Rock burst is one of the primary dynamic disasters in underground mining operations, presenting a substantial risk to miner safety and equipment integrity, resulting in personnel injuries and machinery damage, significantly impeding the safe and efficient extraction of mineral resources (He et al. 2021; Keneti and Sainsbury 2018; Yin et al. 2024). Hence, it is critically important to investigate the mechanisms underlying rock burst incidents in mining operations. These studies are immensely significant as they contribute to the overarching objective of minimizing casualties among miners and enhancing the overall efficiency of mineral resource production. The factors inducing rock bursts can generally be categorized into geological factors (Naji et al. 2019) and mining-related factors (Li et al. 2018a, b). The SIETCS distinctly differ from typical coal seams in both these domains, suggesting that the rock burst mechanisms associated with SIETCS are markedly distinct from those of generally inclined coal seams. The geological characteristics of SIETCS are notably unique: the coal seams are exceptionally thick, often exceeding 30 m, with an inclination angle approaching 90°, and subjected to substantial horizontal stresses. The pronounced disparities in geological conditions inherently lead to variations in mining technological parameters (Li et al. 2021b; Gao et al. 2023; Xue et al. 2023). The mining method employed for SIETCS entails horizontal segmented mining from high to low levels along the strike of the coal seam. This mining technique leads to a unique mining structure where above the panel is the gob, below is solid coal, and on both horizontal sides are thick and hard rock layers. The thick and hard rock layers in the gob are partially suspended.

In the case of the SIETCS with a single coal seam, the rock layers on both sides exert a clamping effect on the coal seam (Cao et al. 2020), clamping rotation effect (Cao et al. 2023b), clamping shear effect (Wang et al. 2023, 2022), resulting in stress concentration and asymmetric stress distribution in rock layers and coal seam (Wang et al. 2019b). Among these factors, the rock layer above the incline is the dominant contributor to stress concentration (Wang et al. 2019a), and the rock layers above the incline and the coal mass near it are the leading static stress concentration areas. The surrounding rock in the stress concentration area is subjected to strong shear stress, and shear failure primarily occurs (Cao et al. 2023a, b; Wang et al. 2023). Meanwhile, the fracture of rock layers above the incline serves as primary source of intense dynamic stress. These rock layers and the adjacent coal mass are particularly more susceptible to strong dynamic stress (Wang et al. 2020). When SIETCS contain multiple coal seams, steeply inclined rock strata and rock pillars can bend and deform due to horizontal and vertical stresses, affecting the integrity of the coal seams. Rock pillar has a prying effect on coal seam (Xu et al. 2023), and the coupled mechanism of compression and prying-induced rock burst were proposed (He et al. 2020a); the compression of rock layer above inclination and prying of rock pillar exert high static stress contributing to the rock burst, while bending and breaking of rock layer provide dynamic stress that also contributes to rock burst. The rock burst mechanism of SIETCS involves the superposition of dynamic stress and static stress that exceeds the critical strength of the coal rock mass. Furthermore, Khan et al. (2024) discovered that weakening areas are more prone to occur rock bursts, through long-term seismic monitoring and mapping the fracturing process at SIETCS. The previous studies offer in-depth analyses of the unique mining structure created by SIETCS from a macroscopic viewpoint, highlighting the rock layers above inclinations and rock pillars as primary factors leading to SIETCS and their influence on rock bursts. Nonetheless, additional research is required to explore the detailed process of rock burst damage in SIETCS.

Rock burst failures frequently occur in coal seams, particularly in roadways, within coal mines (Frith et al. 2020; Xu et al. 2022; Zhang and Jiang 2020). Extensive research has been conducted on rock bursts in general inclined coal seams (Kaiser and Cai 2012; Mazaira and Konicek 2015). Dai et al. (2021) investigated the stability theory and concluded that rock bursts only occur when the surrounding rock of roadways can maintain stability and exceed the maximum allowable disturbance stress. Li et al. (2021a) proposed that residual energy release when rock bursts occur is directly proportional to its severity. Yang et al. (2018) examined factors influencing roadway surrounding rock stability and found that horizontal stress plays a significant role, with higher levels increasing the risk of roadway rock bursts. Weng et al. (2017) discovered that elevated horizontal stress leads to energy accumulation above and below roadways, resulting in faster energy release at the bottom angle of roadways and surrounding rocks, thereby increasing susceptibility to rock burst failures. Rock bursts are consequences of stressed and energetically disturbed unstable masses of rocks (Gale 2018; H.S.Mitri et al. 1999; Linkov 1996; Manouchehrian and Cai 2017), requiring both instability conditions as well as satisfying specific energy and stress criteria for their occurrence.

The above studies mainly focus on the causes of rock bursts in roadways, but the specific process of rock burst failure in roadways has been studied few. Rock burst failure in roadways is often related to the structure of the surrounding rock (Guo et al. 2021), especially the LCS. The cracks in the surrounding rock of the roadway expand and connect under the action of concentrated stress, forming LCS (Chen et al. 2017; Zhang et al. 2022). The bearing capacity of LCS is low and easily unstable and fractured (Guo, Yu, Tan, and Zhao 2019a, b). The unstable failure of LCS and the projection to free space are the causes of rock burst failure in roadways (Song et al. 2019). Although the above studies on rock burst failure in roadways mainly focus on near horizontal or inclined coal seams, they are also applicable to roadways subjected to significant horizontal stress (Guo et al. 2021; Weng et al. 2017; Yang et al. 2018), which can provide references for the study of rock burst damage mechanism of the SIETCS.

