Study on Rational Roadway Layout and Air Leakage Prevention in Shallow Close-Distance Coal Seam Mining

Abstract

To address the issues of roadway instability and severe air leakage in goaf areas during overlapping coal pillar mining in shallow multi-seam coalfields, this study takes the 22,209 working face of Huojitu Shaft in the Shendong Daliuta Mine as the research object. Using the discrete element method (DEM), the optimal layout of roadways in the lower coal seam and the corresponding evolution of overburden fractures were simulated. In addition, the effectiveness of goaf backfilling in controlling overburden air leakage channels was analyzed and verified. The results indicate that the width of coal pillars in the upper seam should be greater than approximately 23 m to ensure that roadways remain in a stress-stable zone. Roadways in the lower seam should be horizontally arranged within a range of 35–55 m from the center of the overlying coal pillar. This layout effectively avoids placing the roadway beneath the high-stress concentration zone or the pressure-relief area of the goaf. After mining the upper coal seam, the overburden collapse zone takes on a “trapezoidal” shape, and mining-induced fractures develop upward to the surface, forming vertical and inclined fracture channels that penetrate to the surface, resulting in severe air leakage in the goaf. Following the mining of the lower seam, the interlayer strata are completely fractured, leading to secondary development of fractures in the overlying old goaf. This results in the formation of a connected fracture network spanning from the surface through the seam goaf linkage. Implementing goaf backfilling measures significantly reduces the vertical settlement of the overburden, prevents the formation of through-layer air leakage channels, and effectively mitigates interlayer air leakage problems during lower-seam mining.

1. Introduction

Shallow coal resources are widely distributed in northern China. Their extraction frequently triggers large-scale surface subsidence, leading to ecological degradation, frequent geological hazards, and environmental deterioration [1,2,3]. Multi-seam mining operations in shallow regions are particularly challenging due to narrow inter-seam spacing and secondary mining disturbances, which complicate the stress distribution of overlying strata and exacerbate air leakage dynamics. Under these conditions, lower-seam mining faces are prone to hazards such as roadway instability and collapse, frequent hypoxia in working faces, and spontaneous combustion in goaf areas [4,5,6]. Consequently, investigating the optimal positioning of mining roadways and the fracture development characteristics in overlying strata during shallow multi-seam extraction is critical for ensuring roadway stability and preventing gas-related disasters.

Extensive research has been conducted on the rational layout of close-distance coal seam roadways from mechanical perspectives, including overburden stress fields, roadway deformation, and spatial arrangement [7,8]. Wang et al. [9] introduced the Dempster–Shafer evidence theory to develop a system optimization model for coal mine roadway layout, establishing a comprehensive factor framework for spatial arrangement. Zhu et al. [10] analyzed roadway deformation patterns at the 2112 working face in Liangjia Coal Mine and proposed a BBSCS method to reinforce roadway roofs and coal seams. Wei et al. [11] optimized the rational layout of lower-seam roadways by comparing stress distribution characteristics in roadways at varying positions. Zhang et al. [12] elucidated the pressure-relief mechanism of roadways beneath goaf areas and investigated stress compositions in the roof, floor, and surrounding strata. Wei et al. [13] analyzed stress variations in lower seams influenced by residual coal pillars and explored optimal roadway layouts for lower-seam mining. Simultaneously, understanding the initiation, distribution, and evolution of overburden fractures during mining is essential for characterizing air leakage pathways [14,15]. Researchers have extensively studied fracture development patterns in shallow multi-seam mining overburden from diverse perspectives, yielding valuable insights [16,17]. Pan et al. [18] investigated the vertical progression of overburden fractures in shallow coal seams and numerically simulated their spatial evolution during mining. Wang et al. [19] spatially analyzed fracture propagation, surface subsidence, and strata collapse, clarifying the transmission mechanisms of overburden fractures. Xu et al. [20] used 3DEC software to simulate the spatiotemporal evolution of overlying strata fractures during coal mining, studied the fractal characteristics of the overlying strata caused by mining, and revealed different time series of fractal properties and spatial fracture patterns of the overlying rock mass. Wang et al. [21] simulated continuous multi-seam mining processes, revealing that the dynamic interaction between collapsed zones and surrounding rock dominates the evolution of overburden load-bearing structures. Zhang et al. [22] proposed a Space Sky Surface (3S) integrated system that effectively monitors the movement of the cover layer in high-intensity mining areas in real time. However, comprehensive research on the reasonable layout of shallow multi seam mining tunnels and the corresponding development of air leakage cracks in overlying rock mining is not yet complete [23,24].

