Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory

Abstract

The continuous downward mining of close-distance coal seam groups faces severe challenges, yet existing research rarely addresses the structural failure mechanisms in groups with three or more layers. To address this, a two-dimensional physical similarity simulation combined with non-contact digital image correlation (DIC) technology and fractal geometry theory was conducted based on the geological conditions of Donghuantuo Coal Mine. This multi-method approach ensured the high-precision capture and validity of the spatiotemporal deformation data. The evolution of overlying strata and fracture networks during the extraction of four close-distance coal seams was quantified. The results indicate that underlying seam mining triggers severe secondary activation of upper goafs, which transforms the classic vertical three-zone structure into a composite trapezoidal failure zone. Driven by structural instability, the maximum subsidence of the overlying strata exhibits a step-like nonlinear growth, increasing dramatically from an initial 0.44 m to 8.70 m. Simultaneously, the topological evolution of the fracture network exhibits an overall nonlinear increase. Specifically, the fractal dimension rose from an initial value of 1.234 to a more stable value of 1.437, featuring two significant surges with growth rates of 8.34% and 3.79% that directly corresponded to spatial goaf connectivity. The mutual verification between the macroscopic displacement jumps and the fracture network evolution confirms the reliability of the obtained results. Ultimately, the mechanical model of the interlayer rock transitions from a rigid load-bearing beam to a loose buffer layer. Based on these mechanisms, a differentiated interlayer support strategy is proposed. High pre-tension and impact-resistant supports must be applied to the upper seams, whereas pressure-relief and flexible yielding supports are required for the lower seams. This study provides theoretical guidance for disaster prevention in close-distance coal seam groups mining.

1. Introduction

Coal is the cornerstone of China’s energy structure, and its safe and efficient mining is crucial for ensuring national energy security [1,2,3]. As shallow and geologically superior single thick coal seam resources become increasingly depleted, coal mining inevitably shifts to deep coal seams and close-distance coal seam groups with complex occurrence conditions [4,5,6]. During the downward mining process of a close-distance coal seam group, the thickness of the rock layers between the coal seams is generally very thin. The mining of the upper coal seam not only generates strong stress superposition effects but also causes irreversible mining damage to the roof structure of the underlying coal seam. During the mining process of the underlying coal seam, there will be frequent occurrences of severe strata behavior, large-scale roof collapse, and asymmetric deformation of the roadway, which seriously threaten the safety of workers and restrict the production efficiency of the mine [7,8,9].

Domestic and foreign researchers have conducted extensive research on the fracture laws and stress evolution characteristics of mining overburden, and have developed classic rock control theories represented by the “Voussoir Beam” theory and the “Vertical Three Zones” model [10,11,12]. In recent years, in order to cope with the complex mechanical environment of multi-seam mining, many scholars have conducted in-depth research on the mining response of close-distance coal seam groups using theoretical analysis, numerical simulation, and physical experiments. Early foundational work by Mark et al. [13] systematically analyzed multiple seam stability across numerous case histories, providing critical empirical and numerical frameworks for overall risk assessment. Building on this, researchers have explored stress redistribution and geomechanical responses in detail. Suchowerska et al. [14] utilized finite element modeling to identify the variables affecting vertical stress changes under supercritical longwall panels, while Zhang et al. [15] highlighted geotechnical considerations and sequencing strategies for concurrent pillar recovery to minimize seam interactions. To ensure safe extraction, Das et al. [16] proposed methodologies for contiguous coal seams that guarantee the stability of the parting and surface structures. Furthermore, since multi-seam mining inherently exacerbates surface subsidence, Suchowerska Iwanec et al. [17] and Pongpanya et al. [18] investigated the geomechanics of subsidence induced by single and multi-seam extractions, emphasizing the importance of constitutive models and optimal panel dimensions in predicting and mitigating surface damage.

In terms of the mechanism of overlying strata fracture, Ning et al. [19] established a mechanical model for secondary activation of overlying strata based on the mechanical response characteristics of the goaf overlying strata, and derived a recursive formula for separation distance. Tan et al. [20] and Song et al. [21] proposed four types of overburden structure models based on the evolution law of interlayer hard rock bearing structure, elucidating the disaster mechanism of interlayer rocks under multiple mining operations. In terms of close-distance coal seam mining, Huang et al. [22] established a roof structure analysis model based on the mining practice of shallow-buried coal seam groups, revealing the fundamental reasons for the occurrence of roof step subsidence and severe strata behavior. Chai et al. [23] investigated the evolution law of mining-induced stress in extremely close-distance coal seam groups based on valley terrain. Xie et al. [24] and Jia et al. [25] analyzed the characteristics of asymmetric roof fracture and surrounding rock failure of roadway under repeated mining of highly inclined coal seam groups based on the geological background of highly inclined coal seams, combined with physical similarity simulation and 3DEC discrete element analysis, and proposed asymmetric coupling support technology. Miao et al. [26] and Yu et al. [27] discussed the reasonable layout and strong support, strong pressure relief, and collaborative control technology of mining tunnels under repeated mining of close-distance coal seam groups. Miao et al. [28] quantitatively analyzed the maximum instability depth of overlying strata based on on-site drilling measurements and theoretical derivation, and determined the feasibility of upward mining. Wu et al. [29] studied the spatial distribution characteristics of void fraction in overlying strata under repeated mining, providing a basis for the capacity evaluation of underground reservoirs in mines. Ju et al. [30] and Shabanimashcool et al. [31] respectively deepened their understanding of cumulative damage and rock stability in multi-seam mining from the perspectives of coupling large mining height supports with surrounding rocks and analysis of layered roof.

