Research on the Fissure Development and Seepage Evolution Patterns of Overburden Rock in Weakly Cemented Strata Under Repeated Mining

Abstract

This paper investigates the repeated disturbance of weakly cemented overburden rock caused by closely spaced coal seam mining, focusing on the effect of water infiltration on the strength degradation of weakly cemented mudstone. The study compares the fissure and fissure distribution characteristics of the overburden rock under seepage conditions. It also examines the dynamic evolution of seepage parameters during repeated mining and their impact on the overburden rock’s bearing capacity and structural stability. The findings are as follows: (1) After water infiltration, the clay mineral content in weakly cemented mudstone decreases, leading to a significant reduction in strength, increased microcrack development, and a moisture content increase from 0% to 3.27%. Uniaxial compressive strength decreases by 59.83%. (2) In the absence of seepage effects, the fissure development zone in the overburden rock changes from a positive trapezoidal shape to an inverted trapezoidal one, with a water-conducting channel forming first on the setup entry side. When seepage is considered, the fissure development in the weakly cemented overburden rock significantly increases, and the location of large-scale fissure initiation and expansion is advanced by 80 m. (3) During coal seam mining, excavation of the upper seam reduces the pore water pressure in the roof, causing the region of reduced pore pressure to shift from a trapezoidal to an “M” shape. As mining progresses to the lower seam, a seepage channel forms near the setup entry and expands. (4) Under repeated mining conditions, seepage field evolution in the overburden rock triggers the migration and transmission of formation water and pore pressure. The sustained influence of fissure water infiltration and seepage pressure accelerates the development of the water flowing fracture zone. As the overburden rock experiences renewed fracturing and caving, secondary fissure formation intensifies the movement of formation water. Consequently, the bearing capacity and water-resistance properties of the overburden rock are gradually degraded, significantly increasing the extent of structural damage within weakly cemented mining overburden rock.

1. Introduction

The coal measure strata in the western mining areas of China typically display characteristics such as late diagenesis, weak cementation, low strength, and high-water sensitivity. Coal mining disrupts the stress and seepage fields in the overburden rock. Specifically, water infiltration during coal seam extraction causes strength degradation, stress redistribution, and dynamic changes in pore pressure. These factors reduce the bearing capacity and structural stability of overburden rock, increasing the risk of catastrophic instability. As a result, problems such as significant coal resource waste and frequent safety accidents occur.