Therefore, this study establishes a DEM model for shallow multi-seam mining based on the 22,209 working face in Huotuji Mine. First, post-mining stress distributions in the upper seam were analyzed. The stress characteristics of lower-seam roadways at varying horizontal positions relative to overlying residual coal pillars were investigated, enabling the proposal of optimal roadway layouts under these conditions. Subsequently, the overlying strata collapse and fracture development patterns during contiguous multi-seam extraction were simulated, identifying the evolutionary mechanisms and spatial distributions of primary air leakage channels. Finally, the efficacy of goaf backfilling in mitigating strata subsidence and suppressing fracture development was comparatively evaluated.

2. Review

The Huojitu mine, part of the Shendong-Daliuta coal mining area, is situated in Shenmu City, Shaanxi Province. The mine contains two coal seams: #12 and No. 22, with the latter being the current primary mining target. The #22 coal seam has an average burial depth of 120.28 m and a thickness of 4–5 m. Overlying this, the #12 coal seam exhibits an average burial depth of 99.73 m and a thickness of approximately 6 m. The inter-seam spacing between the #22 and #12 coal seams averages 30.64 m, and they can thus be classified as a close-distance coal seam group. As the #12 seam has been largely mined out, abundant residual coal pillars remain in the goaf areas. This configuration poses two critical engineering challenges: (1) optimizing roadway layout in the #22 seam and (2) preventing surface air leakage. This study focuses on the #22 coal seam’s 22,209 fully mechanized mining face. The burial depth of this working face ranges from 67.2 to 116.9 m (average: 70.43 m), with a coal seam thickness of 4.2 m and dip angles of 1–3°. The immediate roof and floor of the working face consist of siltstone. Above the face lie the mined-out areas of the #12 coal seam’s 12,201 and 12,203 working faces, with an inter-seam spacing of approximately 24.5 m. The stratigraphic column of the 22,209 working face is illustrated in Figure 1.

3. Numerical Modeling

A two-dimensional discrete element method (DEM) numerical model for shallow multi-seam mining was developed using PFC^2D (5.0) software, based on stratigraphic data from the 22,209 working face in Huojitu Mine, Shendong mining area (Figure 2). The model spans 350 m in length and 100 m in height, comprising 22 strata layers. Particles were generated with a minimum diameter of 0.35 m and a maximum-to-minimum size ratio of 1.66, totaling 43,585 discrete elements. Particle movement was confined within the cuboid boundaries of the model. A full-seam-height mining method was implemented, sequentially extracting the upper and lower coal seams from left to right. The model was solved at 1 m mining increments.

To accurately capture material mechanical behavior, mesoscopic parameters in the particle flow model were calibrated to match macroscopic rock properties. The calibration process followed Wang et al. [25], focusing on two parameter categories: (1) particle properties: density, friction coefficient, Young’s modulus, and normal-to-shear stiffness ratio; (2) parallel bonds: Young’s modulus, normal-to-shear stiffness ratio, tensile strength, shear strength, and friction angle. Calibration was validated through agreement between simulated and experimental stress–strain curves and failure modes, ensuring DEM parameters effectively represented macroscopic coal-rock behavior. The calibrated mesoscopic parameters for each stratum are summarized in

Table 1. Microparameters of calibrated DEM model strata.

