1 Introduction

Characterized by shallow buried closely spaced multi-seam, Jurassic Coalfield in western China is one of seven largest coalfields in the world (Fig. 1). Because of the high-intensity mining of closely spaced multi-seam, the underground concentrated stress is serious, and the surface of ground, characterized by developed fractures and uneven subsidence [1], has already been damaged. Consequently, revealing the characteristics of three-field (fracture field, displacement field and stress field), putting up the determination method of rational CPSD on the basis of three-field coupling control, has important significance to safe and green mining [2]. As mining has developed, the mining of lower second or third main coal seam has gradually entered. Affected by factors such as the mining layout and geological structure of upper coal seams, various coal pillar types are left in upper coal seam goaf area (oblique coal pillars, vertical coal pillars, parallel coal pillars, etc.). Strong coal pillar mining often occurs when lower coal seam passes through coal pillars. Mine pressure phenomenon causes frequent accidents such as pressure frame and production shutdown, which seriously threatens efficient and safe production of mining areas [3,4,5]. According to incomplete statistics, in the past ten years, there have been more than 10 accidents involving strong mine pressure under coal pillars in Shendong mining area, causing huge economic losses. For example: when the 43,207 island working face of Yujialiang Coal Mine passed through overlying coal pillar area, support safety valve opened within 5 min, 40 supports were crushed to damage; Shigetai 31,201 working face had a accident when passing through the overlying room to mine coal pillars, causing the 112 supports on the working face to be crushed, and the column shrinkage was as high as 1500 mm. When the working faces are arranged overlappingly, lower and upper coal pillars overlap, resulting in obvious ground pressure along the roadway or working face middle part. In severe cases, tunnel stability is destroyed, causing bottom drums and coal slabs, seriously affecting safe and efficient mining.

Fig. 1
figure 1

The distribution of coal mines in the Jurassic Coalfield

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Recently, researchers have performed lots of research work referring to stress field, displacement field and fracture field. As for stress field, studies mainly could be classified into two groups, one is related to stress distribution characteristics and rational width of coal pillar in single-seam mining, through hydraulic pillows site observation, Zhang et al. [5, 6] considered design of water-protection pillar in shallow coal seam mining and studied pillar stress distribution and overlying strata tensile stress, and the rational pillar width of 15 m was given. Combining with theoretical analysis and numerical simulation, Li [7] revealed vertical stress characteristics under various width of pillar. Another group is about closely spaced multi-seam mining. Zhang et al. [8] studied ultra-closely spaced multi-seam mining and found stress distribution characteristics under upper pillar, and lower roadway position determination method was put up. Fang et al. [9] analyzed lower roadway stress by numerical simulations, and pointed out that the horizontal distance of lower roadway and upper pillar should be larger than 25 m. Kong et al. [10] revealed the floor failure depth and stress distribution of upper seam mining, and found that the stress under upper pillar was unsymmetrical. Based on the room-and-pillar shallow buried closely multi-seam mining conditions, Guo et al. [11] investigated roof stress distribution of lower face, and found that the stress concentration coefficient of upper pillar is 1.2. Zhang et al. [12] proposed the definition of variation rate coefficient in stress fields under the upper pillar, and the rational staggered distance of roadway was given. Studies referring to displacement field are mainly focused on the movement and subsidence of surface. Based on the surface movement observation of Hanjiawan coal mine, Yu et al. [13] analyzed the parameters of strata movement, the movement deformation and failure characteristics of surface were obtained. Based on the top coal caving in shallow seam, Li et al. [14] revealed the surface movement characteristics by surface observation station in Wanli coal mine. Guo et al. [15] studied the static and dynamic subsidence curves of surface in dip direction and advancing direction, the subsidence and deformation law and integral prediction parameters were obtained. Based on the observation data of surface in Halagou coal mine, Chen et al. [16] investigated dynamic movements and deformation characteristics of surface. Xu et al. [17] studied surface periodic movement characteristics, and a set of calculation parameters and angular parameters of surface subsidence were given by fitting method. Combining with “three-under” mining and existing subsidence theory and practice in China, Liu et al. [18] summarized the surface movement, predicting methods and control of surface subsidence.

