Large mining-height longwall panels have become increasingly prevalent in China in recent years1,2 However, coal resource loss associated with coal pillar retention remains a major issue in the extraction of thick coal seams under large mining-height conditions. To mitigate this problem, pillarless gob-side entry retaining has been widely adopted in Chinese coal mines as an alternative roadway layout3,4,5,6. This approach not only reduces coal resource loss but also decreases roadway development requirements, resulting in significant economic benefits7,8,9.

Gob-side entry retaining is subjected to both intense strata movement within the goaf and the advanced abutment pressure induced by the subsequent working face10,11,12. Consequently, substantial research efforts have been devoted to gob-side entry retaining technologies13,14,15. Gong Peng et al. examined the effects of key roadside support parameters, including wall width and thickness, on roadside displacement, support resistance, and stress evolution16. Some scholars investigated the stress distribution and evolution characteristics of the roof surrounding rock in retained entries under repeated mining-induced dynamic loading17,18,19. Other scholars also proposed an integrated control approach combining roof-cutting control of surrounding rock, reinforcement support, and hydraulic fracturing in the advanced working zone20,21,22,23. Feng Chao et al. established an equivalent bearing constitutive model for the goaf and derived the support resistance expression of the backfilling body in fully mechanized caving pillarless gob-side entry retaining under roof-cutting conditions24. Huang Zhizeng et al. developed a lateral cantilever beam mechanical model for gob-side entry retaining and analyzed the influence of roof cutting and roadside backfilling on beam stress behavior25. Wang Kai et al. proposed a collaborative surrounding rock control system for gob-side entry retaining arranged along the coal seam roof in soft thick coal seams26. Wang En et al. introduced a “five-in-one” collaborative control concept for surrounding rock stability in dual-entry retaining27. Wang Fangtian et al. established a mechanical structural model of the retained-entry roof and proposed a calculation method for roof subsidence28. Zhao Mengye et al. developed a mechanical model for in-entry support and derived corresponding methods for calculating support resistance29. These studies have provided an important foundation for the development of gob-side entry retaining technologies. But they did not summarize the common failure modes of the roadside filling wall, nor did they give the theoretical calculation formula or guidance for the optimal roof cutting height and roof cutting angle range. Otherwise, under high-production and high-efficiency mining conditions in large mining-height panels, existing entry-retaining systems remain inadequate, and current surrounding rock control methods are unable to fully satisfy the requirements of high-intensity, high-efficiency mining.

Previous studies have paid limited attention to roof-cutting pressure-relief in gob-side entry retaining with roadside backfilling under large mining-height conditions. Roof cutting can effectively reduce the direct loading imposed by the overlying strata structure on the retained entry, thereby improving the stability of the surrounding rock. The core of roof cutting and pressure relief technology is to actively cut off the mechanical connection between the roof of the roadway and the key rock strata on the side of the goaf, so as to promote the directional caving of the roof and fill the goaf, thus reducing the stress concentration of the surrounding rock of the retaining roadway. The common technical types can be divided into the following four categories: deep hole pre-splitting blasting roof cutting and retaining technology, directional hydraulic fracturing roof cutting and retaining technology, high pressure water jet roof cutting and retaining technology and mechanical rock breaking roof cutting and retaining technology. The deformation behavior of the surrounding rock under conditions with and without roof cutting is illustrated in Fig. 1.

Fig. 1
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Deformation of the surrounding rock in the retained entry with and without roof cutting.

In this study, a deviatoric stress–based index is introduced to investigate roof-cutting pressure-relief mechanisms in gob-side entry retaining with roadside backfilling under large mining-height conditions. The failure mechanisms of the surrounding rock of the retained entry are systematically analyzed, and a targeted synergistic control strategy combining enhanced support and roof-cutting pressure relief is proposed. Field mine-pressure monitoring is further employed to evaluate the performance of roadside support and to assess the safety and stability of the retained entry, thereby confirming the reliability of the proposed control system.

Engineering background

Gob-side entry retaining with roadside backfilling is planned for the transportation roadway of the 2507 working face. The retained entry is a rectangular roadway with a cross-section of 4.4 m × 3.2 m and is excavated along the coal seam floor. In the 2507 working face area, the coal seam thickness is approximately 6.3 m, and the burial depth is about 530 m. The layout of the working face and the occurrence of the coal and rock strata are shown in Fig. 2.

Fig. 2
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Layout of the working face and distribution of the coal–rock strata.

