Experimental study on rock strata movement and stope stress distribution law under mining height regulation

When pressure relief mining of a non‐all‐coal protective layer, to realize the accurate regulation of the pressure relief range and pressure release value of mining height, it is necessary to carry out research on the migration of overburden and the stress distribution law of stope under mining height control. Through similar material simulation experiments, four groups of similar material simulation experimental models were established. The laws of rock strata movement and stress distribution at 2, 4, and 5 m mining heights under different geological conditions, and the laws of rock strata movement and stress distribution at 5 and 8 m mining heights under the same geological conditions were obtained. The research results show that the geological conditions and mining height do not affect the evolution of the rock strata movement range. The highest position of the strata movement and deformation is linearly related to the advancing distance of the coalface. The maximum height of rock strata movement increases abruptly when encountering the key strata, but it still has a linear relationship. The highest position of each expansion of the fracture zone has an exponential relationship with the advancing distance of the coalface. When the key strata are encountered, the maximum height of the fracture zone increases abruptly. The change in mining height only affects the distribution range of the “three zones.” The mining height controls the stress distribution of stope by adjusting the “three zones” of the roof. With the increase in mining height, the caving zone and fracture zone range increase. The corresponding pressure relief range and pressure relief value show the change law of first increasing and then decreasing. There is an optimal mining height, which can make the pressure relief effect of stope best.


| INTRODUCTION
2][3][4][5] By mining adjacent coal seams or strata pressure relief can reduce the stress of the protected coal seam area and ensure the safe mining of resources.The mining height plays a significant role in stress regulation.2][13][14][15][16] However, the studies focus on the summary of the stress distribution law after mining at a given mining height, lacking a comparative analysis under different mining heights.There are relatively few studies on how mining height affects rock strata movement and stress distribution in stope.Therefore, it is of great theoretical significance to study the regulation mechanism of mining height on rock strata movement and pressure relief range.
8][19][20][21] The above studies focused on the summary of the rules, and did not study the intrinsic mechanism of different stress distributions caused by different shifting deformation laws of rock layers due to different mining heights.The main methods used are numerical simulation, theoretical analysis, and mathematical statistics.In terms of numerical simulation, Tang et al. 22 studied the regional variation characteristics of abutment pressure in front of coalface under different mining depths and thicknesses in the Lianghuai mining area; Zhu et al. 23 analyzed the stress in the region of coal seam thickness change, drew the energy distribution map in the region of coal thickness change by numerical simulation, and proposed the rock burst control measures; Xie et al. 24,25 simulated and analyzed the influence of different mining heights and widths on the distribution law of separated strata and fracture after strata movement; Li et al. 26 analyzed the characteristics of coal wall of large mining height coalface under different mining heights through FLAC 3D simulation; and Chen et al. 27 studied the effect of different thicknesses of mined coal seams on the deformation degree of the floor.In terms of theoretical analysis, Xie et al. 28 established a viscoelastic damage model of abutment pressure distribution and analyzed the abutment pressure distribution of solid coal in front and side of coal wall at different mining heights; Liang et al. 29 proposed that there are two structural forms and six movement forms in the key strata of the large-height mining face with large mining height by theoretical analysis method; and Zhao et al. 30 created a fractal dimension model and analyzed the fractal characteristics of overlying strata at different mining heights.Wen et al. 31 established the roof structure model of large mining height stope by studying the roof strata movement law of fully mechanized mining with large mining height (top coal caving).In terms of mathematical statistics, Feng et al. 32 used statistical principles to determine the distribution state of the initial incoming pressure step and periodic incoming pressure step data in the mining area of Shendong.The numerical simulation adopts a continuous or discontinuous calculation method, which is difficult to reflect the influence of mining height change on rock strata movement and stope stress distribution.In the theoretical analysis, there are more calculation models to establish the stress distribution law of the mining height and in front of the coalface, and fewer stress analysis models in the pressure relief area of goaf after coalface pushing.Mathematical statistics are closely related to the geological conditions of the sample area, and their applicability needs to be further verified.Therefore, it is necessary to carry out research to lay a foundation for establishing the calculation model of stress distribution in the pressure relief area behind the coalface at different mining heights.
This study aims at the unclear effect of mining height control on rock strata movement and stress distribution law in existing studies, carried out study from the perspective of the difference of strata movement and deformation under different mining heights.By comparing the difference of strata movement deformation and stress distribution with different mining heights, the mechanism of stress distribution in stope regulated by mining heights is given.The research results can provide a theoretical basis for the design of mining height in pressure relief mining.

| Overview of the experimental coalface
Working Condition 1: At the 22201 coalface of Shaqu mine, the simulated mining height is 2 m and 2# coal seam is mined.The 2# coal seam where the coalface is located is located in the middle part of Shanxi formation.The average thickness of the coal seam is 1.07 m and the average inclination is 4°.The structure is simple without gangue or occasionally containing 1 layer, and the coalface elevation is +396 ~+486 m.The column diagram of the coal seam where the coalface is located and the lithology description of rock strata are shown in Figure 1A.
Working Condition 2: At the 1402 coalface of Xinan mine, the simulated mining height is 4 m and 4# coal seam is mined.The average burial depth of this coalface is 107 m, and the average thickness of the coal seam is 9.3 m, which is mined in layers.The inclination of the coal seam is 3 ~9°, and the average is 6°, which is a near horizontal coal seam.The column diagram of the coal seam is shown in Figure 1B.
Working Condition 3: At the +400-level shaft 10 slot coalface of Daanshan mine, the simulated experimental mining height is 5 m, mining the upper and lower coal seams of shaft 10.The coal thickness on axis 10 is 0.9 ~3.2 m, the average coal thickness is 3 m, and the coal seam inclination is 19°.The average thickness under axis 10 is 2 m and the spacing is 0.02 ~2.00 m.There is a phenomenon of local merger between the two layers and the simulation experimental design scheme is a two-layer joint mining.The comprehensive column diagram of the coal seam roof and floor is shown in Figure 1C.Based on this geological condition, the mining height of 8 m is simulated.The 5 m coal seam and the roof with a thickness of 3 m above are mined, and comparing and analyzing the movement deformation and stress redistribution law of the strata are when the mining height is different under the same geological condition.