The macro and micro-mechanical property changes of weakly cemented mudstone after water infiltration are crucial for understanding the structural instability of weakly cemented strata. Li et al. [1,2] observed that the uniaxial compressive strength and elastic modulus of mudstone decrease as clay mineral content increases, revealing the multi-scale characteristic s of mudstone’s physical, mechanical, and physicochemical properties. They systematically studied the water–rock interaction mechanism at different scales. Sun et al. [3,4] developed a variation degree index system based on the stress–strain relationship of rocks with varying cementation degrees, emphasizing that the high disintegration potential of weakly cemented mudstone is the primary factor driving the increased height of the water-conducting fissure zone in western mining areas. Zhao [5] identified that the main mechanism of rock fissure–water physics interactions is the reduction of the basic friction angle and the strength of contact protrusions. Wang et al. [6] found that water–rock interactions promote micropore development, damaging the rock’s structural integrity and leading to strength degradation. Miao et al. [7] demonstrated that the failure of weakly cemented sandstone is caused by the rapid accumulation of small pore ruptures, leading to macroscopic fissures. Skrzypkowski et al. [8] found that after water infiltration, the mechanical properties of rock masses, such as mechanical strength and deformation resistance, significantly deteriorate. Han et al. [9] established a quantitative relationship for rock strength degradation due to water immersion. Understanding the fissure characteristics and fissure development patterns of mining overburden rock under seepage conditions is essential for studying the bearing capacity and structural stability of weakly cemented overburden rock. Fan [10] proposed criteria for rock instability and mechanisms of fissure propagation based on the impact of osmotic pressure differences on rock fissure stress thresholds and permeability. Zhang et al. [11] introduced a fluid–solid coupling simulation method that updates coal–rock permeability and strength in real-time during water immersion caused by mining. Zhang et al. [12] developed a stress-damage–seepage coupling model for weakly cemented mudstone based on mechanisms affecting pore structure and mineral composition and studied the permeability evolution of mining weakly cemented overburden rock. Wang et al. [13] analyzed the characteristics of water-conducting fissures and the evolution of coal water-conducting fissure zones during coal seam mining. Xu et al. [14] suggested that the main cause of fissured zone formation ahead of the working face is bearing capacity degradation, noting that more permeable areas correspond to shear fissure development zones under decompression. Manoj Khanal et al. [15] observed that permeability above mined coal seams increases by over six orders of magnitude, and different mining sequences lead to varying rupture patterns and degrees of permeability change. Li et al. [16] found that vertical fissure seepage is positively correlated with fissure development and total water inflow but negatively correlated with rock permeability. Xu et al. [17] optimized the method for calculating the development height of the water-conducting fissure zone by establishing a relationship between the bearing strength of the voussoir beam and the height of the caving zone. Poulsen et al. [18] proposed fissure strength and porosity indicators, determining the height of the enhanced permeability fissure zone above the longwall goaf. Under the influence of repeated mining, the degradation and attenuation of overburden-bearing capacity and water-resistance properties become more pronounced, increasing the likelihood of structural instability. Li et al. [19] studied pressure manifestation laws and roof failure characteristics in closely spaced coal seams. Li Y et al. [20] used UDEC to simulate multi-seam mining, clarifying the impact of upper seam mining on the fissure and displacement fields of lower seams. The authors’ research team [21,22,23] has studied the mechanical response mechanisms of weakly cemented coal–rock bodies under stress, water conditions, and seepage characteristics. They identified the cooperative motion behavior of weakly cemented mining strata overburden rock and the distribution patterns of mining fissures. Additionally, they proposed a quantitative method to characterize the stability and damage of impermeable and aquifer layers during mining.

This paper builds on previous studies by focusing on repeated mining closely spaced coal seams in the weakly cemented coal measure strata at the Ili No. 4 Mine of Xinwen Mining Group. Based on the area’s geological characteristics, the research employs mechanical testing, micro-testing, and numerical simulations to assess how the dynamic evolution of seepage during coal seam mining affects the bearing capacity and structural stability of weakly cemented overburden rock. The results enhance the understanding of fissure development and seepage evolution in weakly cemented strata overburden rock, offering both theoretical insights and engineering guidance for the repeated mining of such strata.

2. Strength Degradation Patterns of Weakly Cemented Mudstone

2.1. Experimental Preparation

The mudstone samples for the experiment were obtained from the roof of the 21-1 coal seam at the Ili No. 4 Mine of Xinwen Mining Group by on-site core drilling. The samples were prepared into standard specimens and subjected to different moisture contents using a constant temperature and humidity chamber and a drying oven. According to exploration reports from the study area, the roof rock mass of the 21-1 coal seam demonstrates good homogeneity and relatively stable mechanical properties. Before beginning the main experiments, three naturally moist mudstone samples were subjected to uniaxial compression tests under identical conditions. These preliminary tests confirmed the mudstone’s homogeneity and ensured the reliability of subsequent results. The measured strengths were 13.14 MPa, 13.82 MPa, and 14.34 MPa, respectively. The stress–strain curves of these specimens also showed good consistency, as presented in Figure 1. Therefore, the mudstone samples used in this experiment were considered to possess stable mechanical properties, fully satisfying the requirements for accuracy and reliability in the testing process.

The testing equipment used was the MTS-E45.605 electronic universal testing machine, as illustrated in Figure 2. The molds can be replaced during the experiment to meet various testing requirements. Uniaxial compression, Brazilian splitting, and shear tests were conducted on mudstone samples with varying moisture contents. Field measurements and prior research [21] show that the overburden rock of the 21-1 coal seam typically fractures at an angle of about 60°, so the shear angle was set accordingly to 60° during shear testing.

2.2. Experimental Results Analysis

The uniaxial compression test results are presented in

Table 1. Uniaxial compression test results.

