*3.3. Technical Basis for Controlling Strong Strata Behaviors*

To avoid the occurrence of strong strata behaviors' manifestation in 42107AHR and 42107 working face during the mining of panel 42107, we decided to adopt hydraulic fracturing technology to relieve concentrated stress according to the above analysis. According to the relevant literature and our rich engineering practice experience, hydraulic fracturing roof strata to reduce the strong mining pressure in stope should be based on the key stratum theory of rock strata control. The working condition of the project site is very complicated (the overlying coal seam 2-2 is mined out, but there is 300~360 m fault protection coal pillar locally, the coal pillar is 20 m in the section of the coal working face of 2-2 coal, and the overlying strata are also affected by the fault, all of which lead to a complex overlying rock stress environment. In addition, it is difficult to establish a realistic theoretical model due to the large number, thickness variation, and strength difference of rock strata), as shown in Figure 3. Therefore, we only establish a simplified theoretical model under ideal conditions for schematic analysis. There are some differences between the calculation results of this model and the actual working conditions, but it can be used for reference.

The key strata control the deformation movement of local and whole strata, and the mechanical properties of the key stratum are important factors affecting the periodic breaking step. The stress state of the key stratum before breaking and turning was simplified into a pure bending fracture model of the beam structure, as shown in Figure 6. According to Figure 4, the relationship between the tensile strength of the key stratum and its periodic breaking step can be qualitatively analyzed.

**Figure 6.** Mechanical model of the elastic beam structure.

According to the mechanics of materials, the maximum normal stress of a pure bending beam occurs at the furthest distance from the neutral axis. The relationship between the maximum tensile stress and the maximum bending moment can be obtained as shown in Equation (1).

$$
\sigma\_{\text{max}} = M\_{\text{max}} h / (2I\_{\text{x}}) \tag{1}
$$

where *σ*max is the maximum tensile stress (MPa), *M*max is the bending moment (kN·m), Iz is the moment of inertia for the *Z*-axis in the rectangular section (m4), and h is the thickness of the key stratum (m).

$$M\_{\text{max}} = -qy\_{\text{max}}^2 / 2 \tag{2}$$

where *q* is the interface compressive stress on the key stratum (MPa), and *y*max is the maximum roof spacing (m).

$$I\_{\mathbf{z}} = bh^3 / 12 \tag{3}$$

where *b* is the width of the rectangle interface (m). Take *b* = 1 m, substitute Equations (2) and (3) into Equation (1), and the relationship between the periodic breaking step distance of the key stratum and its tensile strength can be obtained. If *σ*max = RT, then:

$$y\_{\text{max}} = h \sqrt{\frac{RT}{3q}}\tag{4}$$

where RT is the tensile strength of the key stratum (MPa).

According to Equation (4), the larger the tensile strength of the key stratum is, the larger the maximum suspended roof distance is. In the process of panel propulsion, too large overhead distance will make the hydraulic bracket and coal body bear more load. This will lead to working face pressure frame, wall caving, and roadway deformation. If the strength of the key stratum can be weakened, the strong strata behaviors can be controlled to a certain extent. Therefore, the selection of a hydraulic fracturing horizon is the key to controlling strong strata behaviors.

In the mining process of panel 42107, subcritical stratum 1 and 2 will form the fracture structure of "cantilever beam + masonry beam", which will affect the stope strata behaviors. According to the geological parameters of the overlying strata of coal seam 4-2 combined with Equation (4), it can be calculated that the periodic weighting distance of subcritical stratum 1 is 36.75 m, and that of subcritical stratum 2 is 53.25 m. Subcritical stratum 2 is the key to influencing the stope strata behaviors.

#### **4. Hydraulic Fracturing Technology and Scheme**

#### *4.1. Design Basis of Hydraulic Fracturing Program*

Many scholars have studied the law of strata behaviors in the process of advancing the working face [32–35]. They obtained the "square position" characteristic of the working face [36]. That is, when the advancing length of the working face is an integer multiple of the width of the working face, the strata behaviors will be more severe [37,38].