Property	Parameter	Mudstone	Sandy Mudstone	Fine-Grained Sandstone	Medium-Grained Sandstone	Coarse-Grained Sandstone	Siltstone	Coal
particle	Density (Kg/m3)	2300	2400	2600	2500	2400	2400	1650
	$R_{min}$				1.2
	$R_{max}$/$R_{min}$				1.3
	$E_c$ (GPa)	7.23	11.25	15.87	13.42	9.75	18.88	3.34
	$\mu$	0.5	0.5	0.6	0.5	0.5	0.55	0.45
	$k_n/k_s$	2.0	2.0	1.8	1.8	2.0	1.7	2.15
bond	$\overline{E_c}$ (GPa)	7.23	11.25	15.87	13.42	9.75	18.88	3.34
	$\overline{{\sigma }_c}$ (MPa)	11.05	14.25	24.63	16.62	7.51	18.82	4.21
	$\overline{{\tau }_c}$ (MPa)	11.15	14.25	24.53	16.83	7.51	18.92	4.31
	$\overline{k_n/k_s}$	2.0	2.0	1.8	1.8	2.0	1.7	2.15
	$\phi$	35	35	40	35	35	40	42

Note: D is the density; $E_c$ is the Young’s modulus of particle contact; $\overline{E_c}$ is the parallel bonding Young’s modulus; $\mu$ is the coefficient of friction of particles; $k_n/k_s$ is the particle stiffness ratio; $\overline{{\sigma }_c}$ is the parallel bonding tensile strength; $\overline{{\tau }_c}$ stands for Parallel Bond Shear Strength; $\phi$ is the parallel bonding friction angle.

, Figure 3 shows the calibration results of coal and fine-grained sandstone.

4. Results

4.1. Optimal Roadway Layout

4.1.1. Rational Width of Section Coal Pillars in Upper-Seam Mining

Figure 4 illustrates the vertical stress distribution in the stope following extraction of the #12 coal seam’s 201 working face. Since our research focuses on the stress distribution during the mining process of the 201 working face under shallow buried multi-coal-seam geological conditions, without involving specific external factors, we assume that the rock is isotropic. During mining of the 201 working face, the original stable stress state was disrupted, leading to stress redistribution and localized concentration around the excavated area. Post mining of the upper seam, stress release occurred in the floor strata beneath the goaf, forming a low-stress zone. Conversely, stress concentration developed near the lateral coal pillars of the lower close-distance seam due to overlying goaf pressure, propagating downward along the seam floor. Simultaneously, distinct stress concentration zones emerged on both sides of the 201 goaf, with vertical stress gradually recovering to in situ levels as distance from the goaf increased. As shown in Figure 4, vertical stress in the outer coal seam decreased progressively with distance from the goaf boundary, stabilizing at in situ stress levels approximately 23 m from the goaf edge. Thus, sufficient pillar width must be reserved between roadways in the same seam, and new roadways should avoid stress concentration zones induced by adjacent mining activities. For the Huojitu Mine, section coal pillars should exceed 23 m in width. Current design for Huojitu Mine sites adopts 30 m-wide pillars, which satisfies this criterion.

4.1.2. Optimal Positioning of Lower-Seam Roadways

As shown in Figure 5, mining of the overlying #12 coal seam induced significant stress concentration in section coal pillars. This resulted in a trapezoidal stress distribution pattern in the underlying strata, where the spatial extent of stress concentration expanded with depth while its magnitude diminished. Stress distribution in the #22 coal seam beneath the #12 seam was monitored (Figure 6). The stress magnitude peaked directly below the pillar center, exhibiting a symmetrical decline toward both sides. At 35 m from the pillar center, stress levels fell below in situ values due to pressure relief from the overlying goaf.

Figure 7: Vertical stress variations in the roadway surrounding rock at different distances from the #12 coal pillar center. At 5 m from the pillar center (directly beneath the section coal pillar), both left- and right-side surrounding rock exhibited elevated vertical stresses. At 15 m (adjacent to the pillar’s right edge), the left-side strata experienced marginally increased vertical stress, while the right-side strata, located within the pressure-relief zone of the 203 goaf, showed a significant stress reduction. At 35 m (the critical boundary of the #12 pillar’s influence zone), roadway surrounding rock stress decreased substantially. Beyond this distance, stress continued to decline slightly but at a diminishing rate. Notably, excessive proximity to the upper goaf interior risks overburden stress accumulation, significantly increasing roadway loads. Therefore, roadways should be positioned horizontally within 35–55 m of the residual upper pillar center to balance stress mitigation and operational safety.