Sample ID	Moisture Content (%)	Peak Strength (MPa)	Peak Strain (%)	Elastic Modulus (GPa)
UN-1	0	18.52	1.48	2.30
UN-2	1.32	14.20	1.31	2.08
UN-3	2.86	10.97	1.14	1.88
UN-4	3.27	7.44	0.65	1.70

. As the moisture content increased from 0% to 3.27%, the uniaxial compressive strength of the mudstone samples decreased from 18.52 MPa to 7.44 MPa, indicating a degradation of 59.83%. Meanwhile, the peak strain and elastic modulus decreased by 56.08% and 26.09%, respectively.

The Brazilian splitting test results can be found in

Table 2. Brazilian splitting test results.

Sample ID	Moisture Content (%)	Tensile Strength (MPa)
LN-1	0	1.04
LN-2	1.44	0.76
LN-3	3.45	0.37

. As the moisture content increased from 0% to 1.44% and then to 3.27%, the tensile strength of the mudstone decreased from 1.04 MPa to 0.76 MPa and 0.37 MPa. The increase in moisture content caused a significant reduction in tensile strength, making the mudstone more prone to tensile failure and the formation of tensile cracks.

The results of the 60° shear tests are summarized in

Table 3. Results of 60° shear test results.

Sample ID	Moisture Content (%)	Normal Stress (MPa)	Shear Stress (MPa)
SN-1	0	5.9	10.3
SN-2	1.90	5.3	9.2
SN-3	3.31	2.6	4.4

. As the moisture content increased from 0% to 3.31%, the normal stress and shear stress decreased from 5.9 MPa and 10.3 MPa to 2.6 MPa and 4.4 MPa, respectively, resulting in strength reductions of 55.93% and 57.28%.

The failure characteristics of the mudstone samples from the three sets of mechanical tests are presented in Figure 3. Results indicated that water infiltration significantly re-duced the strength of weakly cemented mudstone, promoting the formation of distinctive macro-damage patterns. In uniaxial compression tests, the specimens exhibited uneven shear surfaces accompanied by numerous secondary tensile fissures. Similarly, several secondary fissures appeared around the main fissures in Brazilian splitting and 60° shear tests.

2.3. Water-Induced Strength Degradation Patterns

The experimental results were linearly fitted to derive the strength reduction curve function for weakly cemented mudstone, as expressed in Equations (1)–(4). Figure 4 illustrates the relationship between uniaxial compressive strength $U_n$, tensile strength $L_n$, normal stress $N_n$, shear stress $S_n$, and moisture content in weakly cemented mudstone. As moisture content increases, the rock softens and expands, which accelerates the development of microcracks and pores, undermining the rock’s structural integrity and leading to strength degradation.

$$U_n=-2.543x+18.10R^2=0.9437$$

(1)

$$L_n=-0.194x+1.04R^2=1.0$$

(2)

$$N_n=-0.959x+6.265R^2=0.8206$$

(3)

$$S_n=-1.715x+10.945R^2=0.8242$$

(4)

3. Microscopic Damage Characteristics of Weakly Cemented Mudstone

3.1. Experimental Preparation

To investigate strength degradation and structural damage in weakly cemented mudstone under water–rock coupling, rock samples were sliced under both dry and saturated conditions. Powdered samples, with particles smaller than 200 mesh, were also prepared. The slices were examined using a Sigma 300 high-resolution field emission scanning electron microscope (SEM) to observe microscopic structural changes, the device is from Karl Zeiss Microscope Co., Ltd., Oberkohen, Germany. Changes in the mineral composition of the powdered samples were analyzed using an ARL9900 X-ray fluorescence spectrometer, the device is from Karl Zeiss Microscope Co., Ltd., Waltham, MA, USA. The testing equipment is shown in Figure 5.

3.2. Microscopic Structural Test Results

The microscopic structures of mudstone samples under different moisture conditions are presented in Figure 6. In their natural state, the mudstone surfaces are rough, with tightly packed particles, some of which are raised in blocky formations. The rock exhibits foliated cementation with few cracks and pores, as shown in Figure 6a. After immersion, the mudstone displays a marked increase in porosity, a weakening of cementation, and an accumulation of cementing material. Plate-like clay minerals accumulate on the particle surfaces, as seen in Figure 6b.

