Characteristics of overlying strata and mechanisms of arch beam failure in shallowly buried thick bedrock coal seams: A case study in western China

The Yangjiacun coal mine, located in western China, exhibits distinctive characteristics, such as its shallow depth, thick bedrock, thin loose layer, and unique roof strata breaking movement. In this research, a comprehensive analysis of previous exploration data, subsequent mechanical testing, key stratum theory, natural equilibrium arch theory, and simulation tests and numerical simulations has been conducted to investigate the failure behavior of the overlying rock strata in the Yangjiacun Coal Mine. Specifically, the load‐bearing capacity of the key stratum structure in the bedrock and the morphological properties of the arch‐beam structure have been thoroughly examined. A mechanical model of the roof rock arch‐beam structure has been established, and an analytical formula for the limit size and stability criterion of the arch‐beam structure has been derived. The mining‐induced rock arch‐beam structure plays a crucial role in transferring the load of the upper strata to the surrounding goaf, thereby preventing subsidence and deformation of the overlying strata. However, the arch‐beam structure periodically experiences instability during the advancement of the working face, gradually progressing towards the upper layer until the main key stratum is breached and the arch‐beam structure collapses. Based on the characteristics of weathered rock and the arch‐beam failure model in the Yangjiacun Coal Mine, quantitative backfilling mining methods have been proposed to either compensate for the mining void created by coal extraction or limit the size of exposed key strata through strip mining, ensuring the stability of the R6 key stratum. This study represents a significant contribution to understanding the intricate failure behavior of overlying rock strata in coal mines and provides a valuable framework for the responsible and efficient extraction of coal resources in similar environments.

As coal resources in eastern China become depleted, the energy base has shifted westward, with 80% of the large coal bases belonging to western coal fields.The western coal fields are characterized by shallow coal seams, with most less than 200 m deep and a low basic load ratio, and diverse topography and geomorphology, including mountain areas, gully development areas, plain areas, and regions with a large surface undulation. 1,2The exploitation of shallowburied coal seams is associated with the following problems: the collapse of the overlying rock layer to the surface after exploitation can lead to a step-like or cut-off type of destruction, with obvious roof compression and the risk of support death accidents, exacerbating soil and water loss and desertification and seriously affecting the ecosystem. 3The extraction of underground coal resources, particularly in shallow-buried coal seams, often causes strata deformation and fracturing, which can result in rock movement and mine pressure.To explain this phenomenon, scholars worldwide have proposed numerous models of stope structure, as shown in Figure 1. 4 Currently, the key stratum theory is a relatively mature theory that is recognized by the mining industry.
By incorporating key stratum theory and numerical simulation techniques, Xie et al. 5,6 put forward the concept that the surrounding rock of the stope forms a stress shell composed of high-stress beams.This stress shell functions as the critical force chain system for the enveloping rock mass in the working face, responsible for bearing and transmitting the stope pressure.Xu et al. 7,8 conducted a comprehensive investigation of shallow coal seam mining and developed a discriminant formula for identifying unstable overlying strata structures.The discriminant method they proposed characterized three types of overlying strata commonly observed in shallow coal seams, providing a useful tool for identifying key strata structures.Liu et al. 9 investigated the characteristics and mechanisms of fracture propagation in shallow coal seams during underground mining in western China, utilizing field observations and numerical simulations.Their research focused on factors such as the interface range between coal and rock and stress characteristics.Xie et al. 10 proposed a new numerical model for predicting fracture height in shallow coal seams during re-mining based on laboratory experiments and numerical simulations, and verified the validity of the model through field observations.Lin et al. 11 investigated the factors influencing ground subsidence during shallow coal mining in western China using field investigations and numerical simulations, including coal-rock properties, coal seam depth, and mining methods.
Wayite 12 conducted on-site monitoring of support forces and rock movements during shallow longwall mining at Angus Pullies Coal Mine in New South Wales, the resulting damage appeared as a "bottle plug" shape that extended to the surface, leading to a maximum subsidence coefficient of 85% with a breaking angle ranging from 76°to 90°.Holla and Bizen et al. 13 observed the movement behavior of roof strata in a longwall working face of a shallow coal seam, finding that the roof strata broke and moved rapidly behind the working face, with the height of the caving zone being nine times the mining height.Soni 14 examined the surface subsidence pattern in thin bedrock conditions through field measurements.Banerjee et al. 15 studied the movement pattern of the overburden rock layers in short-wall mining through field monitoring.Unver et al. 16 analyzed the surface movement pattern resulting from thick coal seam mining.Singh and Singh 17 investigated the behavior of a support system and roof strata during sublevel caving of a thick coal seam.Ramesh and Ram 18 predicted subsidence due to coal mining in the Raniganj coalfield in West Bengal, India.
Mining of shallow coal seams is a challenging task that requires a thorough understanding of the overburden rock collapse patterns, mine pressure emergence, and surface subsidence.Through underground observations conducted during the mining period at Yangjiacun coal mine, it has been observed that, following the complete collapse of the main roof, the overlying loose layers do not collapse entirely.This observation suggests that the overlying rock formations do not exhibit a homogeneous response characterized by rapid strata separation, fracturing, and collapse in tandem with the coal seam extraction.Rather, a certain level of structural integrity is observed, indicating the presence of a composite key stratum structure in the overlying rock formations at Yangjiacun coal mine.Drawing upon preliminary physical exploration data and subsequent mechanical test data, this study employs a combination of key stratum theory and stress arch theory.By utilizing numerical simulation and similar physical experiments, a detailed analysis of the roof failure patterns during coal extraction at Yangjiacun coal mine is conducted.The main purpose of this paper is to fully understand the uniqueness of the breaking movement of the roof strata in the coal mining face under the conditions of shallow buried depth, thin loose layer and thick bedrock, aiming to ensure safe and efficient production in similar mining environments.