Failure mechanisms of gob-side entry retaining with roadside backfilling under large mining-height conditions

Effects of strong mining-induced disturbance in large mining-height panels

In the absence of active roof pre-splitting and pressure-relief measures, large mining heights lead to intense coal extraction and pronounced movement of the overlying strata during face advancement30,31. As a result, both the range and magnitude of mining-induced stresses are substantially increased. With continued face advancement and the progression of gob-side entry retaining, roadway maintenance becomes increasingly difficult in the head entry ahead of the working face as well as in the retained-entry section behind the face. Under such conditions, the surrounding rock of the roadway is highly susceptible to deformation, instability, and failure32.

Instability and failure of narrow roadside backfill

Roadside wall support alone cannot maintain the long-term stability of the retained entry. Owing to the limited ability of the roadside backfill to provide sufficient yielding capacity while sustaining adequate load-bearing resistance, the backfill is prone to different instability modes once the load imposed by the overlying strata exceeds its strength. Several representative instability modes are shown in Fig. 3.

Fig. 3
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Common failure modes occurring under single roadside backfilling wall support conditions.

  1. (1)

    In gob-side entry retaining, the roadside backfill often exhibits relatively high strength. Owing to mining-induced stress redistribution among the working face, roadway surrounding rock, and goaf-side strata, stress concentration readily develops within the backfill. When the concentrated stress exceeds the strength capacity of the yielding wall, cracking or even splitting failure may occur, which poses a potential threat to safe production at the working face33.

  1. (2)

    When the strength of the roadside backfill is insufficient to sustain the load imposed by the overlying strata, deformation occurs at the roof–floor interfaces and along the lateral sides of the wall. Such deformation compresses the retained-entry space, leading to pronounced roof displacement, a reduction in the roadway cross-sectional area, and a further degradation of roadway stability.

  1. (3)

    When the roadside backfill exhibits relatively high strength whereas the roadway roof is composed of weak coal, intense mining-induced disturbance from face advancement is likely to induce shear failure in the roadway roof. Under such conditions, roof deformation is markedly aggravated and, in severe cases, may lead to localized roof falls34.

These observations indicate that roadside backfilling wall support alone is unable to accommodate the movement of the overlying strata structure under large mining-height conditions. Consequently, roof-cutting pressure-relief technology is adopted to pre-split key strata blocks above the roadway, thereby limiting their rotational subsidence and maintaining the stability of the roadway surrounding rock.

Theoretical analysis of roof-cutting pressure relief in gob-side entry retaining under large mining-height conditions

Theoretical calculation of roof-cutting height

To ensure that the collapsed roof strata can fully fill the goaf, the roof-cutting height is determined as follows:

$$\:H=(H\text{M}-H1-H2)/(K-1)\:$$
(1)

where H is the roof-cutting height (m); HM is the coal seam thickness (m); H1 is the roof subsidence (m); H2 is the floor heave (m); and K is the rock bulking factor.

The immediate roof of the roadway consists of mudstone, while the main roof is sandy mudstone. Based on the lithological characteristics of both the immediate and main roofs, the rock bulking factor was taken as 1.42. The maximum mining height of the working face is 6.3 m. By neglecting roof subsidence and floor heave, the roof-cutting height was determined to be H = 15 m.

Theoretical calculation of roof-cutting angle

After slit blasting, the key block becomes prone to sliding instability along the cutting plane. Once key block B slips, the load acting on the overlying strata above the roadway is reduced, which enhances roadway stability23. Let the cutting angle be denoted by α, and the forces acting on key block B at the contact point are analyzed. The sliding instability of block B must satisfy the following condition:

$$(T\: {\rm cos}\: \alpha\: - R\: {\rm sin}\: \alpha ) {\rm tan}\: \phi \leq\: R\:{\rm cos}\alpha\: +\: T\: {\rm sin}\: \alpha$$
(2)

where T is the horizontal thrust (kN), R is the shear force (kN), and φ is the internal friction angle. After simplification, the expression can be written as:

$$\alpha \geq\: \phi - {\rm arctan}\frac{R}{T}$$
(3)

The calculation formulas for R and T are as follows:

$$R\:=\:qL,T =\frac{qL^2}{2(h-\Delta S)}$$
(4)

where:

Q is the load intensity of the main roof (kN/m);

L is the length of the main-roof block (m);

H is the thickness of the main-roof stratum (m); and.

SB is the subsidence of rock block B (m).