Although research has laid the foundation for the disturbance mechanism of overlying strata, most existing theories are primarily applicable to single or double-seam extraction. They cannot accurately explain the structural failure mechanisms in close-distance coal seam groups with three or more layers [13,18]. In conventional single/double seam mining, the disturbed strata typically self-adjust to maintain a stable “voussoir beam” hinged structure [10]. However, under continuous downward multi-seam mining, the mechanical environment undergoes fundamental changes. Specifically, the downward extraction induces severe stress field superposition from residual pillars and boundary goafs [13,14]. Furthermore, the excavation of underlying seams acts as a dynamic trigger for continuous goaf secondary activation, causing previously stabilized upper goafs to re-fracture and continuously subside [19].

Therefore, regarding multi-seam mining as a simple superposition of single mining effects fails to fully elucidate the nonlinear cumulative damage of the overlying strata system [7,9]. Especially for the thin interlayer rock sandwiched between adjacent goafs, the mechanisms of strata instability and dynamic fracture connectivity—under the dual constraints of severe stress field superposition and multi-layer goaf secondary activation—still lack precise quantitative research [17,25]. On the other hand, most physical similarity simulations use point contact monitoring methods such as surface displacement meters or total stations [32,33]. These devices are highly prone to failure upon rock collapse and cannot monitor the dynamic evolution of the internal fracture network in a full-field and continuous manner.

To address the complex mechanisms of close-distance coal seam mining, this study utilized the close-distance coal seam group of layers 8, 9, 11, and 12₋₁ in Donghuantuo Mine as the engineering background. The primary purpose is to quantitatively reveal the evolutionary characteristics of overlying strata instability and fracture network connectivity. Under this framework, physical similarity simulation experiments were conducted and innovatively combined with non-contact Digital Image Correlation (DIC) technology. The continuous simulation and full-field monitoring of the displacement and fracture fields throughout the downward mining process of the four coal seams were successfully achieved. Consequently, this study uncovers the structural transformation mechanisms of the overlying strata and discovers a step-like increase in the subsidence displacement. Furthermore, the growth law of the fractal dimension of the fracture network was quantitatively analyzed using fractal geometry theory, and a differentiated interlayer support strategy was proposed. The study provides theoretical guidance for the safe and efficient mining and disaster prevention of close-distance coal seam groups.

2. Materials and Methods

2.1. Geological Background

Donghuantuo Coal Mine is located in Lubei District, Tangshan City, Hebei Province, China. Its geographical location is 117°57′32″ E to 118°03′18″ E and 39°33′42″ N to 39°40′51″ N. The mining area is 40.5 km², with a production capacity of 4.5 Mt/a. There are a total of 9 coal seams that can be mined, among which 8, 9, 11, and 12₋₁ coal seams are the close-distance coal seam group that can be mined. The distance between 8 coal seam and 9 coal seam is 6 m, the distance between 9 coal seam and 11 coal seam is 8.7 m, and the distance between 11 coal seam and 12₋₁ coal seam is 9.9 m. Mining operations are carried out sequentially from top to bottom. At present, the mining of 8, 9, and 11 coal seams is nearing completion, while the 12₋₁ coal seam is still in the mining stage.

2.2. Physical Model Setup

This study was based on the engineering and geological background of Donghuantuo Coal Mine, and conducted physical similarity simulation research on the mining areas of 8, 9, 11, and 12₋₁ coal seams. Based on the actual situation and research characteristics of the close-distance coal seam group mining in Donghuantuo Coal Mine, a two-dimensional plane physical similarity simulation platform was adopted. The size was 2400 × 200 × 1300 mm (length, width, height), and the area of speckle deployment was 2000 mm × 1000 mm in the middle of the simulation platform. The physical similarity simulation experimental system is shown in Figure 1. Due to the limitation of the size of the model experiment, similar simulation experiments could not simulate all the top and bottom coal and rock layers. The counterweight block simulated the pressure of the overlying strata, and during the laying process, the thickness of each rock layer was ensured to be uniform and stable as much as possible.