| Selection of model geometric similarity ratios
To satisfy the requirement of experimental accuracy, the simulation similarity conditions are simplified by the main similarity factors according to the three theorems of similarity simulation.The similarity conditions between the experimental model and the prototype are mainly geometrical similarity conditions, kinematic similarity conditions and dynamic similarity conditions.The size of this simulation experiment platform was 2 m × 0.2 m × 1.5 m.According to the occurrence characteristics of coal and rock strata, coal seam mining height, coalface parameters, and the size of simulation experiment platform, the geometric similarity ratio of the model was determined to be 1:100.The similarity of the simulation experiment finally determined is shown in Table 1.

| Similar simulated material ratios
According to the ratio table of similar materials in the laboratory, the materials were mainly sand, lime, gypsum, cement, mica, and other materials.The ratio numbers were Engineering background for similar simulation experiments.
T A B L E 1 Similarity ratio of the experimental model.

Name Similarity ratio
Geometric similarity ratio Similarity ratio of volume to weight Poisson's ratio similarity ratio selected in the ratio table in combination with the material strength to mix each component material.The above three groups of experimental ratios are shown in Tables 2-4, with the numbers in the ratio number being the sand, lime, and gypsum components in that order.
2.4 | Experimental signal acquisition methods and scheme design

| Experimental signal acquisition
The main equipment of the experiment was a camera, BW-0.5 earth pressure sensors, and a TST-3822 data acquisition instrument.The experiment was carried out to take photographs of the rock movement and deformation structure during the mining process, while the earth pressure sensor was used to collect the mining stress data.
The stress measurement points are arranged as shown in Figure 2. The measurement line is 3 cm from the coal seam floor, the horizontal spacing of each measurement point is 10 cm, a total of 21 measurement points, and 25 m of coal pillars are left on the side of the open-off cut.

Comparison analysis of coalface with different mining heights and different geological conditions
The purpose of the comparative analysis of the coalface with different geological conditions and different mining heights is to obtain the general law of the movement and deformation of the roof strata.It includes the evolution law of overburden movement and deformation range along with mining and the evolution law of rock collapse structure zoning range along with mining.The pictures of the collapse phenomenon and stress test data obtained from mining experiments of Shaqu mine, Xinan mine, and DaanShan mine were analyzed.