This equation describes the subsidence behavior of the main-roof block by incorporating the block length (L), load intensity (q), thickness (h), and the subsidence of block B (S < sub> B). Substitution of the relevant parameters yields α ≥ 10°.

Numerical simulations of key parameters for roof-cutting pressure relief in gob-side entry retaining with roadside backfilling under large mining-height conditions

Model establishment and simulation scheme

A numerical model was constructed based on the in situ geological conditions of the working face, as shown in Fig. 4. The model dimensions are 180 m × 210 m × 90 m (x × y × z). The top boundary was assigned as a stress boundary, whereas the lateral boundaries and the bottom boundary were defined as fixed-displacement boundaries. The deformation and failure behavior of the materials were simulated using the Mohr–Coulomb constitutive model. The lateral pressure coefficient was set to 1.20, and the unit weight γ was taken as 25 kg/m³.

In this paper, the numerical simulation uses FLAC3D software to build a three-dimensional model, using hexahedral structured grid (Hexahedral structured grid). The unit size of the key area (around the roadway section, the influence area of roof cutting blasting, and the roadside filling wall) is set to be 1.0 m × 1.0 m × 1.0 m. This area is the core area with the most severe stress and displacement changes, and the mesh is encrypted to accurately capture the mechanical response. The unit size of the transition area (5 ~ 20 m from the roadway) is set to 3.0 m × 3.0 m × 3.0 m, and the calculation accuracy and efficiency are balanced. This area is less affected by mining and the stress changes gently. Large units are used to reduce the calculation cost. The total number of units in the model is 1,897,200, and the total number of nodes is 2,034,568, which meets the grid resolution requirements of three-dimensional numerical simulation of gob-side entry retaining with large mining height.

Fig. 4
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Numerical calculation model.

The physical and mechanical parameters of each rock stratum (e.g., elastic modulus, Poisson’s ratio, Tensile strength, etc.) is shown in Table 1.

Table 1 Table of bed thickness and physical and mechanical parameters.
Full size table

The stress state of the coal–rock mass can be described by the combined representation of the deviatoric stress tensor and the spherical stress tensor. Let the three mutually orthogonal principal stresses be denoted as σk (k = 1, 2, 3), σ1≥σ2≥σ3. Accordingly, the stress state of the surrounding rock in the coal–rock mass can be expressed in matrix form as follows:

$$\left(\begin{array}{c}\sigma_1\:\: 0\:\: 0\\ 0 \:\:\sigma_2\:\: 0\\ 0\:\: 0\:\:\:\sigma_3\end{array}\right) = \left(\begin{array}{c}\sigma_m\:\: 0\:\: 0\\ 0 \:\:\sigma_m\:\: 0\\ 0\:\: 0\:\:\:\sigma_m\end{array}\right) + \left(\begin{array}{c}s_1\:\: 0\:\: 0\\ 0 \:\: s_2\:\: 0\\ 0\:\: 0\:\:\: s_3\end{array}\right)$$
(5)

In Eq. (1), σm represents the component of the spherical stress tensor, which is adopted in this study as an index for characterizing spherical stress. Its expression is given by:

$$\sigma_{\rm m} = \frac{1}{3}(\sigma_1 + \sigma_2 + \sigma_3)$$
(6)

sk denotes the components of the principal deviatoric stress tensor (sk=σk-σm), where s1 represents the maximum principal deviatoric stress.

$$s_1=\sigma_1-\frac{1}{3}(\sigma_1 + \sigma_2 + \sigma_3)$$
(7)

These results indicate that deviatoric stress integrates the effects of multiple principal stresses and thus provides a comprehensive measure for evaluating the surrounding rock response to roof-cutting pressure-relief parameters. On this basis, deviatoric stress is selected as the key analytical index, and numerical simulations are performed to investigate the effects of different roof-cutting pressure-relief parameters on stress reduction, with the aim of identifying the optimal roof-cutting scheme.

Analysis of key roof-cutting pressure-relief parameters for gob-side entry retaining

Determination of roof-cutting height

The deviatoric stress contours of the roadway surrounding rock under different roof-cutting heights are shown in Fig. 5. It is indicated by Fig. 5 that the peak deviatoric stress zone in the surrounding rock of the retained entry is primarily located in the upper portion of the immediate roof on the solid coal side. In the absence of pressure-relief measures, severe deformation of the roadway surrounding rock is observed, characterized by pronounced roof subsidence and inward convergence of the sidewalls, with the peak deviatoric stress reaching 14 MPa. After roof-cutting pressure relief is applied, the degree of deviatoric stress concentration within the surrounding rock is markedly reduced. Furthermore, an increase in roof-cutting height leads to a progressive decrease in the peak deviatoric stress, indicating an enhanced pressure-relief effect.