Besides, the fracture field of single-seam and multi-seam mining was carried out by scholars, which was mainly studied by physical simulation and numerical simulation. Huang et al. [19,20,21] revealed the characteristics of mining-induced “upwards fractures” and “downwards fractures”, and the height of fracture development in shallow seam strip-filling mining was determined. Using physical simulations, Li et al. [22] investigated stress, displacement and fracture evolution of single-seam and multi-seam mining, and the generation, extension, compaction, stretch-draw and recompaction of fractures was revealed. Xue et al. [23] revealed fracture evolution in shallow buried multi-seam mining with thin bedrock, and mining-induced fractures were studied quantificationally by fractal percolation theory. Cheng et al. [24] explored fracture and stress fields of surrounding rocks in closely spaced multi-seam mining. In addition, combining with physical simulation, numerical simulation and theoretical analysis. Other scholars analyzed the mining-induced overburden failure [25,26,27] and fractures, surface fracture characteristics under high intensity mining, and the stress and fracture field of floor was studied [28,29,30,31,32]. Kumar analyzed the design and stability of coal pillar [33]; Cao analyzed the abnormal mechanism of mine pressure behavior under the influence of overlying concentrated coal pillar [34]; He analyzed the evolution characteristics of mining-induced stress in the stage of coal pillar [35]. Overall, the studies above are mainly related to one or two field, in fact, on the basis of shallow buried closely spaced multi-seam mining conditions, coal pillar is the primary factor which causes concentrated stress, tensile fractures and uneven subsidence, and the CPSD of multi-seam obviously affects three-field evolution.

Combining engineering practice and physical simulation, the three-field (fracture field, displacement field and stress field) evolution was studied in this paper, and the effect of CPSD on concentrated stress, fractures and subsidence of overlying strata and surface was determined, finally, three-field coupling control-based rational CPSD was determined, and it will be applied in Shendong mining area. In the long run, it can promote green and safe mining in hallow buried closely spaced multi-seam.

2 Physical simulation experiment

2.1 Engineering background

From 2022 to 2025, Shendong Company has a total of 82 working faces involved in different types of coal pillar mining, as shown in Table 1. There are four types of coal pillars: parallel coal pillars, vertical coal pillars, oblique coal pillars and room-and-pillar coal pillars, as illustrated in Fig. 2a. Lower coal seam working face for mining through coal pillars is divided into four types according to the mining height: less than 2, 2–3.5, 3.5–5 and more than 5 m. Among them, working face with mining heights of 2–3.5 m account for the largest proportion, followed by working face with mining heights less than 2 m, and working face with mining heights greater than 5 m accounting for the smallest proportion, as shown in Fig. 2b.

Table 1 Some statistical results of coal pillar mines in Shenfu mining area in 2023
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Fig. 2
figure 2

Statistical analysis of classification: a coal pillar type; b the thickness of lower coal seam mining

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The actual measurement study found that when passing through vertical coal pillars of the middle or head of machine, it has little impact on transportation tunnel stability during mining process, and has little impact on mine pressure distribution pattern on working face. Incoming pressure is mainly concentrated on the working surface. The middle part of the face and the rear section of the return air trough machine.

When passing through the vertical coal pillar at machine rear, vertical coal pillar has a stronger effect on return air channel tunnel stability, as shown in Fig. 3. With the emergence of a certain strong mine pressure, partial safety valves are opened during the pressure in the rear section of the machine, and local gangue leaks and other phenomena. The advanced support unit of the return air duct was broken in a large area, the tunnel was seriously deformed, the anchor cable failed, the bottom drum was large, and the auxiliary support of the return air duct was severely bulged. It is necessary to determine a reasonable staggered distance of coal pillars to decrease coal pillar stress concentration.