Comparison analysis of coalface with different mining heights under the same geological conditions
The purpose of the comparative analysis of coalface with different mining heights under the same geological conditions is to obtain the law of the influence of mining height on the movement and deformation of roof strata with coalface.It includes the influence of mining height on the evolution of roof moving deformation range and the zonation range of roof caving structure.Daanshan mine was selected as the engineering background, and the design mining heights were 5 and 8 m, respectively.The pictures of the caving  During the experiment, the model was photographed when the overburden rock was damaged.The process of strata movement and deformation and the formation of zonation could be obtained by sequencing according to the photographed time.To make the analysis clear, the caving sequence of each rock block during the formation of the caving structure is marked in the picture.Among them, the dividing line of roof caving rock at 2 m mining height is fuzzy, so the picture is sketched.The mining height of Shaqu mine 22201 coalface is 2 m.According to the analysis of physical and mechanical properties of roof strata, the roof does not contain key strata structure.During the advancing process of the coalface, the sketch of the moving damaged rock strata is shown in Figure 3A~G.
The open-off cut formed in the first excavation of the coalface as shown in Figure 3A.The experimental setting | 1535 of the excavation step was 5 m and the mining height was 2 m.Stress measurement points were arranged under the mining layer to monitor the stress distribution in stope during the caving process.
When the coalface is advanced to 65 m, as shown in Figure 3B, the roof collapses for the first time, and the caving strata range from 0 to 2 m.The caving strata on the side of the coalface has slipped and is not hinged with the roof strata.There is a hinged phenomenon on the side of the open-off cut, but the hinged structure is disjointed from the caving strata.The caving structure is similar to a trapezoid, trapezoid bottom side length is 60 m, height is 2 m.
When the coalface is advanced to 70 m, as shown in Figure 3C, the roof strata formed an articulated structure, and the articulated structure did not fracture obviously.It shows that the rock beam has a certain bending deformation ability.When its deflection does not reach the ultimate deflection, the rock beam is still distributed in a continuous state.The articulation range is 2 ~6.5 m above the roof, and the strata zoning in this range conforms to the structural characteristics of bending subsidence zone.Roof movement deformation range is trapezoid, trapezoid bottom angle is roof caving angle.The length of the trapezoidal bottom side is 70 m, and the height range is 2 ~6.5 m.When the coalface is advanced to 90 m, as shown in Figure 3D, the hinged structure of the roof is further expanded in the strike and vertical directions.The range of bending subsidence structure is extended from 2 ~8.5 m to 2 ~17.0 m.In the curved subsidence strata structure, the separation fractures exist at the top of the bending deformation and on the coalface side and the open-off cut side.No separation fracture was found in the compaction area of goaf.Due to the small mining height, the deformed strata have never reached the limit deflection and are still bent and deformed.The deformation range is trapezoidal, the length of the bottom edge is 90 m, and the height range is 2 ~17 m.
When the coalface is advanced to 105 m, as shown in Figure 3E, the range of the bending subsidence structure is further extended to 2 ~26.5 m.The bottom edge of the deformed trapezoidal range is 90 m long and the height range is 2 ~26.5 m.At this time, the moving range of the roof does not change in the strike, but expands upward with trapezoidal structure in the vertical direction according to the caving angle.
When the coalface is advanced to 115 m, as shown in Figure 3F, the range of bending subsidence structure extends to 2 ~33.5 m.The bottom edge of the moving deformation trapezoid is 115 m long, the height range is 2 ~33.5 m.The strata movement range increases in both strike and vertical direction, and the roof movement deformation reaches the upper boundary of the similar model.
When the coalface is advanced to 130 m, as shown in Figure 3G, the vertical deformation range no longer increases, and the trapezoidal bottom edge is extended to 130 m, as the roof movement deformation reaches the upper boundary of the model.
The above experimental process shows that when the mining height is 2 m, the roof movement deformation space is small, only 2 m above the roof of the coalface will collapse and break.Then, with the increase of mining strike length, the strata overhang length increases, the deflection increases, and the rock movement deformation does not damage and forms an articulated structure.The range from 2 to 33.5 m above the coalface can be regarded as a bending subsidence structure.The strata movement and evolution in the mining process are described according to the sequence, which can be decomposed into the geometric shape description in Figure 4.
Figure 4 shows the sequence of roof strata movement at a mining height of 2 m, where numbers 1-12, respectively, represent the sequence of roof strata movement, starting from 1 to 12.The roof strata is regarded as a beam structure, and beam 1 reaches the limit span in the early stage of excavation and its deflection reaches the limit deflection.When the rock beam is damaged, its self-gravity stress is transformed into the compaction stress on the goaf floor.Beam 2 then fails, when beam 3 reaches the limit span and beam 6 does not, so the failure ends at the bottom of 6.As the coalface continues to advance, beam 4 collapses and beam 5 collapses, at which point beam 6 reaches the limit span.At the same time, the limit span of beam 7 is <6 and bending deformation occurs with six movements, and then the deformation failure process is so on.The analysis of the moving deformation sequence in the figure shows that the roof moving deformation is affected by the limit span of the roof rock beam.At the same time, when the first caving occurs in the caving process, the rock strata fracture does not form an articulated structure, whereas the roof forms a curved structure when the beam 3 bends.By comparison it can be found that the strike length of beam 3 is larger than beam 1 when it is bent and deformed.When the bending deformation of the rock beam is a certain space in the vertical direction, its ultimate deflection changes as the strike length of the rock beam increases.When the deflection is greater than the space allowed for caving, a curved structure is formed.For beam 1, when it reaches the limit span, the corresponding deflection is small, and the rock beam is damaged.
The similar simulation experiment of Shaqu mine shows that when the mining height is small and the roof does not contain the key strata, the deformation evolution process of the roof strata is relatively uniform, and the deformation range is basically "trapezoidal," and the similar shape is gradually expanding during the expansion of the trapezoidal range.The experimental phenomenon shows that when the mining height is small, the distribution range of the roof caving zone is small, and when the roof does not contain the key strata structure, the movement deformation evolution of the roof strata is more uniform.

