The Catastrophic Failure Mechanisms and the Prevention of Dynamic Pressure-Related Hazards During Mining Under an Interval Goaf Through an Isolated Coal Pillar in Shallow and Closely Spaced Coal Seams

Abstract

Given the potential for dynamic load-induced support crushing that may occur during mining under an interval goaf through an isolated coal pillar (ICP) in shallow closely spaced coal seams, this paper systematically explored this issue through a case study of the 30,103 working face at the Nanliang Coal Mine. We employed a combined approach of similarity simulations, theoretical analyses, numerical simulations, and field measurements to investigate the catastrophic failure mechanisms and prevention strategies for dynamic pressure-related hazards encountered when mining a lower coal seam that passes through an ICP. The findings indicated that the synchronous cutting instability of the interlayer effective bearing stratum (IEBS) and double-arch bridge structure of the ICP roof were the primary causes of dynamic load-induced support crushing at the working face. A mechanical model was developed to characterize the IEBS instability during mining under an interval goaf. The sources and transmission pathways of dynamic mining pressure during mining passing through the ICP were clarified. The linked instability of the double-arch bridge structure of the ICP roof was induced by IEBS failure. The UDEC numerical model was utilized to elucidate the instability of the IEBS during mining in the lower coal seam and to analyze the vertical stress distribution patterns in the floor rock strata of the interval goaf. A comprehensive prevention and control strategy for roof dynamic pressure, which includes pre-releasing concentrated stress in the ICP, strengthening the support strength of the working face, and accelerating the advancement speed was proposed. The effectiveness of this prevention and control strategy was validated through actually monitoring the characteristics of mining pressure data from the 30,103 working face following pressure relief. The findings provide valuable insights for rock stratum control of shallow and closely spaced coal seam mining under similar conditions.