Fig. 3
figure 3

On-site measurement and of strong pressure during passing coal pillar

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Shendong mining area mainly mines No. 1–2 seam (upper seam) and No. 2–2 seam (lower seam), with 1.84 and 5 m average mining height and 35 m average interburden strata thickness between the two seams. The thickness of bedrock is 70 m, while that of soil layer is 94.7 m. Overall, the seams are nearly flat, and the geological conditions are simple. According to drilling histogram (Fig. 4), the parameters of coal seams and strata are listed in Table 2. Lower and upper coal seam working faces are arranged close to overlapping, and the reasonable coal pillar stagger is determined through research.

Fig. 4
figure 4

Drilling histogram of the working face

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Table 2 Borehole mechanical and physical parameters
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2.2 Experimental design

2.2.1 Establishment of physical simulation model

The physical simulation model has the dimensions of 5 m (length) × 0.2 m (width) × 1.35 m (height) (Fig. 5), with strength ratio of 1:300, bulk density ratio of 1:1.5, and geometric ratio of 1: 200. Due to raw material properties and lithology, calcium carbonate and gypsum were used as cementitious materials and sand was adopted as aggregate.

Fig. 5
figure 5

Physical simulation model

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2.2.2 Observation system of the model

  1. (1)

    Stress observation system

Mining-induced stress was monitored using CL-YB-152 force test sensors, four zones were set up in the model, their position is depicted in Fig. 3, and the observation purposes are shown below:

  • Zone No.1 (Zone No.3): after No. 1–2 seam mining, redistributed stress was monitored in longwall face middle.

  • Zone No.2: after No. 1–2 seam mining, upper pillar concentrated stress was monitored.

  • Zone No.4: after No. 1–2 seam mining, transmitted stress of No. 1–2 seam on No. 2–2 seam was monitored.

  1. (2)

    Displacement observation system

Displacement observation lines were arranged in the model, the position and observation purpose is shown below:

  • Line No.1: 4 cm from No. 1–2 seam roof, No. 1–2 seam immediate roof displacement was monitored.

  • Line No.2: 20 cm from No. 1–2 seam roof, No. 1–2 seam main roof displacement was monitored.

  • Line No.3: 40 cm from No. 1–2 seam roof, bedrock-red soil displacement of interface was monitored.

  • Dial indicators: 20 dial indicators are adopted on model surface to monitor ground surface displacement.

  1. (3)

    Fracture observation system

Mining-induced fractures (ground surface and overlying strata fractures) were monitored using BJQF-1 fracture width gauge. Figure 6 shows the part of the experimental apparatus.

Fig. 6
figure 6

Part of the experimental setup: a total station; b dial indicator; c BJQF-1 fracture width gauge

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2.2.3 Model excavation

Firstly, No. 1–2 seam was mined, and the three-field (fracture field, displacement field and stress field) was monitored. Then, the lower No. 2–2 seam was excavated, during the mining, it can simulate eleven situations: aligned pillars, CPSD of 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90 m (Fig. 7). In addition, the characteristics of three-field evolution based on CPSD were revealed.

Fig. 7
figure 7

Excavation situations of CPSD

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3 Result and discussion

3.1 Characteristics of stress field

3.1.1 Stress field of No. 1–2 seam mining

According to Zone No.1-No.4 and as shown in Fig. 8, after No. 1–2 seam mining, the following conclusions were drawn:

Fig. 8
figure 8

No. 1–2 seam mining stress distribution

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Maximum vertical stress is 20.56 MPa, concentrated at upper pillar center, and gradually reduces to both sides.

Vertical stress in longwall face middle is larger than other positions of the face, and its maximum value, 3.12 MPa, about 0.87 times the No. 1–2 seam virgin in-situ stress (3.6 MPa).

Affected by the No. 1–2 seam mining, No. 2–2 seam stress is redistributed, combining with vertical stress curve and No. 2–2 seam virgin in-situ stress (4.5 MPa), high stress area (red area) caused by the upper pillar and the middle of upper longwall face is depicted in Fig. 6, concentrated stress transmission angle of upper pillar and the middle of longwall face are 25° and 29° respectively.

The arrangement of lower roadway should avoid the red area, when the roadway is placed in position ①, the CPSD is the minimum, when the roadway is placed in position ②, the CPSD is the maximum, the roadway width is 5 m, consequently the rational CPSD is 21–55 m.