Pores were identified through image recognition software, and binary processing was applied to calculate the porosity before and after water immersion, as shown in Figure 7. In the natural state, the mudstone porosity was 9.74%, which increased to 25.22% after saturation, marking a 1.5-fold increase. The dissolution of certain hydrophilic minerals within the mudstone significantly enhanced internal porosity and microcrack development.

3.3. Mineral Composition Content Test Results

The XRD test results for mudstone under different moisture conditions are shown in Figure 8. The mudstone mainly consists of quartz particles as the framework, with clay minerals such as kaolinite, montmorillonite, and chlorite acting as the cementing and filling materials.

The analysis of the mineral composition content in mudstone is summarized in

Table 4. Mineral composition content analysis of mudstone.

Moisture Condition	Quartz Content (%)	Clay Mineral Content (%)	Clay Minerals
			Kaolinite (%)	Montmorillonite (%)	Chlorite (%)
Drying	64.87	35.13	4.15	11.05	19.93
Saturation	76.20	23.80	3.28	10.52	10.00

. In its natural state, the clay minerals, including kaolinite, montmorillonite, and chlorite, accounted for 35.13% of the mudstone. After water saturation, this decreased to 23.80%. Clay minerals are highly water sensitive; they swell and soften when exposed to water, which weakens the rock’s structure. This leads to phenomena such as softening, dissolution, expansion, and disintegration, all contributing to the reduction in mudstone strength.

3.4. Microscopic Structural Evolution of Rock Mass Under Water Infiltration

The microscopic tests indicated that water infiltration significantly influences the microstructural evolution of weakly cemented mudstone. Specifically, water infiltration primarily induces micropore development and dissolution of cementitious materials, changing the cementation type from original contact-based cementation (between quartz particles and clay minerals) to predominantly pore-based cementation between aggregates, with only limited contact cementation remaining, as shown in Figure 9. The shift in cementation type significantly affects the microstructure of weakly cemented mudstone, greatly weakening the cementing strength responsible for structural stability and reducing constraints on pore expansion and micro-fissure growth. Consequently, the mudstone’s mechanical strength and deformation resistance deteriorate noticeably. In the mechanical tests described in Section 2, samples with higher moisture content exhibit a pronounced decrease in bearing capacity and a marked increase in damage.

4. Fissure Development Characteristics of Overburden Rock in Weakly Cemented Strata Under Repeated Mining

4.1. Numerical Calculation Model Construction

To assess the impact of multiple mining operations and seepage-induced weakening on the failure characteristics, damage extent, and fissure development patterns of weakly cemented overburden rock, this paper models the seepage problem as a fissure seepage issue. UDEC 7.0 numerical simulation software is used, incorporating a strain-softening model to study the water-softening effect on rock blocks. Based on mine exploration data and earlier investigations [21], the mechanical parameters of each stratum were determined for the numerical calculations. The physical and mechanical parameters of the layers and joints in the model are outlined in

Table 5. Mechanical parameters of coal measure strata in the research area.

Lithology	Density (kg·m−3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Cohesion (MPa)	Tensile Strength (MPa)	Internal Friction Angle (°)	Thickness (m)
Gravelly sandstone	2300	6.0	5.47	2.0	2.0	50	30
Mudstone (aquifer)	2300	2.5	1.1	2.4	1.2	40	5
Gravel–sandstone	2200	24.0	10.5	2.1	0.9	28	20
Mudstone	2400	6.5	3.2	2.2	1.1	33	10
Sandy mudstone	2600	18.5	9.0	1.3	0.85	30	10
Mudstone	2400	6.5	2.5	1.1	0.8	32	10
Coal 21-1	1240	6.7	2.2	1.1	0.7	35	5
Medium sandstone	2430	8.5	4.0	1.6	1.1	35	8
Sandy mudstone	2600	19.5	8.5	1.5	1.7	30	12
Mudstone	2500	7.8	3.5	2.8	1.4	33	5
Coal 23-2	1240	8.5	2.5	1.6	1.1	40	10
Siltstone	2330	21.5	9.3	1.86	1.6	45	10

.