Analysis of overburden movement and deformation process of 1402 coalface in Xinan mine
The experimental simulation mining height was 4.0 m.The analysis of the physical and mechanical parameters of the roof strata shows that the roof of the coalface contains two key strata, which are medium sandstone located in the range of 2.8 m ~29.2 m above the roof and medium sandstone in the range of 38.8 ~46.8 m above the roof.
Figure 5A shows the collapse structure when the coalface is advanced to 105 m, the roof strata have collapsed twice, and the collapse has not formed an articulated structure.When the coalface continues to advance to 115 m, the overhanging rock beam and rock beam 3 above the coalface simultaneously collapse.Due to the formation of an articulated structure of small rock blocks above the coalface, the caving rock beam 3 forms different structures on the left and right sides of the roof, and the rock beam 3 slides and fails on the left and is articulated on the right.At the same time, rock beam 3 forms the upper and lower parts, the upper left and right sides of Figure 5B are hinged, and the roof strata form the articulation for the first time.When the articulated structure is formed in the roof strata, the subsequent caving strata evolve with an articulated structure in the vertical direction.
When the rock beam 4 collapses (Figure 5C), the rock beam is articulated, which further explains the evolution characteristics of the above-mentioned articulated structure in the vertical direction.When the coalface advanced 145 m (Figure 5D), the small rock beam in front of the rock beam 2 fell.Rock beam 4 collapses, and the rock beam is articulated at this time, further illustrating the evolution characteristics of the aforementioned articulated structure in the vertical direction.With the advancement of the coalface, the vertical height of the articulated Structure 4 is further expanded, and the failure range of the roof rock beam is further expanded.The strike length of the damage range does not change, and the hinged range is extended to 24 ~44 m in the vertical direction.The caving zone range remains unchanged, still ranging from 0 to 24 m.
In Figure 5E, the rock beam 5 collapses, followed by the rock beam 6 above it.With the coalface advancing, the moving failure range of overburden develops further, and the moving failure range increases in both strike and vertical directions.In the vertical direction, the range of caving zone remains unchanged, the range of articulation structure of fracture zone increases to the maximum, and the bending subsidence structure appears.With the further advance of the coalface, as shown in Figure 5F, the roof moving deformation structure further expands.However, when the fracture zone reaches the maximum and forms a bending subsidence zone, the fracture zone will remain constant with the advance of the coalface.
By summarizing the evolution law of rock movement deformation structure during the advancing process of 4 m mining height coalface, it can be found that the movement zoning characteristics of overlying rock at 2 m mining height are obviously different.The range of the roof caving zone is small when the mining height is 2 m.The roof of Xinan mine contains two key strata.In the process of advancing with the coalface, when the span of the key strata reaches the limit span and the free space below is not enough to support the rock beam, the key strata breaks.After the key strata is broken, the overburden collapsed on a large scale.Therefore, when the mining height increases and the key strata is included, the caving evolution of the roof strata has a mutability.The formation of "three belts" in the roof also shows nonuniform characteristics.The layers 4 and 7 of the rock beam are the key strata distribution layers.It can be seen from the caving structure feature map that when the caving structure develops to the key strata, the three-zone distribution can be stable in a short time.When the limit span of the key strata is reached, the range of caving zone and fracture zone changes abruptly.
Analysis of the process of overburden rock movement and deformation at the +400 horizontal shaft 10 slot coalface of Daanshan mine The DaanShan mine is mined at a height of 5 m above and below Axis 10, and there is a siltstone with a thickness of 28 m above the roof, which is the main key strata.Figure 6 shows the evolution process of the deformation structure of the main rock strata with the advance of the coalface.Figure 6A shows the caving structure when the coalface advanced to 55 m.Due to the coal seam roof contains a key strata with a thickness of 22 m, the maximum caving height of the roof is 4 m at the initial pressure step.When the coalface continues to advance to 75 m, as shown in Figure 6B, the rock beam 3 collapses.The strata where the rock beam 2 is located reaches the limit fracture distance, and the rock beam breaks and collapses, and the collapse range is about 8 m.
As shown in Figure 6C, with the continuous advance of the coalface, the caving range continues to extend in the vertical direction when the strata where the rock beam 5 is located reaches the limit fracture distance.However, the rock beam 5 does not form an articulated structure, the rock beam 5 bends and deforms, and the roof moves and deforms into the bending subsidence zone.According to the above analysis, the formation of roof structure and caving space are related to the limit deflection of the rock beam itself.The strata in which the rock beam 5 is located is the key strata of the rock beam.The collapse space of the rock beam 5 decreases after the collapse of the rock beams 1-4 due to the swelling of the strata itself.At the same time, due to the increase in the span of the rock beam, its deflection value increases, which meets the conditions for forming a curved structure.For rock beams 1 and 2, although they have the same mechanical properties as rock beam 5, their corresponding caving space is larger, so they enter the caving zone.It can be seen that the characteristics of roof caving zoning are closely related to roof lithology and caving free space.And the roof moving deformation zoning does not necessarily have a "three-zone" structure.In Figure 6D, the horizontal advancing distance continues to increase and the range of the curved structure in the vertical direction does not change.
When the advancing distance is increased to 115 m, as shown in Figure 6E, the curved rock beam 5 expands horizontally, and then the block 7 bends and sinks.The strike and vertical direction of the bending subsidence zone increase with the advance of the coalface.When the coalface advances 130 m, the mining ends, as shown in Figure 6F.It can be seen that there are only two developed structures on the roof.The caving zone eventually reaches 8 m and the bending subsidence zone eventually develops to the surface.
The mining height of Daanshan mine is 5 m, and there is a main key strata above the coal seam.The experimental results show that the roof caving zone ends below the main key strata.With the advancing distance of the coalface, the main key strata does not break.The upper strata controlled by the main key strata eventually formed a curved subsidence structure.The above experiments show that when the key strata structure is broken due to mining, the range of the roof caving structure increases.The structure of caving zone and fracture zone that can be formed in the roof is obvious.When the key strata structure does not break, the strata above the key strata structure are all in the bending subsidence zone, which will show the structural characteristics of "two zones."

| Quantitative analysis of the process of overburden movement and deformation
Based on the above pictures of the evolution of the distribution range of the moving deformation of strata and the moving deformation zone, the moving deformation law of overburden is quantitatively described.Taking the advancing distance of the coalface as the independent variable and the height of the maximum moving range of the corresponding rock stratum as the dependent variable, the movement and deformation rule of overburden with the advancing of the coalface are analyzed.
2 m mining height without key strata rock movement deformation evolution The mining height of Shaqu mine is 2 m, there is no key strata structure in the roof, and the main structure is a bending and sinking structure.The relationship between the advancing distance of the coalface and the moving deformation range is obtained, as shown in Figure 7.
Figure 7 shows that when the roof is no key strata structure, the movement range of the roof strata has a linear relationship with the advancing distance of the coalface.The moving range is similar to trapezoidal expansion, and the initial caving distance is 60 m.After the first caving, the increase of the advancing distance of the coalface when the roof caving occurs corresponds to the length of the horizontal line in the figure.The length of the horizontal line in the figure is relatively close, indicating that the moving deformation area of the strata is evenly expanded.