(1)

Similar parameters were determined. Based on the occurrence conditions of the overlying strata in the area and the size parameters of the laboratory model frame, a plane stress model was used for physical simulation experiments. The geometric similarity ratio of the model was 1:150. Geometric similarity ratio ${\alpha }_l=l_m/l_p=1:150$, Time similarity ratio ${\alpha }_t=t_m/t_p=4:49$, Gravity similarity ratio ${\alpha }_{\gamma }={\gamma }_m/{\gamma }_p=2:3$, Uniaxial compressive strength, elastic modulus, and cohesion similarity ratio ${\alpha }_R={\alpha }_E={\alpha }_C={\alpha }_l{\alpha }_{\gamma }=1:225$, Similarity ratio of internal friction angle ${\alpha }_{\varphi }=r_m/r_p=1$, Gravity acceleration similarity ratio ${\alpha }_g=g_m/g_p=1$, and Similarity ratio of applied forces ${\alpha }_f=f_m/f_p={\alpha }_g{\alpha }_{\gamma }{\alpha }_l^3=1.975\times {10}^{-7}$. Among them, l_m, γ_m, t_m, r_m, g_m, and f_m are the geometric dimensions, bulk density, time, internal friction angle, gravitational acceleration, and applied forces in similar simulations; l_p, γ_p, t_p, r_p, g_p, and f_p are relevant data measured on the model.

(2)

The selection and proportioning of similar materials were determined. Similar materials mainly included river sand, lime, gypsum, and mica flakes. River sand was used as aggregate, gypsum and lime were used as bonding materials, and mica flakes were used as separation materials between rock layers. Black ink was added to simulate coal seams. Based on the physical and mechanical parameters and similar conditions of each rock layer, the ratio of coal seam physical similarity testing materials with different rock characteristics was determined. The proportioning scheme of the physical similarity model is shown in

Table 1. Proportioning scheme of the physical similarity model.

Serial Number	Rock Character	Buried Depth/m	Thickness/m	Model Thickness/cm	Material Quality/kg
					River Sand	Lime	Gypsum
25	Siltstone	480.80	16.50	11.00	213	13.3	13.3
24	Fine sandstone	497.30	19.50	13.00	254.7	14.2	14.2
23	Siltstone	516.80	18.00	12.00	235	13	13
22	Fine sandstone	534.80	12.00	8.00	156.6	8.7	8.7
21	Siltstone	546.80	4.50	3.00	58.5	3.25	3.25
20	Fine sandstone	551.30	5.85	3.90	76.5	4.8	4.8
19	Siltstone	557.15	7.70	5.10	99	5.5	5.5
18	Claystone	564.85	7.80	5.20	56	24.9	31.1
17	Fine sandstone	572.65	6.45	4.30	82.7	5.2	5.2
16	8 coal seam	579.10	3.60	2.40	46.8	3.1	2.1
15	Siltstone	582.70	6.00	4.00	76.5	4.8	4.8
14	9 coal seam	588.70	2.60	1.70	33.3	2.2	1.5
13	Fine sandstone	591.30	4.00	2.60	49.8	3.1	3.1
12	Siltstone	595.30	4.70	3.10	60.3	3.35	3.35
11	11 coal seam	600.00	1.80	1.20	23.4	1.6	1
10	Fine sandstone	601.80	3.30	2.20	44.5	2.8	2.8
9	Siltstone	605.10	6.00	4.00	77.4	4.3	4.3
8	12−1 coal seam	611.10	2.40	1.60	30.4	2	1.4
7	Siltstone	613.50	3.60	2.40	46.8	2.6	2.6
6	Fine sandstone	617.10	16.80	11.20	213	13.3	13.3
5	12−2 coal seam	633.90	4.64	3.10	90	6	4
4	Siltstone	638.54	10.50	7.00	137	7.6	7.6
3	Fine sandstone	649.04	19.50	13.00	254.7	14.2	14.2
2	Siltstone	668.54	15.00	10.00	196.2	10.9	10.9
1	Fine sandstone	683.54	22.50	15.00	294	16.4	16.4

.

Before constructing the physical model, standard cylindrical specimens of the mixed materials were prepared and subjected to uniaxial compression tests. Through iterative adjustments of the ratio, it was ensured that the comprehensive mechanical properties of the proportioned materials strictly satisfied the predetermined similarity ratio.

