In order to study the key parameters of gobside entry retaining by roof cutting in closedistance 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.
2.1. Theoretical calculation
(1) The presplitting blasting height
The maximum vertical distance between the slit formed after directional presplitting 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:
HP = (HM  HR  HF) / (K − 1) (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) The angle of presplitting blasting hole
Before the working face is mined, presplitting 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.
The condition of rock stability is as follows:
[T cosα + (R  F) sinα]∙tanφ=(R  F) cosαT sinα (2)
So α = arctan(( R – F ) ⁄ T )  φ
$$\left\{\begin{array}{c}f=\tau \times h\times 1\\ R=\rho ghd\end{array}\right.$$
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° to 15°, the roof strata of the roadway can remain stable.
(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.
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 presplitting blasting, the damage cracks formed after the blasting of two adjacent presplitting blasting holes should be connected. Therefore, the spacing of the presplitting blasting hole should meet the following relationship:
d ≤ 2(rb+Rs) (5)
substituting Eq. (4) into Eq. (5) can be obtained as follows:
$$d\le 2{r}_{b}\left[1+{\left(\frac{10\lambda {P}_{b}}{\left(1{D}_{0}\right){\sigma }_{t}+{\sigma }_{0}}\right)}^{\frac{1}{\alpha }}\right]$$
6
According to the actual conditions of the 1010201 ventilation roadway, the field parameters were chosen as follows: rb=0.02m, λ = 0.9, Pb=200MPa, D0 = 0.05, σt = 2.5MPa, σ0 = 8.6MPa, α = 2. According to the aforementioned theoretical calculation, the spacing of the presplitting 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 LSDYNA.
(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.
Lithology

Bulk
(GPa)

Shear
(GPa)

Friction
(°)

Tension
(MPa)

Density
(103kg/m3)

Cohesion
(MPa)

Overlying strata

13.1

18.5

36

3.92

2.5

3.88

Muddy siltstone

9.7

14.5

33

1.92

2.1

1.88

Silty mudstone

12.1

20.5

36

2.92

2.3

3.25

Carbon mudstone

10.1

18.5

34

2.32

2.1

2.28

Muddy siltstone

9.7

14.5

33

1.92

2.1

1.88

C2 coal seam

8.1

12.2

31

1.69

1.4

1.71

Muddy siltstone

9.7

14.5

33

1.92

2.1

1.88

C3 coal seam

8.1

12.2

31

1.69

1.4

1.71

Silty mudstone

12.1

20.5

36

2.92

2.3

3.25

Siltstone

11.1

15.5

35

2.52

2.2

2.55

Construction of numerical model based on geological conditions of Yuwang Coal Mine. The strike length of the model is 270m, the dip length is 200m, and the height is 60m. The length of the working face is 200m, and the width of the mining roadway is 5m. In order to avoid the impact of excavation disturbance, a 30m 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 8.6MPa on the upper boundary. The numerical model is shown in Fig. 4.
(2) LSDYNA
The blasting numerical model is composed of rock unit, air unit, explosive unit and energygathering pipe unit. The inner diameter of the energygathering 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 nonreflecting boundary condition. The material parameters of the model unit are shown in Table 3 and Table 4.
Table 3
Parameters of the explosive.
Density
(g/cm3)

Detonation
Velocity (m/s)

Ferocity
(mm)

Detonation
Transmission (m)

Diameter (mm)

Power (mL)

AntiExplosive
Coupling (cm)

1.1

2600

≥ 10

≥ 50

42

≥ 250

≥ 3

Table 4
Parameters of the energygathering pipe.
Density(g/cm3)

Bulk (GPa)

Shear (GPa)

Yield Stress (MPa)

Poisson Ratio

1.43

43

3.2

≥ 50

42

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 energygathering 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.