m mining height contains key strata movement deformation evolution
The mining height of Xinan mine is 4 m, and the roof has two key strata structures.The structure of the roof is caving zone, fracture zone, and bending subsidence zone.According to the pictures of caving structure evolution process, the relationship between the advancing distance of the coalface and the range of overburden movement and deformation is obtained, as shown in Figure 8.
Figure 8 shows that with the advance of the coalface, the increase of the moving deformation range is still approximately linear with the increase of the advancing distance of the coalface after the initial caving.The transverse distance shown in 1, 2, and 3 in Figure 8 is shorter, which can be approximately considered as the increase in the moving range caused by the same break.The vertical height of the corresponding moving deformation range here is the layer where the key strata is located.Therefore, when the roof strata contains the key strata structure, the rock strata movement is similarly trapezoidal expansion, and the trapezoidal expansion ratio is closely related to the key strata structure.When the strength of the key strata is large, the range of overlying rock movement and deformation increases abruptly.However, the expansion area of moving deformation is still similar to the previous expansion area, that is, the increase of the advancing distance of the coalface is linearly related to the increase in the vertical direction of the moving deformation range, and it is consistent with the slope before the rupture of the key strata.
Movement and deformation evolution of strata containing key strata at 5 m mining height The mining height of Daanshan mine is 5 m, and there is a key strata structure near the roof of the mining layer.When the roof collapses for the first time, the key strata are bent and deformed, and then the roof overburden expands in the strike and vertical direction | 1541 of the coalface in a curved deformed structure.After the first caving, the increase of the advancing distance of the coalface is approximately linear with the increase of the displacement and deformation range of the strata.The evolution of the moving deformation range is more uniform, as shown in Figure 9.

Evolution of overburden movement and deformation at different mining heights in the same coalface
The design mining height of Daanshan mine is 5 m, and the roof contains a key strata, which is close to the mining layer.To obtain the influence of mining height on roof moving deformation structure, based on the stacking conditions of the same model, the evolution law of roof moving deformation structure at 8 m mining height was analyzed, as shown in Figure 10.Influence of mining height on the evolution of overburden movement deformation range.
The evolution law of roof strata movement and deformation range at different mining heights under the same mining conditions is shown in Figure 11.Under different mining height conditions, the evolution law of roof strata movement deformation is similar.However, there are differences in the zoning range of overburden caving structure.At the mining height of 8 m, the moving deformation of the strata above roof have caving zone, fracture zone, and bending subsidence zone.When the mining height is 5 m, the caving zone is only 4 m above the roof, and the rest is a curved subsidence structure.

| Evolution of overburden movement and deformation law
After the initial collapse of the roof strata, with the increase of coalface advancing distance, the difference between the coalface advancing distance and the initial pressure step is linearly related to the increase of the height of the moving deformation range of strata.It can be expressed in the following relation: where L is the coalface advancing distance, m; Ls is the initial pressure step; h w is the vertical range of movement deformation of roof; h ′ w is the vertical range of roof movement deformation during the initial pressure; and η is the proportional coefficient.
The evolution process of overburden movement deformation is regarded as trapezoidal evolution.The increase in the evolutionary range is proportional to the increase in the vertical direction, indicating that the trapezoid is expanding in a certain proportion during the evolutionary process.When the roof does not contain the key strata structure, the expansion ratio is basically unchanged.When the roof contains the key strata and evolves to the bottom of the key strata, the movement deformation range of the strata expands abruptly.The ratio of the increment of deformation range to increment of advance distance remains unchanged.The overall law of moving deformation of the roof strata can be represented by Figure 12, which is consistent with the literature. 33ring the evolution process, the range of the trapezoid of moving deformation increases with a certain proportional coefficient.When the deformation range is developed below the key strata, the limit span of the key strata causes the trapezoidal range of the moving deformation to increase abruptly.As shown in Figure 12, the lengths are not equal during advancing, but the trapezoids are similar.From the geometric relationship in the figure, it can be seen that the ultimate span of each strata determines the strike to expand the step of the trapezoid range.Line A in the figure is parallel to the boundary of the moving deformation range of strata.The change in the height of the moving deformation range is always proportional to the change in the strike range, that is, η constant.As can be seen from the geometric relationship in Figure 12, where θ is the caving angle of the roof strata.Under different mining height conditions, the moving deformation range of roof overburden rock is close.The mining height only affects the distribution range of the zonal structure within the range of a certain state moving deformation.In Figure 12, within the same overburden movement range, the distribution range of caving zone, fracture zone and bending subsidence zone at a mining height of 5 m is different from that at a mining height of 8 m.