DIC technology was used for the non-contact continuous monitoring of the physical models to capture the spatiotemporal evolution characteristics of the overlying strata displacement under multiple mining operations. Compared to traditional point displacement sensors, DIC technology overcomes the limitations of limited measurement points and susceptibility to rock collapse. It can accurately reproduce the full field displacement evolution process of the model surface [33]. The experiment utilized the Image J 1.54g, VIC-2D (Version 7.0) analysis system developed by Correlated Solutions (Columbia, SC, USA), with monitoring equipment including digital cameras and non-contact full area displacement measuring instruments. The digital camera was used to record the evolution of the fracture and crack network of each rock layer during the mining process, while the non-contact full area strain gauge was used to monitor the displacement changes of each rock layer. To ensure image matching accuracy and eliminate environmental interference, the layout of the DIC monitoring system strictly followed the following regulations:

(1)

The quality of the speckle field directly determined the calculation accuracy of DIC. After the surface of the model was uniformly coated with white matte primer, black scattered spots of different sizes were randomly drawn manually. The speckle diameter was strictly controlled within 2 to 5 pixels to ensure optimal gradient identification by the software subset. To accurately capture the development of cracks and deformation of tunnels, speckle refinement was carried out in the expected fracture and strong disturbance areas to ensure high contrast and randomness of the grayscale distribution.

(2)

To eliminate testing errors caused by changes in natural lighting angles, the experiment was conducted in a dark room environment. Two sets of high-brightness photography lights were used for symmetrical supplementary lighting to ensure a uniform and shadow-free light field on the surface of the model. The high-definition digital camera continuously tracked and captured the experimental process at a fixed frequency of once every 5 s.

(3)

Due to the limitations of the camera field of view and the influence of boundary effects, the model’s surroundings were excluded, resulting in an effective full field monitoring area of 2000 mm × 1000 mm.

2.3. Excavation Procedure

After the model was laid, it was maintained in its natural state for one week. To accelerate the evaporation of internal moisture and ensure structural stability, a strategy of dismantling the channel steel formwork at intervals was adopted. After the model reached the design test strength and stabilized, all formwork was removed. Subsequently, white matte latex paint was evenly applied on the surface of the model as the DIC speckle base color, and black spots were sprayed on the white background. At the top of the model, a similar load equivalent to a 480.80 m thick overlying strata is applied.

Based on the actual engineering background of Donghuantuo Mine, this experiment adopted a downward mining strategy: 8 coal seam → 9 coal seam → 11 coal seam → 12₋₁ coal seam. To truly reproduce the changes under close-distance coal seam mining, the entire simulation process was divided into the following stages. First, the 8 coal seam was mined by sequentially excavating the 3084 working face and 3086 working face from right to left. Then, the 9 coal seam was mined by excavating the 3094 and 3098 working faces from right to left in sequence. Next, the 11 coal seam was mined by excavating the 3014 and 3018 working faces from right to left in sequence. Finally, the 12₋₁ coal seam was mined by excavating the 3024 working face from right to left. As shown in Figure 2.

Standardized excavation cycles were strictly followed in the mining simulation. The fixed step distance for each advancement was 5 cm, corresponding to an actual advancement of 7.5 m. After the excavation of each working face was completed, the model was left to stand for 0.5 h to ensure that the internal stress of the model was fully transmitted. The crack network was fully developed, and the rock structure deformation was stable. During mining, the DIC system performed continuous high-frequency image acquisition to capture the deformation characteristics of the entire field.

3. Results and Analysis

3.1. Characteristics of Overlying Strata Fractures and Displacements

The cumulative damage behavior during the downward mining process of the close-distance coal seam group was accurately captured. By combining high-definition macroscopic fracture images with DIC full area displacement cloud maps, an in-depth analysis was conducted on the rock fracture morphology and displacement field evolution characteristics of each coal seam mining stage. It should be noted that to facilitate direct guidance for on-site engineering practice, all displacement and dimensional values mentioned in the following analyses are the actual engineering conversion values. These values were calculated by converting the raw displacement data measured on the physical model based on the geometric similarity ratio.

3.1.1. 8 Coal Seam Mining

As the initial working face, the deformation and fracture behavior of the overlying strata in the 3084 working face of the 8 coal seam exhibited typical evolutionary characteristics of single coal seam mining, as shown in Figure 3. In the early stage of working face advancement (Figure 3a), the roof rock layer mainly underwent bending deformation with minimal subsidence. When the advancing distance reached 67.5 m (Figure 3b), the main roof reached the limit span and underwent the first rupture, causing the collapse of the overlying strata. At this stage, due to differences in mechanical properties such as elastic modulus and tensile strength among different rock layers, the asynchronous rotation and subsidence behavior of each rock layer leads to obvious horizontal delamination cracks in the surrounding rock.

As the working face continued to advance (Figure 3c), the fracture network further expanded upwards, and the overlying strata gradually evolved the classic three-zone structural characteristics in the vertical direction. Namely, the caved zone, fractured zone, and continuous deformation zone. By combining the DIC full field displacement cloud map, it could be observed that the subsidence displacement field of the roof presented a local ellipsoidal distribution of subsidence basins in space. During the initial rupture of the main roof, the maximum displacement of the entire field was 0.09 m. When the working face advanced to the design position, the roof fracture structure tended to stabilize, and the maximum cumulative displacement of the entire site increased to 0.44 m. The deformation feature of the initial working face provided an initial damage and stress environment for subsequent close-distance multi-seam downward mining.

