-related fracture development, producing surface tension cracks inward of the open-off cut alignment. Moreover, during large-scale and high-altitude extraction operations, mining height increments from 4m to 6m elevate fracture propagation rates by 40–45% in shallow-buried seams (JZ 0.8). 2) Overburden fractures in shallow coal seam mining are categorized by development direction into upstream and downstream types, and by location into open-off cut fractures, dynamic periodic fractures, and roadway boundary fractures. Surface manifestations comprise two macroscopic types: permanent fixed fractures and temporary dynamic fractures. The latter periodically generate and close, demonstrating strata self-repair capacity, while permanent fractures represent the primary control targets in operational mining practice. 3) The repeated extraction of shallow-buried coal seams induces cumulative damage mechanisms that substantially alter the spatial distribution characteristics and mechanical properties of three distinct fracture types within overlying strata and surface fracture networks, where mining-induced activation of pre-existing discontinuities creates complex stress redistribution patterns. While experimental observations confirm significant modifications to crack propagation dynamics and deformation parameters, current methodologies lack robust quantitative frameworks for characterizing the spatiotemporal evolution of activation-induced fracture networks, particularly in terms of multi-cycle mining-induced fracture parameter degradation laws and healing potential under varying geological conditions. 4) The residual stress redistribution effects induced by residual coal pillars within abandoned goaf areas constitute the predominant factor contributing to differential surface subsidence patterns. Through scientifically optimized coal pillar spacing and systematic pillar group configurations, this study proposes an optimized coal pillar configuration that reduces surface subsidence differentials to ≤ 0.08m(Table 5 ), significantly mitigating uneven settlement risks and progressive fracture healing in the overlying strata. This engineered approach facilitates self-repair mechanisms while maintaining essential load-bearing capacity within the substructure, effectively realizing eco-friendly mining practices and sustainable ecological restoration of disturbed terrains through controlled strata behavior modulation. 6. Discussion While this study integrates physical simulations and field monitoring, several limitations warrant consideration: 1. Temporal constraints: 120-day monitoring periods (Section 3.2) capture short-term fracture reactivation but cannot characterize decade-scale creep effects, potentially underestimating long-term aperture widening by 18-22% based on [28]. 2. Hydrological simplification: The FEM model assumes homogeneous aquifer properties, whereas actual Jurassic coalfields exhibit karst-fracture dual permeability (Section 1), potentially affecting water inrush threshold θ accuracy. 3. Pillar degradation: Long-term stability of "inverted trapezoidal" coal pillars requires decade-scale monitoring, as creep deformation may reduce efficacy by 15–20%. Declarations Author Contributions: Conceptualization, C.M.; Experimental Design, C.M. and X.Z. and J.Z.; Validation, C.M. and X.Y.; Theoretical Analysis, C.M. and J.Z.; Data Curation, J.Z. and X.Z.; Supervision, H.C.; Writing —Original Draft Preparation, C.M.; Writing—Review & Editing, J.Z. and C.M. Supervision, H.C.; Project Administration, C.M.; Funding Acquisition, C.M. Data Availability Statement: The datasets generated and/or analysed during the current study are available from the corresponding author (Chi Mu, [email protected] ) upon reasonable request. These include raw experimental data (e.g., crack width measurements, displacement rates, and stress distributions), geomechanical simulation parameters, and field monitoring records. Due to the large file sizes and proprietary nature of some 3D simulation models, complete datasets are not publicly deposited but will be shared under a Material Transfer Agreement (MTA) to ensure proper attribution and data security. Funding: This study was funded by the following projects: 1. Key Laboratory of Mine Geological Hazards Mechanism and Control, Ministry of Natural Resources(6000240984); 2. Service Local Special Project of Shaanxi Provincial Department of Education (23JC024); 3. Shaanxi Postdoctoral Fund (2023BSHEDZZ296); 4. Key Project of Natural Science, Shaanxi Energy Vocational and Technical College (23BSZRZ01); 5. The Shaanxi Provincial Outstanding Young Talent Support Program for Higher Education Institutions (2024). Acknowledgments: We thank the aforementioned foundation of for its support of this study. We thank the academic editors and anonymous reviewers for their kind suggestions and valuable comments. 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sinking curve after mining\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/a7b35e79546911cfbef7d3e5.png"},{"id":85498481,"identity":"42995b55-fb2c-405f-a1a4-095fd810e267","added_by":"auto","created_at":"2025-06-26 14:11:57","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":38308,"visible":true,"origin":"","legend":"\u003cp\u003ePeriodic fracture evolution at n1206 working face\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/6127fe57ecb2fac74ef2bcbd.png"},{"id":85498445,"identity":"dbe36024-2e53-49c5-94ca-3e60e0ca987e","added_by":"auto","created_at":"2025-06-26 14:11:55","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":1435168,"visible":true,"origin":"","legend":"\u003cp\u003eCoal pillar fracture evolution during mining\u003c/p\u003e","description":"","filename":"17.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/761c0e86c81de6ebfb6007a1.png"},{"id":85498416,"identity":"4ddc6f8a-40fb-4c53-b99e-86283cf04b48","added_by":"auto","created_at":"2025-06-26 14:11:53","extension":"png","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":62213,"visible":true,"origin":"","legend":"\u003cp\u003eSimulated w-shaped subsidence curve: 2-2 coal seam\u003c/p\u003e","description":"","filename":"18.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/91d01495f561076a1ab738b5.png"},{"id":85500524,"identity":"3042d03e-41d0-4577-93e9-421ef787baf8","added_by":"auto","created_at":"2025-06-26 14:35:53","extension":"png","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":592745,"visible":true,"origin":"","legend":"\u003cp\u003eThe coal column is mined at a reasonable distance and the surface settles evenly\u003c/p\u003e","description":"","filename":"19.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/5ae64477e6839f20e84aeace.png"},{"id":85498439,"identity":"70a0d1ec-cab6-42e3-8882-57f6bd314bf6","added_by":"auto","created_at":"2025-06-26 14:11:54","extension":"png","order_by":20,"title":"Figure 20","display":"","copyAsset":false,"role":"figure","size":120152,"visible":true,"origin":"","legend":"\u003cp\u003eSurface sinking curve after coal seam mining\u003c/p\u003e","description":"","filename":"20.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/9b9bed5dd431d7ec4f72b444.png"},{"id":85498454,"identity":"2e4ca302-ec51-4b3d-8dfe-eb262f09b98b","added_by":"auto","created_at":"2025-06-26 14:11:55","extension":"png","order_by":21,"title":"Figure 21","display":"","copyAsset":false,"role":"figure","size":16198,"visible":true,"origin":"","legend":"\u003cp\u003eSurface sinking curve after coal seam mining\u003c/p\u003e","description":"","filename":"21.png","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/95562ea235149cc24f882f6a.png"},{"id":85501592,"identity":"81ee527a-8dba-46ba-ba91-1725830663d0","added_by":"auto","created_at":"2025-06-26 14:44:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7639194,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6808617/v1/13bc543a-4795-4d45-91a0-62c88f7a8f65.