Fig. 5
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Deviatoric stress distribution of the surrounding rock under different roof-cutting heights.

When the roof-cutting height is 5 m, the cutting line does not penetrate into the main roof stratum, and severe deformation of the surrounding rock of the retained entry persists. The deformation characteristics are nearly identical to those observed under the no–roof-cutting condition. At this roof-cutting height, the peak deviatoric stress is reduced to 13 MPa; however, the overall level of stress concentration remains high. These results indicate that a roof-cutting height of 5 m is insufficient to ensure the stability of the roadway surrounding rock.

When the roof-cutting height is increased to 10 m, the cutting line penetrates into the main roof stratum, resulting in a noticeable reduction in surrounding rock deformation compared with the no–roof-cutting and 5 m cases. Although the peak deviatoric stress decreases to 12 MPa, the surrounding rock remains in a relatively high stress state, and the deviatoric stress concentration zone is still extensive. Consequently, this roof-cutting height is insufficient to ensure the long-term stability of the retained entry.

When the roof-cutting height is increased to 15 m, the cutting line penetrates approximately 70% of the main roof stratum. Both the surrounding rock deformation and the peak deviatoric stress are further reduced, indicating that roof cutting effectively alleviates the stress acting on the retained entry. This condition is favorable for maintaining the long-term stability of the roadway; therefore, a roof-cutting height of 15 m was selected for field application.

Determination of roof-cutting angle

The deviatoric stress distribution in the surrounding rock under different roof-cutting angles is shown in Fig. 6. As shown in Fig. 6, when the roof-cutting angles are 0°, 5°, 10°, and 15°, the peak deviatoric stresses on the solid coal rib are 8, 13, 12, and 11 MPa, respectively, whereas those on the roadside wall are 14, 10, 11, and 12 MPa. At a roof-cutting angle of 0°, although the solid coal rib exhibits the lowest peak deviatoric stress, the stress concentration zone is the most extensive, with a large area near the roof remaining in a highly concentrated stress state. Under this condition, the overlying load is mainly borne by the yielding wall. As the roof-cutting angle increases from 0° to 15°, the deviatoric stress on the solid coal side progressively decreases, accompanied by a gradual increase in deviatoric stress within the yielding wall, indicating a redistribution of load between the two sides.

Fig. 6
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Deviatoric stress distribution of the surrounding rock under different roof-cutting angles.

When the roof-cutting angle increases from 5° to 10°, the changes in deviatoric stress on both the solid coal side and the roadside wall side are relatively minor. As the angle is further increased from 10° to 15°, the reduction in deviatoric stress remains limited, whereas the bearing pressure on the yielding wall increases by approximately 1 MPa. At a roof-cutting angle of 15°, the deviatoric stress is more uniformly distributed between the solid coal side and the yielding wall side, and this angle is also more favorable for overlying strata caving. Accordingly, a roof-cutting angle of 15° was selected for field implementation.

Determination of the width of the roadside backfilling body

The deviatoric stress distribution in the surrounding rock under different widths of the roadside backfilling body is shown in Fig. 7. It is indicated by Fig. 7 that the deviatoric stress in the surrounding rock of the roadway roof and floor remains at a relatively low level, whereas the deviatoric stress distribution along the two sidewalls varies significantly with different roadside backfill widths.

Fig. 7
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Deviatoric stress contour distribution of the surrounding rock under different wall widths.

When the roadside filling width is 0.5–1.0 m, severe deformation occurs in both the roadway and the roadside wall, and the deviatoric stress concentration is mainly transferred to the solid coal side, indicating insufficient load-bearing capacity of the roadside wall. When the width is increased to 1.5–2.0 m, deviatoric stress concentrations develop on both the roadside wall and the solid coal side, accompanied by a pronounced enhancement in the load-bearing capacity of the roadside wall compared with the 0.5–1.0 m cases. Under these conditions, the roadside wall plays a critical role in supporting the overlying strata. Accordingly, a roadside backfill width of 1.5 m was selected.