1. Introduction

The coal resources in the northern Shaanxi mining area are characterized by shallow burial, thin bedrock, multiple minable layers, and small interlayer spacing (10 to 40 m) [1]. Between 2004 and 2012, many mines adopted the “interval mining method” after resource integration [2,3], which leaves numerous strip-like coal pillars of varying sizes in the upper coal seam goaf, resulting in an extensive shallow interval goaf [4]. As the upper coal seam is nearly exhausted, the working face in the lower coal seam will inevitably face challenges during mining under these interval goafs. Production practices have demonstrated that when mining the lower coal seam under the overlying goaf and residual coal pillars (RCPs), there is a high risk of synchronous transient collapse of the overburden rock (OR) structures in the upper and lower coal seams. This may lead to incidents such as coal wall spalling, roof caving, and support crushing, which jeopardize the safety of the working face [5,6,7,8,9,10]. Xu et al. [11,12] classified the structures of OR key strata in shallow coal seams and analyzed the types of support crushing encountered in mining operations. Feng et al. [13] identified the simultaneous fracture of the upper voussoir beam and lower cantilever beam structures of thick interlayer key strata in shallow and closely spaced coal seams as the most dangerous condition. Du et al. [14] pointed out that the changes in geological conditions of the OR are a major cause of strong dynamic pressure-related hazards caused by the sliding of interlayer key strata during the entry of the working face into the coal pillars. Xie et al. [15] introduced the concept of interlayer equivalent main roofs for multi-coal seam mining, which effectively explained the small periodic weighting intervals in thick and hard main roofs. Chen et al. [16] developed a double-plasticized foundation boundary mechanical model for the main roof plate structure by considering the elastoplastic deformation of coal and the support capacity weakening of coal pillars on both sides. Subsequently, they analyzed the fracture characteristics of the main roof during the entry and exit of underlying coal seam into and from the coal pillar. Chen et al. [17] established a fracture constitutive model to simulate and analyze the shear failure characteristics of the roof and coal pillar during the fracture process. Feng et al. [18] studied the catastrophic failure mechanism of chain-type impact instability caused by coal pillar groups during room-and-pillar mining in shallow and closely spaced coal seams. Wang et al. [19,20] and Bai et al. [21] investigated the movement and destruction characteristics of the ORs during mining under shallow room-and-pillar goafs. They established a sharp point mutation model for the stability of room-type coal pillars and revealed the patterns of sudden instability in RCPs during room-and-pillar mining in shallow coal seams. Yang et al. [22] developed a model for the movement of the ORs when mining under shallow room-and-pillar goafs. Huo et al. [23] studied the distribution characteristics of the stress field under shallow and closely spaced room-style goafs. Zhu et al. and Xu et al. [24,25,26] investigated the catastrophic failure mechanism of the dynamic instability of coal pillar groups during room-and-pillar mining in shallow and closely spaced coal seams. Ju et al. [27,28] examined the mechanisms of dynamic load mining pressure in the working face during the mining of shallow and closely spaced coal seams through inclined coal pillars. They also proposed corresponding prevention and control measures such as regulating the advancing speed of the working face, promoting the movement of key strata, and reducing the space of overhanging roofs. Wang et al. [29] proposed methods for the pre-filling of goafs and blasting pressure relief to limit the settlement of the OR structure and to weaken the stress on coal pillars in the upper coal seam. Yang [30,31] established a mechanical model to characterize the fracture instability of composite cantilever beam structure and analyzed the instability mechanism of roof structure in large mining height working face. Wu et al. [32] suggested a method for controlling strong mining pressure-related hazards during the entry of concentrated coal pillars into the working face using advanced hydraulic roof cutting. Wang et al. [33] studied the characteristics of a W-shaped voussoir beam hinged structure formed by the fracture and rotation of the main roof strata induced by the instability of a temporary coal pillar (TCP) in a shallow interval goaf. Zhu et al. [34] investigated the collaborative bearing characteristics of “coal pillar groups-goaf rubble” in shallow insufficiently caved zones, and revealed the stability and stress distribution patterns of coal pillars in such zones. Yuan et al. [35] used numerical simulations to study the bearing characteristics of coal pillars in an interval goaf, the distribution of concentrated stress in the floor, and the load transfer patterns in pre-splitting and blasting pressure relief.

The aforementioned research findings enrich and enhance understanding of the instability mechanisms of OR structures in room-and-pillar goafs during longwall fully mechanized mining in shallow and closely spaced coal seams. They provide insights into the catastrophic mechanism of dynamic pressures when mining under an interval goaf of shallow and closely spaced coal seams through ICPs. This paper employed a combined approach of similarity simulations, theoretical analysis, numerical modeling, and field measurements to study the instability patterns of the IEBS when a longwall working face passes through the ICPs of an overlying interval goaf in shallow and closely spaced coal seams. It reveals the mechanism of dynamic load mining pressure when the working face of the lower coal seam passes the ICPs. The findings offer references for mined strata control under similar conditions.

2. Working Face Conditions

The 30,103 working face of the Nanliang Coal Mine in Northern Shaanxi mining area is located at a depth of 130 m, with a bedrock thickness of 63 m and a loose layer thickness of 67 m. The 3-1 coal seam is 4 m thick. The 2-2 coal seam, which is 2.2 m thick, is situated at a depth of 97 m. It is underlain by a 25 m thick bedrock layer and overlain by an 8.5 m thick weathered rock layer. The interlayer spacing between these two coal seams is 30 m. The interlayer rock strata (IRS) primarily consist of siltstone, fine sandstone, and sandy mudstone. The 2-2 coal seam employs an interval mining method, while the 3-1 coal seam utilizes longwall mining. The spatial relationship and distribution of these two coal seams are illustrated in Figure 1 and Figure 2.

3. Structural Instability Characteristics of the OR During Mining Under an Interval Goaf

3.1. Simulation Scheme

This section aimed to investigate the structural instability of the OR when mining under an interval goaf through ICPs and to elucidate the deformation and instability patterns of the IEBS during the mining of the lower coal seam. The model design is presented in Figure 3, with dimensions of 3000 mm × 200 mm × 1350 mm (length × width × height). The material mixing ratios are detailed in

Table 1. Thickness and mixing ratio of model rocks.