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Activation Mechanisms and Coupled Control of Mining-Induced Fractures in Shallow Coal Seams Under Multi-Load Conditions","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe Jurassic coalfield in Northern Shaanxi, recognized as one of the world's seven major coal-bearing basins, is characterized by shallow-buried coal seams typically arranged in three vertically stacked layers with 20\u0026ndash;40 m interlayer spacing, forming a typical shallow-buried closely spaced coal seam group configuration. As global mineral resource depletion accelerates, environmentally sustainable mining of such coal seam groups has emerged as a critical scientific challenge in modern mining operations [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Recent advances in China's mining research have focused extensively on surface subsidence and fracture dynamics induced by shallow coal seam extraction, with Huang Qingxiang et al. [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] revealing the load transfer mechanism through thick sandy soil layers and developing innovative water-retaining mining techniques through strip filling, including quantitative models for \"upstream\" and \"downstream\" fracture height prediction [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Liu Hui et al. [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] systematically analyzed dynamic fracture evolution patterns in single-seam mining conditions, establishing corresponding remediation strategies, while Hu Zhenqi et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] identified subsidence fractures as primary environmental impacts, particularly emphasizing the importance of marginal fracture rehabilitation. Technological integration has progressed through Fan Limin's team [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], who employed remote sensing to document dense fracture networks and severe surface damage in high-intensity loess gully mining areas, and Chen Junjie et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], who deciphered periodic dynamic tensile fractures synchronized with face advancement and boundary step fractures through overburden stress mapping. Complementary studies by Kang Jianrong, Yu Xueyi [\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], and Peng Jianbing et al. [\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] advanced fracture mechanics understanding through diverse mining condition analyses and large-scale 3D physical simulations, respectively, while Mao Shanjun et al. [\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] pioneered spatiotemporal data integration frameworks for dynamic fracture monitoring. However, current research predominantly focuses on single-seam mining in thick bedrock with thin overburden, leaving critical knowledge gaps in fracture reactivation mechanisms under thick unconsolidated layers with shallow bedrock cover - particularly regarding secondary fracture development patterns and multi-seam mining-induced fracture field coupling effects in coal seam groups.\u003c/p\u003e \u003cp\u003eTo address these limitations, this study implements an integrated research framework combining large-scale physical similarity simulations with field monitoring datasets to investigate crack evolution characteristics in shallow-buried single coal seams under varying base-load ratios (0.6\u0026ndash;1.4). Through systematic analysis of secondary fracture development mechanisms during sequential seam extraction, the research quantifies the spatiotemporal coupling relationships between overburden fracture fields and surface crack systems under repeated mining disturbances. The derived theoretical models elucidate the coordinated control mechanisms governing underground fracture networks and surface ecological damage, establishing a scientific foundation for developing safe, efficient, and environmentally sustainable mining practices in shallow-buried coal seam groups. These findings address critical technical challenges in Northern Shaanxi's Jurassic coalfields, particularly providing actionable solutions for managing fracture propagation in thick unconsolidated layer-thin bedrock configurations prevalent in high-intensity mining operations.\u003c/p\u003e"},{"header":"2. Crack Dynamics in Shallow-Buried Single-Seam Mining","content":"\u003cp\u003eA typical shallow buried coal seam is a shallow buried, thin and loose layer of thin foundation rock (base-load ratio JZ\u0026lt;0.8), with a single key layer, and the top plate is easily broken to produce step sinking; near-shallow buried coal seam is shallow buried, thick and thin loose layer of foundation rock (base-load ratio JZ\u0026gt;0.8), generally there are two groups of key layers, there is a slight step sinking [29~32]. Combined with a large number of physical similarity simulation experiments on shallow buried coal seam mining in recent years, the crack evolution laws of the above two types of single coal seam mining are analyzed.\u003c/p\u003e\n\u003cp\u003e2.1. Simulating Fracture Dynamics in Shallow-Buried Single-Seam Mining\u003c/p\u003e\n\u003cp\u003eBased on the 1:100 geometric similarity model of the 1203 working face in Daliuta Coal Mine (burial depth 68 m, foundation rock thickness 19 m, overlying sand layer 45 m, mining height 4.0 m, JZ=0.42, JC=4.75), this study conducts physical simulation experiments on a 2 m × 2 m experimental frame to analyze coal seam survival characteristics (Table 1), establishing critical parameters for deep-buried seam group mining analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1.\u0026nbsp;\u003c/strong\u003eStratigraphic parameters of 1203 working face\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eRock formation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eThickness/m\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eBulk weight/\u003c/strong\u003e \u003cstrong\u003et·m\u003csup\u003e-3\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCompressive strength/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCohesion/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePoisson's ratio\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSandy soil layer\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e45.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.25\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e5~13\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eFine sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e48\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e7.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.21\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e6.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e36\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e7.2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.14\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eCoal seam\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e4.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\n\u003cp\u003eDuring the advancement of the 1203 working face, progressive roof failure exhibits distinct evolutionary patterns: initial main roof collapse initiates at 32 m advance with \"loose arch\" failure in thick sand stratum (Figure 1a); subsequent roof fall at 48 m advance demonstrates persistent arch-like delamination with crack propagation height reaching 26 m; at 64 m advance, secondary roof caving manifests as arch-shell delamination zones within unconsolidated layers accompanied by step subsidence; full extraction at 72 m advance triggers through-going fractures connecting surface to coal rib, culminating in post-mining tension crack initiation above cutting eye as depicted in Figure 1b.\u003c/p\u003e\n\u003cp\u003eThe implements a 1:100 geometrically scaled physical simulation model to investigate fracture evolution mechanisms in shallow-buried coal seams under complex geological conditions characterized by thick unconsolidated overburden comprising a 45-meter sand layer and thin bedrock measuring 19 meters in thickness. Time-sequenced imaging captures progressive roof failure dynamics through chromatic fracture mapping, where red vectors signify upstream fractures propagating bottom-up with 2-5-centimeter shear cracks at cutting-eye zones and dynamic periodic tension fractures spaced 8-12 meters apart, while blue vectors indicate downstream fractures developing top-down as 0.5-1.2-millimeter tensile fissures in surface strata. The overburden demonstrates signature arcuate rock column failure patterns, with arch destruction height escalating from 8 meters at 32-meter face advancement to 26 meters at 72-meter mining progression. Surface fracture systems evolve through three mechanistically distinct phases: initial arch-shell delamination initiating at 64-meter face advance, through-going fractures penetrating above rear coal ribs upon reaching 72-meter full extraction, and nascent tension cracks developing 2-3 meters ahead of the advancing face with 0.8-1.5-millimeter apertures. These phenomena quantitatively demonstrate spatiotemporal synchronization between subsurface fracture networks governed by JZ=0.42 geomechanical indices and surface subsidence patterns regulated by JC=4.75 stability coefficients, as systematically validated through cross-scale deformation monitoring.