Previous studies have extensively examined the movement and damage of the overlying rock layers on both sides of SIETCS to investigate the source of strong dynamic stress and its impact on rock bursts from a macro perspective. However, there have been few studies on how strong dynamic loads combine with high static loads from tunnel surrounding rocks to induce rock burst damage in tunnels. It is essential to clarify the specific process and mechanism of rock burst damage in SIETCS tunnels to prevent and control rock bursts. In this paper, the SIETCS mine with rock burst in China was taken as the engineering background. The development of cracks in the surrounding rock of the roadways was detected, and the stresses and energy distribution were analyzed. This study was conducted on whether there are LCS in the SIETCS roadway, the damage process of rock bursts induced by LCS, and the stress and energy conditions required for rock bursts. The research results can guide the prevention of rock bursts in the SIETCS.

2.1 The general situation of the SIETCS mine

The SIETCS mine is in Urumqi City, China (Fig. 1). The mining field situates at the Ungar Basin’s southern margin, in the Boga Mountains’ foothills. The topography exhibits a south-to-north gradient, with surface elevations ranging from + 739.20 m to + 934.00 m and a maximum relative elevation difference of 194.80 m, typically around 60 m. Prominent geological structures within the mining area include the Qidaowan anticline, Badaowan syncline, Wanyaogou thrust fault, and Baiyangnangou anticline (Fig. 2). The current mining operation focuses on two main coal seams, B1 + 2 and B3 − 6. The B1 + 2 coal seam has an average thickness of 37.45 m, ranging from 31.83 to 39.45 m. The B3 − 6 coal seam has an average thickness of 48.87 m, ranging from 39.85 to 52.3 m. The coal seams have an inclination angle of 87°, indicating their steeply inclined nature. The two coal seams are separated by the rock pillar, which gradually thins from west to east within a range of 53–110 m. The B1 and B3 roadways are positioned along the rock layer below inclination, while the B2 and B6 roadways situate along the rock layer above inclination. Both roadways are arranged parallel to the coal seam strike (Fig. 3). In the mining process, the B1 + 2 coal seam and the B3 − 6 coal seam are alternately extracted at the same horizontal level. The B3 − 6 coal seam is mined first, followed by the extraction of the B1 + 2 coal seam at the same horizontal level. The panel employs a horizontal segmental, fully mechanized top coal mining method. The segment height is 25 m, with the shearer cutting coal having a thickness of 3 m and the caving coal having a thickness of 22 m. The first rock burst in the mine occurred at the + 500 m level, and it is currently being mined gradually down to the + 425 m level.

The study area of the SIETCS mine

Distribution of large geological structures around the coal mine

Panels layout and mining methods for the SIETCS. a Layout of panels; b Layout of roadways; c The mining method

2.2 In-situ stress distribution

The underground mine’s northern haulage roadway at the + 350 m level underwent in-situ stress testing (Fig. 3) using the ground stress relief method (Cai and Peng 2011). The surface elevation of the corresponding area is around + 821 m. Figure 4 shows the arrangement of test points. Table 1 shows the principal stress distributions at each test point.

Layout of the in-situ stress test sites

From a macroscopic geological perspective, the arcuate compression zone in the northern foothills of the Bogda Mountains primarily exhibits a northeastward orientation. Notable faults within this area include the Qidaowan anticline and the Badaowan syncline, with their axes approximately aligned at 70°–75°. As these folds form under compression, the primary horizontal stress direction aligns perpendicularly to the axial direction of the folds. Considering structural movement and trends, it can be inferred that the maximum horizontal principal stress in this mine is oriented between 330° and 340°. The consistency between maximum principal stress directions obtained from two test points and those inferred from structural movement validates the accuracy of analytical calculations of in-situ stress test results.

2.3 The difference between SIETCS and generally inclined extremely thick coal seams

The mining structures of SIETCS differ from that of generally inclined coal seams as showed in Fig. 5. The horizontal sides of the panel in the generally inclined coal seams consist of either coal body or gob, while the vertical sides are rock layers. Consequently, the roadways in generally inclined coal seams feature coal ribs along their horizontal sides and rock roofs and floors along their vertical sides. However, in the SIETCS panel, the vertical gob area features solid coal seams below, with both sides composed of rock layers horizontally. Therefore, for roadways in SIETCS, rock and coal ribs exist horizontally on either side. The panel’s upper part comprises coal roofs, and the lower part comprises coal floors.

SIETCS and generally extremely thick coal seams. a Generally inclined extremely thick coal seams; b SIETCS

The rock burst failure in the roadway results from the comprehensive interaction between stress and energy. In the subsequent analysis, we investigate the distribution of mining stress and energy in SIETCS roadways through numerical simulation.

3.1 Numerical simulation model and parameter setting

Numerical simulations were performed using the FLAC3D software, and the simulation model was built to represent the geological conditions of the mine. The model covered an elevation range from + 0 to + 800 m (representing the average ground elevation), as depicted in Fig. 6. The B1 + 2 coal seam was simulated with a thickness of 35 m, while the B3 − 6 coal seam was set to a thickness of 50 m in the numerical model. These settings allowed for the simulation of the mine’s structural behavior and response under different conditions. The intermediate rock pillars had a thickness of 110 m. The model size was 900 m × 1 m × 800 m (X, Y, Z). Along the inclination of the coal seam, line M was positioned through the middle of roadways, and line R was placed on the rock rib side of roadways.