Number	Lithology	Thickness of Stratum	Ratio (Sand: Gypsum: Calcium Carbonate)
1	Loess layer	20	Sand: Loess = 20:1
2	Loess layer	47	Sand: Loess = 20:1
3	Sandy mudstone	3	837
4	Fine sandstone	2	746
5	Siltstone	5	837
6	Fine sandstone	5	746
7	Siltstone	8	837
8	2−2 coal seam	2	2.1
9	Sandy mudstone	2	828
10	Siltstone	13	837
11	Fine sandstone	6	746
12	Fine sandstone	8	837
13	Mudstone	1	828
14	Miltstone	3	837
15	Sandy mudstone	1	837
16	3−1 coal seam	4	628
17	Siltstone	5	837
Total		135

. The similarity constants for geometry [36], bulk density, displacement, stress, and time are 100, 1.6, 100, 160, and 10, respectively.

3.2. Instability Characteristics of the OR in Interval Mining

Reference [33] discussed the interval mining method of the 2⁻² coal seam and the stress variation characteristics of the RCPs in the interval goaf were discussed. In the early stage of the formation of the interval goaf, the immediate roof strata caved as mining proceeded, while the main roof rock strata remained stable. Over time, the load on the TCPs exhibited an overall trend of “uniform increases—accelerated accelerates—sudden instability”. Due to the narrow width of the TCPs, they experienced creep under the concentrated load, which gradually degraded their load-bearing capacity. Eventually, the TCPs experienced sudden instability, thereby triggering fractures and instability of the main roof rock strata in the interval goaf. Following the fracturing of the roof OR in the mining strip, these combined with the soil caving arch to form a “trapezoidal-semicircular arch” fissure zone, which created conditions conducive to mining under the interval goaf of the lower coal seam, as presented in Figure 4.

3.3. Instability Characteristics of the OR During Mining Under the Interval Goaf

A 15 m boundary coal pillar was retained in the model. As displayed in Figure 5, when the working face advanced to 49 m, the lower key stratum (LKS) experienced sliding instability behind the support. The caving height of the goaf roof rock strata reached 12.9 m, exhibiting a “trapezoidal” shape. As presented in Figure 6, the working face advanced to 82 m, it was 7 m past the 1# unstable TCP. After moving the support, the LKS experienced reverse rotation and sliding instability, while the upper key stratum (UKS) fractured forming a hinge structure. This caused the hinged goaf roof rock blocks in the upper coal seam to rotate and sink, while secondary instability occurred in the OR structure, leading to a further expanded caving range. The support resistance reached 8200 kN.

When the working face advanced to 135 m, as illustrated in Figure 7, the LKS experienced sliding instability, and the UKS underwent rotational shear fractures along the support top. Fractures in the OR developed towards the left boundary of the ICP, with support resistance reaching 7885 kN. As the working face advanced to 150 m, as shown in Figure 8, it was 7 m past the boundary of the ICP (4 m behind the support). The support resistance suddenly increased, and the IRS experienced shear failure in front of the support, with the cutting height of the ICP measuring 0.5 m. The support resistance reached 10,934 kN, resulting in support crushing and difficulty in moving the support. The analysis indicates that when the working face passed through the ICP, the effective bearing area of the IRS above the support gradually decreased, which made it difficult for the cantilever structure formed by the IRS to bear the concentrated stress transmitted by the double-arch bridge structure of the ICP. This led to the sliding instability of the IRS along the coal wall of the working face and the rotational instability of the hinged rock blocks in the double-arch bridge structure of the roof, which formed a caving and fragmentation zone that penetrated through the goaf in both the upper and lower coal seams.