Based on these parameters, a two-dimensional mining model with a length of 500 m, width of 135 m, and depth of 350 m was developed. Coal pillars, each 150 m wide, were placed on both sides of the working face to eliminate boundary effects, with an entry set on the left side. The lateral pressure coefficient was set to 1. According to the burial depth of 350 m, both vertical and horizontal stresses were calculated as 8.75 MPa. The left and right lower boundaries of the model were impermeable, and a 1 MPa pore pressure zone was introduced above the aquifer layers to simulate water movement through the rock. The thicknesses of the 21-1 coal seam and 23-2 coal seam are 5 m and 10 m, respectively. The working face is 200 m long, with full-height mining conducted in one pass, and excavation proceeds 40 m at a time.

In order to ensure the reliability of the mining model, the uniaxial compressive, tensile, and shear strength reduction functions derived in Section 2.3 (Equations (1)–(4)) were incorporated into the model. For the analysis of the aquifer and the roof of the coal seam, a triangular block grid method was used to divide the layers randomly. The Mohr–Coulomb criterion was applied for block failure, and a Coulomb slip was used for contact surface failure. After calibration, the numerical simulation results for uniaxial compressive strength matched well with the laboratory test results. Additionally, the triangular block discretization accurately reproduced the irregular failure surfaces typical of weakly cemented mudstone, as illustrated in Figure 10.

The model utilizes the FISH language built into UDEC 7.0 software to monitor and modify the mechanical parameters of joints by iterating through them. This enables the simulation of water-softening effects and the associated changes in the physical and mechanical parameters of rocks at varying moisture contents. The numerical calculation model and the initial pore pressure distribution are shown in Figure 11.

4.2. Failure Characteristics of Overburden Rock Under Repeated Mining Without Seepage

Repeated mining, caused by the extraction of two coal seams, leads to failure modes in the overburden rock that exhibit sequential structural characteristics: a caving zone, fissure zone, and bending-subsidence zone, from bottom to top [24]. Figure 12 illustrates the deformation and failure of the overburden rock caused by the extraction of the 21-1 coal seam.

As the working face of the upper coal seam advances by 40 m, the roof of the 21-1 coal seam experiences its initial caving. The caving height in the immediate roof reaches 5 m, while the mining-induced effect extends to 15 m. Despite this, the overall structure of the overburden rock remains relatively stable. When the working face reaches 80 m, the fine sandstone layer in the immediate roof fully caves. At this point, the caving height increases to 10 m, and the fissure zone develops to a height of 55 m. The rock layers above the immediate roof maintain their integrity and a trapezoidal fissure development zone forms within the 55 m height range of the overburden rock at the working face. As excavation deepens, the number and extent of fissures increase significantly. Shear fissures mainly occur in high-shear-stress areas, such as at the contact surfaces between the coal seam and surrounding rock. In contrast, tensile fissures are more prevalent within the coal seam and at the top of the strata. When the working face advances to 200 m, the breaking length of the fine sandstone layer increases further before stabilizing. The mining impact on the overburden rock extends to the top of the model, forming an inverted trapezoidal fissure development zone. The transition between the trapezoidal and inverted trapezoidal areas primarily consists of newly formed sliding failures.

After extracting the upper coal seam, mining continues in the lower coal seam. At this point, the rock layers between the two coal seams are in an unloaded state, and no caving in the roof of the lower coal seam is observed within a certain excavation range, similar to that in the upper coal seam. When the working face advances to 80 m, both the old roof and immediate roof completely cave and become compressed, with the caving height reaching 17 m. As excavation progresses, the roof of the lower coal seam, consisting of mudstone and sandy mudstone, caves, and numerous vertical fissures develop, penetrating through the immediate roof mudstone layer. At 200 m, the overburden rock becomes unstable due to the extraction of the lower coal seam. The height of overburden rock failure reaches 75 m, with multiple fissures forming and extending through the overlying strata. The water-conducting fracture zone expands laterally, causing bending and failure of the gravelly sandstone and mudstone impermeable layers. Many cavities and permeating channels form within the rock layers and a complete water-conducting channel forms above the setup entry on the left side.