3.1.2 No. 1–2 and No. 2–2 seam mining stress fields

Figure 9 shows relationship between lower pillar maximum vertical stress and different CPSD, when the CPSD is 20–50 m, smaller maximum vertical stress is observed, and it is well consistent with the results obtained by the No. 1–2 seam mining.

Fig. 9
figure 9

Lower pillar maximum vertical stress with different CPSD

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According to the analysis above, stress field control-based rational CPSD is 21–50 m.

3.2 Characteristics of displacement field

3.2.1 No. 1–2 seam mining displacement field

During No. 1–2 seam mining, overlying strata caved from bottom to top as the face advanced, and the overlying strata and ground surface subsidence were obtained (Fig. 10). From the figure, the conclusions can be summarized as below:

Fig. 10
figure 10

No. 1–2 seam mining ground surface and overlying strata subsidence

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Compared with the overlying strata subsidence curve, the surface subsidence curve is more gentle, the displacement of red soil has continuity.

The maximum subsidence value of immediate roof and surface is 1.8 m and 1.098 m respectively. In addition, the coefficient of mining influence is 0.58.

3.2.2 Displacement field of No. 1–2 and No. 2–2 seams mining

Eleven situations of CPSD were simulated, the surface subsidence curves were obtained (Fig. 11), and the overlying strata movement and failure was revealed (Fig. 12). From the figures, the conclusions can be summarized as below:

Fig. 11
figure 11

Surface subsidence curves with different CPSD

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Fig. 12
figure 12

Overlying strata movement vs. CPSD: a aligned pillars; b CPSD of 10 m; c CPSD of 20 m; d CPSD of 40 m; e CPSD of 60 m; f CPSD of 80 m

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The surface subsidence above pillar is small, while the largest subsidence is witnessed in longwall face middle. Consequently, pillar has control effect on surface subsidence.

When the CPSD is smaller than 20 m, the uneven subsidence is obvious (Fig. 12a), and the maximum subsidence drop height is 3.52 m (aligned pillars).

When the CPSD is 40 m, the interburden strata fractures along the shear falling zone (Fig. 12d), the upper pillar and its overlying strata present fully subsidence, in addition, the uneven surface subsidence decreases.

According to the analysis above, rational CPSD on the basis of displacement field control has to be larger than 40 m.

3.3 Characteristics of fracture field

3.3.1 No. 1–2 seam mining fracture field

Following No. 1–2 seam mining, characteristics of fracture field are shown in Fig. 13. The caving angle of bedrock is 65°, red soil caving angle is 70°, and surface fractures are formed above the model. Transformed into actual value, the surface fracture width is 0.222–0.255 m.

Fig. 13
figure 13

Characteristics of fracture field of No. 1–2 seam mining: a characteristics of fracture field; b surface fracture ①; c surface fracture ②

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3.3.2 Fracture field of No. 1–2 and No. 2–2 seams mining

During No. 1–2 and No. 2–2 coal seam mining, the fracture field evolution characteristics based on different CPSD were revealed. Surface fracture ① above the boundary pillar (Fig. 13) is permanent fracture which can not be controlled effectively [2, 19, 36], whereas, surface fracture ② is controlled through CPSD rational arrangement, consequently surface fractures ② evolution with CPSD was mainly studied in this paper.

Characteristics of fracture field (surface fractures ②) of No. 1–2 and No. 2–2 seams mining were shown in Fig. 14. Figure 15 shows the relationship between width of fractures ② and CPSD. From the figures, when CPSD is smaller than 30 m and pillars are aligned, surface fractures width is relatively larger (when coal pillars are aligned the width is 0.646 m, when the CPSD is 10 m, the width is 0.521 m), as the CPSD increases and larger than 30 m, the width of surface fracture ② decreases obviously (basically smaller than 0.122 m). It decreases by 81.1%. As the CPSD continues increases, the fracture width presents unobvious change.