Subsequently, during the mining period of the 3086 working face adjacent to the same coal seam, the evolution of the overlying strata spatial structure showed significant lateral support pressure concentration and secondary activation effect in the goaf, as shown in Figure 4. In the early stage of the 3086 working face, thanks to the bearing and control effect of the overlying high-strength fine sandstone layer, the direct roof did not experience instability or collapse. However, the local deviatoric stress field induced by mining shifted laterally, causing the already stable overlying strata structure of the 3084 goaf to be reactivated.

The DIC full area displacement monitoring clearly captured this deformation behavior. When the 3086 working face advanced to 52.5 m (Figure 4a), although the roof of this working face was still in a relatively stable state, lateral disturbance had caused a large number of newly separated fractures and secondary sliding collapses in the rock layers above the 3084 goaf. The maximum subsidence displacement at its center rose to 0.93 m. When the working face advanced to 60 m (Figure 4b), the main roof of the 3086 working face reached the limit span, and the first fracture occurred, causing the overlying strata to be completely cut off. This strong dynamic fracturing process caused significant stress impact on adjacent goaf areas, further exacerbating the nonlinear settlement of the overlying strata in the 3084 working face, causing its maximum displacement to surge to 1.148 m.

As the working face continued to advance and entered the periodic collapse stage, a typical cantilever beam structure was formed in front of the working face. When the extraction of the 3086 working face was completed (Figure 4c), the fractured rock blocks in the 3084 goaf were again fractured and rotated to sink under the influence of secondary mining, and the fracture zone rapidly expanded towards the upper rock layers, with a maximum displacement of 2.07 m in this area. The stress arch structure above the mining area underwent reconstruction, and the settlement center of the entire site eventually shifted towards the main mining area of the 3086 working face, with the maximum displacement of the entire area increasing to 3.00 m.

3.1.2. 9 Coal Seam Mining

As a continuous coal seam of the 8 coal seam, the mining of the 9 coal seam was in a more complex stress and structural environment. The movement of overlying strata and the evolution of fracture networks were influenced by both the damage in the goaf above and the support pressure.

The monitoring of overlying strata movement and displacement during the mining period of the initial working face 3094 in the 9 coal seam is shown in Figure 5. In the initial stage of advancing the working face (Figure 5a), the interlayer rock structure remained basically stable. The maximum displacement center of the entire site was still above the 3086 goaf, with a subsidence displacement of 3.08 m. When the 3094 working face advanced to 60 m (Figure 5b), the main roof reached the limit span, and the first collapse occurred. It is worth noting that the initial collapse step distance of the initial working face in coal seam 9 was 60 m, which was less than 67.5 m for coal seam 8, with a reduction of 11.11%. The reason was that the roof of the 9 coal working face was within the disturbance range of the 8 coal mining, resulting in crack damage, reduced rock strength, and a decrease in the basic collapse step distance.

When the mining face passed under the return airway of the 3086 working face, the mining stress activated the floor cracks caused by the mining of the upper coal seam, resulting in a large number of new cracks expanding diagonally upwards. These cracks cut through a 6 m thick sandstone layer, causing instability and failure of residual coal pillars in the 8 coal seam (Figure 5c). The three goaf areas of 3084, 3086, and 3094 were connected, and the overlying strata structure changed. The isolated local subsidence funnels were fully connected and merged, forming a composite subsidence basin spanning three working faces. The maximum displacement of the entire area moved above the 3094 working face, and the maximum displacement of the entire area eventually increased to 3.56 m.

As a continuous working face of the 9-coal seam, the mining of the 3098 working face brought severe challenges to the overlying strata system. This area was not only directly affected by the excavation of this working face, but also subjected to multiple disturbances from the lateral support pressure of the adjacent 3094 goaf and the residual stress of the 8-coal double goaf.

The monitoring of overlying strata movement and displacement during the mining period of the 3098 working face is shown in Figure 6. When the working face advanced to 60 m (Figure 6a), the roof strata bent and subsided, with a maximum displacement of 3.75 m in the entire area. When the working face advanced to 67.5 m (Figure 6b), the interlayer rock layers were damaged under shear and tension, and the overlying strata structure reached its bearing limit, with the basic roof experiencing its first rupture. Secondary instability and subsidence occurred in the rock blocks above the goaf of coal seam 8. The residual bearing structure of the interlayer rock mass was destroyed, and the fracture network expanded both horizontally and vertically.

When the working face advanced to 127.5 m (Figure 6c), the failure mode of the overlying strata structure changed. The layered rock mass, which originally had bearing capacity, gradually transformed into a fragmented block structure similar to loose media after four mining damages. Under the compaction effect of gravity and the self-weight of the overlying strata, the maximum displacement of the entire area moved above the 3098 working face and eventually increased to 5.19 m.