Surrounding rock control strategies and technologies for roof-cutting pressure relief in gob-side entry retaining with roadside backfilling under large mining-height conditions

Control strategies

One of the main challenges in gob-side entry retaining lies in the requirement for the roadway support system to provide sufficient active support capacity to withstand the intense effects of repeated mining disturbances. In addition, the fracture position of the overlying roof strata above the retained entry is difficult to predict, which can significantly affect the performance of roadside support. In view of these challenges, a combined control strategy integrating enhanced support and roof-cutting pressure relief is adopted for gob-side entry retaining in the 2507 working face.

The combined control strategy of “enhanced support and roof-cutting pressure relief” aims to ensure roadway stability through the coordinated application of support reinforcement and stress reduction measures. The support component primarily involves strengthening the roadway roof and ribs using structural elements such as rock bolts and cables, wire mesh, single hydraulic props, and concrete roadside walls, thereby preventing roadway instability. Roof-cutting pressure relief is implemented by blasting through thick and hard roof strata above the roadway to deliberately modify the overlying strata structure, which mitigates stress concentration around the roadway and effectively reduces the roof load.

“Enhanced support and roof-cutting pressure relief” act synergistically to maintain the stability of the surrounding rock of the retained entry. The mechanism of this combined control strategy is reflected in two complementary aspects. First, the overall support capacity is strengthened through the integration of active and passive support systems, in which load-bearing is provided by elements such as rock bolts, anchor cables, and single hydraulic props, thereby maintaining the integrity of the retained-entry space. Second, roof-cutting blasting is employed to modify the fracture position at the ends of the overlying strata, inducing controlled breakage of the roof at a predetermined location. This promotes early sliding and caving of the overlying rock mass, reduces the load acting on the roadway roof, and prevents the formation of a long-span beam–like roof structure, which would otherwise lead to pronounced stress concentration adjacent to the retained entry.

Another key technical challenge in gob-side entry retaining is the mismatch between the entry-retaining rate and the working face advance rate. This issue is addressed by adopting high-performance yielding concrete support in combination with compatible construction procedures, thereby enabling full-process mechanized implementation. In addition, an integral roadside casting technique is applied to achieve effective isolation between the retained entry and the goaf, which significantly mitigates air leakage from the goaf. Temporary support is provided by protective support frames during the construction process to ensure construction safety and stability.

Surrounding rock control technology and scheme for the retained entry

Prior to the advance of the shearer, roof mesh installation is completed, followed by the installation of anchor cables ahead of the gangue-retaining support. The roof-cutting height and angle are set to 15 m and 15°, respectively. The roadside wall has a width of 1.5 m and a concrete strength grade of C40. The roof anchor cables are Φ22 × 8.3 m, arranged with a spacing and row spacing of 1.8 m, which may be adjusted according to site-specific roof conditions. Additional roof reinforcement anchor cables with a length of 10.3 m are installed, with a pretension force of no less than 150 kN. The rib anchor cables are Φ22 × 5.3 m, with a spacing of 0.9 m and a row spacing of 1.8 m. The roof and rib bolts are Φ20 × 2.2 m, arranged with both spacing and row spacing of 0.9 m. The detailed support layout and design parameters are illustrated in Fig. 8.

Fig. 8
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Layout scheme and design parameters of the gob-side entry retaining system.

The width of the yielding roadside wall was set to 1.5 m. A combined support configuration consisting of 1.6 m π-shaped steel beams and single hydraulic props (one beam with two props) was adopted, with a spacing of 900 mm between adjacent props. the reason for selecting the combined support of π-shaped steel beam + single hydraulic prop is that the support scheme is highly adapted to the engineering application scenario. The specific analysis is as follows: (1) In this study, the 2507 working face transportation crossheading is affected by mining. The initial roof subsidence of the retaining roadway is 118 mm, and the convergence of the roadway is about 80 mm. The combined support system of π-shaped steel beam + single hydraulic prop has the characteristics of ' rigid support + flexible adjustment’ : π-shaped steel beam can effectively expand the roof control range and disperse the roof load; the single hydraulic prop can realize the dynamic adjustment of support resistance through pressure relief and fluid replenishment, adapt to the deformation process of surrounding rock, and avoid the failure of support system due to large deformation of surrounding rock. (2) The roof of this study area is dominated by sandy mudstone, with thin fine sandstone locally. After roof cutting, the roof caving block is uneven, and local suspended roof is easy to appear. The π-shaped steel beam + single hydraulic prop can flexibly adjust the spacing of the prop arrangement according to the roof collapse situation, and encrypt the prop in the suspended roof area to ensure the support strength. (3) The combined support system of π-shaped steel beam + single hydraulic prop has the advantages of low cost and reusable materials. During grouting, the π-shaped beams and single hydraulic props are removed after serving their temporary support function. In addition, W-shaped steel straps are installed at the roof spanning the yielding wall to provide reinforced support. The roadside construction layout is illustrated in Fig. 9.