Fig. 14
figure 14

Characteristics of fracture field of No. 1–2 and No. 2–2 seams mining: a aligned pillars; b 10 m CPSD; c 30 m CPSD; d 40 m CPSD; e 50 m CPSD; f 80 m CPSD

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Fig. 15
figure 15

Surface fractures width with different CPSD

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According to the analysis above, the rational CPSD to realize control of surface fractures② should be larger than 30 m.

3.4 The model of reasonable coal pillar staggered distance

Previous studies have found that concentrated stress on upper coal pillar floor is not transmitted in a straight line, but the existing theoretical models are mostly simplified to use a certain transmission angle to determine the range of decompression. Reasonable coal pillar staggered distance model to avoid upper coal pillars transmitting concentrated stress was put up, if it is simply determined by “misplaced coal pillar group structure” boundaries, results will be biased. Based on the simulation experiments and theoretical analysis in text, in coal seam group mining, coal pillar floor in upper coal seam left section is the interval rock layer between two coal seams. Therefore, coal pillar floor depth x is regarded as the thickness of different interburden spacing, when the interburden (x) is constant, the vertical stress distribution law of lower coal seam top under section coal pillar uniform load can be obtained. In addition, as the interburden increases, the influence of interval rock layer self-weight becomes obvious, therefore, when solving the vertical stress at the top of lower coal seam in coal seam group mining, self-weight of the interval rock layers should be considered. The corrected vertical stress σx1 is:

$$ \sigma_{{x{1}}} = \sigma_{x} + \gamma x $$
(1)

In the formula: σx1 is the corrected vertical stress, MPa; γ is interval rock layer average bulk density, kN/m3; x is the thickness of different interburden, m.

At present, most scholars at home and abroad believe that to avoid upper and lower coal pillar stress superposition in coal seam mining under coal seam group, the coal seam tunnels in coal seam group lower part has to be arranged outside the high stress zone where coal pillars in upper coal seam section transmit downward. If the original rock stress at lower coal seam is σ, then conditions for lower coal seam roadway to avoid concentrated stress are:

$$ \sigma_{x1} \le \sigma $$
(2)
$$ \sigma = \gamma x + \gamma_{1} H_{1} + \gamma_{2} H_{2} $$
(3)
$$\begin{gathered} \sigma_{x} = \frac{2q}{\uppi }\int_{{{ - }\frac{1}{2}a}}^{{\frac{1}{2}a}} {\frac{{x^{3} {\text{d}}\xi }}{{[x^{2} + (y - \xi )^{2} ]^{2} }} = } \hfill \\ \frac{q}{\uppi }\left[ {{\text{arctan}}\frac{{y + \frac{1}{2}a}}{x} - {\text{arctan}}\frac{{y - \frac{1}{2}a}}{x} + \frac{{x(y + \frac{1}{2}a)}}{{x^{2} + (y + \frac{1}{2}a)^{2} }} - \frac{{x(y - \frac{1}{2}a)}}{{x^{2} + (y - \frac{1}{2}a)^{2} }}} \right] \hfill \\ \end{gathered}$$
(4)

In the formula: σ is the original rock stress at lower coal seam, MPa; γ1 is red soil layer average bulk density, kN/m3; H1 is the thickness of red soil, m; γ2 is overlying strata average bulk density, kN/m3; H2 is the thickness of overlying strata, m. q is concentrated stress of coal pillar, MPa; a is the width of coal pillar, m; y is the horizontal distance from coal pillar center, m.

According to Eqs. (1)–(4), there is:

$$\begin{gathered} \frac{{\gamma_{1} H_{1} (a + H_{1} {\text{cot}}\alpha_{1} ) + \gamma_{2} H_{2} (a + 2H_{1} {\text{cot}}\alpha_{1} + H_{2} {\text{cot}}\alpha_{2} )}}{{a\uppi }} \cdot \hfill \\ \left[ \begin{gathered} {\text{arctan}}\frac{{y + \frac{1}{2}a}}{x} - {\text{arctan}}\frac{{y - \frac{1}{2}a}}{x} + \hfill \\ \frac{{x(y + \frac{1}{2}a)}}{{x^{2} + (y + \frac{1}{2}a)^{2} }} - \frac{{x(y - \frac{1}{2}a)}}{{x^{2} + (y - \frac{1}{2}a)^{2} }} \hfill \\ \end{gathered} \right] \le \gamma_{1} H_{1} + \gamma_{2} H_{2} \hfill \\ \end{gathered}$$
(5)

In the formula: α1 is the breaking angle of red soil layer, °; α2 is the breaking angle of overlying strata, °.