3.1.3. 11 Coal Seam and 12₋₁ Coal Seam Mining

As the third layer of coal seam mining, the mining environment of the 3014 working face became a passive loading state affected by the overlying loose goaf. The monitoring of overlying strata movement and displacement during this stage is shown in Figure 7.

During the advancement process of the 3014 working face (Figure 7a,b), the roof exhibited slow bending and sinking characteristics, which were different from the severe collapse caused by the mining of 8 and 9 coal seams. The reason was that the overlying strata had undergone repeated mining of the first two layers of coal, and the original load-bearing beam structure had transformed into loose and broken. The broken roof did not experience a sudden collapse.

As the working face continued to advance (Figure 7c), the mining pressure relief space further expanded. The delamination cracks repeatedly expanded and contracted. When the mining of the working face was completed, the deformation of the overlying strata mainly manifested as the overall downward movement and internal compaction of the overlying composite subsidence basin. The maximum displacement of the entire area increased to 5.70 m. This stage marked a shift in the dominant movement pattern of overlying strata in close-distance coal seam groups from brittle fracture to compaction dominated by fragmented and expansive rock masses.

Subsequently, it entered the mining stage of the 3018 working face. The monitoring of the overlying strata fracture and displacement during the advancement of the 3018 working face is shown in Figure 8.

DIC displacement monitoring detected a process from local damage to structural failure. When the working face advanced to 15 m (Figure 8a), slight subsidence occurred in the interlayer rock layers, and the overlying strata structure remained stable. When the working face continued to advance to 30 m (Figure 8b), two conjugate shear fractures rapidly developed diagonally upwards from the working face. These two shear cracks not only cut through the interlayer rock layers, but also caused the instability of the residual coal pillars above. This event marked the connection of three goaf areas.

As the working face entered the periodic collapse stage (Figure 8c), the overlying strata structure still exhibited a fragmented block group structure without obvious overall bearing units. The maximum settlement center shifted towards the 3018 working face, and the cumulative maximum settlement displacement increased to 7.88 m.

The 12₋₁ coal seam was the bottom final mining layer of the close-distance coal seam group, and the overlying strata bearing system had suffered cumulative damage from the continuous downward mining of three layers of coal. Figure 9 shows the monitoring of overlying strata movement and displacement during the advancement of the 3024 working face.

In the initial stage of advancing the 3024 working face (Figure 9a), although the overlying strata had been highly fragmented and transformed into fragmented and expansive particles, the roof and floor structure of this coal seam remained stable, and no obvious cracks or collapses were observed. During the initial stage of bottom coal mining, the residual self-bearing capacity of the original rock did not completely disappear. When the working face advanced to 52.5 m (Figure 9b), the main roof reached its ultimate span. The overlying loose rock mass experienced an overall decline, with the maximum displacement of the entire area increasing to 8.03 m.

As the working face continued to advance (Figure 9c), the fracture network was completely formed. The interlayer penetrating fractures that originated in the deep part were connected to the residual delamination fractures in the shallow part, and the development height of the water-flowing fractured zone reached directly to the top of the model. The depth of each subsidence funnel in the goaf was fused to form a clearly defined trapezoidal failure zone. The maximum displacement of the entire area ultimately increased to 8.70 m.

3.2. The Displacement Evolution Characteristics of Overlying Strata

The displacement cloud maps of each working face after mining are shown in Figure 10. The maximum displacement of the entire area after seven mining operations was extracted. After the successive mining of working faces 3084, 3086, 3094, 3098, 3014, 3018, and 3024, the maximum subsidence displacement of the overlying strata reached 0.44 m, 3.00 m, 3.56 m, 5.19 m, 5.70 m, 7.88 m, and 8.70 m, respectively. The step-by-step displacement increments were 2.56 m, 0.56 m, 1.63 m, 0.51 m, 2.18 m, and 0.93 m, respectively.

The maximum displacement curve of the overlying strata after mining on each working face is shown in Figure 11. The subsidence displacement of the overlying strata exhibited a step-like nonlinear growth pattern. The monitoring data revealed three significant stages of displacement surges during the mining process. The first significant increase occurred during the mining of the 3086 working face, where the maximum displacement jumped from 0.44 m to 3.00 m. The second increase occurred during the mining of the 3098 working face, rising from 3.56 m to 5.19 m. The third increase was observed during the mining of the 3018 working face, escalating from 5.70 m to 7.88 m.

Furthermore, comparing the displacement distribution patterns in Figure 10a–g reveals clear changes in the displacement gradients. During the mining of coal seams 8 and 9, clear color variation boundaries were present in the displacement cloud maps, indicating high displacement gradients. Conversely, during the mining of coal seams 11 and 12₋₁, the displacement contour lines became noticeably thinner and narrower, indicating a more continuous and expansive deformation pattern across the entire overlying strata.