4.2. Fracture Development Patterns

4.2.1. Upper-Seam Mining

Figure 8: Simulated overburden collapse during mining of the #12 coal seam’s 201 working face. As shown in Figure 8a, when the 201 working face advanced to 56 m, the immediate roof (Layer 11) experienced initial collapse, triggering the first weighting event. Mining-induced fractures propagated upward, with collapsed zones exhibiting higher porosity. The main roof (Layer 10) remained largely unaffected, retaining lower porosity. At 74 m advancement, failure of the key stratum (Layer 10) induced the first periodic weighting of the main roof, with a weighting interval of 18 m. Collapsed zones adopted a trapezoidal geometry, accompanied by pronounced lateral fractures. Vertical fractures emerged in central goaf areas, extending upward to Layer 07, while high-porosity zones and bed separation cracks dominated goaf margins. During the third periodic weighting at 105 m, the key stratum (Layer 07) prevented fracture propagation to overlying strata (Layers 07–05). Cyclic collapse occurred between Layers 11 and 08, expanding horizontally rather than vertically. The fourth periodic collapse at 121 m caused vertical expansion of collapsed zones through Layers 07–05. Microscopic fractures proliferated in Layers 04–02, with porosity maps revealing interconnected vertical fractures along goaf margins, forming inclined fracture channels extending to the bed separation zone between Layers 05 and 04. At 138 m advancement, pronounced bending subsidence occurred in upper Layer 05. Minor bed separation between Layer 02 and surface Layer 01 connected preexisting inclined fracture channels, enabling fracture propagation to the surface. This suggests potential air leakage pathways from the #12 goaf to the surface, allowing atmospheric air ingress. Throughout the mining process, periodic weighting events recurred at intervals of 13–20 m.

Figure 9 illustrates the comprehensive overburden collapse pattern, fracture development, and air leakage pathways following mining operations at Panels 201 and 203 in #12 coal seam. Based on previous analysis of rational roadway layout, a 30 m inter-panel coal pillar was established between Panels 201 and 203. The overburden collapse mechanism at Panel 203 followed patterns similar to those observed at Panel 201 (as detailed in prior excavation analysis) and will not be reiterated here. As shown in Figure 9, mining-induced fractures propagated to the ground surface following completion of extraction operations in #12 coal seam.

4.2.2. Lower-Seam Mining

Mining of the #22 coal seam’s 209 working face commenced after completion of the overlying #12 seam. Based on optimal roadway layout analysis, the 209 working face roadway was positioned 45 m horizontally from the center of the upper section coal pillar. Figure 10 depicts the overburden collapse during mining of the 209 working face. At 52 m advancement, Layers 19–17 fractured and collapsed, while mining-induced cracks developed in Layers 16–13. High-porosity “oblique zones” formed along both sides of the goaf, yet porosity did not extend to Layer 16 or the interburden strata, indicating no interconnected air leakage pathways between seams at this stage. At 70 m advancement, the face traversed the overlying section coal pillar. Excessive interburden stress caused complete fracture of the interlayer strata. Vertical cracks propagated through the central and peripheral zones of the goaf, connecting to the abandoned #12 goaf. This established cross-seam air leakage pathways, allowing oxygen-depleted gas from the #12 goaf to flow downward into the 209 working face goaf. Porosity distribution maps confirmed that high-porosity zones extended to the #12 goaf. Notably, the high-porosity oblique zones on both sides of the section pillar became interconnected, enabling surface air leakage to penetrate both upper and lower goaf areas through this channel. Cumulative mining thickness increased overburden subsidence, inducing a “reactivation effect” on the abandoned upper goaf. Fractured strata in the upper goaf underwent secondary crack development, with microscopic fractures proliferating and overall porosity rising significantly. At 87 m and 110 m advancements, the interburden experienced its 2nd and 4th periodic fractures, respectively. The influence zone of lower-seam mining expanded progressively, with porosity increases spreading across the abandoned upper goaf. Simultaneously, strata above the section coal pillar fractured extensively, accompanied by marked porosity elevation.