| General situation of 22 mining area in Yangjiacun Coal Mine
Yangjiacun Coal Mine is situated in Dongsheng District, Ordos City, Inner Mongolia Autonomous Region, with the center of the mine field approximately 11 km from the center of the district.The mining area spans 1629.5 m in strike length and 4132.3 m in tendency length, covering a delineated area of 4.367 million m 2 .The coal seam being mined is the 2-2 upper coal seam with an average thickness of 5.28 m and a dip angle ranging from 1°to 5°.The coal seam structure is simple and it is free from water and gas, with a stable occurrence.The roof of the 2-2 upper coal seam is primarily composed of finegrained sandstone and sandy mudstone, while the floor mainly consists of mudstone, although the roof and floor of the coal seam exhibit significant variability across the mining area.With a ground elevation ranging from +1444.75 to +1545.14 m and an average depth of 100 m, the Yangjiacun coal mine has a resource reserve of 3.478 million tons and a recoverable reserve of 2.609 million tons.Figure 2 depicts the location of the mine and the rock assignment map of the working face.
The structural form of Yangjiacun coal mine field is consistent with the structural form of regional | 3319 coal-bearing strata.The overall structure is a monocline structure inclined to the southwest, and the formation attitude is gentle, with a tendency of 220-260°.During the excavation of the working face entry, two faults were exposed, which were numbered F1 and F2.The F1 fault is a normal fault with a dip angle of 145 °, a dip angle of 65 °, and H = 1.3 m, which is a non-water-conducting fault.The F2 fault is a normal fault with a tendency of 158 °, a dip angle of 48 °, and H = 1.5 m, which is an impermeable fault.
The hydrogeological conditions in this area are relatively simple.There is no strong aquifer in the roof and floor of the working face, only a weak aquifer.The aquifer affecting the mining is mainly the fine-grained coarse sandstone aquifer above 20 m of the roof of the 2-2 upper coal seam.The aquifer is a direct water-filled aquifer with poor recharge conditions and medium water-rich fractures.It is expected that after the mining of the working face, with the collapse of the roof in the goaf, the cracks will go straight to the surface, which will cause water gushing from various sandstone confined water aquifers and Quaternary loose aquifers in the roof, which is manifested as water gushing from the goaf after the working face, which will have a certain impact on mining.