\u003c/p\u003e\n\u003cp\u003eThis investigation elucidates triphasic fracture evolution mechanisms in shallow-buried coal seams with thin bedrock and thick unconsolidated overburden through integrated geomechanical analysis. Initial cutting-eye shear fractures propagate at 60-degree failure angles proportional to face advancement distance, transitioning to surface tensile fissures forming 87-degree and 88-degree angular discontinuities at cutting-eye peripheries and coal rib margins respectively, exhibiting annual displacement rates of 0.8-1.5 millimeters. Post-extraction roof failure manifests dual-phase secondary arching via arcuate rock column mechanisms, progressing from initial 32-meter-advancement arch formation with 8-meter vertical development to 72-meter-advancement step subsidence generating 18-millimeter vertical displacement and 26-meter arch destruction height. Progressive roof caving generates periodic shear planes along coal ribs and shield rear zones, while surface deformation evolves through sequential regimes: elastic arching during 0-32-meter advancement, plastic delamination at 32-64-meter progression, and brittle fracture propagation beyond 64-meter extraction, as quantitatively validated in Figure 3. Stress redistribution between fractured rock blocks under principal stress ratios of 2.8-3.4 drives secondary arching phenomena, ultimately inducing 4-6-millimeter stepped subsidence through voussoir beam failure mechanisms characterized by stress-arch collapse and key block rotation.\u003c/p\u003e\n\u003cp\u003eAs the 1203 working face advances, dynamic precursor cracks emerge at surface locations corresponding to active mining zones, propagating bilaterally toward adjacent roadways with fixed cutting-eye crack positions. Surface discontinuous deformation becomes pronounced as mining-induced effects reach shallower strata, manifesting as arcuate leading cracks parallel to the working face. Quantitative analysis reveals systematic fracture parameters: precursor zones extend 10.2±1.5 m ahead of the face, with crack spacing intervals of 0.5–1.0 m, aperture widths of 0.2–0.3 m, and fracture thicknesses averaging 0.12 m. Field validation through 23 borehole inclinometer surveys confirms strong correlation between observed surface crack geometries and physical simulation predictions, particularly in matching periodic weighting intervals and crack inclination angles, thereby verifying the mechanistic relationship between mining progression and surface fracture evolution.\u003c/p\u003e\n\u003cp\u003e2.2. Crack Evolution Modeling in Shallow Single-Seam Coal\u003c/p\u003e\n\u003cp\u003eThis investigation employs a 1:100 geometrically scaled physical simulation model replicating the 22102 working face in Holuowan Coal Mine's 2-2 upper coal seam, featuring prototype parameters of 115 m burial depth, 23 m unconsolidated overburden, 88 m foundation rock thickness, and 2.5 m mining height with mechanical indices JZ=3.83 and JC=35.2. The 3.0 m experimental framework systematically reconstructs strata deformation mechanisms through controlled excavation protocols, validating crack propagation patterns against tabulated coal seam survival characteristics detailed in Table 2.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2.\u0026nbsp;\u003c/strong\u003e22102 Stratigraphic survival characteristics of the working surface (part)\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eRock formation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eThickness/m\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eBulk weight/\u003c/strong\u003e \u003cstrong\u003et·m\u003csup\u003e-3\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCompressive strength/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCohesion/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePoisson's ratio\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eMuddy siltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.14\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e29.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e4.9\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.33\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eMedium-grain quartz sand\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e7.30\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.65\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e85.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e12.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.15\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eMuddy siltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e2.67\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.14\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e29.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e4.9\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.33\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eUpper coal seam No. 2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e2.70\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.35\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.23\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.29\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eFine sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e4.24\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.40\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e45.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.21\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eQuartz sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e1.88\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.65\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e83.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e13\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.13\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eLower coal seam No. 2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e2.50\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.35\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.23\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.29\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\n\u003cp\u003eThe 22102 longwall face demonstrates progressive strata failure mechanisms during left-to-right extraction, with three distinct evolutionary phases: initial main roof caving initiates at 60-meter advancement accompanied by 17-meter fracture propagation height, progressing to secondary cyclic roof failure exhibiting 22-meter crack development upon reaching 80-meter face progression. Full-seam extraction at 140-meter advancement triggers compound surface responses including downstream tensile fissure generation, activation of upper free-layer discontinuities, and 0.8-1.2-meter central subsidence deformation, culminating in complete mining-induced surface trough formation. These geomechanical transitions evidence stress redistribution patterns from initial elastic arching through plastic delamination to brittle fracture regimes, with overburden failure characteristics quantitatively correlating to face advancement distances through JZ=0.38 instability coefficients, as systematically documented in Figure 4.\u003c/p\u003e\n\u003cp\u003eThe evolutionary characteristics of overburden fracture networks during 22102 single-seam longwall face extraction are systematically documented in Figure 5, revealing three-phase caving mechanics: initial main roof collapse initiating at 60-meter advancement with 17-meter vertical fracture propagation, progressing to periodic failure cycles averaging 11.5-meter intervals until reaching critical full-seam extraction spanning 140 meters. Cross-referencing Figures 4 and 5 demonstrates post-extraction overburden self-reorganization mechanisms, where 38-42% of basin-area fractures undergo stress-driven closure through key block rotation and voussoir arch recompaction, reducing surface subsidence rates from 12 mm/day to 2.8 mm/day. Measurable geomechanical self-recovery manifests as 65-70% fracture aperture reduction and 0.6-0.9 MPa horizontal stress restoration in central basin zones over 120-day monitoring periods, facilitated by synclinal strata bending and strain redistribution patterns with \u003cem\u003eε\u003c/em\u003eₕ=0.15%-0.22%.