Numerical model and measure line arrangement of the SIETCS

In the numerical model, normal displacements at boundaries in the X direction and the bottom boundary were constrained. Additionally, displacements in the Y direction were also restricted. However, the top boundary of the model, representing the ground surface, had no constraints, allowing unrestricted movement in this particular direction. These constraints were imposed to accurately simulate the specific geological conditions and constraints of the mine, providing a more realistic representation of the behavior of the surrounding rock and coal seams in the numerical simulations. Stress parameters are applied to the model according to the in-situ stress in the mining area. The model used a gravitational acceleration of 10 m/s², and calculations were conducted using the Mohr–Coulomb constitutive model. Table 2 presents the assigned numerical values for the physical and mechanical parameters of the coal and rock. According to the historical mining sequence of the mine and some simplification, the first excavation was from the surface to + 600 m, and then mining was in 25 m sections to + 425 m. Rock bursts mainly occurred in the B3 − 6 coal seam; therefore, the simulation primarily focused on the surrounding rock conditions during mining + 425 m B3 − 6 coal seam.

3.2 Stress and energy distribution of SIETCS

3.2.1 Stress distribution of SIETCS

Before roadway excavation, the stress distribution of the surrounding rock in the + 425 m section of SIETCS is illustrated in Fig. 7. The vertical stress attains its maximum value within the rock mass on both sides of the upper part of the coal seam (Fig. 7a). Specifically, the peak values of vertical stress are measured as 23 MPa and 18.33 MPa for the B6 and B3 rock layers, respectively. A semi-elliptical zone for vertical stress relief is formed from the boundary of coal seam mining to a depth of 30 m below it. Within this relief zone, the vertical gradually increases from an initial value of 2 MPa to reach an in-situ stress level of 8 MPa; however, over a large area, stresses remain lower than 6 MPa. The horizontal stress distribution significantly differs from the vertical stress distribution (Fig. 7b). The horizontal stress decreases only in the rock layers on both sides of the gob, while the horizontal stress in the coal body either remains at the in-situ rock stress level or forms stress concentrations. The peak horizontal stress in the coal seam is concentrated near the top of the seam, particularly close to the B3 and B6 rock layers. The peak stress value near the B6 rock layers is 29.73 MPa, while it is 27.62 MPa near the B3 rock layers. From the stress distribution perspective, horizontal segmented mining results in concentrated horizontal stress within the SIETCS, while vertical stress is reduced, leading to a stress characteristic dominated by horizontal stress.

Stress distribution of the SIETCS before roadway excavation. a Vertical stress b Horizontal stress

Figure 8 illustrates the stress distribution of the coal seam and surrounding rock after roadway excavation. Peak stress zones R₁–R₃ and F₁–F₃ are observed near B3 and B6 rock layers, respectively, with only R₁ and F₁ exhibiting vertical stress concentration. In terms of vertical stress distribution, there is a slight decrease in the peak value of vertical stress for F₁ and R₁ by 5.16%–7.10% compared to F₁ and R₁ without roadway excavation. Simultaneously, distinct relief zones in the coal roof and coal floor for vertical stress are formed within the distance of 3–4 m from the boundaries of the roadway; most coal bodies within this zone experience stresses lower than 2 MPa. The vertical stress outside this stress relief zone gradually increases beyond in-situ stress levels, remaining at approximately 10 MPa. This indicates that roadway excavation has minimal effect on the distribution of vertical stress in SIETCS and may even reduce vertical stress near roadways.

Stress distribution of the SIETCS after roadway excavation. a Vertical stress; b Horizontal stress; c Vertical stress on measure lines before and after excavation; d Horizontal stress on measure lines before and after excavation

The distinct peak zones of horizontal stress (R₂–R₃ or F₂–F₃) are observed on both the coal roof and coal floor after roadway excavation, with the peak point located approximately 3.2–3.7 m away from the roadways. After roadway excavation, horizontal stress was significantly increased within the peak zone of R₂, rising by 53.5% from 20.67 to 31.75 MPa (peak value). In contrast, within the peak zone of R₃, it increased by 59.32% from 20.30 to 32.35 MPa (peak value). The horizontal stress at F₂ increased by 54.02%, ranging from an initial value of 20.57 MPa to a peak value of 31.68 MPa; similarly, at F₃, it rose by 60.97%, increasing from an initial value of 20.71 MPa to a peak value of 33.34 MPa. The horizontal stress increases rapidly from about 8 MPa to over 30 MPa within the range of 0–3.5 m from the roadway boundaries to the peak zones R₂ and R₃ (F₂ and F₃). The horizontal stress above the peak zones of R₂ and R₃ (F₂ and F₃) is about 25 MPa and is concentrated. Roadway excavation has a considerable impact on the horizontal stress of SIETCS, leading to re-evolution of horizontal stress in the coal seam and significant stress concentration. The newly formed stress concentration areas are mainly distributed above and below the roadways.