| Basic mechanical properties of coal seams and roof strata
This study focused on the upper coal seam, gangue, and roof of the 2-2 section in the 2206 working face of Yangjiacun Coal Mine as the experimental research objects.Specimens were collected from the working face near the belt transport corridor and made in the laboratory.Brazilian splitting tests and uniaxial compression tests were conducted on coal and rock using MTS servo machines and Shimadzu AX-G250 testing machines.
Figure 3A,C,E illustrate the uniaxial tensile curve of the roof, coal seam, and gangue, respectively.The experimental results reveal that the roof of the upper coal seam 2-2 exhibits a tensile strength range of 1.225-1.960MPa, with an average of 1.564 MPa.Moreover, the uniaxial tensile strength of 2-2 coal seam is 0.841-1.507MPa, with an average value of 1.296 MPa.Similarly, the uniaxial tensile strength of gangue is found to be in the range of 1.951-2.527MPa, with an average value of 2.178 MPa.It is evident that the rock specimens of 2-2 coal seam are relatively soft.
Figure 3B,D,F depict the full stress-strain curves of uniaxial compression obtained by the test.The main failure modes of rock specimens are observed to be conical failure and cylindrical splitting failure.The experimental results indicate that the roof of the upper coal seam 2-2 has an uniaxial compression strength range of 7.796-16.847MPa, with an average value of 16.234 MPa, an elastic modulus of 2770.26MPa, and a Poisson's ratio of 0.138.Similarly, the uniaxial compressive strength of 2-2 upper coal seam is found to be in the range of 9.159-11.932MPa, with an average value of 10.402 MPa, an elastic modulus of 1302.94MPa, and a Poisson's ratio of 0.1220.Furthermore, the uniaxial compressive strength of gangue in 2-2 upper coal seam ranges from 8.512 to 23.937 MPa, with an average value of 9.176 MPa.The elastic modulus, and Poisson's ratio of gangue were found to be 1415.77MPa and 0.2829, respectively.
After analyzing the initial exploration coring data, the strata parameters for Yangjiacun coal mine are presented in Table 1.Through load calculations for each rock layer, three key strata layers have been identified in the overlying strata of the coal seam: the R3 layer consisting of fine-grained sandstone, the R6 layer comprising medium-grained sandstone, and the R8 layer composed of conglomerate.Drawing on the understanding of key strata, the overlying strata in Yangjiacun coal mine can be classified as having a composite key strata structure.Specifically, the R3 layer of fine-grained sandstone and the R6 layer of medium-grained sandstone above the coal seam are considered sub-critical strata, while the R8 layer of conglomerate is regarded as the main key stratum. 19,20

| Structure characteristics of coal seam strata in Yangjiacun Coal Mine
After analyzing the geological exploration data and mechanical test data, the coating rock structure of Yangjiacun Coal Mine is characterized by the following aspects: 1.The roof rock of the working face displays apparent sedimentary characteristics and distinct soft-hard interaction structure.The roof of 2-2 coal seam mainly consists of mudstone, fine sandstone, and medium sandstone.The sandstone layers exhibit high water content, large porosity, and high compression-tensile strength ratio, which belong to typical brittle fractureprone strata.2. The basic roof is composed of mudstone and sandy mudstone, which exhibit relatively thick single layer thickness of sandstone strata and relatively thin single layer thickness of sandy mudstone strata.The proportion of sandstone strata is relatively high.| 3321 According to the statistics in Table 1, the covered sandstone and conglomerate rock strata account for about 74% of the total thickness of 2-2 coal seam, while the mudstone rock strata accounts for about 26%, indicating that the water barrier performance of covered rock strata is poor.The mining effect is characterized by poor anti-deformation ability, poor anti-disturbance ability, and weak regeneration and isolation ability.3. Based on preliminary drilling results, it has been observed that both mudstone and sandstone in the upper rock have a higher content of clastic minerals and a lower content of clay minerals, indicating typical brittle material characteristics.This feature is unfavorable for restraining the development of rock failure height and reducing water inflow from the bedrock aquifer.