4.3. Failure Characteristics of Overburden Rock Under Repeated Mining with Seepage

The failure characteristics of weakly cemented overburden rock, and the fissure development patterns under repeated mining with seepage are illustrated in Figure 13.

When the 21-1 coal seam advances to 40 m, compared to the scenario without seepage, the number of fissures in the fine sandstone immediate roof increases significantly, and the caving range expands. The fissure zone extends to 75 m, and the caving height of the immediate roof reaches 10 m, which is 5 m more than the no-seepage case. At 80 m of excavation, the fine sandstone in the immediate roof fully caves, and interlayer separation occurs between the overlying gravelly sandstone and mudstone. Due to pore water pressure, the number of fissures in the overburden rock increases significantly. In the no-seepage weakening model, the number of fissures in the overburden rock increases substantially when the upper coal seam is mined to 160 m. Seepage causes fissures to initiate and expand on a large scale, 80 m earlier than in the no-seepage case. As the upper coal seam is mined to 120 m, delamination occurs in the mudstone layer 50 m above the coal seam, and the separation between the gravelly sandstone and mudstone closes under the compaction of the strata. As mining continues to 200 m, primary fissures expand laterally while vertical fissures continue to increase. The overburden rock gradually sinks and compacts, and the caved rock fills the goaf. This leads to the formation of delamination and fractured fissures.

When the lower coal seam is mined to a depth of 40 m, delaminating fissures between the mudstone aquifer and the impermeable layer continue to develop. The goaf of the upper coal seam is nearly fully compacted, and lateral delaminating fissures expand. The immediate roof of the lower coal seam caves, and a few lateral fissures form in the upper sandy mudstone. At this stage, the mining-induced influence of the lower coal seam has not yet reached the base of the medium sandstone layer of the 21-1 coal seam. As mining progresses, pore water pressure has a significant impact on fissure development. Compared to the no-seepage model, both the rate and height of fissure zone expansion increase substantially. When the lower coal seam is mined to 80 m, delamination occurs in the medium sandstone layer of the upper coal seam’s floor. The mining influence extends further, affecting the roof of the upper coal seam. At 120 m of excavation, due to the fracturing and bending subsidence of the overburden rock in the lower coal seam, the opening of delaminating fissures between the mudstone aquifer and impermeable layer increases, and the fissure zone continues to expand. When mining reaches 200 m, the roof gradually collapses and compacts, and the overburden rock undergoes deformation, delamination, and instability. The mining impact on the overburden is more pronounced on the left side.

5. Evolution of Seepage in Weakly Cemented Strata Under Repeated Mining

5.1. Pore Pressure Distribution

Before mining, the stress and seepage fields in the strata are in a natural equilibrium. However, coal mining disrupts this equilibrium, particularly the balance of the pore pressure field. The interactions between the stress, seepage, and fissure fields cause both the pore opening and the number of pores to increase significantly.

Figure 14 presents the distribution of pore pressure under repeated mining in weakly cemented strata. Mining causes a reduction in pore pressure around the goaf, disturbing the equilibrium in the strata. As a result, pore pressure migrates from high-pressure areas to low-pressure areas. When fissures open to a certain extent, the speed of pore pressure migration increases dramatically. When the working face of the 21-1 coal seam reaches 40 m, the unloading effect from mining reduces the pore water pressure around the goaf, creating a pressure difference in the strata. This causes the high pore pressure above the goaf to gradually migrate downward. As the working face advances to 160 m, the area of reduced pore pressure continues to expand, shifting from a trapezoidal shape to an “M” shape. The downward migration of high pore pressure becomes more pronounced. In the strata beneath the 23-2 coal seam, a region remains unaffected by the pore pressure changes. After the upper coal seam is mined, the 23-2 coal seam is then extracted. When the lower coal seam advances to 40 m, the reduction zones from both seams merge. As mining progresses to 160 m, the fracturing of the overburden rock and the formation of water-conducting fractures create a seepage channel on the left side of the model near the setup entry, which continues to expand. The overburden rock in this area is particularly affected by the mining operations.