4. Instability Patterns of the OR Structure During Mining Under the Interval Goaf

4.1. Definition of the IEBS

The IEBS refers to the rock layer that governs the fracturing, movement, and mining pressure manifestations of rock layers in the lower coal seam roof during mining in closely spaced coal seams, where the interlayer hard rock plays a critical role [37]. The presence and instability characteristics of hard rock layers within the IEBS significantly influence the mining pressure experienced in the lower coal seam. As depicted in Figure 9, the thickness of the IEBS (h_e) can be defined as the IRS thickness (h_cj) minus the thickness of the floor rock damaged by upper coal seam mining (h_s) and the thickness of rock in the caving zone during lower coal seam mining (h_m), i.e.,

$$h_{\mathrm{e}}=h_{\mathrm{cj}}-h_{\mathrm{s}}-h_{\mathrm{m}}$$

(1)

4.2. Temporal and Spatial Evolution Patterns of Failure and Instability of the IEBS

4.2.1. Fracturing Characteristics of IEBS Fracture During Mining Under a Stress Release Zone

As shown in Figure 10a, as the working face of the lower coal seam advanced, once the hanging distance exceeded its limit span, the interlayer LKS fractured and caved, while the UKS remained stable since its hanging distance was smaller and did not reach its limit breaking distance. As the working face continued to advance, the LKS gradually formed a new cantilever beam structure, which experienced periodic fractures as the cantilever length increased. Given that the UKS remained stable, the load on the OR of the goaf roof in the upper coal seam was mainly supported by the horizontal cooperative load-bearing structure formed by W-shaped voussoir beam and double-arch bridge. When the hanging length of the interlayer UKS reached its limit span, the UKS underwent rotational fracture and instability. This caused the caving rock in the upper coal seam goaf to cave again, creating a new subsidence space in the goaf roof rock strata of the upper coal seam. Additionally, this triggered secondary activation instability in the W-shaped voussoir beam structure of the upper coal seam goaf, which led to further rotational deformation of the hinged rock blocks within the W-shaped voussoir beam structure in the first mining strip and caused another caving of the OR, as presented in Figure 10b. During the periodic weighting in the working face of the lower coal seam, when only the LKS experienced instability, the UKS bore the load from the W-shaped voussoir beam and double-arch bridge structures of the goaf roof in the upper coal seam. The mining pressure in the working face appeared to be mitigated, and the support resistance remained relatively low. This condition was referred to as a small periodic weighting. When the interlayer UKS underwent rotational instability, which was accompanied by the secondary instability of the W-shaped voussoir beam structure in the goaf roof of the upper coal seam, the mining pressure in the working face became pronounced, resulting in a significant increase in the support resistance. This condition was termed a large periodic weighting. After every 1 to 2 small periodic weightings on the interlayer LKS, the interlayer UKS synchronously fractured, resulting in one instance of large periodic weighting. During the mining under the stress release zone of the lower coal seam, the structural fracturing instability of the IEBS exhibited an asynchronous caving of the lower cantilever beam and upper voussoir beam. The mining pressure in the working face alternated between small and large periodic weightings.

4.2.2. Fracture Characteristics of the IEBS During Mining Under the Stress Concentration Zone

In the case of mining under the stress concentration zone (the double-arch bridge structure of the ICP) of the lower coal seam, when the working face entered the ICP, the IRS remained in the stress release zone of the upper coal seam mining section. At this time, the cantilever structure formed by the interlayer UKS supported the load of the OR. As the working face advanced, the manifestations of the mining pressure were mild, and the support resistance was low. When the working face reached the middle of the ICP, as shown in Figure 11a, the interlayer LKS experienced rotational instability along the back of the support, while the interlayer UKS underwent rotational instability toward the goaf side along the left boundary of the ICP. The mining-induced fractures in the lower coal seam developed upwards and penetrated into the fractures in the OR of the overlying interval goaf, resulting in further goaf caving in the upper coal seam and the development of fractures in the OR that directly penetrated to the surface.