| SIMULATION TEST OF ARCH BEAM STRUCTURE FAILURE
3.1 | Similar simulation test scheme

| Laying scheme
In a similar material simulation test, similar materials (sand, calcium carbonate, gypsum, water, etc.) were used.According to the coal seam histogram and the mechanical properties of coal and rock, similar models were made according to the similar material theory and similarity criteria.The proportioning number was selected according to the compressive strength of the laying rock, and the amount of sand, gypsum, calcium carbonate and water was calculated according to the size of the model and the thickness of the rock (Table 2).

| Observation scheme 1. Rock subsidence observation
The experimental model is divided into a grid with dimensions of 10 cm×10 cm, and longitudinal and latitudinal lines are marked using ink as reference points.Reflector markers are inserted at the grid intersections to facilitate displacement observation.In the upper part of the coal seam, a total of 10 displacement observation lines are laid out, with 18 measuring points along each line.The layout scheme is illustrated in Figure 4.An industrial measurement system is employed to accurately measure the specific data related to rock movement.

Stope stress observation
The stress within the fully mechanized mining stope is observed using resistance strain miniature pressure sensors, which are connected to TS3890A static resistance strain gauges.Figure 4 depicts the arrangement of the stress sensors.The supporting software is connected to a computer via a USB interface, enabling direct reading of micro strains.In this experiment, 30 stress sensors are placed in two layers of key strata (main) and the bottom plate, divided into three groups: series 1 (1-1# to 1-10#), series 2 (2-1# to 2-10#), and series 3 (3-1# to 3-10#).The layout of the sensors is illustrated in Figure 4.  Additionally, the evolutionary trend of pressure during model excavation is recorded.

| Mining-induced rock failure process
The simulation tests conducted on the caving method provide insights into the rock failure process induced by mining.Initially, the direct roof undergoes development along with the mining process, and the failure subsequently Layout of industrial measurement system, sensors, and static resistance strain gauges.
WANG ET AL.
| 3323 extends to the R3 fine-grained sandstone layer, resulting in the formation of a temporary stable arch structure (refer to Figure 5).Following this, separation occurs beneath the R3 fine-grained sandstone layer, causing the vault to expand horizontally.Meanwhile, a beam failure process of the R3 fine-grained sandstone layer is observed, involving stages of separation, bending, fracture, rotation, and collapse.After the initial fracture of the R3 layer, rapid failure propagates to the R6 medium-grained sandstone layer, and a similar beam failure process is observed, including delamination, bending, fracture, rotation, and collapse.The upper soft rock then rapidly collapses to the lower part of the main key layer, which is the R8 conglomerate.Due to the strong support provided by the main key layer, long-term and extensive delamination occurs in the lower part of the R8 conglomerate (as depicted in Figure 6).As a result, the vault continues to expand horizontally until the R8 conglomerate ultimately breaks, leading to the closure of the delamination and the occurrence of surface subsidence (as shown in Figure 7).

| Mining fracture development law
1. Subsidence curve Data analysis was carried out on five displacement measuring lines, namely L1, L2, L3, L4, and L5, which were positioned above coal seam 20, 40, 60, 80, and 100 m, respectively.These measurements were taken during the shallow seam mining process of 2-2, where the strata collapse sequentially from the bottom to the top.The subsidence curve of the rock demonstrates a transformation from a "V-shaped" profile to a "flatbottom pot-shaped" profile, with the flat bottom region expanding continuously.However, the curve displays a serrated shape, which is symmetrical with the center line of the goaf (as depicted in Figure 8).Analysis of the data obtained from the measuring points near the open-off cut and the stop line of the working face revealed a mutation point of the subsidence value at both ends of the subsidence curve, indicating the occurrence of step subsidence.Overall, the subsidence behavior of the rock is characterized by a transformation in shape and the occurrence of step subsidence.