Figure 11 illustrates the comprehensive fracture development and air leakage pathway formation in stabilized overburden following extraction at Panel 209 of #22 coal seam. Severe overburden damage occurred after mining the lower #22 coal seam. Comparative analysis with Figure 9 reveals a significant increase in microfracture density and overall porosity within the overburden. Numerous, intricate air leakage pathways developed between the ground surface and the goaf area of #22 coal seam. The primary leakage channels were identified as oblique fractures along the goaf margins and zones between periodic overburden breakages.

Following the completion of lower-coal-seam extraction, interconnected vertical fracture zones were formed in the interlayer region, exhibiting an average width of 35 cm and a maximum vertical development height of 82 m. These fracture zones penetrate into the overlying goaf area. Symmetrically distributed oblique fractures were observed along the goaf margins, with a width of 25–30 cm on each side, extending over 80 m along the coal seam strike.

5. Discussion

5.1. Fracture Development Control Measures

As established, thin interburden layers and substantial cumulative mining thickness in shallow, closely spaced coal seams induce complete interlayer failure and secondary activation of overlying abandoned goafs. This process intensifies overburden fragmentation, accelerates fracture/leakage pathway development, and amplifies surface-to-goaf air leakage. The utilization of solid waste materials for underground backfilling has emerged as a pivotal strategy in sustainable coal mining practices. Unfilled goafs created by coal extraction generate substantial voids, triggering sequential subsidence, fracturing, and collapse of overlying strata from lower to upper levels, accompanied by drastic stress redistribution and displacement. Fractured strata reorganize during collapse, creating extensive bed separations, voids, and fractures. These unstable structures pose critical safety risks while severely damaging overlying aquifers and surface ecosystems. Solid waste backfilling significantly modifies overburden deformation patterns and structural responses, effectively reducing strata damage and fundamentally altering the spatiotemporal evolution of fractures. A numerical model simulating fracture development in overlapping shallow coal seams was implemented, incorporating backfill blocks (5 m intervals, 3 m height, 60% backfill ratio) during lower-seam panel advancement. This simulation investigated overburden failure mechanisms, fracture propagation, and leakage pathway formation under backfilled conditions.

Figure 12 presents simulated overburden collapse patterns and fracture generation processes during backfill mining at Panel 22,209 of Huojitu Mine.

Figure 12a reveals that initial overburden fracture and collapse occurred at a panel advance of 52 m, with pronounced subsidence observed in Layers 19 to 17. The presence of backfill blocks substantially mitigated deformation severity. Comparative analysis with Figure 10a demonstrates significant reduction in interlayer bed separations and oblique fractures along goaf margins under backfilled conditions. Microscopic fractures propagated upward to Layers 16–14. At 70 m advancement, microfractures extended to the floor of Coal Seam #12, resulting in complete interlayer failure. Backfill blocks effectively restricted settlement height of fractured overburden. Vertical high-porosity pathways were constrained, with only multiple transverse interlayer channels developing (porosity ~0.2). Comparison of Figure 10b and Figure 12b indicates markedly reduced porosity in fractured overburden and non-interconnected leakage pathways between upper and lower goafs under backfilling. Porosity remained stable at ~0.2 during subsequent advances (87 m and 110 m), though spatial propagation occurred in both vertical and lateral directions. Oblique leakage pathways along abandoned upper goafs progressively interfaced with lower channels. Nevertheless, compared to unfilled conditions, both pathway development intensity and leakage magnitude were significantly reduced.

Figure 13 illustrates the spatial distribution of overburden fractures and air leakage pathways following mining completion at Panel 209 in coal seam #22 under backfilled goaf conditions. Figure 13a reveals sustained high fragmentation intensity in the backfilled goaf overburden, particularly within periodic weighting zones where microfracture clustering is pronounced. However, backfill blocks significantly reduced overburden settlement height, enabling recompaction of fractured strata. Consequently, overall porosity remained stable, with limited connectivity to overlying goafs through low-porosity leakage pathways (high airflow resistance). Comparative analysis of Figure 8, Figure 10 and Figure 12 demonstrates that lower-seam backfilling effectively mitigates reactivation effects on overlying abandoned goafs during mining. Fracture density and porosity distribution in overlying legacy goafs showed negligible variation under lower-seam backfilled mining conditions. The specific changes in porosity are shown in

Table 2. Table of changes in porosity before and after filling.