As the working face advanced to the middle and rear parts of the ICP, the load supported by the double-arch bridge structure in the goaf roof of the upper coal seam was transmitted to the IRS via the ICP. With continuous advancement of the working face, the effective bearing area of the IRS under the ICP decreased, while the load per unit area on the IEBS increased. When the working face was 7 m past the ICP, as shown in Figure 11b, the actual load per unit area on the IEBS exceeded the limit strength of its cantilever structure. This resulted in an overall cutting failure along the coal wall of the working face, with a cutting angle of approximately 75°, thereby leading to a sudden increase in support resistance. Meanwhile, as the double-arch bridge structure of the ICP rotated toward the goaf side, the hinged arch beam structure on the right side of the ICP experienced reverse rotational instability. Consequently, the entire OR underwent rotational deformation toward the goaf side, and noticeable vertical tensile cracks appeared on the surface ahead of the working face. The mining-induced shear and tensile cracks that formed on both sides of the OR of the ICP gradually closed as the double-arch bridge structure became unstable.

5. Catastrophic Failure Mechanism of IEBS Instability During the Working Face Passing Through the ICP

As shown in Figure 12a, as the working face entered the phase of mining under the ICP, the load from the double-arch bridge structure of the upper coal seam goaf and its inverted trapezoidal OR was transmitted to the IRS via the ICP. With the continuous advancement of the working face, the effective bearing area of the IRS under the ICP decreased, while the load per unit area on the IEBS increased. When the actual load per unit area on the IEBS exceeded the limit strength of its cantilever structure, an overall cutting failure occurred along the coal wall of the working face, with a cutting angle of approximately 75°, thereby resulting in a sudden increase in support resistance. Figure 12b illustrates the movement of the OR due to IRS instability. The overload instability of the IEBS, along with the rotational movement of the double-arch bridge structure of the ICP toward the goaf side, caused the hinged arch beam structure on the right side of the ICP to experience reverse rotational instability. Consequently, the entire OR underwent rotational deformation towards the goaf side, and noticeable vertical tensile cracks occurred on the surface ahead of the working face. The mining-induced shear and tensile cracks formed on both sides of the OR of the ICP gradually closed as the double-arch bridge structure became unstable.

5.1. Calculation of Overburden Load on the ICP

Due to the instability of the TCPs in interval mining, the OR caved to the surface, and the ICP roof carried the entire inverted trapezoidal load from the surface, thereby creating a stress concentration zone. The concentrated load on the ICP can be treated as an equivalent uniformly distributed load q₃, as illustrated in Figure 13. By considering the fracture characteristics of rock and soil layers in the Yushenfu mining area, as well as the geometric features of load-bearing rock layers above the coal pillars, the distribution equation for the uniform load on the ICP can be derived.

$$q_3={\displaystyle \frac{{\gamma }_0{h}_0{l}_m+{\gamma }_1{h}_1(l_m+h_1/\tan {\phi }_1)+{\gamma }_2{h}_2(l_m+h_1/\tan {\phi }_1+h_2/\tan {\phi }_2)}{l_m}}$$

(2)

where γ₀h₀ is the self-weight load of the immediate roof, which is 0.042 MPa; γ₁h₁ is the self-weight load of the OR, which is 0.736 MPa; γ₂h₂ is the self-weight load of the overlying soil, which is 1.206 MPa; φ₁ and φ₂ are the fracture angles of the rock and soil layers, respectively. Based on the rock layers in the Yushenfu mining area, φ₁ and φ₂ were taken as 67° and 80°, respectively. Based on the parameters of overburden rocks and soils in the Nanliang Coal Mine, the equivalent uniform load q₃ on the ICP was calculated to be 4.69 MPa, with a stress increase coefficient of 2.36.

