1 Introduction

Gob-side entry retaining by roof cutting is a new technology proposed by Chinese scholars in recent years (Wang et al. 2018a; Yang and Zhang 2022; He et al. 2021). This technique cuts off the roof strata of the roadway and gob using bidirectional concentrated blasting technology. After the working face is mined, the roof strata of the gob collapses and forms a new rib (Hu et al. 2022a; Zhu et al. 2020; Wang et al. 2018b). Compared with traditional technology for gob-side entry retaining, this method has completely changed the core idea of replacing coal pillars with backfill and removed the need for roadside backfill. The technology of gob-side entry retaining by roof cutting solves the problem of the stress concentration of the backfill, and the construction process is simpler (Zhu et al. 2023; Zhang et al. 2022; Hu et al. 2022b). To date, this method has been successfully applied under a variety of working conditions such as thin coal seams, thick coal seams, hard roofs, composite roofs, shallow burials, and high gas mines and has achieved good economic benefits (Wang et al. 2021a, 2022a; Yang et al. 2020). However, the technology of gob-side entry retaining by roof cutting is less commonly applied under the condition of a close-distance seam group.

The stress concentration caused by the upper coal seam has a great impact on the mining of the lower coal seam under the condition of a close-distance seam group (Wang et al. 2022b; Li et al. 2022; Shang et al. 2023). This stress can easily cause large deformations or the disappearance of surrounding rocks on the roadway and cause safety accidents. By adopting the technology of gob-side entry retaining by roof cutting, the coal pillar of the upper coal seam is removed, which solves the problem of stress concentration on the coal pillar.

1.1 Process flow of gob-side entry retaining by roof cutting

The process flow of gob-side entry retaining by roof cutting (Yang et al. 2021; Bian et al. 2022; Hu et al. 2020) is shown in Fig. 1, (1) Reinforce the roof strata of roadway along gob with high-strength anchor. (2) Pre-splitting blasting the roof strata of roadway by directional blasting technology. (3) After the working face is mined, the gangue retaining composed of U-shaped steel and reinforced mesh is arranged at the edge of the gob. In order to ensure the stability of the roof strata, the roadway is temporarily supported by single pillar. (4) When the gob-side roadway is stabilized, the single pillar can be removed, and only the gangue retaining is retained.

Fig. 1
figure 1

The process flow of gob-side entry retaining by roof cutting

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1.2 Key technology of gob-side entry retaining by roof cutting

The core idea of gob-side entry retaining by roof cutting is to replace the coal pillar with a collapse gangue (Long et al. 2022; Tian et al. 2022a, b). Directional presplitting is used to blast the roof strata of the roadway before mining (Wang et al. 2022c, 2023a; He et al. 2021). When the working face is mined, the roof strata can collapse over time and fill the gob. The new rib of the roadway is then formed under the action of the gangue retaining (Wang et al. 2023b; Chen et al. 2022, 2023). Therefore, the key technology of gob-side entry retaining by roof cutting is pre-splitting blasting (Chen et al. 2021; Li 2021; Li et al. 2021). The optimum presplitting blasting height should ensure that the collapsed gangue can fill the gob after mining to effectively support the overlying strata, make the overlying strata reach a stable state as soon as possible, and reduce the impact of dynamic pressure on the surrounding rocks of the roadway (Guo et al. 2021a; Wang et al. 2021b; Xue et al. 2023). The optimum angle and spacing of the pre-splitting blasting hole should enable the roof strata of the gob to collapse as soon as possible after mining, reduce the unsupported roof of the goaf, and minimize the disturbance effect of the roof strata collapsing on the surrounding rock (Guo et al. 2021b; Wang et al. 2022d; Gao et al. 2023).

2 Methods

In order to study the key parameters of gob-side entry retaining by roof cutting in close-distance seam group, taking the Yuwang Coal Mine as the engineering back ground. The coal bearing strata in the Yuwang Coal Mine are the C2 to C19 coal seams of the Longtan Formation, with a total thickness of 32.28 m. The spacing of each coal seam is shown in Table 1.