When the geological conditions are certain, the minimum value of y to avoid upper coal pillar concentrated stress can be obtained through Eq. (5). The geometric relationship between the offset distance of coal pillar and y to avoid concentrated stress is illustrated in Fig. 16. As presented in the figure, vertical stress at point M is original rock stress, and the minimum decompression coal pillar offset distance Lσmin can be obtained as:

$$L_{\sigma \min } \ge y - \frac{a}{2} + b$$
(6)
Fig. 16
figure 16

Minimum decompression coal pillar offset

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In the formula: Lσmin is the minimum decompression coal pillar offset based on concentrated stress control, m; b is the width of roadway, m.

According to Eqs. (5) and (6), the minimum decompression coal pillar offset Lσmin to avoid concentrated stress can be obtained. However, its solution process is still relatively complex. As presented in Fig. 17, in order to solve practical engineering problems, a refined decompression model with staggered coal pillar spacing is established. In the figure, a is coal pillar width in upper coal seam section, m; a2 is coal pillar width in the lower coal seam section, m; b is lower coal seam tunnel width, m; and γH is original rock stress, MPa. The abscissa is horizontal distance from upper coal pillar center, ordinate is interburden spacing (a multiple of coal pillar width in upper coal seam section), and the red line is original rock stress contour. The following conclusions can be drawn:

  1. (1)

    Concentrated stress directly under coal pillar in the section is maximum. Calculated according to coal pillar width a = a2 = 20 m, the interburden is 10 m (0.5a), 20 m (a), 30 m (1.5a), 35 m (1.75a) respectively), maximum vertical concentrated stress transmitted to lower coal seam is 4.6, 3.1, 2.2 and 1.9 times the original rock stress respectively.

  2. (2)

    Taking the original rock stress line as the dividing line (red stress contour), the area inside it is the high stress bubble area, and vertical stress is greater than original rock stress; the area outside it is the low stress area, and vertical stress Less than original rock stress.

Fig. 17
figure 17

Refined minimum decompression coal pillar offset model to avoid concentrated stress transmitted by upper coal pillar

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For shallow-buried close-distance coal seam mining, lower coal seam roadway should be arranged in a low-stress area. When mining conditions are constant (the interburden is determined), the minimum decompression coal pillar offset Lσmin based on concentrated stress control can be obtained.

In terms of upper coal seam (1–2 coal seam) and lower coal seam (2–2 coal seam), other mining conditions are constant (bedrock thickness 81.9 m, soil layer thickness 94.7 m), by changing the interburden to analyze its impact on the reasonable coal pillar staggered distance, due to the distance between upper and lower coal seams is generally within 45 m. Therefore, the interburden range selected for the study is 5–45 m. Under different interburden conditions, the reasonable minimum decompression coal pillar offset to reduce concentrated stress in lower coal pillar changes with the interburden as illustrated in Fig. 18. Figure 18 shows that the change curve of reasonable minimum decompression coal pillar with the interburden is a “parabola”, and the fitting relationship between the two is:

$$L_{{\sigma {\text{min}}}} = - 0.01x^{2} + 0.7014x + 4.5898$$
(7)
Fig. 18
figure 18

Reasonable minimum pillar staggered distance changes with interburden

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According to the relationship between the reasonable minimum decompression coal pillar offset and the interburden, the “parabola” curve can be divided into two sections:

① Reasonable minimum decompression stagger increase section

When the layer distance (Under coal seam mining conditions), the change in interburden has a significant impact on the reasonable minimum decompression coal pillar offset. When the interburden is 35 m, the reasonable minimum decompression coal pillar offset is the largest and should be greater than 16.9 m.