Inclination curve
The variation law of the inclination curve can be described as follows: the inclination increases gradually from the basin's edge to the inflection point of the surface subsidence curve, and decreases gradually from the inflection point to the maximum subsidence point.At the maximum subsidence point, the inclination is observed to be "0."Conversely, the inclination is observed

F I G U R E 7
Step sinking of the surface.
to be the largest at the inflection point, with two maximum inclinations in opposite directions (as depicted in Figure 9).These observations indicate that the inclination of the subsidence surface exhibits a complex variation pattern with multiple points of maximum inclination.Of particular interest is the inflection point, which appears to represent a critical transition point in the subsidence process.Further analysis is required to fully comprehend the underlying mechanisms responsible for these observed patterns.

Underlying mechanisms
Rock deformation is primarily governed by the behavior of the key rock beam.When the rock fractures, it undergoes rapid and cohesive deformation.In the context of mining operations, the movement of key layer and sub-key layer fractures drives the displacement of the overlying rock layer above the goaf.This displacement results in the formation of detached and broken fissures within the overlying rock layer.With an increase in mining scale, these separated fractures migrate upward under the influence of the key strata and interact with the overlying rock layers, exhibiting combined movements.These fractures develop at a faster rate compared to the broken fractures and are responsible for the observed deformation patterns.

| Stress distribution law of roof and floor
In different advancing stages of the working face, the evolution of stress variables after the stability of the roof and floor rock mass along the strike of the coal seam has zoning characteristics.As depicted in Figure 10, the stress variable exhibits a gradual upward trend during the initial stages of mining operations.This trend continues until the point of initial roof collapse, at which point there is a marked spike in the stress variable.As the mining process continues, the stress variable gradually stabilizes until the key strata is breached, leading to another increase in the stress variable.Eventually, as full extraction of the mineral deposit is achieved, the stress variable declines over time.
As shown in Figure 11, taking the 2-2 sensor (positioned as indicated in Figure 4) as an example, the stress variable exhibits a significant mutation when the main key layer and sub-key layer are fractured.After the mining, the stress gradually recovered, but did not reach the initial stress state.It can be hypothesized that the behavior of the rock strata during the entire mining process exhibits the characteristics of an arch structure followed by a beam structure.The pattern of behavior exhibited by the rock strata can be described as follows: rock weight in low arches (directly roof initial weighting), rock weight in low arches and high beams (main roof initial weighting), and rock weight under a breaking angle of a cantilever beam (main roof periodic weighting).

| Analysis of arch-beam failure process
Assuming that the natural equilibrium arch axis is a quadratic curve, as shown in Figure 12. 21,22 Taking any point M (x, y) on the arch axis, according to the condition that the arch axis cannot withstand tension, the bending moment of all external forces on point M should be zero.
where q is the uniform load generated by the self-weight of the rock mass above the arch axis; T the balance the horizontal thrust of vault section; and x, y the M point axial coordinates.
There are two unknowns in the above equation, and it is necessary to establish an equation to obtain its solution.According to the static equilibrium equation, the horizontal thrust in the above equation is equal to the horizontal thrust acting on the arch foot, and the direction is opposite, namely: According to Prussian theory, the horizontal thrust at the foot of an arch must meet certain requirements due to its tendency to produce horizontal displacement and alter the internal force distribution of the arch as a whole: The horizontal thrust acting on the arch foot must be less than or equal to the maximum friction generated by the vertical reaction to maintain the stability of the arch foot.In addition, for safety, after the horizontal thrust is reduced by half, order T = qa f 2 1 , the arch axis equation can be obtained by substituting it into Equation (1): Obviously, the arch axis equation is a parabola.According to this equation, the height of any point on the arch axis can be obtained.
When the sidewall is stable, x a = , y b = , we can obtain: where b is the maximum height of natural equilibrium arch when the middle-sidewall is stable; a the maximum span of natural balanced arch when the sidewall is stable; and f the an empirical coefficient with dimension 1.In practical application, the integrity of rock mass and the influence of groundwater should be taken into account at the same time, where 1.2 is taken.
According to Prussian theory, the limit span can be calculated as follows: where b is the thickness of soft rock, 14 m; h the thickness of coal seam, 4 m; and ϕ the angle of internal friction.According to experience, 30°is taken and calculated a 1 = 19.1 m.As shown in Figure 13, when the rock beam in the layer breaks (σ σ = t max , σ t is ultimate tensile strength), the limit span is Therefore, the critical working face advancing length when the hard rock stratum in the first layer breaks is L m where L m is the critical working face advancing length when the rock stratum is broken; h i -thickness of the stratum; φ q , φ h the front and rear fracture angles of rock strata; l m− max the limit span of the rock beam in the second layer when fracture occurs.
Based on the relevant data of the R3 rock strata, it is determined that the ultimate span of the R3 rock beam is 13.4 m when fracture occurs, while the advancing distance of the working face is 22.3 m.After the initial failure of the natural equilibrium arch in the weak rock stratum, periodic failures of the arch will continue to occur until the initial failure of the R3 rock stratum takes place, provided that the collapse length of the key stratum above is not reached.Similarly, for the R6 and R8 strata, a similar arch-beam failure process occurs.