Table 1 The spacing of each coal seam
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2.1 Theoretical calculation

2.1.1 The presplitting blasting height

The maximum vertical distance between the slit formed after directional pre-splitting blasting and the horizontal plane of the roadway roof is the presplitting blasting height. The optimum presplitting blasting height should ensure that the collapsed gangue of the roof strata can fill the gob. Therefore, the optimum presplitting blasting height can be calculated by the following formula:

$${\text{H}}_{{\mathrm{P}}} = \left( {{\text{H}}_{{\mathrm{M}}} - {\text{H}}_{{\mathrm{R}}} - {\text{H}}_{{\mathrm{F}}} } \right)/\left( {{\text{K}} - {1}} \right)$$
(1)

where HP is the optimum presplitting blasting height in m; HM is the mining height in 2.4 m; K is the bulking coefficient in 1.4; In order to obtain the maximum value of the height of roof cutting, the volumes of roof convergence and floor heave were not taken into consideration, i.e., HR = HF = 0. The presplitting blasting height was 6.0 m, which was calculated using Eq. (1).

2.1.2 The angle of presplitting blasting hole

Before the working face is mined, pre-splitting blasting should be undertaken on the roof strata of the roadway. If the angle of presplitting blasting is too small, this angle will not be conducive to the stability of the roadway roof strata, whereas if the angle is too large, it will not be conducive to the collapse of the gob roof strata. The mechanical model of roof strata after presplitting blasting can be simplified as Fig. 2. The rock block A is located above the roadway and rock block B is located above the gob.

Fig. 2
figure 2

Schematic diagram of rock mass occlusion

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The condition of rock stability is as follows:

$$\left[ {{\text{T }}\cos \upalpha + \left( {{\text{R}} - {\text{F}}} \right)\sin \upalpha } \right]\tan {\upvarphi } = \left( {{\text{R}} - {\text{F}}} \right)\cos \upalpha - {\text{T }}\sin \upalpha$$
(2)

So α = arctan((R − F)/T) − φ

$$\left\{ {\begin{array}{*{20}l} {f = \tau \times h \times 1} \hfill \\ {R = \rho ghd} \hfill \\ \end{array} } \right.$$
(3)

where T is the horizontal force of the rock block in kN; R is the load of block B in kN; F is the working resistance of a single prop in kN; h is the height of roof cutting in m; φ is the friction angle of the rock block in °; α is the angle of the presplitting blasting borehole in °; f is the shear force of the rock block in kN; ρ is the density of the rock strata in kg/m3; and d is the width of the roadway in m.

For convenient calculation, a component of T can be approximated as f where “f = T sin α”. According to the actual conditions of the 1010201 ventilation roadway, the field parameters were chosen as follows: φ = 27°, τ = 0.13 MPa, ρ = 2500 kg/m3, h = 6 m, and d = 5.0 m. According to the aforementioned theoretical calculation, when the angle of the presplitting blasting borehole is in the range of 10°–15°, the roof strata of the roadway can remain stable.

2.1.3 The spacing of the presplitting blasting hole

Directional blasting use an energy gathering device to make the detonation wave cut off the roof strata of the roadway along a given direction. The blasting mechanics model of presplitting blasting hole is shown in Fig. 3.

Fig. 3
figure 3

The blasting mechanics model of presplitting blasting hole

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According to the attenuation law of explosion stress wave, the damage range of explosion can be calculated as follows:

$$R_{s} = r_{b} \left[ {\frac{{10\lambda P_{b} }}{{\left( {1 - D_{0} } \right)\sigma_{t} + \sigma_{0} }}} \right]^{{\frac{1}{\alpha }}}$$
(4)

where Rs is the damage range of explosion in m; rb is the radius of the presplitting blasting hole in m; λ is the coefficient of lateral pressure; Pb is the pressure peak after explosive charge in MPa; D0 is the initial damage of rock mass; σt is the tensile strength of the roof strata in MPa; σ0 is the protolith stress in MPa; α is the attenuation index of explosive stress wave in rock mass.

To ensure the effect of the pre-splitting blasting, the damage cracks formed after the blasting of two adjacent pre-splitting blasting holes should be connected. Therefore, the spacing of the pre-splitting blasting hole should meet the following relationship:

$${\text{d}} \le {2}\left( {{\text{r}}_{{\mathrm{b}}} + {\text{R}}_{{\mathrm{s}}} } \right)$$
(5)

substituting Eq. (4) into Eq. (5) can be obtained as follows:

$$d \le 2r_{b} \left[ {1 + \left( {\frac{{10\lambda P_{b} }}{{\left( {1 - D_{0} } \right)\sigma_{t} + \sigma_{0} }}} \right)^{{\frac{1}{\alpha }}} } \right]$$
(6)

According to the actual conditions of the 1010201 ventilation roadway, the field parameters were chosen as follows: rb = 0.02 m, λ = 0.9, Pb = 200 MPa, D0 = 0.05, σt = 2.5 MPa, σ0 = 8.6 MPa, α = 2. According to the aforementioned theoretical calculation, the spacing of the pre-splitting blasting hole is 0.512 m.