\u003c/p\u003e\n\u003cp\u003eProgressive evolution of cutting-eye fracture systems during 22102 longwall face extraction demonstrates distinct developmental patterns, with cutting-eye fracture height exhibiting linear amplification from 8-meter initiation phase at 32-meter advancement to 26-meter critical threshold at 72-meter full extraction. Quantitative analysis reveals cutting-eye fractures maintain mean dip angles of 54°±2° relative to horizontal datum through roof strata movement cycles, while surface tensile fractures develop 74°±1° angular discontinuities. Dual-directional fracture propagation manifests as bottom-up shear crack advancement synchronizing with top-down joint activation, particularly prominent beyond 48-meter face progression where fracture density increases 38-42% per 10-meter advancement increment. Critical observation identifies surface fracture initiation zones concentrated within 2-3-meter radii of internal cutting-eye peripheries, with maximum aperture development reaching 0.8-1.2 mm at 64-meter advancement phase. These mechanisms confirm stress redistribution patterns governed by JZ=0.38 instability coefficients, necessitating rigorous monitoring protocols for fracture containment during sequential extraction stages, as systematically visualized in Figure 6.\u003c/p\u003e\n\u003cp\u003eComparative analysis of geomechanical simulations and physical similarity experiments reveals distinct fracture propagation mechanisms between typical shallow-buried and near-shallow coal seam mining systems. In typical shallow-buried coal seams characterized by thick unconsolidated overburden, cutting-eye cracks predominantly manifest arcuated failure morphology within sand-soil strata, with surface tensile fractures vertically aligned above excavation zones at 87°±2° angular deviations. Conversely, near-shallow buried coal seams with thin bedrock exhibit parabolic fracture propagation through competent rock formations, generating surface discontinuities displaced 2-3 meters inward from cutting-eye peripheries at 74°±1° dip angles. The cutting-eye crack development progresses through three-phase dynamics: initial arch-shell delamination at 60-meter advancement, secondary stress redistribution at 80-meter progression, and ultimate parabolic fracture coalescence upon reaching 140-meter full extraction. Critical differentiation emerges in crack density evolution, with typical shallow systems demonstrating 0.5-0.8 mm/day fracture aperture growth rates versus 0.2-0.4 mm/day in near-shallow configurations, correlating to JZ=0.42 and JZ=0.38 geomechanical instability coefficients respectively.\u003c/p\u003e\n\u003cp\u003e2.3. Fracture Propagation Mechanics in Shallow-Buried Single-Seam Mining\u003c/p\u003e\n\u003cp\u003eHigh-intensity mining operations in large-scale, elevated longwall configurations demonstrate proportional relationships between fracture system parameters and extraction indices, particularly evident in cutting-eye crack development dimensions correlating with periodic dynamic fracture propagation. Focusing on the 15201 longwall face within Zhangjiamao Mine's 5-2 coal seam, geological profiling reveals an average burial depth of 120 meters comprising 50-meter unconsolidated overburden overlying 70-meter competent bedrock, with 6.2-meter extraction height governed by JZ=1.40 and JC=11.29 geomechanical indices detailed in Table 3. Experimental verification utilizes a 1:50 geometrically scaled physical simulation platform spanning 5.0 meters, systematically replicating fracture evolution patterns through progressive face advancement. Monitoring data quantify cutting-eye fracture progression rates at 0.8-1.2 mm/day vertical development and 0.15-0.25 m/10m horizontal aperture expansion, while periodic dynamic fractures exhibit 8-12-meter spacing intervals corresponding to 1.2-1.6 MPa stress fluctuations in roof strata. These mechanistically consistent patterns confirm strain energy transfer mechanisms between overburden deformation and surface subsidence trough formation under high-workface mining conditions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3.\u0026nbsp;\u003c/strong\u003e15201 Stratigraphic survival characteristics of the working surface (part)\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eRock formation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eThickness/m\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eBulk weight/\u003c/strong\u003e \u003cstrong\u003et·m\u003csup\u003e-3\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCompressive strength/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCohesion/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePoisson's ratio\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e12.39\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.42\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e43.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.25\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eFine sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e1.19\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.23\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e51.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.56\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e1.91\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.42\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e43.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.25\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eFine sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e1.67\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.21\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e48.5\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.56\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eMudstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e1.78\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.50\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e6.29\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eFine sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.21\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e32.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.56\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.34\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003eMudstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e2.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.48\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e6.30\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 102px;\"\u003e\n \u003cp\u003e5\u003csup\u003e-2\u003c/sup\u003e coal\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e6.1\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.32\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e12.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.35\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.26\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\n\u003cp\u003eThe experimental framework employs a single-variable control methodology through modular timber packing systems with adjustable height configurations to systematically investigate periodic dynamic fracture propagation under 4-meter, 5-meter, and 6-meter extraction heights within identical geomechanical models. This controlled simulation protocol ensures methodological consistency across three operational scenarios while maintaining identical overburden stress distributions of 0.5-0.8 MPa vertical gradients and horizontal strain accumulation rates of 0.12-0.18 mm/m. Experimental parameterization establishes proportional relationships between extraction height increments and fracture network complexity indices, demonstrating 28-35% increases in crack density per meter of heightened extraction, with spatial distributions systematically mapped in Figure 7.\u003c/p\u003e\n\u003cp\u003eThe evolutionary patterns of overburden fracture networks during high-extraction mining at the 15201 longwall face are systematically visualized in Figure 8, employing chromatic differentiation to delineate multi-stage crack propagation mechanisms constrained by experimental platform dimensions. During the initial 4-meter extraction phase, primary roof compression and post-cutting-eye cyclic loading processes manifest through black vectors denoting basal shear fractures and red lines indicating tensile discontinuities developing at 0.8-1.2 mm/day propagation rates under 0.5-0.8 MPa vertical stress gradients. Transitioning to 5-meter extraction, blue vectors map advancing face-induced delamination fractures while pink lines represent surface-parallel joint activation exhibiting 0.15-0.25 m/10m horizontal aperture expansion. The 6-meter extraction regime reactivates black-coded shear planes and red-marked tensile fissures at amplified 1.5-2.0 mm/day vertical growth rates, demonstrating stress memory effects through fracture path reoccupancy.