| Conceptual drawing of structural damage of arch beam
The mechanical properties test of the coal and rock mass of the 2-2 upper coal seam reveals that the roof rock strength of the 2-2 upper coal seam is low, leading to direct roof fall during coal extraction operations.The collapse of the rock strata occurs from bottom to top as the space for movement of the overlying strata is created.The inherent fragmentation and expansion of the rock mass contributes to the temporary stability of the roof structure once it has been damaged to a certain extent.The red area in Figure 14A represents the stress rise zone, which constitutes the virtual stress arch structure.
During the first weighting stage of the mining process at the working face, the span of the upper "beam" of R3 continues to increase as the mining area expands.By the end of the first weighting stage, the upper "beam" of R6 reaches its maximum length, leading to a noticeable weighting process, as depicted in Figure 14B,C.After the R6 upper "beam" is breached, the R8 lower "arch" experiences periodic damage, and the R8 upper "arch" reaches its maximum length of breakage, resulting in a noticeable weighting process, surface cracking, and the formation of step subsidence, as shown in Figure 14D,E.
Similar simulation tests found that the arch under the "arch" and the arch on the "beam" both failed after the coal seam was mined.The size of the arch is continuously developed until the upper beam breaks and the lower "arch" is also destroyed.The arch structure is temporarily stabilized at the bottom of the key layer with the coal seam advancing, following the evolution process of "gestation-development-decline-transition."The evolution of the arch structure develops upward jump with the change of the bearing characteristics of the key layer.Similar findings can be found in the numerical simulation.As shown in Figure 15, with the increase of the advancing scale, the principal stress of each key layer will always experience a stage higher than the surrounding stress, that is, the stress shell stage, also follows the evolutionary process of the stress shell's "inoculationdevelopment-decay-transition," and the plastic zone distribution also presents a trend of step transition.