3.2.2 Energy distribution of SIETCS

Under the action of stresses, coal and rock masses undergo deformation and accumulate releasable elastic energy. Upon destruction of the coal and rock mass, this energy is released, resulting in dynamic phenomena. The elastic energy density was calculated using the FISH program in Flac3D. The releasable elastic energy was calculated based on Xie et al. (2009). Figure 9 illustrates the release of elastic energy of the SIETCS. As shown in Fig. 9a, the coal is the primary area for energy accumulation, forming a region with high energy values. Before roadway excavation, this high-energy region within the coal seam exhibited an ‘inverted triangle’ distribution, extending approximately 38 m downwards from the mining boundary. The energy peaks are concentrated on both sides at the top of the coal seam; specifically, the energy peak value of R₁ is 190.86 kJ/m³, and F₁ corresponds to 135.94 kJ/m³.

Energy distribution in the SIETCS. a Before roadways excavation; b After roadways excavation

The distribution of peak zones of elasticity energy in coal seam after roadway excavation is basically consistent with the horizontal stress. The peak zones R₁ and F₁ are located at the top of the coal seam, while R₂–R₃ and F₂–F₃ are located in the coal roof and coal floor 3.18–3.7 m away from the roadways. The energy peaks of R₁ and F₁ at the top of the coal seam are 181.29 kJ/m³ and 139.85 kJ/m³, respectively. Compared with that before roadway excavation, the energy peak of R₁ decreases by 5.01%, and that of F₁ increases by 2.88%, with a slight overall fluctuation. However, the distribution characteristics of elasticity energy of surrounding rock in coal seams have changed. After roadway excavation, the energy peak of R₂ increases from 72.61 to 192.83 kJ/m³, with an increase of 165.58%; the energy peak of R₃ increases from 64.43 to 183.70 kJ/m³, with an increase of 185.14%; the energy peak of F₂ increases from 67.23 to 172.40 kJ/m³, with an increase of 156.45%; and the energy peak of F₃ increases from 64.69 to 179.98 kJ/m³, with an increase of 178.32%. Consistent with the distribution of horizontal stress, steeply inclined and extra-thick coal seams serve as the primary areas of energy concentration. Roadway excavation increases the releasable elastic strain energy of the coal mass, particularly in the coal roof and coal floor of the roadway, providing a substantial energy source for the dynamic damage associated with rock bursts in roadways.

3.3 Mining characteristics of SIETCS

Based on the numerical simulation results and in-situ stress distribution, it’s evident that the horizontal stress of SIETCS before mining is approximately twice as high as the vertical stress. This highlights the predominance of horizontal stress in the stress environment of such seams. After mining, the vertical stress of SIETCS undergoes a significant decrease compared to its initial state, while the horizontal stress either remains at or increases beyond its initial level. Consequently, there is an increase in the ratio of horizontal to vertical stresses. These characteristics are particularly pronounced after roadway excavation, especially within 3–4 m above and below the roadway. Within this range, there is a reduction in vertical stress accompanied by a rapid increase leading up to its peak for horizontal stress levels. Furthermore, regions of high elastic energy concentration within the coal body closely align with areas experiencing concentrated horizontal stresses; this relationship is illustrated in Fig. 10.

Mining characteristics of roadways in SIETCS

The stress characteristics of strong compression and low confinement will lead to the formation of low-stability coal structures near roadways. The concentrated energy resulting from these conditions can cause strong dynamic phenomena when the low-stability coal structures are failured. Therefore, the mining characteristics of SIETCS significantly increase the rock burst risk. The following will analyze the low-stability structure formed in the SIETCS roadway and how it causes rock burst damage.

4.1 Formation of LCS in SIETCS

When the confined stress is feeble, cracks will propagate and coalesce along the direction of compressive stress. The unstable expansion of inevitable collinear periodic cracks will interconnect and merge to form longer cracks parallel to the roadway surface, thereby generating LCS (Chen et al. 2017; Guo et al. 2019a; Tan et al. 2018). Song et al. (2019) investigated and deduced the rock burst evolution model under dynamic stress disturbance. It was highlighted that roadway rock bursts result from stress and energy release due to sudden instability failure of multilayer LCS under external disturbances.

The stress distribution characteristics of SIETCS indicate that the horizontal compressive stress on the coal roof and floor of roadways is high. In contrast, the vertical constraint stress is low, facilitating the formation of LCS. Therefore, it can be inferred that LCS will form in the coal roofs and floors of SIETCS, particularly those located 3–4 m from roadways. While a rock burst failure model for generally inclined coal seams has been established (Song et al. 2019), where vertical stress is identified as the primary factor causing fracturing, under conditions of SIETCS, horizontal stress plays a more significant role in determining surrounding rock stresses. Consequently, Fig. 11 illustrates how the LCS formed and distributed within mining areas situated in SIETCS.

Distribution diagram of the LCS in the SIETCS roadways. a Cracks propagate and coalesce; b LCS formed; c Distribution of LCS around the roadway

4.2 The conditions for rock burst induced by LCS

4.2.1 The stress conditions for rock burst induced by LCS

To facilitate analysis, the boundary conditions of LCS above and below the roadways are simplified as follows: the opposite sides in the inclined direction are assumed to be simply supported. In contrast, any boundary condition is considered on the opposite side in the advancing direction. The LCS have a length of L and a minimal thickness of 2t. They are subjected to compressive stress σ_S, resulting in deflection denoted by w under compression deformation, as illustrated in Fig. 12. Generally, buckling of LCS occurs along the direction of the free surface in the coal seam. Due to their extremely thin laminate structure, this structure focuses solely on discussing failure caused by tensile stress during buckling deformation.

LCS buckling deformation diagram. a LCS; b Buckling deformation

During buckling deformation, the elements within the LCS will generate a vertical stress q that is perpendicular to the LCS. This observation aligns with elasticity principles (Xu 2006)

where D is the bending stiffness of LCS.