5.2. Development Patterns of Overburden Rock Fissures and Damage Characteristics

The fissure development characteristics of overburden rock under the influence of seepage damage are illustrated in Figure 15a. As excavation progresses, the number of fissures increases in a stepped pattern. After mining the upper coal seam and continuing mining in the lower coal seam, the formation of new fissures surpasses the closure or intersection of existing fissures. Consequently, the number of fissures continues to increase, though at a slower rate. Initially, the length of fissures increases rapidly before stabilizing. Once mining of the upper coal seam concludes, the fissure development length approaches its peak, and the fissure length becomes stable.

Adopting fissure number and fissure length as indicators of overburden rock damage, Figure 15b shows that prior to excavation, the overburden rock is unaffected by mining, resulting in zero damage. In the early stages of excavation, the mining influence on the overburden is minimal, and the damage level is low. The fissure number increases rapidly, while the increase in fissure length is significantly slower, which does not align with actual observations. Therefore, the fissure length instability indicator is considered a better representation of the overburden rock’s initial damage process. As the simulation progresses, the overburden damage level continues to rise. Once mining of the upper coal seam ends and mining of the lower coal seam begins (excavation step 5), the damage level of the overburden increases steadily but at a slower rate. The fissure length stabilizes, which does not correspond with the fissure development characteristics described in Section 4.3. As a result, the fissure number instability indicator is regarded as a more accurate representation of the overburden’s damage in the later stages of mining.

5.3. Overburden Rock Subsidence and Flow Characteristics

Monitoring points are placed every 20 m horizontally across the overburden rock to track subsidence and flow rates. The subsidence of the overburden rock during repeated mining is presented in Figure 16. The subsidence curve is asymmetric, with the peak closer to the left side of the model. As mining of the upper coal seam progresses to 40 m, 120 m, and 200 m, the maximum subsidence of the overburden rock reaches 0.09 m, 1.04 m, and 4.64 m, respectively. As mining of the lower coal seam continues, the subsidence increases further. By the end of the excavation, the total subsidence reaches 10.4 m. The most significant increase in subsidence occurs between 120 m and 200 m of mining. During this phase, large-scale fractures and caving occur in the immediate roof above the goaf, destabilizing the overburden rock and driving further subsidence. Additionally, the downward migration of the high-permeability pressure zone and the continued development of water-conducting fractures intensify the migration of the formation of water, exacerbating the fracture and instability in the weakly cemented strata. This leads to a substantial increase in roof caving and subsidence rates.

The monitoring results of overburden rock subsidence and flow rate are presented in Figure 17. The flow rate curve shows significant fluctuations above the goaf, with greater fluctuations observed on the left side compared to the right side. The inflow and outflow of fissure water on both sides of the overburden are generally balanced, causing the flow rate to approach zero. The flow rate peaks at a horizontal coordinate of 220 m (70 m from the setup entry), reaching 3.06 × 10⁻⁴ m³/s. At this point, the subsidence of the overburden rock is at its maximum. As fissures develop and connect, and overburden rock failure and caving increase, the migration of formation water intensifies. Fissure water infiltrates the rock, and the continued action of high-permeability pressure promotes the formation of a water-conducting fracture zone, ultimately degrading and reducing the overburden rock’s bearing capacity and water-resistance properties.

5.4. Discussion on Seepage-Fissure Coupling Mechanisms in Overburden Rock Under Repeated Mining

Double-seam mining causes continuous stress accumulation and dynamic equilibrium in the overlying rock mass [25]. As stress-concentration zones expand, existing fissures lengthen, and new fissures form [26]. This process sharply reduces the overburden’s bearing strength and significantly weakens its structural stability, making it more prone to fracture. Without seepage, fissure development and propagation depend mainly on the rock mass’s mechanical properties and the mining-induced stress. As the working face advances, the overlying rock fails sequentially from the bottom upward, with fissures extending continuously [27]. Along the goaf boundary, fissure numbers initially rise and then stabilize, whereas in the central portion of the overburden, fissure growth first increases and then decreases.