From the perspective of "square position", we should design a long fracturing borehole when the panel advances to integral multiples of the width of the working face. The design can promote the full collapse of the hard roof and weaken the "square position". The vertical height of the long fracturing borehole should be close to the upper boundary of the siltstone (subcritical stratum 2) to ensure its collapse. The horizontal width of the long fracturing borehole should ensure that the fracture range covers the width of the working face. The short fracturing borehole is designed to prevent cantilever beams from forming in the hard rock above the lateral coal pillar. The short fracturing borehole shall be at least 10 m within the siltstone bed. The horizontal length of the borehole is determined according to the depth of the borehole and the construction angle of the drilling equipment.

The spacing of the fracturing borehole is set according to the effective fracturing radius of the borehole [39,40]. The fracturing radius of the borehole is 3.2~4.3 M when the water injection pressure is 12~16 MPa under the condition of crustal stress and surrounding rock strength. Therefore, 1/3 (7 m) of the average periodic weighting was selected as the fracturing borehole spacing. In this way, sufficient fracturing can ensure that the edge of the goaf caved in time after mining, so as to reduce the stress concentration.

#### *4.2. Parameter Design of Implementation Scheme*

Based on the above analysis, hydraulic fracturing drilling holes were arranged to the roof through 42107AHR, 42107BTR, and 42108 AHR. The vertical depths of the long and short holes are 38 m and 30 m, respectively. The drilling hole opening is about 2.2 m away from the floor.

Single-hole multiple fracturing technology is adopted, and the part of drilling in the coal seam is not fractured. The fracturing boreholes are arranged before mining, and the fracturing operation is carried out ahead of the working face in the process of mining. The sealing pressure of hydraulic fracturing is 12~16 MPa. The hydraulic fracturing roof fracturing process is divided into three steps, that is, drilling, sealing, and fracturing, respectively.

The hydraulic fracture drill holes were drilled by ZDY1200S fully hydraulic drilling rig along the right rib of 42107AHR (Fracturing drilling A), the left rib of 42107BTR (Fracturing drilling C), and 42108AHR (Fracturing drilling B) to the roof of panel 42107, respectively, with a diameter of 56 mm, as shown in Figure 7.

**Figure 7.** Hydraulic fracturing drill layout plan of hydraulic fracture drill holes.

In 42107AHR, a total of 18 hydraulic fracturing drill holes with a length of 150 m are arranged. The six drill holes in the first group are arranged within the range of 80~140 m from the open-off cut of panel 42107, as shown in Figure 6a. In 42108AHR, a total of 15 hydraulic fracturing drill holes with a length of 150 m are arranged. The three drill holes in the first group are arranged within the range of 120~140 m from the open-off cut of 42107, as shown in Figure 6b.

The second group of three drill holes is 300 m away from the open-off cut, and then each group is 300 m away from the other. The spacing of the drill holes in the group is 7 m, the drilling angle is 17◦, and the vertical height of the drill holes is 38 m. In 42107BTR, a total of 65 hydraulic fracturing drill holes with a length of 43 m are arranged in the range of 560~1080 m away from the open-off cut, the drilling angle is 51◦, and the vertical height of the drill holes is 30 m, as shown in Figure 8c. The area where the borehole is located in

the coal seam will not be fractured (red section). To prevent the loose gangue from leaking in front of the hydraulic bracket, the yellow section of the borehole will not be fractured. The green part is fractured every 3 m, and the fracturing duration is 10–15 min.

**Figure 8.** Schematic diagram of hydraulic fracturing technical scheme parameters: (**a**) parameters of hydraulic fracture drill holes in 42107AHR, (**b**) parameters of hydraulic fracture drill holes in 42108AHR, and (**c**) parameters of hydraulic fracture drill holes in 42107BTR.

#### **5. On-Site Industrial Test Monitoring**