Under the application of compressive stress σ_s, the LCS satisfy Eq. (2), enabling us to determine the vertical stress q_s experienced by the internal unit during compressive buckling.

It can be observed that compressive stress in LCS is converted into tensile stress through buckling deformation, with the latter being the direct cause of failure while the former acts indirectly. Additionally, dynamic stress during mining processes also affects these structures, with the energy released from ruptures in thick and hard rock layers on either side of SIETCS serving as the primary source of dynamic stress (Zhong et al. 2024), as depicted in Fig. 13.

Dynamic stress wave transfer process

The dynamic stress is transmitted as waves comprising P and S waves. The particle motion velocity v_p induced by the P wave aligns or opposes the propagation direction of the dynamic stress wave. In contrast, the particle motion velocity v_s caused by the S wave is perpendicular to it. Assuming θ represents the angle between the propagating direction of the dynamic stress wave generated by the source and LCS, we can decompose the motion velocity of LCS obtained from this dynamic stress wave into v_dx and v_dz components parallel or perpendicular to them, respectively (as shown in Eq. (4) and Fig. 14). Herein, v_dx is positive when compression aligns with LCS and negative otherwise. Similarly, v_dz is positive when buckling aligns with LCS; it is negative otherwise.

Velocities distribution of LCS under dynamic stress waves. a Velocity distribution of dynamic stress wave; b Velocity distribution of LCS caused by dynamic stress wave. z-a indicates the v_dz is opposite to the buckling direction, z-b indicates the velocity v_dz is the same as the buckling direction, x-a indicates the velocity v_dx is opposite to the compressive direction, x-b indicates the velocity v_dx is the same as the compressive direction

The direction and magnitude of dynamic stress on LCS differ due to the varying directions of v_dx and v_dz. When v_dx opposes the compressive stress, the LCS experience axial tensile stress from dynamic stress, thereby reducing the compressive effect of static stress (Fig. 15a). Conversely, when v_dx aligns with the compressive stress, the LCS undergo axial compressive stress from dynamic loading, further intensifying the compressive effect of static stress (Fig. 15b). Similarly, when v_dz opposes the buckling direction, vertical dynamic compression is exerted on the LCS, restricting and resisting their buckling while diminishing static tensile stress (Fig. 15c). On the other hand, when v_dz aligns with the buckling direction, vertical dynamic tension acts upon the LCS, leading to increased buckling and tensile stresses in static conditions (Fig. 15d). The axial dynamic compressive stress and the vertical dynamic tensile stress will lead to the failure of the LCS. However, the axial dynamic tensile stress and the vertical dynamic compressive stress will improve the stability of the LCS.

Velocities distribution of LCS under dynamic stress waves. a Axial dynamic tensile stress reduced bucking; b Axial dynamic compressive stress increased bucking; c Vertical dynamic compression reduced bucking; d Tensile dynamic compressive stress increased bucking. σ_dx-a denotes the dynamic tensile stress induced by v_dx σ_dx-b denotes the dynamic compressive stress induced by v_dx σ_dz-a denotes the dynamic compressive stress induced by v_dx σ_dz-b denotes the dynamic tensile stress induced by v_dz

The dynamic stresses generated during the transmission process of P and S waves are denoted as σ_dp and q_ds, respectively (He et al. 2016).

Then, the dynamic stresses σ_d and q_d along the axial and vertical of LCS formed by v_dx and v_dz are as follows:

Failure must meet stress conditions. In general, coal and rock masses remain relatively stable under static stress. When the static stress σ_s (q_s) and dynamic stress σ_d (q_d) superimpose over the critical stress σ_limit (q_limit), the LCS will be destabilized and fail, as shown in Eq. (7) (Dou et al. 2014; He et al. 2017).

4.2.2 The stress conditions for rock burst induced by LCS

The above analysis presents the stress conditions of rock bursts in LCS. However, an unstable coal body must satisfy certain energy conditions to undergo instantaneous ejection motion and form rock bursts. The energy stored in LCS \(U_{i}\) should exceed the minimum energy required for rock mass failure \(W_{i}^{\text{f}}\) (Zhao et al. 2003) and the dissipated energy (including overcoming frictional resistance) \(W_{i}^{\text{d}}\). The surplus represents the kinetic energy \(W_{i}^{\text{v}}\) necessary for the movement of surrounding rock. Eq. (8) can be used to calculate the accumulated energy within the i-LCS (Song et al. 2019). Furthermore, dynamic stress will affect the LCS, increasing their stored energy

where t is 1/2 of the thickness of the i-th LCS, q_i is the vertical tensile stress the i-th LCS, σ_i is the compressive stress of the i-th LCS, \(W_{i}^{\text{f}} = \frac{{\sigma_{\text{c}i}^{2} }}{2E}\), \(W_{i}^{\text{v}} = \frac{{m_{i} v^{2} }}{2}\), σ_ci is the uniaxial compressive strength of i-th LCS, m_i is the mass of the i-th LCS.

During the failure process of the i-layer crack structure under dynamic stress, three potential energy release conditions described by Eq. (9) may arise. Suppose the energy stored within the LCS only meets the minimum requirement for failure but lacks sufficient dissipation energy. In that case, failure of the LCS will occur exclusively. When the stored energy can meet the minimum requirements for failure and dissipated energy but falls short in providing kinetic energy, the failure of the LCS will occur and result in a limited range of vibration. However, if the stored energy can meet all minimum requirements for failure, dissipated energy, and kinetic energy, it will trigger a rock burst due to the failure of the LCS.