5.2. Determining the Combined Effect of the IEBS

The stress characteristics of the sub-key and main key strata in the second and fourth rock layers above the 30,103 working face of the Nanliang Coal Mine under a concentrated load were evaluated using the combinatorial key stratum (CKS) theory [38], with the calculated results presented in Equation (3).

$${\displaystyle \frac{{\displaystyle \sum _{i=1}^n{\rho }_{i}g{h}_i}·{\displaystyle \sum _{i=n+1}^m{E}_i{h}_i{}^3}}{({\displaystyle \sum _{i=n+1}^m{\rho }_{i}g{h}_i}+q)·{\displaystyle \sum _{i=1}^n{E}_i{h}_i{}^3}}}=0.25<1$$

(3)

where E_i (i = 1, 2, …, m) is the elastic modulus of the i layer, h_i (i = 1, 2, …, m) is the thickness of the i layer, ρ_ig is the bulk density of the i layer, q is the weight of the loose layer.

The calculations indicated that the second and fourth rock layers in the OR of the 30,103 working face meet the criteria for the CKS in the shallow coal seam. Based on the parameters of the OR in the working face, the relevant parameters for the CKS are calculated as follows:

The thickness of the CKS is h_zu = 21.6 m.

The load on the CKS S is:

$$q_4=q_{\mathrm{zu}}{(x)}_{m+1}={\displaystyle \frac{E_{\mathrm{zu}}{h}_{\mathrm{zu}}^3({\displaystyle \sum _{i=z}^m{\rho }_{i}g{h}_i+q})}{{\displaystyle \sum _{i=z}^m{E}_i{h}_i^3}}}=4.9 \mathrm{MPa}$$

(4)

The periodic weighting interval of the CKS is [39]:

$$l_{\mathrm{zu}}=h_{\mathrm{zu}}\sqrt{{\displaystyle \frac{{\sigma }_t\psi }{3q_{\mathrm{zu}}}}}=10.5 \mathrm{m}$$

(5)

where the tensile strength of the CKS was 3.5 MPa; Ψ is the influence coefficient for the number of the CKS, which is taken as 1; q₄ is the equivalent uniform load on the CKS, which is 4.9 MPa.

5.3. Analysis of the Instability Characteristics of the Fractured Rock Column in the CKS

A structural model was established to describe the instability of the fractured rock column in the CKS, as illustrated in Figure 14, where E₁ and E₂ represent the key rock blocks in the two hard rock layers above the working face. The CKS is composed of these two rock blocks and the IRS. Under the action of the concentrated load q₄, both key rock blocks fractured simultaneously, resulting in a columnar cutting fracture. During the mining under the concentrated coal pillar, the columnar fractures controlled the fracture movement of the entire OR due to the significant fractured thickness of the CKS.

According to the CKS theory, the condition for the key rock column to avoid sliding instability is as follows:

$$T\tan \varphi \ge R_0$$

(6)

Based on the horizontal thrust T acting on the rock column, the support force R_O is required to maintain rock column stability and the periodic fracture l_zu of the rock column, and the maximum load $q_4^{\prime }$ on the OR that prevents the sliding instability of the key rock column can be derived by

$$q_4^{\prime }\le {\displaystyle \frac{{(2\tan \varphi +3\sin \alpha )}^2[{\sigma }_c]\psi }{8n}}$$

(7)

where tanφ is the friction coefficient between the rock columns, which can be taken as 0.6; [σ_c] is the uniaxial compressive strength of the CKS, and n is the ratio of the compressive strength to tensile strength of the CKS, with n = 10.