Advancing Distance	Observation Indicators	Before Filling	After Filling	Drop
52 m	Interlayer fissure	0.25–0.30	0.15–0.18	40–42%
0 m	Vertical through passage	0.35–0.40	≤0.20	43–50%
	Horizontal interlayer channel	0.28–0.32	0.18–0.22	33–36%
87 m	Periodic break zone	0.38–0.45	0.20–0.25	43–47%
110 m	Activation area of old goaf	0.40–0.50	0.18–0.22	55–60%

.

5.2. Settlement Characteristics

Vertical displacement of marker particles at varying elevations above upper- and lower-seam roofs was monitored pre- and post-mining. Figure 14 presents settlement curves for distinct overburden strata following lower-seam extraction. As shown in Figure 14a, post-mining settlement curves exhibit a concave profile with steep flanks. Proximal strata experienced deeper subsidence and more severe collapse, attenuating with increasing distance from the seam. Distant strata displayed reduced subsidence magnitude and spatial extent due to load-bearing effects from collapsed lower layers. Collapse completeness correlated inversely with vertical distance from the mined seam.

Figure 14b depicts settlement patterns under backfilled conditions, revealing substantial modifications compared to unfilled scenarios in shallow multi-seam mining. First, backfilling reduced maximum settlement magnitudes across all strata through enhanced recompaction, flattening the concave profile. Second, settlement curves exhibited minor fluctuations rather than abrupt discontinuities at profile edges. Notably, near-surface strata maintained minimal deformation, demonstrating backfilling’s efficacy in controlling surface subsidence and leakage pathway development.

5.3. Field Application

Based on research findings, the following technical guidelines are proposed for adjusting mining plans: (1) Pillar width optimization: strictly maintain the residual pillar width in the upper seam ≥23 m to ensure lower-seam roadways remain within the stress-stable zone outside the pillar projection. (2) Roadway layout selection: lower-seam roadways should be horizontally arranged within 35–55 m on both sides of the overlying pillar centerline. Real-time monitoring using borehole stress gauges during construction is required to avoid entering high-stress concentration zones or goaf pressure-relief areas. (3) Backfill reinforcement: prioritize paste backfill technology for upper-goaf treatment, requiring a backfill rate exceeding 85% and backfill strength ≥5 MPa to suppress mining-induced fracture development. This study provides methodologies for addressing roadway layout in shallow coal seams and overburden fracture development. However, due to variations in the properties of different coal seams, field applications require case-specific analysis. The proposed solutions should be adapted to formulate appropriate prevention measures based on actual geological conditions.

6. Conclusions

(1) Analysis of vertical stress distribution outside the upper-seam goaf revealed a gradient attenuation pattern with increasing distance from the goaf boundary. Vertical stress recovered to the in situ stress threshold at 23 m, necessitating inter-panel coal pillars exceeding 23 m to ensure roadway placement within stable in situ stress zones. (2) When the lower-seam roadway was 5 m from the overlying panel pillar center, both sides exhibited high vertical stresses due to direct superposition. At 15 m spacing (adjacent to the pillar’s right edge), left-side stresses increased marginally under concentrated loading, while right-side stresses dropped sharply within the goaf pressure relief zone. Stress decay became critical at 35 m (stress influence boundary), with further distance (>35 m) yielding gradual stress reduction. Optimal roadway positioning is recommended within 35–55 m of legacy pillar centers. (3) Post mining of upper seams, mining-induced fractures propagated to the surface, causing severe air leakage under atmospheric pressure differentials. Lower-seam extraction intensified overburden damage, with microfracture density and overall porosity increasing markedly. Complex leakage networks formed between surface and goaf, dominated by oblique fractures along goaf margins and zones of periodic overburden failure. Thin interburden and high cumulative extraction thickness in shallow multi-seam mining triggered complete interlayer failure and secondary activation of abandoned goafs, exacerbating overburden fragmentation and leakage pathway development. Backfilling in lower-seam goafs significantly reduced reactivation effects on overlying abandoned goafs. Fracture density and porosity distribution in legacy goafs remained largely unchanged under backfilled mining. Settlement curves demonstrated pronounced modifications, including reduced maximum subsidence magnitudes and collapse intensity.