| Evolution of fracture zone development height at coalface of different mining heights
To further analyze the evolution law of caving structure distribution in the zonal range, the evolution law of fracture zone with mining during the advancing process of different coalfaces was compared and analyzed.The mining height is 2 m (Figure 13A), the fracture zone range of the roof is small, and the fracture zone range does not change during the evolution of caving structure.After the first collapse of the roof of the coalface, the range of fracture zone is approximately exponential with the advance of the coalface.After the initial collapse of the roof of coalface, the fracture zone range evolves approximately exponentially as the coalface advances, as shown in Figure 13B.When the critical layer is encountered, the range of fracture zone is unchanged, and the corresponding advancing distance of the coalface is 120 m.When the key strata is broken, the range of fracture zone suddenly increases, and then the range of fracture zone stabilizes 60 m above the roof without changing.In Figure 13C, during the initial stage when the coalface is advanced from 55 m to 75 m, the extent of the fracture zone increases exponentially.Then, with the advance of the coalface, the fracture zone reaches the maximum value and remains constant.The development height of the fracture zone here is the total height of the caving zone and fracture zone above the roof.

| Development and evolution regularity of fracture zone in the same geological conditions and different mining heights
As shown in Figure 14, under the same geological conditions, as the coalface advances, when the mining height is 5 m, the key strata above the roof are bent and deformed.The extension of the fracture zone is terminated.When the mining height is 8 m, the key strata are broken, and the range of fracture zone suddenly increases.With the continuous advance of the coalface, the range of fracture zone increases exponentially.
When the mining height is relatively small, such as the mining height of 2 m, the structure range of caving zone and fracture zone formed by the roof is small.With the increase in mining height, it is conducive to the expansion of the caving zone and fracture zone, but the range increase of the fracture zone is controlled by the key strata.

| Influence of mining height on caving structure evolution
The caving zone range and fracture zone range are collectively referred to as fracture zone development height.Comparing the evolution law of fracture zone development height and movement deformation range at different mining heights, it can be seen that different mining heights corresponding to the movement deformation range and evolution law of overburden are consistent.The corresponding "three zones" distribution in this range are different.F G U R E 14 and evolution of fracture zone under the same geological conditions and different mining heights.

| Influence of mining height on the distribution range of overburden "three zones"
The same coalface has the same evolution law of movement deformation range along with mining overburden, and the range zoning is different.As shown in section a in Figure 15, the stress values transferred to the floor at this stage of the two mining heights can be regarded as the same.When the advancing distance of the coalface is in the range of a + b, the force of overburden on the floor on the coalface with larger mining height is still the self-weight of caving zone and fracture zone.However, the overburden forces on the floor of the coalface with small mining height include the self-gravity stress of caving zone and fracture zone and the self-gravity stress of overburden transferred by bending subsidence zone.When the advancing distance is greater than a + b range, both bending subsidence zone load the floor.Due to the different mining heights, the caving zone range is different, which makes the loading force of the curved rock beam on the floor different.When the mining height is small, the range of the fracture zone in the caving zone is small.The pressure of the bending subsidence zone directly acting on the goaf and transferring to the goaf is large, which is not conducive to pressure relief.When the mining height increases, there is a certain structure of the caving zone and fracture zone, and the caving zone and fracture zone support the bending subsidence zone.The coordinated deformation of the curved rock beam can mobilize the self-bearing capacity of strata in the bending subsidence zone, reduce the stress transfer from the bending subsidence zone to the goaf, and enhance the pressure relief effect.When the mining height increases further, the range of the caving zone and fracture zone increases.Although the ability to coordinate deformation can be further improved, the range of bending subsidence zone is reduced.The stress of overburden is more transferred to goaf through the caving zone and fracture zone, which leads to the weakening of pressure relief.
In the early stage of mining, the largest position where the difference of compaction stress between overburden and goaf is that the coalface advances from a to a + b range.In the same movement deformation range at the initial stage of mining, the stress value transferred from the coalface with a larger mining height to the floor is less than or equal to that of the coalface with a smaller mining height.When the movement deformation range of strata extends to the bending subsidence zone, the zonation range of roof evolves stably.The stress value transferred from the coalface to the floor at different mining heights is controlled by the mining height, and there is a mining height that makes the pressure relief effect of the goaf best.

| Engineering measurement and verification
Surface movement deformation is a reflection of the structural deformation of the overlying rock mass after coal seam mining.The surface subsidence value is the vertical component of the surface movement and deformation, which reflects the migration of the working point of the observation line in the vertical direction.A negative sinking value indicates that the measurement point is sinking.In this study, the monitoring and analysis of surface subsidence in the process of coalface mining are compared, and the measured data of the surface subsidence field are compared with the simulated experimental data.The surface subsidence values are plotted as curves, as shown in Figure 16.Due to the size limitation of the physical model, the impact of overburden failure on load transmission, and the lack of consideration of water and sand flow, the subsidence value obtained by similar simulation experiments is slightly smaller than the measured data.However, the overall evolutionary trend maintains a high degree of similarity.The correctness of the influence of mining height on overburden movement is indirectly verified.As shown in Figure 17, when the coalface is advanced to 30 m, the stress in the middle of the goaf is unloaded from 12 to 8 MPa.When advancing to 60 m, as the roof is still not caving at this time, the pressure relief value of the goaf further increases, meanwhile the abutment pressure on both sides of the coalface increases.When the coalface is advanced to 80 m, the roof bends and sinks.The roof subsidence structure reaches the bottom, and part of the floor is compacted.However, its compaction range is 10 ~20 m, and the peak abutment pressure decreases compared with that when it is pushed to 60 m.When the coalface is advanced to 100 m, the compaction range is about 40 ~50 m, and the compaction effect of goaf is stronger than that of mining 80 m.The peak abutment pressure of the coalface decreases and the peak relief pressure on the coalface side increases.When the coalface is advanced to 110 m, the pressure relief value in the goaf is close to that at 100 m, and the peak value of floor pressure relief strength does not change significantly.The peak abutment pressure is basically unchanged, indicating that mining tends to be stable at this time.Only the compaction range changes, the stress in the compaction area of the goaf restores to the stress value of the original rock and the pressure relief effect is weak.The advance of the coalface to 130 m further indicates that when the overburden of the coalface is moved and destabilized, the abutment pressure develops with a stable value.During the development process, the stress peak and the pressure relief range of the coalface side do not change, only the compaction range increases.