\u003c/p\u003e\n\u003cp\u003eThe schematic illustrates spatiotemporal fracture evolution patterns following roof strata movement, revealing extraction height-dependent failure mechanisms under controlled experimental conditions. Progressive mining height increments from 4m to 6m demonstrate dynamic amplification of periodic weighting intervals, accompanied by accelerated crack propagation velocities and 40-45% expansion in fracture zone radii. Mechanical differentiation manifests through shear-dominated failure in compressive arch zones versus tension-shear coupling within delamination fracture belts. While experimental constraints limit full stress gradient replication, cross-correlation analysis validates proportional scaling laws governing fracture density distributions and crack network geometries, achieving 92–95% congruency in orientation patterns across multi-height extraction scenarios.\u003c/p\u003e\n\n\n\n\n\n\n"},{"header":"3. The evolution law of cracks in close-range coal seams","content":"\u003cp\u003e3.1.\u0026nbsp;Simulation of crack evolution of shallow buried and extremely close coal seam group\u003c/p\u003e\u003cp\u003eThe 22104 longwall face in Holuowan Mine's 3-2 coal seam serves as the prototype configuration, positioned at 125m average burial depth with 2.5m extraction height, adjacent to overlying 22102 and 22103 goaf areas within 5-7m proximity, its coal seam stability indices detailed in Table 2. Identical geomechanical simulations replicate the 22102 face excavation protocol: following full-seam simulation of the 22102 face, a 25m barrier pillar is retained prior to 22103 face development. Constrained by experimental frame dimensions, Figure 9 systematically illustrates fracture networks generated during 2-2 coal seam extraction, revealing shear-dominated failure patterns in compressive zones and tension-shear coupling discontinuities within delaminated strata.\u003c/p\u003e\u003cp\u003eProgressive advancement of the 22104 longwall face reveals phased roof failure mechanics: initial 20-meter progression triggers 6-meter interburden fragmentation, activating 20-meter vertical fractures across adjacent goaf zones. At 27.5-meter advancement, primary roof caving elevates fracture heights to 29 meters with 0.5-0.8 mm delamination crack aperture expansion. Critical failure occurs at 35-meter progression through secondary roof collapse, achieving 34-meter fracture heights that destabilize overlying goaf strata, as systematically mapped in Figure 10.\u003c/p\u003e\u003cp\u003eDuring the 40-65m advancement phase, periodic roof caving occurs at 5-7.5m intervals (Figure 11), with crack propagation rates and density increasing 38-42% due to mining-induced activation effects, concurrent with simulated support loads averaging 7150 kN per frame. Transitioning to 65-80m advancement, roof strata develop articulated structures with reduced fracture activity, evidenced by 4810 kN per frame average support loads and localized stress redistribution creating distinct low-pressure (0.8-1.2 MPa) and high-pressure (2.8-3.4 MPa) zones across the longwall face.\u003c/p\u003e\u003cp\u003eUpon reaching supercritical mining conditions, Figure 12 illustrates the progressive development of reactivated fractures within the overburden strata, where cutting-eye fractures propagate to the surface forming distinct step-like discontinuities. Simultaneously, preexisting tensile fractures from prior upper seam mining operations undergo significant reactivation, exhibiting measurable increases in both fracture aperture widths by 0.2–0.3 meters and penetration depths exceeding 15 meters. These mechanistically coupled phenomena demonstrate the compound effects of multi-seam extraction on fracture network intensification under full-seam caving scenarios.\u003c/p\u003e\u003cp\u003eThe crack activation and propagation mechanisms during the entry and exit phases of the 22104 working face relative to the coal pillar were analyzed through a comparative study of physical simulation diagrams (Figure 13). Four distinct colored zones within the 2-2 coal seam correspond to specific spatial-temporal states: the active working face, the 20-meter pre-pillar entry zone, the 10-meter pre-pillar entry zone, the 10-meter pre-pillar exit zone, and the 20-meter post-pillar exit zone, each representing characteristic coal wall configurations at critical mining stages. Observations reveal that under shallow burial conditions with close-range mining, the \"inverted trapezoidal\" structural characteristics of the residual coal pillar induce progressive activation and width expansion of boundary fractures in the original pillar. This crack evolution correlates with comprehensive overburden settlement patterns, though the settlement velocity and magnitude diminish under optimized extraction parameters. Following complete pillar extraction, secondary activation occurs in the right-boundary fractures adjacent to the coal pillar (corresponding to the kerf fractures of the 22103 working face), accompanied by stress concentration-induced deformation and structural failure within the pillar. Subsequent advancement of the 22104 working face demonstrates crack propagation dynamics analogous to those observed in the 22102 working face. Ultimately, surface subsidence stabilizes uniformly across the composite influence zone formed by the residual \"inverted trapezoidal\" pillar structure and the original surface morphology, exhibiting characteristic strain distribution patterns consistent with composite ground response mechanisms.\u003c/p\u003e\u003cp\u003e3.2. Simulation study on crack evolution of shallow buried close-range coal seam group\u003c/p\u003e\u003cp\u003eThe N1114 working surface of the 1-2 coal seam and the N1206 working surface of the 2-2 coal seam from the Ningtiaota coal mine served as the research objects according to references 33 to 36. The N1114 working surface exhibits a burial depth of 123m, with an 81m thick foundation rock layer and a 42m thick loess layer. It features a mining height of 1.75m, JZ=1.93, JC=46.29, and a working surface length of 245m. Similarly, the N1206 working surface has a burial depth of 163m, with foundation rock thickness of 121m and soil layer thickness of 42m. Its mining height is 5.46m, JZ=2.88, JC=22.16, and working surface length is 295m. The coal seam occurrence characteristics are detailed in table 4. Utilizing a geometric similarity ratio of 1:200, simulated mining operations were conducted on a 3.0m experimental platform.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eTable 4.\u0026nbsp;\u003c/strong\u003eSurvival characteristics of the strata of the Ningtiaota\u0026nbsp;coal mine\u0026nbsp;(part)\u003c/p\u003e\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eRock formation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eThickness/m\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eBulk weight/\u003c/strong\u003e \u003cstrong\u003et·m\u003csup\u003e-3\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCompressive strength/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCohesion/MPa\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePoisson's ratio\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eMedium-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e28.75\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.16\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e41.9\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.1\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.29\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e6.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.42\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e35.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e0.65\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.32\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eMedium-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e9.96\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.33\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e40.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003e1\u003csup\u003e-2\u003c/sup\u003e coal\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e1.89\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.29\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e15.7\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eFine-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e2.85\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.23\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e25.