2.2 Numerical model

The height and angle of presplitting blasting are analyzed via FLAC3D, and the distance between presplitting blasting holes is analyzed using LS-DYNA.

2.2.1 FLAC3D

The mechanical parameters of the roof and floor strata of the C2 coal seam in Yuwang Coal Mine are shown in Table 2.

Table 2 The mechanical parameters of the roof and floor strata
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Construction of numerical model based on geological conditions of Yuwang Coal Mine. The strike length of the model is 270 m, the dip length is 200 m, and the height is 60 m. The length of the working face is 200 m, and the width of the mining roadway is 5 m. In order to avoid the impact of excavation disturbance, a 30 m boundary is reserved on both sides of the mining roadway. Set constraints on the left, right and the lower boundaries of the model, and load a vertical stress of 8.6 MPa on the upper boundary. The numerical model is shown in Fig. 4.

Fig. 4
figure 4

The numerical model

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2.2.2 LS-DYNA

The blasting numerical model is composed of rock unit, air unit, explosive unit and energy-gathering pipe unit. The inner diameter of the energy-gathering pipe model is 3.6 cm, the outer diameter is 4.2 cm, and the explosive diameter is 3.2 cm. The model is simplified as a plane stress state, and 0.5 cm is taken in the thickness. In the calculation process, the explosive adopts the central point initiation mode, and the air boundary condition is set as the non-reflecting boundary condition. The material parameters of the model unit are shown in Tables 3 and 4.

Table 3 Parameters of the explosive
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Table 4 Parameters of the energy-gathering pipe
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The model adopts the mapping mesh division method. Here local mesh refinement is conducive to the transfer of detonation pressure, enabling the air and rock mesh near the explosive area to be refined. The grid quality of the energy-gathering pipe and explosives has no obvious influence on the simulation effect, so uniform grid division is adopted. The meshes of each element are shown in Fig. 5.

Fig. 5
figure 5

Meshing of the model of a Air unit. b Rock unit. c Energy-gathering pipe unit. d Explosive unit

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3 Results and discussion

3.1 The presplitting blasting height

According to the aforementioned theoretical analysis, the stress distribution and displacement of surrounding rock on the roadway are analyzed with the presplitting blasting height of 5, 6, 7, and 8 m. The results of numerical simulation are shown in Figs. 6, 7, 8 and 9.

Fig. 6
figure 6

The vertical stress distribution of the roadway with the presplitting blasting height is a 5 m. b 6 m. c 7 m. d 8 m

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Fig. 7
figure 7

The horizontal stress distribution of the roadway with the presplitting blasting height is a 5 m. b 6 m. c 7 m. d 8 m

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Fig. 8
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The vertical displacement of the roadway with the presplitting blasting height is a 5 m. b 6 m. c 7 m. d 8 m

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Fig. 9
figure 9

The horizontal displacement of the roadway with the presplitting blasting height is a 5 m. b 6 m. c 7 m. d 8 m

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As shown in Fig. 6, when the presplitting blasting height is 5 m, the maximum value of vertical stress is 21.3 MPa, and the stress concentration is 7.5 m away from the roadway; when the presplitting blasting height is 6 m, the maximum value of vertical stress is 15.3 MPa, and the stress concentration is 8.1 m away from the roadway; when the presplitting blasting height is 7 m, the maximum value of vertical stress is 15.6 MPa, and the stress concentration is 8.5 m away from the roadway; when the pre-splitting blasting height is 8 m, the maximum value of vertical stress is 17.0 MPa, and the stress concentration is 8.7 m away from the roadway.

As shown in Fig. 7, when the presplitting blasting height is 5 m, the maximum value of horizontal stress is 16.2 MPa; when the presplitting blasting height is 6 m, the maximum value of horizontal stress is 15.5 MPa; when the presplitting blasting height is 7 m, the maximum value of horizontal stress is 15.6 MPa; when the presplitting blasting height is 8 m, the maximum value of horizontal stress is 15.8 MPa.