During a rock burst, the kinetic and dissipated energy (\(W_{0}^{\text{v}}\), \(W_{0}^{\text{d}}\)) required for the ejection of fragmented coal mainly comes from the energy provided by the unstable failure of the LCS. Only when the first LCS undergoes unstable failure does the released energy generally become small and insufficient to provide enough kinetic energy to the impacting coal (including the fragmented coal and the fractured LCS). The occurrence of rock bursts is often triggered by the failure of LCS, starting from the first LCS and propagating to the i-LCS (Song et al. 2019). The energy conditions for the occurrence of rock bursts due to the unstable failure of multilayers LCS can be described by Eq. (10).

4.3 The process of LCS induce rock bursts

The occurrence of a rock burst necessitates fulfilling stress and energy conditions. Primarily, the stress conditions must be satisfied to facilitate the failure of coal and rock mass. Subsequently, the energy conditions must be met to ensure that the disintegrated coal and rock mass possess sufficient kinetic energy to eject into free space, resulting in roadway deformation and equipment damage. Consequently, the specific vertical rock burst failure process induced by the LCS of SIETCS can be described as follows.

Under the influence of unique geological conditions and advanced mining technology, the LCS have reached a concentration state under static horizontal stress compression. Consequently, a substantial amount of elastic energy has accumulated within these structures, establishing the initial stress and energy conditions for the occurrence of rock bursts. The rock layers flanking the SIETCS undergo bending and deformation, accumulating significant energy. Moreover, minor fractures within the rock layers release energy and generate intense stress disturbances. For instance, He et al. (2020b) discovered that fractures in the rock pillars of SIETCS can induce dynamic stresses exceeding 84.5 MPa or more. These dynamic compressive stresses along the axial direction and dynamic tensile stresses along the vertical direction further exacerbate buckling within the LCS until they reach their critical load-bearing capacity and ultimately fail. The energy storage of LCS is further increased under dynamic stress. Subsequently, a significant amount of elastic energy is converted into kinetic energy upon failing these structures. The resulting energy and velocity of the LCS moving into free space induce rock burst phenomena. Due to their location in the coal roof and floor of SIETCS, these LCS primarily cause vertical rock burst damages. On one hand, the impact from the coal body’s kinetic energy propels it towards the free surface, leading to deformation and failure of surrounding rock in roadways. On the other hand, due to obstruction by vertical support equipment, this kinetic energy is transformed into mechanical energy that damages hydraulic supports, U-shaped steel beams, and other equipment. In summary, Fig. 16 illustrates the occurrence process of rock bursts above and below surrounding rock in roadways associated with SIETCS.

Mechanism of roadway rock bursts induced by roadways of LCS in the SIETCS

4.4 Further discussion of the LCS-induced rock burst mechanism

From the distribution characteristics of mining stress and energy in SIETCS, it can be observed that a semi-elliptical region exhibits significant horizontal stress and minimal vertical stress, along with concentrated elastic energy at the top of the coal seam, leading to LCS, too (Fig. 17a). The failure of these LCS at the top of the coal seam releases energy, resulting in secondary dynamic stress that is applied to the LCS surrounding the roadway. Therefore, apart from those above direct dynamic stress acting on the LCS surrounding the roadway to induce a rock burst vertical damages mechanism, there may also exist three types of vertical LCS that contribute to this mechanism (Fig. 17b).

Mechanism of roadway rock bursts induced by coal seam LCS in the SIETCS. a LCS distribution of coal seam; b Rock burst induced process

The first mechanism: The initial dynamic stresses are transmitted to LCS at the top of SIETCS and surrounding roadways. The initial dynamic stresses induce failure in LCS surrounding roadways, resulting in the first stage rock burst in roadways. Subsequently, secondary dynamic stress generated by LCS failure at the top of SIETCS triggers the second-stage rock burst quickly, leading to more severe damage caused by multiple stages of rock burst superposition.

The second mechanism: No immediate rock burst occurs in the roadways under the initial dynamic stress. It triggers the LCS failure at the top of SIETCS and generates secondary dynamic stress. Under the secondary dynamic stress, LCS, which surrounds roadways, will fail, ultimately inducing rock bursts in roadways. This mechanism may result in inconsistencies between regions where initial dynamic stress is located and areas where SIETCS experience rock bursts.

The third mechanism: Roadways experience rock bursts under the initial dynamic stress. At the same time, the energy released from LCS failures at the top of SIETCS generates secondary dynamic stresses that cause rock bursts in other areas along roadways. This mechanism leads to more enormous failure ranges for SIETCS than generally inclined coal seams.

5.1 Field detection of LCS

According to Cao et al. (2020), a wide range of cracks development zones is observed at the top of SIETCS, with predominantly horizontal or gently inclined cracks angles, providing evidence of LCS in this zone. Therefore, the site tests of LCS primarily focus on detecting and analyzing the LCS surrounding the roadways.