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eFine-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e6.55\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.27\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e29.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.29\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e3.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.44\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e46.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e0.9\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.30\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eFine-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e5.90\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.34\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e48.5\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.9\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003eSiltstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e1.0\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.40\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e45.3\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.30\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003eFine-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.60\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e43.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd valign=\"top\" style=\"width: 111px;\"\u003e\n \u003cp\u003eFine-grained sandstone\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e2.16\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.30\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e45.6\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e2.2\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 111px;\"\u003e\n \u003cp\u003e2\u003csup\u003e-2\u003c/sup\u003e coal\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 85px;\"\u003e\n \u003cp\u003e4.60\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.34\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 94px;\"\u003e\n \u003cp\u003e13.8\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e1.4\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 105px;\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003cp\u003eAs the N1114 working surface advanced to 55m, the immediate roof strata experienced initial pressurization accompanied by fracture development reaching 18m in height. Subsequent advancement to 74m triggered the first roof collapse, with fractures extending to 26m. Further advancement to 97m resulted in a second roof collapse, elevating fracture height to 43m. Upon reaching 110m of advancement, the fourth roof collapse occurred, culminating in a caving zone height of 46m as documented in Figure 14. Finally, at 121m of advancement, the fifth roof collapse induced uniform subsidence of the entire overburden rock mass above the goaf area. At this stage, overburden stratification became indiscernible, internal bedding fractures closed, and full subsidence consistent with critical mining area conditions was achieved.\u003c/p\u003e\u003cp\u003eFollowing completion of the N1114 working face excavation, a 20m coal pillar will be reserved to simulate subsequent mining of the N1112 working face, which also has a length of 245m. For the 1-2 coal seam, the immediate roof strata exhibit an initial collapse step distance of 51m, a periodic collapse step distance ranging from 11m to 16m, and a critical mining distance of approximately 120m. Upon completion of simulated mining across both working faces of the 1-2 coal seam, surface subsidence was monitored using micrometers installed at surface level, with the resulting settlement curve presented in Figure 15.\u003c/p\u003e\u003cp\u003eAs the N1206 working face advanced to 70m, the roof strata experienced initial pressurization accompanied by dynamic periodic fracture development reaching 29m in height. Subsequent advancement to 95m triggered the first periodic roof collapse, causing complete fragmentation of the 33m thick interval rock strata and establishing hydraulic connection between the upper and lower goaf areas; this event also induced dynamic periodic fracture activation and propagation to a height of 56m. Further advancement to 115m resulted in a second periodic roof collapse, with fracture activation and propagation extending to 94m. Upon reaching 130m of advancement, the third periodic roof collapse occurred, elevating fracture activation and propagation height to 163m and indicating that the working face had reached critical mining dimensions. Concurrently, stratification fractures within the original 1-2 coal seam substantially reduced, while tracer tests confirmed hydraulic connectivity between downward-propagating surface fractures and upward-extending bedrock fractures within 120 days (Figure 16).\u003c/p\u003e\u003cp\u003eUpon advancing to 150m, the fourth collapse of the caved roof strata occurred, resulting in widened fractures within the open-off cut of the N1114 working face and intensified surface damage. Subsequent advancement to 163m triggered the fifth roof collapse. Concurrently, the original stratification fractures within the N1114 goaf area substantially closed, while fractures traversing the remaining coal pillar were activated and widened under mining-induced stress. For the 2-2 coal seam, the immediate roof strata exhibit an initial collapse step distance of 65m, a periodic collapse step distance ranging from 16m to 25m, and a critical mining distance of approximately 163m.\u003c/p\u003e\u003cp\u003eDuring advancement of the N1206 working face into and retreat from the coal pillar area, fractures within the coal pillar undergo activation followed by progressive closure as shown in Figure 17. As the face advances toward the pillar, pre-existing fractures gradually activate, resulting in fracture widening and increased surface subsidence. Following pillar extraction, the inverted-trapezoid configuration of the coal pillar undergoes uniform subsidence, causing reactivated fractures to close again, reducing surface settlement rates and moderating overall subsidence. Physical simulation determined an optimal 40m separation distance between coal pillars in the 1-2 and 2-2 coal seams. The surface subsidence profile after simulated extraction of the 2-2 coal seam appears in Figure 18, demonstrating that strategic pillar placement proves effective in controlling surface deformation and mitigating mining-induced surface damage.\u003c/p\u003e"},{"header":"4. The effect of coal seam group and coal column group structure","content":"\u003cp\u003eThe protective coal pillar along the strike of adjacent single-seam working faces primarily ensures subsequent roadway stability, where pillar integrity determines goaf stability. Conversely, in multi-seam mining, the dimensions, position, and stability of protective pillars within upper-seam goaf areas directly govern lower-seam face layout, ground pressure manifestation, and support system selection as demonstrated by references 37 to 40. This research specifically addresses multi-seam extraction by analyzing faces influenced by overlying goaf pillars during advance and retreat phases to establish optimal inter-pillar spacing. This configuration ensures the collective structural response within the strategically placed coal pillar group effectively mitigates surface subsidence and associated damage.\u003c/p\u003e\u003cp\u003eFollowing simulated extraction of the 1\u003csup\u003e-2\u003c/sup\u003e coal seam at Ningtiaota, a 20m coal pillar remains intact. Subsequent mining of the 2\u003csup\u003e-2\u003c/sup\u003e coal seam located 40m deeper, followed by extraction of the 3\u003csup\u003e-1\u003c/sup\u003e coal seam, induces progressive healing of original fractures within overlying seams through goaf compaction and subsidence. Maintaining 80m separation between 3\u003csup\u003e-1\u003c/sup\u003e and 2\u003csup\u003e-2\u003c/sup\u003e seam pillars achieves uniform surface subsidence documented in Figure 19. Dial gauge measurements enabled reconstruction of the tri-seam post-mining subsidence profile presented in Figure 20.\u003c/p\u003e\u003cp\u003eFigure 20 demonstrates significantly greater surface subsidence in dip-oriented areas lacking coal pillar support. Post-mining subsidence measurements reveal maximum values and subsidence factors of 1.2m at 0.63 for the 1\u003csup\u003e-2\u003c/sup\u003e seam, 4.0m at 0.62 for the 2\u003csup\u003e-2\u003c/sup\u003e seam, and 5.5m at 0.60 for the 3\u003csup\u003e-1\u003c/sup\u003e seam. During 3\u003csup\u003e-1\u003c/sup\u003e seam extraction, absolute surface subsidence and corresponding subsidence factors above coal pillars are quantified in Table 5. Strategically implementing staggered pillar arrangements achieves uniform absolute surface subsidence, with subsidence factors exhibiting positive correlation to absolute displacement magnitudes.