As shown in Fig. 8, when the presplitting blasting height is 5 m, the maximum value of vertical displacement is 27.1 mm; when the presplitting blasting height is 6 m, the maximum value of vertical displacement is 22.0 mm; when the presplitting blasting height is 7 m, the maximum value of vertical displacement is 20.8 mm; when the presplitting blasting height is 8 m, the maximum value of vertical displacement is 30.0 mm.

As shown in Fig. 9, when the presplitting blasting height is 5 m, the maximum value of horizontal displacement is 19.6 mm; when the presplitting blasting height is 6 m, the maximum value of vertical displacement is 22.0 mm; when the presplitting blasting height is 7 m, the maximum value of vertical displacement is 20.8 mm; when the presplitting blasting height is 8 m, the maximum value of vertical displacement is 30.0 mm.

The numerical simulation indicates that the presplitting blasting height has a great influence on the stress distribution and deformation of the roadway. Considering the stress distribution of the roadway, the deformation of surrounding rock, and the engineering quantity, the optimal height for presplitting blasting is 6 m.

3.2 The angle of presplitting blasting hole

According to the aforementioned theoretical analysis, the stress distribution and displacement of roadway surrounding rock are analyzed with the angle of presplitting blasting hole are 5°, 10°, 15° and 20°, respectively. The results of numerical simulation are shown in Figs. 10, 11, 12 and 13.

Fig. 10
figure 10

The vertical stress distribution of the roadway with the angle of presplitting blasting hole is a 5°. b 10°. c 15°. d 20°

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Fig. 11
figure 11

The horizontal stress distribution of the roadway with the angle of presplitting blasting hole is a 5°. b 10°. c 15°. d 20°

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Fig. 12
figure 12

The vertical displacement of the roadway with the angle of presplitting blasting hole is a 5°. b 10°. c 15°. d 20°

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Fig. 13
figure 13

The horizontal displacement of the roadway with the angle of presplitting blasting hole is a 5°. b 10°. c 15°. d 20°

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As shown in Fig. 10, when the angle of presplitting blasting hole is 5°, the maximum value of vertical stress is 21.64 MPa, and the stress concentration is 7.5 m away from the roadway; when the angle of presplitting blasting hole is 10°, the maximum value of vertical stress is 15.15 MPa, and the stress concentration is 8.1 m away from the roadway; when the angle of presplitting blasting hole is 15°, the maximum value of vertical stress is 15.06 MPa, and the stress concentration is 8.5 m away from the roadway; when the angle of presplitting blasting hole is 20°, the maximum value of vertical stress is 15.3 MPa, and the stress concentration is 8.6 m away from the roadway.

As shown in Fig. 11, when the angle of presplitting blasting hole is 5°, the maximum value of horizontal stress is 26.45 MPa; when the angle of presplitting blasting hole is 10°, the maximum value of horizontal stress is 15.87 MPa; when the angle of presplitting blasting hole is 15°, the maximum value of horizontal stress is 15.79 MPa; when the angle of presplitting blasting hole is 20°, the maximum value of horizontal stress is 15.97 MPa.

As shown in Fig. 12, when the angle of presplitting blasting hole is 5°, the maximum value of vertical displacement is 220.27 mm; when the angle of presplitting blasting hole is 10°, the maximum value of vertical displacement is 217.11 mm; when the angle of presplitting blasting hole is 15°, the maximum value of vertical displacement is 208.87 mm; when the angle of presplitting blasting hole is 20°, the maximum value of vertical displacement is 227.29 mm.

As shown in Fig. 13, when the angle of presplitting blasting hole is 5°, the maximum value of horizontal displacement is 192.52 mm; when the angle of pre-splitting blasting hole is 10°, the maximum value of vertical displacement is 144.73 mm; when the angle of presplitting blasting hole is 15°, the maximum value of vertical displacement is 144.18 mm; when the angle of presplitting blasting hole is 20°, the maximum value of vertical displacement is 144.66 mm.

The numerical simulation indicates that the angle of presplitting blasting hole has a great influence on the stress distribution and deformation of the roadway. Considering the stress distribution of the roadway, the deformation of surrounding rock and the engineering quantity, the optimal angle of presplitting blasting hole is 15°.

3.3 The spacing of the presplitting blasting hole

To determine the optimal spacing of the presplitting blasting hole, the stress evolution process of shaped charge blasting and the development law of blast cracks with blast hole spacing of 400, 500, and 600 mm are compared and analyzed. The numerical simulation are shown in Figs. 14, 15, 16, 17, 18 and 19.