3.3. Evolution Characteristics of the Fractal Dimension of the Fracture Network

The rock fracture network of each mining stage was extracted using digital image processing technology, and the box counting method based on fractal geometry was introduced to analyze the fractal dimension of the fracture network. The topological characteristics of the fracture network were quantitatively evaluated [34]. As shown in Figure 12, the fitting coefficient R² of each working face after mining was greater than 0.99, indicating that the mining-induced fracture network possessed significant fractal characteristics in its spatial distribution.

After the successive mining of working faces 3084, 3086, 3094, 3098, 3014, 3018, and 3024, the fractal dimensions of the fracture network increased to 1.234, 1.337, 1.344, 1.395, 1.410, 1.422, and 1.437, respectively. The corresponding stage-by-stage growth rates were 8.34%, 0.52%, 3.79%, 1.08%, 0.85%, and 1.05%.

The variation curve of the fractal dimension throughout the entire mining process is shown in Figure 13. Based on the monitoring data, the evolution of the fractal dimension exhibited a distinct nonlinear trend, characterized by two significant surges followed by a slow and stable growth phase. Specifically, the fractal dimension showed a remarkable increase after the mining of the 3086 working face and the 3098 working face. In contrast, during the subsequent mining of the 11 and 12₋₁ coal seams, the growth rate of the fractal dimension significantly decreased and fluctuated around 1%.

4. Discussion

4.1. Instability Mechanisms of Overlying Strata in Close-Distance Coal Seam Groups Mining

The downward mining of close-distance coal seam groups is fundamentally distinct from the simple linear superposition of single-seam mining effects. Rather, it is a complex, non-linear cumulative damage process governed by the mechanical transition of the interlayer rock. Based on the similarity simulation results, the overlying strata undergo a continuous degradation from rigid discontinuous deformation to flexible continuous deformation. During the extraction of the upper coal seams, the interlayer rock maintains sufficient stiffness to act as a rigid load-bearing beam, exhibiting classical “vertical three-zone” characteristics. However, as mining progresses downwards, the strong dynamic load effect not only collapses the immediate seam but also triggers the secondary activation of the upper goafs. After multiple mining disturbances, the interlayer rock completely loses its beam-bearing characteristics, transforming into a highly fragmented, low-cohesive loose buffer medium. Ultimately, the deep fusion of isolated local subsidence funnels creates a composite subsidence basin, forming a macroscopic trapezoidal failure zone spanning the entire mining area.

Driven by this structural evolution, the damage behavior exhibits remarkable cross-scale consistency between macroscopic subsidence and fracture network topology. The subsidence displacement of the overlying strata demonstrates a step-like nonlinear growth. This is mirrored by significant surges in the fractal dimension of the fracture network. The first major structural failure occurs when the main roof undergoes large-scale breakage during the full extraction of the first coal seam. This event not only transforms the single goaf into multiple connected goafs, causing the initial displacement surge, but also drives the topological transformation of isolated local cracks into whole-layer connectivity, resulting in the first significant surge in the fractal dimension.

As mining proceeds to the underlying seams, a secondary disturbance stage initiates. The localized deviatoric stresses completely shear through the interlayer rock between the upper and lower goafs. This spatial penetration induces massive joint pressure relief and generates a large number of oblique fractures. Consequently, the overlying system experiences a simultaneous secondary jump in both macroscopic subsidence displacement and the fracture fractal dimension.

In the final stages of continuous mining, the system enters a damage saturation stage. At this point, the cumulative damage sustained by the overlying strata system reaches its maximum. The mechanical behavior of the highly fragmented rock mass shifts from brittle fracturing to frictional sliding and the compaction of loose particles. Because the roof strata lack the physical continuity to form stable “voussoir beam” hinged structures, no large-scale new fracture networks are generated. Therefore, the fractal dimension growth tends to stabilize, and the final increases in subsidence are primarily driven by the volumetric compaction of the gaps between the fragmented rock blocks rather than new structural failures.

4.2. Differentiated Disaster Prevention and Control Strategies

Differentiated engineering guidance strategies for disaster prevention and control of close-distance coal seam groups were proposed. The disaster monitoring window moves forward in time and space, and the most dangerous stage is during the mining period of the continuous working face between the first and second coal seams, rather than the mining period of the bottom 11 and 12₋₁ coal seams. This is because the overlying strata structure undergoes drastic changes during the early stages, causing water-flowing fractured channels to expand rapidly. Therefore, when deploying on-site gas extraction drilling and roof grouting projects, the key monitoring window must be focused on the mining period of the upper seams. Once mining enters the 11 or 12₋₁ coal seams, although the subsidence is massive, the water channels and fracture networks have already essentially stabilized.