Under seepage conditions, the overburden rock fractures result from the combined effects of double-seam mining-induced stress and the pore pressure and hydration damage linked to the evolving seepage field. In weakly cemented strata, water exposure accelerates rock damage and deterioration. Repeated mining disturbances intensify both the stress and seepage fields, substantially increasing overburden permeability and facilitating fluid flow, thereby expediting the dynamic evolution of fissure and seepage fields [28]. When pore pressure acts on overburden fissures, an additional stress field develops at the fissure tips. Once this pressure surpasses a certain threshold, tensile stress at the fissure tips rises significantly, amplifying fissure propagation. The pore stress field generated by the pressure gradient also guides and controls fissure growth [29,30,31]. As shown in Figure 18, fissure water driven by the pressure gradient elevates internal rock pressure, promoting fissure development and continual extension along the fissure tips. This process creates numerous secondary fissures. Fissures typically extend along principal stress directions and weak cementation zones, eventually intersecting pre-existing fractures and forming more complex fissure networks. These networks further enhance rock mass permeability [32,33].

During repeated mining, stress field redistribution triggers the initiation and expansion of fissures in the overlying weakly cemented coal measure strata. As these fissures grow, they further accelerate the seepage field’s dynamic evolution. Simultaneously, the seepage field drives the migration and transfer of formation water and pore pressure, causing hydration damage to the overburden and generating numerous additional fissures. Consequently, the evolving and interacting stress, fissure, and seepage fields under double-seam mining considerably increase both fissure development and overburden structural failure in weakly cemented strata.

6. Conclusions

This study combines mechanical testing, microscopic analysis, and numerical simulation to investigate the strength degradation of weakly cemented mudstone after water infiltration. It compares the failure characteristics and fissure development patterns of overburden rock in weakly cemented strata under repeated mining, both with and without seepage effects. The study also examines the impact of seepage dynamics on the bearing capacity and structural stability of overburden rock during repeated mining. The main findings are as follows:

(1)

Weakly cemented mudstone is highly water sensitive. After water infiltration, the clay mineral content decreases significantly, and the development of microcracks increases substantially. The uniaxial compressive strength, tensile strength, and shear strength of the samples decline sharply, with the moisture content increasing from 0% to 3.27%. Specifically, the uniaxial compressive strength, peak strain, and elastic modulus decreased by 59.83%, 56.08%, and 26.09%, respectively.

(2)

In the absence of seepage, the fissure development zone in the overburden rock shifts from a trapezoidal to an inverted trapezoidal shape as mining progresses through both coal seams. A complete water-conducting channel first forms above the left-side setup entry, and the expansion area consists of fissures caused by overburden rock-sliding failure. With seepage effects, the presence of pore pressure and fissure water significantly accelerates the development of fissures in the overburden rock, further expanding the caving range. Seepage causes large-scale fissure initiation and expansion to occur 80 m earlier than without seepage.

(3)

During coal seam mining, the excavation of the upper coal seam reduces the pore water pressure in the roof, causing the pressure reduction zone to shift from a trapezoidal to an “M” shape. The high pore pressure above the goaf migrates downward. By the time mining of the lower coal seam is complete, a seepage channel forms near the left-side setup entry and continues to expand. The overburden rock in this area is particularly affected by mining disturbance.

(4)

Under repeated mining conditions, both the number and length of fissures in the overburden rock continue to increase. After mining of the upper coal seam, the increase in fissure number slows while fissure length stabilizes. At 70 m from the setup entry, both the fissure water flow rate and overburden subsidence reach their peak. The dynamic evolution of the overburden seepage field drives the migration of formation water and pore pressure. Water infiltration causes hydration damage to the overburden structure while migrating pore pressure leads to the formation of numerous new fissures. Persistent fissure water infiltration and seepage pressure facilitate the expansion of the water flowing fracture zone. Subsequent overburden fracturing and secondary fissure propagation further intensify water migration, diminishing both the bearing capacity and water-resistance property of the overburden. As a result, weakly cemented overburden rock in mining areas sustains markedly increased damage.