Open AccessArticle Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory by Baogui Yang Baogui Yang SciProfiles Scilit Preprints.org Google Scholar , Fei He Fei He SciProfiles Scilit Preprints.org Google Scholar , Sheng Zhang Sheng Zhang SciProfiles Scilit Preprints.org Google Scholar * and Yongliang Li Yongliang Li SciProfiles Scilit Preprints.org Google Scholar School of Energy and Mining Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China * Author to whom correspondence should be addressed. Processes 2026, 14(12), 1852; https://doi.org/10.3390/pr14121852 (registering DOI) Submission received: 24 April 2026 / Revised: 16 May 2026 / Accepted: 30 May 2026 / Published: 8 June 2026 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 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. 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 km 2, 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 . 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 α l = l m / l p = 1 : 150 , Time similarity ratio α t = t m / t p = 4 : 49 , Gravity similarity ratio α γ = γ m / γ p = 2 : 3 , Uniaxial compressive strength, elastic modulus, and cohesion similarity ratio α R = α E = α C = α l α γ = 1 : 225 , Similarity ratio of internal friction angle α ϕ = r m / r p = 1 , Gravity acceleration similarity ratio α g = g m / g p = 1 , and Similarity ratio of applied forces α f = f m / f p = α g α γ α l 3 = 1.975 ୍ଠ 10 − 7 . Among them, lm, γm, tm, rm, gm, and fm are the geometric dimensions, bulk density, time, internal friction angle, gravitational acceleration, and applied forces in similar simulations; lp, γp, tp, rp, gp, and fp 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 . 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 . 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. 3.1.2. 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 . In the initial stage of advancing the working face (a), 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 (b), 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 (c). 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 . When the working face advanced to 60 m (a), 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 (b), 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 (c), 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 . During the advancement process of the 3014 working face (a,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 (c), 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 . DIC displacement monitoring detected a process from local damage to structural failure. When the working face advanced to 15 m (a), 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 (b), 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 (c), 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. 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 (a), 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 (b), 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 (c), 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 . 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 . 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 a–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 , the fitting coefficient R 2 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 . 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. 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. 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. Author Contributions Conceptualization, B.Y.; methodology, B.Y.; validation, B.Y. and Y.L.; formal analysis, F.H.; investigation, S.Z.; resources, Y.L.; data curation, F.H.; writing—original draft preparation, F.H. and S.Z.; writing—review and editing, B.Y.; visualization, S.Z.; supervision, S.Z.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript. Funding This research was funded by the National Natural Science Foundation of China (No. 52574181, No. 52174095). Data Availability Statement The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors. Conflicts of Interest The authors declare no conflicts of interest. References Physical similarity simulation experiment system. Physical similarity simulation experiment system. Mining sequence of the working face. Mining sequence of the working face. Overlying strata fracture and displacement cloud map on 3084 working face. Overlying strata fracture and displacement cloud map on 3084 working face. Overlying strata fracture and displacement cloud map on 3086 working face. Overlying strata fracture and displacement cloud map on 3086 working face. Overlying strata fracture and displacement cloud map on 3094 working face. Overlying strata fracture and displacement cloud map on 3094 working face. Overlying strata fracture and displacement cloud map on 3098 working face. Overlying strata fracture and displacement cloud map on 3098 working face. Overlying strata fracture and displacement cloud map on 3014 working face. Overlying strata fracture and displacement cloud map on 3014 working face. Overlying strata fracture and displacement cloud map on 3018 working face. Overlying strata fracture and displacement cloud map on 3018 working face. Overlying strata fracture and displacement cloud map on 3024 working face. Overlying strata fracture and displacement cloud map on 3024 working face. Displacement cloud map of each working face after mining. ( a) After the mining of working face 3084; ( b) After the mining of working face 3086; ( c) After the mining of working face 3094; ( d) After the mining of working face 3098; ( e) After the mining of working face 3014; ( f) After the mining of working face 3018; ( g) After the mining of working face 3024. Displacement cloud map of each working face after mining. ( a) After the mining of working face 3084; ( b) After the mining of working face 3086; ( c) After the mining of working face 3094; ( d) After the mining of working face 3098; ( e) After the mining of working face 3014; ( f) After the mining of working face 3018; ( g) After the mining of working face 3024. Maximum displacement changes of overlying strata after working face mining. Maximum displacement changes of overlying strata after working face mining. Fractal dimension of the fracture network after mining in each working face. ( a) After the mining of working face 3084; ( b) After the mining of working face 3086; ( c) After the mining of working face 3094; ( d) After the mining of working face 3098; ( e) After the mining of working face 3014; ( f) After the mining of working face 3018; ( g) After the mining of working face 3024. Fractal dimension of the fracture network after mining in each working face. ( a) After the mining of working face 3084; ( b) After the mining of working face 3086; ( c) After the mining of working face 3094; ( d) After the mining of working face 3098; ( e) After the mining of working face 3014; ( f) After the mining of working face 3018; ( g) After the mining of working face 3024. Changes in the fractal dimension of the fracture network during the mining process. Changes in the fractal dimension of the fracture network during the mining process. Proportioning scheme of the physical similarity model. 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 Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. Share and Cite MDPI and ACS Style Yang, B.; He, F.; Zhang, S.; Li, Y. Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory. Processes 2026, 14, 1852. https://doi.org/10.3390/pr14121852 AMA Style Yang B, He F, Zhang S, Li Y. Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory. Processes. 2026; 14(12):1852. https://doi.org/10.3390/pr14121852 Chicago/Turabian Style Yang, Baogui, Fei He, Sheng Zhang, and Yongliang Li. 2026. "Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory" Processes 14, no. 12: 1852. https://doi.org/10.3390/pr14121852 APA Style Yang, B., He, F., Zhang, S., & Li, Y. (2026). Evolution of Overlying Strata and Fracture Networks in Close-Distance Coal Seam Groups Based on DIC and Fractal Theory. Processes, 14(12), 1852. https://doi.org/10.3390/pr14121852 Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here. Article Metrics Article metric data becomes available approximately 24 hours after publication online.