| Evolution law of stope stress in 1402 coalface of Xinan mine
As shown in Figure 18, when the coalface is advanced to 30 m, the stress in the middle of the goaf is reduced to 1 MPa.When the coalface is advanced to 60 m, the roof does not collapse.The pressure relief range is expanded, and the stress value is unloaded to 1.5 MPa.Due to an anomaly at the 5# measuring point, the pressure relief value cannot be greater than when the excavation reaches 30 m.When the coalface is advanced to 90 m, the compaction range is between 60 and 70 m, and the pressure is relieved to 2.12 MPa.At this time, the roof collapses and the goaf is compacted.When the coalface is advanced to 120 m, the stress in the middle of the goaf recovers to 3.8 MPa.When the mining is finished, the

| Influence of caving structure distribution range on stress distribution of floor
The experimental results of Shaqu mine show that when the mining height is small, the pressure relief effect is not obvious when the range of bending subsidence zone is large.The experimental results of Xinan mine show that when the mining height increases, the overburden in goaf is easy to form a caving zone and fracture zone structure.However, the caving zone and fracture zone structure are not conducive to floor pressure relief when no bending subsidence zone is formed and the corresponding pressure relief zone of goaf is small.The experimental results of Daanshan mine show that when the mining height is large and the roof "three zones" are formed, the pressure relief value is large and the range of pressure relief area is large.At the same time, the pressure relief of the floor in goaf of different mining heights under the same geological conditions was compared, indicating that floor pressure relief is related to the distribution range of the caving structure.When the advancing distance of the coalface is small and the moving deformation range is small in the vertical direction, the pressure relief of the goaf with a mining height of 5 m is better than that with a mining height of 8 m.With the advancing of the coalface to 130 m, the mining height of 8 m forms a "three zones" structure.When the range of the caving zone and fracture zone increases compared with the mining height of 5 m, the pressure relief range of floor with a mining height of 8 m is large, but the pressure relief value is reduced.
The increase in mining height is beneficial to the pressure relief of the bending subsidence structure.However, the increase in mining height will reduce the range of bending subsidence structures and increase the range of caving zone and fracture zone.The increase in the range of the caving zone and the fracture zone is not conducive to pressure relief.Therefore, it is necessary to further analyze the influence of mining height on the pressure relief of bending subsidence zone.On this basis, the appropriate mining height is selected to achieve the optimal combination of the distribution range of the caving zone, fracture zone, and bending subsidence zone, so that the range of the pressure relief zone and the pressure relief value reach the maximum.

| DISCUSSION
This study focuses on the influence of mining height as a variable.The general law of the influence of mining height on overburden migration is obtained by comparing different geological conditions and different mining heights.Then the differences of overburden migration caused by the change of mining height under the same geological conditions are compared.The difference from other research is the study in accurate regulation of mining height on pressure relief range and pressure relief value.
Under different geological conditions, the expansion height of the rock strata movement range is positively correlated with the advancing distance of the coalface.The key strata of the roof will lead to a sudden increase in the process of expanding the range of rock strata movement.Under the same geological conditions, the range of collapse structure formed by mining at different mining heights is different, which affects the stress distribution of the floor.The higher the mining height of the coalface, the more overburden subsidence.However, when the mining height exceeds a certain value, the pressure relief effect on the overburden is reduced.Similar conclusions were also made in the literatures. 24,25ccording to the geological conditions of the working face, the optimal mining height can be selected.It can make the distribution range of the caving zone, fracture zone, and bending subsidence zone reach the optimal combination, so that the pressure relief effect is the best.
In the later study, the field test data under various working conditions will be further combined to more comprehensively analyze the stress variation law of overburden migration and pressure relief area with mining height change.

| CONCLUSION
(1) The evolution law of overburden movement and deformation range under different mining heights and different geological conditions is obtained.After the initial breakage of the roof strata with the advance of the coalface, the difference between the advancing distance of the coalface and the initial pressure step is linearly related to the increase of the height of the deformation range of the strata movement.When the roof does not contain the key strata structure, the expansion process of moving deformation range is more uniform.When the roof contains the key strata, the expansion of the strata moving deformation range is affected by the key strata.The extension of the overburden moving deformation range increases abruptly with the breaking of the key strata.The increase in the strike and vertical directions corresponding to the range extension still conforms to the linear relationship.(2) After the initial collapse of the coalface roof, the fracture zone height is approximately exponentially functional as the coalface advances.The height of the fracture zone remains unchanged when the key strata is encountered.When the key strata is broken, the height of the fracture zone changes abruptly.(3) Under the same geological conditions, the evolution law of strata movement and deformation range with the advancing of coalface at different mining heights is approximately the same.The mining height only affects the zoning range of the caving zone, fracture zone, and bending subsidence zone in the range of overburden movement and deformation.(4) The mechanism of mining height influencing the stress distribution of the floor is proposed.When the mining height is small, the bending subsidence structure in the overburden movement deformation range is large, which is not conducive to pressure relief.With the mining height, the range of caving zone and fracture zone increases.The strata in the bending subsidence zone are co-deformed with the strata in the caving zone and the fracture zone.The strata in the bending subsidence zone deform cooperatively with the strata in the caving zone and the fracture zone, and the bearing effect of the bending subsidence zone on the overburden is enhanced.The pressure relief range and pressure relief value of goaf are gradually increased.