| STABILITY ANALYSIS
The Yangjiacun Coal Mine is a crucial coal production center and a quintessential example of a high-strength mining area in China.It is also situated in a typified arid and semi-arid fragile ecological environment in China, making the harmonious development of high-strength coal production and ecological preservation a crucial area F I G U R E 14 Failure process of arch beam structure. of research.[25] 1.The implementation of backfilling mining can mitigate the impact of coal extraction on the surrounding environment by compensating for the void created by mining and limiting the movement damage to the overlying strata.The equivalent mining height is determined based on various factors, including the mining thickness, compression ratio, and other relevant parameters: where S 1 is the roof subsidence before filling; S 2 the filling body under-top; S 3 the filling body compression; and ε the filling body compression rate.
Taking a key layer as a simply supported beam, the maximum deflection value of the roof under uniform load is The thickness of rock strata and residual dilatancy coefficient between the coal seam and the key strata of the structure determine whether there is an empty roof in the lower part of the key strata falling behind the rock and reaches the critical deflection value of the key strata breaking: where Δ i is the height of the free space of the second layer; h j the thickness of the j strata; and k j the fragmentation coefficient of the j layer.
When the free space below the key stratum is less than its maximum deflection, the stratum will remain stable: where ω i,max is the maximum deflection of soft rock.
2. If strip mining is adopted, the length of the key layer is limited.When the face length of the strip working face is smaller than the advancing length of the critical working face, the stability of the key layer can be ensured, and the specific calculation is not repeated here.
Based on similar simulation and theoretical research, it has been observed that during the initial stage of the working face advancement, the bearing effect of the R3 sub-critical layer is significant.However, due to the close proximity between the R3 sub-critical layer and the coal seam, this stage is usually of short duration.As the mining width increases, the concentration of stress arch in the R6 sub-critical layer gradually intensifies.Eventually, when the limit span of the R6 sub-critical layer is exceeded, the sub-critical layer undergoes its initial fracture, accompanied by synchronous fracture of the overlying soft rock.Following the rupture of the R6 subcritical layer, the main critical layer of R8 assumes the bearing role.As the stress surpasses the limit span of the main critical layer, the main critical layer experiences initial fracture, leading to the fracture and communication of the soft rock with the surface.From a perspective of safety and operational feasibility, the principle of mining subsidence control can be defined as maintaining the stability of the R6 key layer.This implies that efforts should be focused on ensuring the stability of the R6 subcritical layer to effectively control subsidence during mining operations.

| CONCLUSION
1. Based on the analysis of previous exploration coring data and indoor mechanical testing, the parameters of the rock strata in the coal mine have been determined.The overlying roof rock exhibits a composite key stratum structure, characterized by sedimentary features in the sandstone and relatively poor deformation resistance in the sandy mudstone.The region is characterized by low atmospheric precipitation and weak aquifers, which increases the risk of water flowing fractured zones in the shallow coal seam and the potential for mine water and sand inrushes.2. Through arch-beam failure simulation tests conducted based on similarity theory, the movement, deformation, and stress variables of the roof rock during coal seam mining have been analyzed.The arch-beam structure significantly influences the movement, deformation, and stress evolution of the weathered rock.As the key strata are progressively damaged, the mining-induced fractures expand, following a process of initiation, growth, and closure.The stress changes exhibit zoning and occur in stages as the arch-beam structure undergoes progressive damage.3. The structural stability of the arch-beam system has been investigated using the fixed beam model and natural equilibrium arch model.Quantitative formulas for calculating the failure process and extent of arch-beam failure in each layer of the roof rock have been derived.Numerical simulations have verified the presence of the arch-beam structure.Combining the results of theoretical calculations, similar simulation tests, and numerical simulations, a conceptual map depicting the stages of arch-beam failure has been established.4. To effectively control surface subsidence, measures such as local backfilling or strip mining can be employed to modify the stress environment at the lower boundary of the arch-beam system.It is worth noting that the R3 sub-critical layer, located near the coal seam, has a limited bearing capacity, while the R8 main critical layer is closer to the surface and breaches rapidly.Consequently, prioritizing the stability of the R6 sub-critical layer is recommended as the key principle for successful surface subsidence control during mining operations.

3. 1 . 3 |
Excavation schemeThe test involves laying a model in a 1:100 proportion with actual size parameters of 190× 20 × 160 m, with a 150 m working face and 20 m coal pillars on both sides.A time similarity ratio of 1:10 is used, and the excavation parameter of similar material is set at 5 cm per 2 h, calculated based on the model size ratio and time similarity ratio.During the excavation process, the first weighting and periodic weighting interval of the working face are observed and recorded, while the deformation, fracture, and collapse process of key strata in the overlying strata are photographed and documented.

F I G U R E 5
Arch failure below R3.F I G U R E 6 Separation layer below R8.

F I G U R E 9
Slope change after mining stability.F I G U R E Stress states zoning characteristics.F I G U R E 11 Stress analysis of 2-2 sensor.
Formation parameters of Yangjiacun Coal Mine.
Parameter table of rock layer laid by similar simulation test.
T A B L E 2