The rotation angle of the fractured rock block in the key stratum is actually determined by the mining height M, the thickness of the immediate roof Σh, the crushing expansion coefficient k_p, and the length of the fractured rock column l_zu.

$$\sin \alpha ={\displaystyle \frac{M-{\displaystyle \sum h(k_p-1)}}{l_{\mathrm{zu}}}}$$

(8)

By substituting M = 2.2 m, Σh = 5.35 m, k_p = 1.3, and l_zu = 10.5 m for the 30,103 working face of the Nanliang Coal Mine into Equation (8), the final rotation angle was determined to be α’ = 3.5°, which was then substituted into Equation (9) to yield

$$q_4^{\prime }\le {\displaystyle \frac{{(1.2+3\sin \alpha )}^2\times 3.5}{8}}=0.82 \mathrm{MPa}$$

(9)

A comparison between Equation (9) and Equation (4) revealed that the actual load borne by the CKS far exceeds the maximum load that prevents sliding instability of the OR. Therefore, during the phase when the working face passed through the ICP, the fractured rock blocks in the CKS inevitably experienced sliding instability. Thus, the columnar sliding instability of the CKS was the primary cause of the sudden increase in support resistance during mining under the ICP of shallow and closely spaced coal seams.

6. Numerical Simulation of the Instability Characteristics of IEBS During Mining Passing Through the ICP

According to the geological conditions of the Nanliang Coal Mine, a numerical simulation model was established using the discrete element UDEC 5.0 simulation software [39,40,41,42,43,44] to investigate the instability characteristics of the IEBS during mining passing through the double arch bridge structure of the ICP. The analysis focused on the fracture and instability patterns of IRS during mining in the lower coal seam. As shown in Figure 15, the simulated thickness of the 2-2 coal seam was 2 m, while the thicknesses of the immediate and main roofs were 2 m and 8 m, respectively, where a longwall interval mining method was employed. The 3-1 coal seam has an average depth of 132 m, a bedrock thickness of 65 m, and a loose layer thickness of 67 m. The simulated thickness was 2.2 m, and a fully mechanized longwall mining method was employed. The average interlayer spacing between the two coal seams was 34 m. The model parameters were adjusted based on laboratory tests of the physical and mechanical properties of the coal rock. The block body followed the Mohr–Coulomb criterion, while the joint surfaces adhered to the Coulomb slip failure criterion. The model dimensions are 300 m in length and 147 m in height, with the top surface as a free face and fixed boundary conditions on the left, right, and bottom sides. A 15 m boundary coal pillar was retained on both sides of the mining model for the 3⁻¹ coal seam, and the model was mined sequentially from left to right, with a mining interval of 2 m.

After the interval mining of the 2-2 coal seam was completed, the TCP experienced sudden instability after a period of time. This triggered the main roof rock to form a W-shaped voussoir beam hinge structure through two rotational hinges. Consequently, the OR underwent fracture instability, creating a “stair-arch” caving fracture zone that eventually developed to the surface, as presented in Figure 15.

As the working face advanced to 164 m, the sixth periodic weighting occurred, with an interval of 17 m, as illustrated in Figure 16a. The working face was 6 m away from the boundary of the ICP, and shear failure occurred in the OR along the support. The roof subsided by 0.69 m, and the support resistance increased sharply, leading to dynamic mining pressure in the working face, with a dynamic load factor reaching 1.83. The analysis indicated that during mining in the lower coal seam, the two interlayer key strata primarily controlled the OR. The ICP bore a significant concentrated load from the OR. The thickness of the IEBS ranged from 17.49 to 18.42 m. Due to the influence of the fracture angle of the rock layers, the caving of the IRS lagged behind the working face by 7 to 9 m. When the working face left the boundary of the ICP, the effective bearing area of the IEBS above the support gradually decreased, which made it difficult for the cantilever beam structure of the IEBS to support the concentrated stress transmitted from the OR. Cutting failure occurred along the coal wall of the working face forming a cutting failure zone from the coal wall to the boundary of the ICP. Simultaneously, as the ICP experienced roof caving failure, the fractured rock blocks on the right side of the double-arch bridge structure rotated backward toward the goaf, which exacerbated the caving of the roof rock strata. When the working face continued to advance to 180 m, the seventh periodic weighting with an interval of 16 m occurred while it was in the third mining strip, as shown in Figure 16b. The interlayer LKS and its accompanying rock layers rotated upward toward the goaf, forming a separation with the UKS. The separation spanned 12 m, with a maximum separation distance of 0.4 m.