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eTable 5.\u0026nbsp;\u003c/strong\u003eAbsolute sinking value and sinking coefficient of coal column position after multi-coal seam mining\u003c/p\u003e\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd style=\"width: 133px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eObservation category\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMining coal seams\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1\u003csup\u003e-2\u003c/sup\u003e coal seam\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003ecoal column\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e2\u003csup\u003e-2\u003c/sup\u003e coal seam\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003ecoal column\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e3\u003csup\u003e-1\u003c/sup\u003e coal seam\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003ecoal column\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd rowspan=\"4\" style=\"width: 133px;\"\u003e\n \u003cp\u003eAbsolute sinking value/m\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e1\u003csup\u003e-2\u003c/sup\u003e coal seam\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e0.18\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e1.20\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e1.21\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e2\u003csup\u003e-2\u003c/sup\u003e coal seam\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e2.98\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e1.85\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e2.79\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e3\u003csup\u003e-1\u003c/sup\u003e coal seam\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e2.05\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e2.18\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e1.49\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eTotal\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e5.21\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e5.23\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e5.29\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd rowspan=\"4\" style=\"width: 133px;\"\u003e\n \u003cp\u003eRelative sinking value/m\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e1\u003csup\u003e-2\u003c/sup\u003e coal seam\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e2\u003csup\u003e-2\u003c/sup\u003e coal seam\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e0.65\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e0.40\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e0.60\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003e3\u003csup\u003e-1\u003c/sup\u003e coal seam\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e0.75\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e0.80\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e0.54\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd style=\"width: 110px;\"\u003e\n \u003cp\u003eTotal\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 116px;\"\u003e\n \u003cp\u003e1.46\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003e1.84\u003c/p\u003e\n \u003c/td\u003e\u003ctd style=\"width: 124px;\"\u003e\n \u003cp\u003e1.78\u003c/p\u003e\n \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003cp\u003eTable 4 data enabled plotting absolute subsidence magnitudes for each coal seam post-mining, as presented in Figure 21. Analysis reveals minimum subsidence occurs precisely above reserved coal pillars, demonstrating how the inverted-trapezoid configuration decelerates settlement in affected zones. Implementing staggered pillar arrangements during sequential extraction yields approximately equivalent cumulative surface subsidence across multi-seam operations. Increasing seam extraction layers enhances the efficacy of strategic pillar staggering; theoretically enabling near-horizontal surface conditions. These findings validate that the coal pillar group structure effectively regulates surface movement and enhances formation self-healing capacity.\u003c/p\u003e\u003cp\u003eBased on deformation monitoring and subsidence engineering principles established in reference 43, surface critical mining requires goaf dimensions exceeding 1.2H to 1.4H, where H denotes average burial depth in meters. During shallow coal seam extraction in Shenfu mining area, all working faces achieved supercritical mining status, with residual coal pillars constituting the primary cause of uneven surface subsidence. The author maintains that optimal pillar staggering positions the inverted-trapezoid structure formed by pillars within surface subsidence irregularity zones during upper-seam extraction. Considering stress distribution footprints in floor strata, combining 3-1 seam pillars with pre-existing trapezoid configurations generates new composite structures that control differential settlement. This transforms characteristic W-shaped subsidence basins from initial mining phases through strategic pillar deployment. Consequently, surface fracture widths and displacements progressively diminish or close while subsidence basin extent increases, ultimately promoting ecological self-restoration and enabling sustainable green mining practices.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003e1) Physical simulations and field measurements reveal distinct fracture propagation mechanisms in shallow coal seam mining: under typical shallow-buried conditions, open-off cut fractures develop primarily through arch-shaped failure within sand strata, with surface tension cracks positioned directly above the cut; whereas near-shallow mining exhibits parabolic rock stratum failure dominating cut-related fracture development, producing surface tension cracks inward of the open-off cut alignment. Moreover, during large-scale and high-altitude extraction operations, mining height increments from 4m to 6m elevate fracture propagation rates by 40\u0026ndash;45% in shallow-buried seams (JZ\u0026thinsp;\u0026lt;\u0026thinsp;0.8), but exhibit diminished effects in near-shallow seams (JZ\u0026thinsp;\u0026gt;\u0026thinsp;0.8).\u003c/p\u003e \u003cp\u003e2) Overburden fractures in shallow coal seam mining are categorized by development direction into upstream and downstream types, and by location into open-off cut fractures, dynamic periodic fractures, and roadway boundary fractures. Surface manifestations comprise two macroscopic types: permanent fixed fractures and temporary dynamic fractures. The latter periodically generate and close, demonstrating strata self-repair capacity, while permanent fractures represent the primary control targets in operational mining practice.\u003c/p\u003e \u003cp\u003e3) The repeated extraction of shallow-buried coal seams induces cumulative damage mechanisms that substantially alter the spatial distribution characteristics and mechanical properties of three distinct fracture types within overlying strata and surface fracture networks, where mining-induced activation of pre-existing discontinuities creates complex stress redistribution patterns. While experimental observations confirm significant modifications to crack propagation dynamics and deformation parameters, current methodologies lack robust quantitative frameworks for characterizing the spatiotemporal evolution of activation-induced fracture networks, particularly in terms of multi-cycle mining-induced fracture parameter degradation laws and healing potential under varying geological conditions.\u003c/p\u003e \u003cp\u003e4) The residual stress redistribution effects induced by residual coal pillars within abandoned goaf areas constitute the predominant factor contributing to differential surface subsidence patterns. Through scientifically optimized coal pillar spacing and systematic pillar group configurations, this study proposes an optimized coal pillar configuration that reduces surface subsidence differentials to \u0026le;\u0026thinsp;0.08m(Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), significantly mitigating uneven settlement risks and progressive fracture healing in the overlying strata. This engineered approach facilitates self-repair mechanisms while maintaining essential load-bearing capacity within the substructure, effectively realizing eco-friendly mining practices and sustainable ecological restoration of disturbed terrains through controlled strata behavior modulation.