3 | 3 . 1 |
INFLUENCE OF MINING HEIGHT ON ROCK STRATA MOVEMENT Influence of mining height on overburden movement and deformation range 3.1.1| Overburden movement deformation evolution process Analysis of overburden movement and deformation process in 22201 face of Shaqu mine

F I G U R E 3
Roof strata movement damages evolution in 2 m mining height.

F I G U R E 5
Roof strata movement damages evolution in 4 m mining height.

F I G U R E 6
Mining height 5 m roof strata movement failure evolution.

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I G U R E 7 Relationship between the moving deformation range of strata with mining height 2 m and the advancing distance of coalface.SHI ET AL.
When the coalface is advanced to 60 m, the immediate roof breaks.The corresponding roof moving deformation range is 5 m, and the moving deformation structure is in the collapse zone.When the coalface continues to advance to 70 m, the roof movement deformation range increases to 14 m.The newly extended moving deformation structure is located in the fracture zone.When the coalface is advanced to 90 m, the roof movement deformation range continues to expand.The moving deformation range extends to 22 m in the vertical direction, and the newly extended moving deformation structure belongs to the fracture zone.Continue advancing to 110 m, and the moving deformation range is extended vertically to 40 m.The moving deformastructure is in the fracture zone, and the fracture range increases further.Continue advancing to 150 m, forming a bending subsidence zone.

F I G U R E 8
Relationship between the moving deformation range of strata with mining height 4 m and the advancing distance of coalface.F I G U R E 9 Relationship between the moving deformation range of strata with mining height 5 m and the advancing distance of coalface.F I G U R E 10 Roof strata movement damages evolution in 8 m mining height.

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I U R E 11 Relationship between the moving deformation range of strata and the advancing distance of coalface with different mining heights.F I G U R E 12 Analysis of caving structure evolution law.

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I G U R E 13 Development and evolution of fracture zone under different geological conditions and mining heights.

F 4 . 1 |
I G U R E 15 Evolution of roof "three zones" at different mining heights.ON STRESS DISTRIBUTION IN STOPE Evolution law of stope stress 4.1.1| Evolution law of stope stress in 22201 face of Shaqu mine The stress distribution of the measured line 3 m below the model goaf when the coalface advances at different distances is given in Figure 17.

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I G U R E 16 Contrasting curves of surface subsidence.F I G U R E 17 Vertical stress distribution curve of the floor during the coalface advancing with a mining height of 2 m.coalface is advanced to 150 m and the floor stress returns to the in-situ stress value.

4. 1 . 4 |
Evolution of stope stress at different mining heights in Daanshan mineThe evolution law of floor stress during the advancing process of coalface with the same geological conditions and different mining heights is shown in Figure20.When the coalface is advanced to 90 m, there are caving zone, fracture zone, and bending subsidence zone in the overburden movement deformation range of goaf in the coalface with a mining height of 5 m.However, there are only caving zone and fracture zone structures in the overburden movement deformation range of goaf in the coalface with a mining height of 8 m.At this time, the pressure relief range and pressure relief value of the goaf at the mining height of 5 m are increased compared with that at the mining height of 8 m.As the coalface continues to advance to 130 m, the range of curved subsidence structure above the goaf of the coalface with a mining height of 5 m increases.The structure range of the caving zone and fissure zone above the goaf at the mining height of 8 m is increased, and the structure of the bending subsidence zone is formed.At this time, the pressure relief range of 8 m mining height is larger than that of 5 m mining height, but the stress recovery value of the goaf increases when it is higher than that at the mining height of 5 m.

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I G U R E 18 Vertical stress distribution curve of the floor during the coalface advancing with a mining height of 4 m.F I G U R E 19 Vertical stress distribution curve of the floor during the coalface advancing with a mining height of 5 m.

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I G U R E 20 Comparison of vertical stress of the floor during the coalface advancing with different mining heights.
Shaqu mine similar material ratio.
Daanshan mine similar material ratio.Excavation model and stress monitoring point layout schematic.
SHI ET AL.
Stress data were extracted for the occurrence time of major caving structures, as shown in Figure19.When the coalface is advanced to 30 m, the pressure relief value in the middle of the goaf reaches 13.3 MPa.With the advance of the coalface, the roof strata do not collapse and the pressure relief is further developed.When the coalface is advanced to 60 m, the pressure is relieved to 13.5 MPa.The pressure relief value does not increase significantly, but the pressure relief width changes.When the coalface is advanced to 90 m, the roof strata collapse and the caving range is loaded on the goaf.The floor stress recovers to 16.6 MPa but does not recover to the initial stress value.The coalface continues to advance, and the caving development enters the stage of bending deformation.When the coalface is advanced to 100 m, the floor stress recovers to 16.9 MPa.When the mining is finished, the floor stress recovers to 17.1 MPa and the floor does not recover to the initial stress value after the final stoppage.