7. Strategies for Dynamic Pressure Prevention

To address the aforementioned dynamic pressure issues, a comprehensive prevention and control strategy involving pre-releasing concentrated stress in the ICP, strengthening the support strength of the working face, and accelerating the advancement speed was proposed. Following the selection of the ICP for loosening and pre-splitting, a loosening blasting was implemented for the ICP in the range of 900 to 1200 m in the 30,103 working face from 10 October to 13 November 2019. The changes in the working resistance of the No. 75 support during mining through this section were monitored, as shown in Figure 17. During the mining process, the immediate roof subsided with the advancement of the working face, leading to the gradual formation of separation and the caving of the OR in the goaf roof. The mining pressure in the working face exhibited alternating small and large periodic weightings, with an average interval of 13 m for small periodic weighting and 19 m for large periodic weighting. The average working resistance of the support during non-weighting was 28.7 MPa, while those during small and large periodic weightings were 37.5 MPa and 32.9 MPa, respectively. The average dynamic load factors during periodic weighting were 1.3 and 1.15 MPa. After the pre-splitting blasting of the ICP, when the working face left the double-arch bridge structure of the ICP, the maximum pressure on the support reached 43 MPa. The volume of coal wall spalling decreased, the safety valve opening rate of the support reduced, and the mining pressure appeared to be alleviated. This indicated that the pre-splitting blasting released the stress concentration in the ICP, reduced the hanging height of the hinge rock blocks in the double-arch bridge structure, and effectively mitigated the dynamic load impact caused by the instability of the double-arch bridge structure in the working face of the lower coal seam.

8. Conclusions

In the production practice of the northern Shaanxi coal mining area, many coal mines will face the situation of coal seam mining under various types of goaf, such as pillar-and-panel goaf, strip goaf, roadway-pillar goaf, and Wongawilli goaf. These insufficiently mined goafs leave a large number of coal pillars, forming diverse overlying rock structures in the goaf. Due to the similarity of the remaining coal pillars in these insufficiently mined goafs, rock mass control in coal seam mining can be mutually referenced. Therefore, this paper focuses on the problem of dynamic mine pressure easily occurring in the coal seam mining face passing through the isolation coal pillars under the shallow-buried close-distance coal seam interval goaf in the northern Shaanxi mining area. Using a combination of similar simulation, theoretical analysis, numerical simulation, and field measurement methods, the main research results are as follows:

(1)

The definition of the IEBS was proposed. The instability characteristics of the IRS during the passage of working face passing through the ICP were studied by performing similarity simulations. It was found that when the working face of the lower seam left the ICP for 7 m (4 m behind the support), the support resistance increased rapidly, and shear failure of the IRS occurred along the front of the support, with a cutting height of 0.5 m for the ICP. The support resistance reached 10,934 kN, resulting in support crushing and difficulty in moving the support.

(2)

The structural instability evolution patterns of the IRS structure in different mining zones of the lower coal seam were examined. During mining under the stress release zone, the structural fracturing instability of the IEBS exhibited an asynchronous caving of the lower cantilever beam and upper voussoir beam. During mining under the stress concentration zone, the IEBS was highly susceptible to cutting failure and support crushing due to the concentrated stress from the double-arch bridge structure in the ICP roof. This pattern was validated through numerical simulations.

(3)

A mechanical model of the double-arch bridge structure with the working face passing through the ICP roof was established. The dynamic pressure load transfer laws during the passage of the working face through the double-arch bridge structure was derived. This revealed that the IEBS met the criteria for the CKS when the working face left the double-arch bridge structure. The excessive load transferred from the ICP to the IRS led to instability, which was the root cause of dynamic pressure-related hazards in the mining working face of the lower coal seam.

(4)

A comprehensive prevention and control strategy involving pre-releasing concentrated stress in the isolation coal pillar, strengthening the support strength of the working face, and accelerating the advancement speed was proposed, which has yielded positive results in application.