\u003c/p\u003e"},{"header":"6. Discussion","content":"\u003cp\u003eWhile this study integrates physical simulations and field monitoring, several limitations warrant consideration:\u003c/p\u003e\n\u003cp\u003e1. Temporal constraints: 120-day monitoring periods (Section 3.2) capture short-term fracture reactivation but cannot characterize decade-scale creep effects, potentially underestimating long-term aperture widening by 18-22% based on [28].\u003c/p\u003e\n\u003cp\u003e2. Hydrological simplification: The FEM model assumes homogeneous aquifer properties, whereas actual Jurassic coalfields exhibit karst-fracture dual permeability (Section 1), potentially affecting water inrush threshold\u0026nbsp;θ\u0026nbsp;accuracy.\u003c/p\u003e\n\u003cp\u003e3. Pillar degradation: Long-term stability of \"inverted trapezoidal\" coal pillars requires decade-scale monitoring, as creep deformation may reduce efficacy by 15–20%.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u0026nbsp;\u003c/strong\u003eConceptualization,\u0026nbsp;C.M.;\u0026nbsp;Experimental Design,\u0026nbsp;C.M.\u0026nbsp;and X.Z. and J.Z.; Validation,\u0026nbsp;C.M.\u0026nbsp;and X.Y.;\u0026nbsp;Theoretical\u0026nbsp;Analysis,\u0026nbsp;C.M.\u0026nbsp;and J.Z.;\u0026nbsp;Data\u0026nbsp;Curation,\u0026nbsp;J.Z. and X.Z.;\u0026nbsp;Supervision,\u0026nbsp;H.C.;\u0026nbsp;Writing\u0026nbsp;—Original\u0026nbsp;Draft\u0026nbsp;Preparation,\u0026nbsp;C.M.;\u0026nbsp;Writing—Review\u0026nbsp;\u0026amp;\u0026nbsp;Editing,\u0026nbsp;J.Z. and\u0026nbsp;C.M.\u0026nbsp;Supervision, H.C.; Project Administration,\u0026nbsp;C.M.; Funding Acquisition,\u0026nbsp;C.M.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement:\u003c/strong\u003e The datasets generated and/or analysed during the current study are available from the corresponding author (Chi Mu, [email protected]) upon reasonable request. These include raw experimental data (e.g., crack width measurements, displacement rates, and stress distributions), geomechanical simulation parameters, and field monitoring records. Due to the large file sizes and proprietary nature of some 3D simulation models, complete datasets are not publicly deposited but will be shared under a Material Transfer Agreement (MTA) to ensure proper attribution and data security.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThis study was funded by the following projects: 1.\u0026nbsp;Key Laboratory of Mine Geological Hazards Mechanism and Control,\u0026nbsp;Ministry of Natural Resources(6000240984); 2. Service Local Special Project of Shaanxi Provincial Department of Education (23JC024);\u0026nbsp;3. Shaanxi Postdoctoral Fund (2023BSHEDZZ296);\u0026nbsp;4. Key Project of Natural Science, Shaanxi Energy Vocational and Technical College (23BSZRZ01);\u0026nbsp;5.\u0026nbsp;The Shaanxi Provincial Outstanding Young Talent Support Program for Higher Education Institutions (2024).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments:\u003c/strong\u003e We thank the aforementioned foundation of for its support of this study.\u0026nbsp;We thank the academic editors and anonymous reviewers for their kind suggestions and valuable comments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest:\u003c/strong\u003e The authors declare no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHuang, Q. X. \u003cem\u003eStudy on roof structure and ground control in shallow seam longwall mining\u003c/em\u003e (China University of Mining and Technology, 2000).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, S. M., Huang, Q. X. \u0026amp; Fan, L. M. 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Y., Hu, Y., Deng, Y. \u0026amp; Mahadevan, S. Supplier selection using AHP methodology extended by D numbers \u003cem\u003eJ\u003c/em\u003e. \u003cem\u003eExpert Syst. Appl.\u003c/em\u003e \u003cb\u003e41\u003c/b\u003e (1), 156\u0026ndash;167 (2014).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Shallow buried coal seams, activated structure, crack development, coupling control, green mining","lastPublishedDoi":"10.21203/rs.3.rs-6808617/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6808617/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eComprehending fracture evolution mechanisms in shallow coal seam mining is fundamental to ensuring operational safety and environmental sustainability. This research integrates physical simulations and field measurements to investigate fracture dynamics within Shenfu Coalfield's three strategically selected mining zones characterized by coal seam thicknesses of 4–6m, interlayer spacings of 20–40m, and burial depths of 68–163m. The study establishes a systematic fracture classification: directionally as upstream fractures propagating upward from excavation zones and downstream fractures extending surfaceward; spatially as open-off cut fractures at 60°–65° inclinations, dynamically periodic fractures synchronized with roof pressure cycles, and roadway-boundary fractures. Surface manifestations comprise permanent fissures exceeding 0.2m width requiring engineered control and temporary fractures demonstrating self-healing through strata recompaction. Key findings reveal a significant correlation between surface fracture spacing and periodic roof pressure intervals with correlation strength R²=0.92 and statistical significance p\u0026lt;0.01. Optimized 40–60m coal pillar spacing reduces boundary fracture propagation by 62% while confining differential surface settlement below 5%. Practical implementation of staggered pillar configurations achieves 78% fracture closure efficiency, permitting ecologically balanced mining with under 3% vegetation disturbance. These outcomes establish a validated predictive framework for overburden stability management while advancing sustainable extraction protocols through science-driven pillar engineering and fracture mitigation strategies.\u003c/p\u003e","manuscriptTitle":"Activation Mechanisms and Coupled Control of Mining-Induced Fractures in Shallow Coal Seams Under Multi-Load Conditions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-26 14:11:45","doi":"10.21203/rs.3.rs-6808617/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-05-05T12:44:04+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-05T04:15:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"19455999388501215378803075984408160961","date":"2026-03-01T07:06:31+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-31T01:47:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"90575238377494972434944663254517824729","date":"2025-12-26T01:27:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-24T01:58:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"283876454266684979153960404491722084426","date":"2025-12-23T00:37:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-07T09:39:09+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-14T06:51:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"30438864819587845825711763109891893655","date":"2025-08-14T15:44:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"172190669495330237338645412528822110838","date":"2025-08-13T04:07:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"165388713540305621982464294791668813885","date":"2025-08-11T17:56:38+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-24T11:51:34+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-24T11:46:31+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-06-18T23:36:25+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-12T07:33:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-06-12T07:08:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9f836191-4469-4f86-98ae-d9a417c5d5ff","owner":[],"postedDate":"June 26th, 2025","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-05-05T12:44:04+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[{"id":50524489,"name":"Earth and environmental sciences/Solid earth sciences/Geodynamics"},{"id":50524490,"name":"Earth and environmental sciences/Solid earth sciences/Geology"},{"id":50524491,"name":"Earth and environmental sciences/Solid earth sciences/Hydrogeology"},{"id":50524492,"name":"Earth and environmental sciences/Solid earth sciences/Mineralogy"},{"id":50524493,"name":"Earth and environmental sciences/Solid earth sciences/Petrology"}],"tags":[],"updatedAt":"2026-05-05T12:57:38+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-26 14:11:45","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6808617","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6808617","identity":"rs-6808617","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}