Fig. 14
figure 14

The stress evolution process with the spacing of 400 mm in a 0.13 ms. b 0.24 ms

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Fig. 15
figure 15

The development law of blast crack with the spacing of 400 mm in a 0.13 ms. b 0.24 ms

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Fig. 16
figure 16

The stress evolution process with the spacing of 500 mm in a 0.15 ms. b 0.32 ms

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Fig. 17
figure 17

The development law of blast crack with the spacing of 500 mm in a 0.15 ms. b 0.32 ms

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Fig. 18
figure 18

The stress evolution process with the spacing of 600 mm in a 0.20 ms. b 0.32 ms

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Fig. 19
figure 19

The development law of blast crack with the spacing of 600 mm in a 0.20 ms. b 0.32 ms

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At the moment after initiation of the explosive, the tensile stress concentration zone appears near the shaped energy holes. When the tensile stress exceeds the tensile strength of the surrounding rock, tensile cracks will occur in the surrounding rock near the energy accumulation hole. After the initial tension crack is formed, under the action of tension stress, the surrounding rock cracks along the horizontal single direction of energy accumulation to form six tension cracks, and the tension stress concentration perpendicular to the crack surface occurs at the crack tip. With continuous propagation of the blast-generated stress wave, the crack tip continues to break and open, resulting in continuous expansion of the crack along the direction of energy accumulation. When the spacing of the presplitting blasting hole is 400 and 500 mm, cracks develop and expand along the direction of the energy accumulation and connect with each other. A fully connected tension crack is formed between adjacent blastholes, and the shaped charge blasting effect is good. When the spacing of the presplitting blasting hole is 600 mm, the tensile crack of adjacent blastholes will stop growing and expanding without extending and penetrating because the strength of the blast-generated stress wave is continuously attenuated during the propagation process, and the tensile stress generated by this wave is continuously reduced. When the tensile stress is lower than the tensile strength of the surrounding rock, the tensile crack stops developing and expanding. According to the results of the numerical simulation, and taking into account the hole construction and blasting cost benefits, the optimum hole spacing is 500 mm.

3.4 Engineering application

According to the above analysis, under the geological conditions of the C2 coal seam in the Yuwang Coal Mine, the best presplitting blasting height is 6 m, the best angle of presplitting blasting hole is 15°, and the best spacing of the presplitting blasting hole is 500 mm. In order to verify the rationality of the key parameters of gob-side entry retaining by roof cutting, the 1010201 ventilation roadway was selected for a field test. In order to determine the effect of directional presplitting blasting, one observation hole was left in the middle of every five presplitting blasting holes for blasting without charging. The pre-splitting blasting holes arrangement and the effect of blasting are shown in Fig. 20.

Fig. 20
figure 20

a The pre-splitting blasting holes arrangement. b The effect of blasting

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As shown in Fig. 20, after presplitting blasting, the hole was sealed within 0–1.5 m, and no directional cracks formed on the hole wall. Within a range of 1.5–6.0 m from the blasting hole, the through-fracture was obvious, which indicated that the effect of presplitting blasting was good. According to the field observations, under the action of presplitting blasting, the roof strata in the gob would fall with mining, with no obvious phenomenon of a suspended roof. As shown in Fig. 19, within a range of 10–20 m behind the working face, the collapsed gangue was able to fill the gob. The field test results showed that presplitting blasting is reasonable with a presplitting blasting height of 6 m, a presplitting blasting hole angle of 15°, and presplitting blasting hole spacing of 500 mm (Fig. 21).

Fig. 21
figure 21

The fall of the gangue in the gob

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4 Conclusions

Based on the engineering background of the Yuwang Coal Mine, the key parameters of gob-side entry retaining by roof cutting in a close-distance seam group were analyzed, and the following conclusions were obtained through a theoretical analysis, engineering analogy, numerical simulation, and field test:

  1. 1.

    When the presplitting blasting height is 6 m, and the angle of the presplitting blasting hole is 15°, the roof strata easily collapse, and the collapsed gangue can fill the gob, which plays an effective support role for the overlying strata.

  2. 2.

    When the spacing of the presplitting blasting hole is 500 mm, a through-fracture can form under the action of the stress wave, which can ensure the collapse of the roof strata after the working face is mined.

  3. 3.

    According to the field test, under the conditions of the above design parameters, the roof strata will collapse with mining, and the gob can be filled within a range of 10–15 m behind the working face. The effect of pressure relief by roof cutting was also found to be good.