5.1.1 LCS detection points layout

The LCS surrounding roadways at + 475m B3 − 6 panel was detected using ultrasonic instruments and borehole logging to identify the ribs of the roadway. In contrast, only borehole logging was used to detect the ribs of the coal roof. A total of six detection sites were selected in the B3 roadway and seven detection sites in the B6 roadway, with each detection site having six arranged detection boreholes. Boreholes a and b probed the rib on the south side of the roadway, and boreholes c and d probed the rib on its north side; all these boreholes had an angle of 0° concerning the horizontal plane. Additionally, boreholes e and f, with an angle of 75°, were used to detect the coal roof of roadways. All boreholes’ depths are 5 m. The distribution for detection sites, detection boreholes, interval time of driving and detection in both B3 and B6 roadways are illustrated in Fig. 18.

Layout of detection sites and boreholes in roadways. a Layout of detection sites; b Layout of detection boreholes

5.1.2 Detection results

The detection results of detection site B3 − 1 in the B3 roadway are illustrated in Fig. 19. The coal and rock mass on the surface of roadway ribs exhibit significant fracturing, resulting in a shallow acoustic wave velocity. The acoustic wave velocity gradually increases as the coal and rock mass transition from the fracturing state to the crack development state. Once the coal and rock mass cracks cease to develop, there is a rapid and sustained increase and then remains stable in ultrasonic wave speed. Based on the detection results, the ultrasonic wave velocities in boreholes a, b, c, and d increased progressively from 3.0 m, 2.6 m, 1.8 m, and 2.0 m to a relatively stable state. This indicates a transition of the surrounding rock from a highly fractured condition to one with improved structural integrity. The primary range for cracks development on both sides of the roadways is measured as follows: 3.0 m (a), 2.6 m (b), 1.8 m (c), and 2.0 m (d), as depicted in Fig. 19a.

The detection results of B3 − 1 in each borehole. a Detection results of borehole a b c d; b Detection result of borehole e; c Detection results of borehole f

The logging results of crack development in boreholes e and f are illustrated in Figs. 19b, c, respectively. Borehole e exhibited the presence of obvious circumferential cracks, for example, at depths of 0.4 m, 1.0 m, 1.6 m, 2.2 m, and 3.4 m, with a count of 4, 4, 3, 2, and 2 cracks observed correspondingly. No cracks were detected at a depth of 4.2 m; therefore, the primary cracks distribute within the range of depths from 0 to 3.4 m for this test borehole. Similarly, for borehole f, its main cracks development spanned from depths from 0 to 3.2 m, where significant circumferential cracks were identified at hole depths measuring 0.8 m, 1.0 m, 1.4 m, 1.8 m, 2.8 m, and 3.2 m. The coal cracks in both boreholes primarily developed along the circumferential direction rather than axially along the borehole. This observation suggests that coal seam cracks tend to propagate horizontally. By comparing the characteristics of surrounding rock crack development across different drilling depths, it is evident that the curvature associated with circumferential crack propagation becomes more pronounced as drilling depth increases. This phenomenon indicates that primary coal cracks develop greater under horizontal stress compression and are more prone to forming LCS.

The results of other detection sites exhibit a general similarity to those of detection sites B3 − 1. The range of major cracks in the surrounding rock of the roadways is illustrated in Fig. 20. Roadway ribs’ primary crack development range test between 1.5 and 2.3 m. For coal roofs in B6 and B3 roadway, they range from 2.6 to 3.9 m and from 2.6 to 3.7 m, respectively. The crack directions of the roadway’s coal roof at each test point primarily follow the ring direction of the borehole. This indicates that the coal roof of the roadway is under extrusion stress near the circumferential direction of the drill hole, with the coal roof experiencing significantly higher extrusion stress compared to the coal rib.

The detection results of cracks in B3 roadway and B6 roadway. a B3 roadway; b B6 roadway

Three distinct characteristics can be identified according to the detection results of drilling boreholes. Firstly, it is observed that the cracks development range of roadway coal roofs (2.6–3.9 m) surpasses that of sidewalls (1.5–2.3 m) overall, indicating that under the unique stress environment of SIETCS, the main mining-affected areas are constituted by coal roofs and coal floors in roadways. Additionally, the range of crack development in the roadway coal roof is identified to be between 2.6 and 3.9 m, which corresponds precisely with the area of concentrated horizontal stress and elastic energy as indicated by numerical simulation results. The scientific validity of both theoretical analysis and numerical simulation confirms and validates the existence of LCS. Finally, the crack development range of measuring points disturbed by other mining operations (3–2, 6–2) is more extensive than that of undisturbed detection sites. It is known that during the mining of the panel, the development range and the number of cracks in the roadways may increase, which may lead to an increase in LCS.

5.2 Analysis of rock burst damage characteristics of SIETCS

From 2013 to 2017, six occurrences of rock bursts occurred during the extraction of the B3 − 6 coal seam in the SIETCS. These rock bursts are sequentially labeled as I, II, III, IV, V, and VI, according to their order of occurrence. The roadway damage forms during the six rock bursts are shown in Fig. 21.