Furthermore, a differentiated support strategy based on damage evolution must be implemented. In the upper coal seam mining stage, impact-resistant reinforcement support is used, and in the lower coal seam mining stage, flexible yielding support is used. During the mining of 8 and 9 coal seams, the fractal dimension increased significantly, and the roof was accompanied by severe fractures of hard rock layers. At this time, the support of the roadway must be centered on resistance to impact and shear, and prevention of strong dynamic load damage. A high pre-tension and high-strength constant-resistance and large-deformation cable support system can be used to absorb the elastic energy released by sudden rock fractures. Entering the stage of mining 11 and 12₋₁ coal seams, the fractal dimension grows slowly, and the overlying strata have become looser and looser. At this point, the roadway is no longer facing dynamic load impact, but rather huge static pressure settlement and continuous floor heave. The strategy of using pressure relief and flexible load-bearing support should be adopted, such as using U-shaped steel collapsible brackets, etc. [35,36,37]. It provides theoretical guidance for safe and efficient mining and disaster prevention and control of close-distance coal seam groups.

Beyond guiding on-site engineering practice, from a scientific perspective, this study significantly advances the fundamental understanding of strata mechanics in multi-seam environments. Methodologically, by integrating non-contact DIC technology with fractal geometry, this research achieves a novel cross-scale quantitative characterization—bridging the gap between macroscopic structural instability and the microscopic topological evolution of fracture networks. Theoretically, it breaks through the limitations of traditional single-seam “vertical three-zone” models. By revealing the mechanisms of step-like nonlinear subsidence and the formation of a composite trapezoidal failure zone driven by goaf secondary activation, these findings provide a universal theoretical framework for evaluating cumulative rock damage, which can be broadly applied to the stability control of complex underground multi-seam extractions globally.

However, it should be pointed out that this study is based on the assumption of two-dimensional plane strain and dry-state physical similarity simulation, which, to some extent, ignores the three-dimensional spatial effects and fluid–structure coupling effects under real working conditions [38,39,40]. The lack of three-dimensional spatial constraints may result in simulated displacement values slightly higher than actual values [41], while the absence of water weakening effects may underestimate the severity of rock mass evolution in water-rich mining areas [42]. Regarding the next steps for further research, we will introduce three-dimensional discrete element numerical simulation and on-site microseismic monitoring techniques [43,44,45,46]. These methods will be used for engineering verification and parameter correction of the transition mechanism from structural beams to loose cushion layers and the topological law of crack networks.

5. Conclusions

This study is based on the downward mining of the close-distance coal seam group in Donghuantuo Coal Mine. Through similarity simulation experiments combined with DIC full-area displacement monitoring technology, the evolution of the overlying strata structure and fracture connectivity laws under the influence of multiple mining operations was revealed. The main findings are summarized as follows:

(1)

The evolutionary mechanism of the overlying strata structure in close-distance coal seam groups was revealed. The mining of the upper coal seam causes damage to the lower rock mass, weakening the structural stability of the lower coal seam, which resulted in an 11.1% reduction in the initial collapse step distance of the 9 coal seam compared to the 8 coal seam. Furthermore, the downward mining triggers the secondary activation of the upper goafs. After multiple mining disturbances, the interlayer rock structure is significantly crushed, transforming the classic vertical three-zone structure into a composite trapezoidal failure zone, while the interlayer rock evolves from a rigid load-bearing beam to a loose buffer layer.

(2)

The subsidence displacement of the overlying strata exhibits a step-like nonlinear growth characteristic. Driven by structural instability rather than simple linear superposition, the maximum subsidence displacement increased dramatically from an initial 0.44 m to a final state of 8.70 m. The change in maximum subsidence displacement is closely related to changes in rock structure. There are three significant increases in subsidence displacement, and the reasons for the increases are different.

(3)

The topological evolution of the fracture network was quantitatively captured. The fractal dimension of the fracture network showed an overall nonlinear increasing trend, rising from 1.234 to a stabilized 1.437. This evolution was characterized by two significant surges with growth rates of 8.34% and 3.79%, which were triggered by the connectivity of the goaf in the same coal seam and the connectivity of the upper and lower goafs, respectively. The fractal dimension eventually stabilized because the overlying strata became highly fragmented, transitioning the movement into sliding between fragments and the compaction of loose particles.

(4)

A differentiated interlayer support strategy for close-distance coal seam groups was proposed. Based on the damage evolution, the support design must be differentiated. For the upper coal seams, a high pre-tension and high-strength constant-resistance large-deformation support system should be applied to resist dynamic impacts and shear forces. Conversely, for the lower coal seams, a pressure-relief and flexible yielding support strategy must be adopted to accommodate the massive static pressure settlement. These differentiated strategies provide significant industrial usefulness and practical guidance for safe and cost-effective disaster prevention in close-distance coal seam groups mining operations globally.