Fig. 9
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Roadside support configuration.

Analysis of mine pressure monitoring results

After the working face reaches squaring, Station A is set up immediately when gob-side entry retaining construction commences. Measuring point of roof on the roof cutting side and on the intact coal side at station A is separately marked with 1# and 2#. Subsequently, Station B is established after the working face advances and the entry retaining is implemented for approximately 50 m, with a distance of 50 m between Station A and Station B. Roof measuring point on the roof cutting side and on the intact coal side at station B is separately marked with 3# and 4#. It can be shown in Fig. 10. The observation period is 70 days, and the observation method adopted is the cross method.

(1) Displacement monitoring of the surrounding rock of the retained entry

The monitoring results of the surrounding rock displacement are also shown in Fig. 10. The roof deformation on both the roof-cutting side and the solid coal side of the retained entry exhibits an initial increase followed by gradual stabilization, with the deformation on the roof-cutting side being greater. After stabilization, the roof deformation on the roof-cutting side remains below 120 mm, indicating that the overall surrounding rock deformation is relatively limited and within the acceptable safety control range.

Fig. 10
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Monitoring results of surrounding rock displacement in the retained entry.

(2) Monitoring of anchor cable loading

The monitoring results of anchor cable loading are shown in Fig. 11. With increasing distance behind the working face, the anchor cable load initially increases to a peak value and then gradually decreases before stabilizing. Overall, the load carried by the roof anchor cables on the roof-cutting side is greater than that on the solid coal rib side. After stabilization, the load of the roof anchor cables on the roof-cutting side is approximately 200 kN, indicating effective anchorage performance and reliable support behavior.

Fig. 11
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Monitoring results of anchor cable loading in the surrounding rock of the retained entry.

(3) Monitoring of roadside wall pressure

The monitoring results of the load acting on the roadside wall are shown in Fig. 12. As shown in Fig. 12, the load acting on the roadside backfilling wall initially increases with increasing distance from the working face, reaches a peak, and subsequently decreases to a stable level. At a distance of approximately 40 m, the wall stress attains a maximum value of about 36 MPa, after which it rapidly decreases and stabilizes at around 28 MPa. This response is mainly attributed to fracture of the main roof above the roadway, which leads to a reduction in the load transferred to the yielding wall. When the distance from the working face exceeds 90 m, the wall stress gradually stabilizes. Moreover, the maximum stress developed in the yielding wall remains below the design strength, indicating that the wall exhibits adequate load-bearing capacity and stable support performance.

Fig. 12
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Monitoring results of the load acting on the roadside wall.

Conclusions

  1. (1)

    Theoretical analyses confirm that the optimal roof-cutting parameters for roadside filled gob-side entry retaining are a roof-cutting height of approximately 15 m and an angle of no less than 10°. This parameter range achieves effective pressure relief, laying a foundation for stable surrounding rock control in large mining height panels.

  2. (2)

    At the optimal roof-cutting height of 15 m, the cutting line penetrates approximately 70% of the main roof thickness. This reduces deviatoric stress concentration on the solid coal rib, enables the flexible formwork wall to exert sufficient bearing capacity, and forms a synergistic load-bearing system between the coal rib and the wall, effectively maintaining roadway cross-section integrity.

  3. (3)

    A 0° roof-cutting angle leads to the largest stress concentration zone (high stress concentrates near the solid coal side roof), with the flexible formwork wall bearing most of the overlying load. As the angle increases from 0° to 15°, the coal rib load decreases gradually while the wall load increases. At 15°, stress distribution between the two becomes uniform, facilitating controlled caving of overlying strata and enhancing long-term stability.

  4. (4)

    A roadside wall width of 0.5–1.0 m causes severe deformation of the coal rib and backfilling wall, with peak deviatoric stress shifting to the coal rib. When the width is increased to 1.5–2.0 m, the wall and coal rib bear comparable loads, and the wall’s supporting resistance is significantly improved. This width matches the intense mining disturbance of large mining height panels, ensuring reliable roadway reuse.