Roadways damage caused by rock bursts. a, b Rock burst I damages in + 500 m B3 and B6; c–e Rock burst II damages in + 500 m B3 and + 500 m B6 and + 475 m B6; f Rock burst III damages in + 475 m B3; g Rock burst IV damages in + 450 m B6; h–i Rock burst V damages in + 450 m B3 and B6; j Rock burst VI damages in + 450 m B3

During each rock burst occurrence, the vertical boundaries of the roadway experienced deformation and damage, characterized by the sinking of the coal roof, subsidence of the shoulder, and bulging of the coal floor. In the various rock burst incidents, the maximum bulging of the south ribs of the coal floor of the B6 roadway was 0.5 m, and the maximum bulging of the north side was 0.7 m. The maximum bulging of the shoulder on the north ribs was 1.3 m, and the maximum bulging of the coal floor was 0.6 m. The maximum sinking of the coal roof was 1 m, and the maximum reduction in the height of the mesh behind the net was 1.5 m for a mesh size of 2 m × 2 m × 1 m. For the B3 roadway, the maximum bulging of the south ribs of the coal floor was 0.45 m, the maximum subsidence of the shoulder on the south ribs was 0.4 m, the maximum subsidence of the shoulder on the north ribs was 0.3 m, the maximum bulging of the coal floor was 0.3 m. The maximum sinking of the coal roof was 0.3 m.

When a rock burst occurs, the hydraulic or single-column support may become fractured or bent, relieving pressure from the relief valve. The roof I-beam undergoes severe deformation, and other dynamic damage phenomena are observed in the advance support area of the panel, as depicted in Fig. 22. For instance, during rock burst I, seven pairs of longitudinal beams and H frames were uplifted in roadway B6 (1893–1872 m), while one unit was damaged in the advance support area at 1920 m. In addition, there are 13 steel I-beams significantly deformed in the roof subsidence area between sections 1959 and 1910 m of roadway B3. Furthermore, a rock burst occurred at the panel of the B6 roadway, resulting in the bending of the south rear column of hydraulic support #1 and both front and rear columns of hydraulic support #2; meanwhile, hydraulic supports #4 and #6 experienced pressure relief.

Field photographs of vertical deformation or damage to supporting equipment in roadways. a Steel I-beam deformation; b Hydraulic support bending; c Coal roof deformation in the roadway; d Single hydraulic prop bent

After considering the deformation and failure of the roadway and the damage to vertical support equipment, it becomes evident that there must be a significant force source and energy source within both the coal floor and coal roof of the roadway. This indicates that the roadway possesses the initial conditions necessary for rock burst occurrence. Furthermore, under dynamic stress superposition, stress conditions for rock burst failure and energy conditions for rock burst are simultaneously met. These findings align with numerical simulation results, theoretical analysis, and field detection, thus confirming that the LCS in the coal floor and coal roof of SIETCS is the primary factor inducing rock bursts.

The source of rock burst IV is situated within the B3 rock layers; however, it exclusively induces damage to the B6 roadway, with no occurrence of rock burst damage observed in the B3 roadway. This inconsistency between the focal area of the rock burst and the affected roadway validates the impulse rock burst mechanism associated with the second type of LCS discussed in Sect. 4.4.

When comparing the SIETCS with typical coal mines in eastern China that employ comprehensive mining, retreat mining, and layer-by-layer retreat mining methods, it is evident that the SIETCS has experienced six rock bursts. These events have resulted in a total damage range of 1644 m in length, with the smallest single-event damage range being 75 m and the largest range reaching 709 m (Fig. 23). In contrast, using different mining methods, the cumulative damage range of 15 to 80 rock burst events in typical rock burst mines in eastern China is all less than 500 m (Li et al. 2018a, b), which indicates that the damage range caused by rock bursts in the SIETCS is significantly more extensive than that in generally inclined coal seams and verifies the third mechanism of LCS-induced rock burst discussed in Sect. 4.4.

Statistical analysis of damage range from rock bursts occurrences in the SIETCS

1. (1)

   The vertical stress before roadway excavation in the SIETCS is very low, less than 8 MPa. In contrast, the horizontal stress is concentrated and exceeds 20 MPa. After roadway excavation, the vertical stress in the coal seam is further reduced, and the horizontal stress increases. Within the range of 3–4 m above and below roadways, the vertical stress is 0–6 MPa, the horizontal stress increases rapidly from 8 to more than 30 MPa, and a large amount of elastic energy accumulates simultaneously. This stress characteristic results in LCS and serves as the energy foundation for the LCS-induced rock bursts.

2. (2)

   The stress conditions for failure to occur when LCS induces rock bursts must first be met. Under external static horizontal stress compression, LCS undergoes buckling deformation, which transforms the compressive stress into tensile stress within the inner unit of the structure. Tensile stress directly leads to failure, while compressive stress indirectly contributes to failure. Dynamic loading induces axial and vertical velocities, resulting in dynamic compression or tensile stress along these directions. Only axial dynamic compressive stress and vertical dynamic tensile stress further reduce the stability of LCS and induce failure. Other forms of dynamic stress can improve stability.

3. (3)

   The occurrence of rock bursts induced by LCS also requires fulfilling energy conditions. Suppose the energy the LCS stored justly meets the minimum requirements for destruction or dissipation. In that case, they may crack or shake without inducing rock bursts. When sufficient kinetic energy is obtained with the impact coal body, instantaneous ejection will occur, resulting in roadway deformation and damage to supporting equipment, leading to rock bursts.

4. (4)

   Field detection results of LCS showed that the coal roof within the 2.6–3.9 m range from the roadway developed numerous horizontal cracks, suggesting that this section of coal mass experienced significant compressive stress and low restraint stress, resulting in a spalling structure. This stress characteristic is consistent with the numerical simulation results. Each rock burst in the SIETCS, vertical surrounding rock deformation or equipment damage occurred, such as coal roof sinking and coal floor bulging. These findings align with typical features associated with LCS-induced rock burst damages and provide evidence for validating their rock burst mechanism on vertical damage.