② Reasonable minimum decompression offset reduction section

When the interburden between 1–2 and 2–2 coal seams is greater than 35 m, the lower coal seam is gradually less influenced by concentrated stress transmitted by coal pillar after upper coal seam is mined.

4 Engineering practice

4.1 No. 1–2 seam mining surface fractures

LW N1110 mines No. 1–2 seam, buried depth is 48–160 m, the coal seam thickness is 1.92 m on average, thickness of bedrock is 48–60 m, and thickness of soil is 0–100 m. After mining, the characteristics of surface fractures are shown in Fig. 19a, the maximum width of surface fractures ② is 0.2 m.

Fig. 19
figure 19

No. 1–2 seam mining surface fractures: a LW N1110 mining surface fractures; b LW N1114 mining surface fractures

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In another engineering practice, LW N1114 mines No. 1–2 seam, the buried depth is 64–156 m, the coal seam thickness is 1.71 m on average, thickness of bedrock is 54–66 m, and thickness of soil is 10–90 m. After mining, the characteristics of surface fractures are shown in Fig. 19b, the maximum width of surface fracture ② is 0.3 m.

Overall, under mining conditions of No. 1–2 seam, the maximum width of surface fracture ② is 0.2–0.3 m, according to physical simulation, it is 0.222–0.255 m, consequently the experimental results are basically consistent with the engineering practice.

4.2 Surface fractures of No. 1–2 and No. 2–2 seams mining

LW N1114 mine No. 1–2 seam and LW N1206 mine No. 2–2 seam, characteristics of surface fractures are shown in Fig. 20, it can be known that the surface fractures develop more seriously than single-seam mining, and the width of surface fractures ② is 0.4–1.5 m.

Fig. 20
figure 20

Surface fractures of No. 1–2 and No. 2–2 seams mining: a surface fractures; b surface fracture site observation

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According to the analysis above, three-field evolution is closely related to CPSD, the rational CPSD avoiding concentrated stress is 21–55 m, displacement field control-based rational CPSD should be larger than 40 m, and the rational CPSD to realize surface fracture control should be larger than 30 m.

Therefore, three-field coupling control-based rational CPSD is 40–55 m, in addition, the longwall faces width of No. 1–2 and No. 2–2 seams, both 245 m, is designed, and parallel arrangement of upper and lower longwall faces will be adopted in subsequent arrangement.

5 Conclusions

According to physical simulation, to avoid concentrated stress, CPSD of No. 1–2 and No. 2–2 seams has optimal range, the minimum CPSD is determined to avoid the transferred concentrated stress by upper pillar, while the maximum CPSD is determined for avoiding higher stress area by upper goaf middle. The concentrated stress transmission angle of upper pillar and longwall face middle are 25° and 29° respectively.

Following No. 1–2 seam mining, maximum surface subsidence is 1.098 m, and the coefficient of mining influence is 0.58. As for No. 1–2 and No. 2–2 seams mining, as the CPSD increases, the surface subsidence above upper pillar gradually increases, which results in the decrease of the subsidence drop height, in addition, the uneven surface subsidence also decreases obviously.

After No. 1–2 seam mining, bedrock and red soil caving angles are 65° and 70°, respectively, surface fracture width② above pillar is 0.222–0.255 m. As for two seams mining, the surface fractures ② can be controlled using CPSD rational arrangement, as CPSD increases, surface fracture width decreases by 81.1%. According to engineering practice, the maximum width of surface fracture ② of No. 1–2 seam mining is 0.2–0.3 m, and the width of two seams mining is 0.4–1.5 m.

Three-field evolution is closely related to CPSD, through rational arrangement of CPSD, the concentrated stress can be avoided, the uneven surface subsidence can be reduced, and the developed surface fractures decrease. Overall, the rational CPSD, determined by the intersection of coupling control of three-field, is 40–55 m, and parallel arrangement of upper and lower longwall working faces will be adopted, research findings will be employed in subsequent Shendong mining area arrangement.