*2.2. Strong Strata Behaviors Survey*

Panel 42106 of coal seam 4-2 is adjacent to panel 42107, as shown in Figure 1. During the mining of panel 42106, the leading area of 42106AHR (reserved roadway of panel 42105) appeared to manifest strong strata behaviors, and the influence range could reach 100 m ahead. Strong strata behaviors' manifestation seriously affected the normal mining of the working face, as shown in Figure 2.

**Figure 2.** Strong strata behaviors' manifestation: (**a**) the hydraulic bracket was buried, (**b**) heaving floor and hydraulic bracket tilt, (**c**) ribs bulge and anchor broken, and (**d**) nets bag and leakage of gangue.

The working resistance of the hydraulic bracket was greater than 23,000 kN, and the coal wall flakes were serious during periodic weighting of the working face. The floor vibrated and caused the coal cutter to pop up and led to the overhead hydraulic bracket being buried, as shown in Figure 2a. The roadway in front of the working face shows the heaving floor and hydraulic bracket tilt, as shown in Figure 2b. The roadway adjacent to the goaf (coal pillar 25 m) was severely deformed within 40 m in front of the working face, with ribs bulging greater than 2 m and the heaving floor greater than 1.5 m, as shown in Figure 2c. The anchor net broke, resulting in the nets bag and leakage of gangue, as shown in Figure 2d.

#### **3. Mechanism and Treatment Principle of Strong Strata Behaviors**

#### *3.1. Main Controlling Factors of Strong Strata Behaviors*

#### (1) Geological conditions

The roof of coal seams 4-2 in Burtai Coal Mine is thicker and stronger, with strong self-sustaining ability. The upper part of the goaf is prone to forming a cantilever roof, which increases the fracturing step of the basic roof. This leads to an increase in the elastic energy accumulated before the roof breaks. In the process of a hard roof breaking or sliding, the elastic energy stored in the roof is prone to suddenly release, inducing strong strata behaviors disaster [27]. When the buried depth of the coal seam is greater than 350 m, the frequency and intensity of strong strata behaviors will gradually increase. The buried depth of coal seam 4-2 is 457 m, which is the internal factor of strong strata behaviors' manifestation.

(2) Influence of mining stress

Under the influence of repeated mining, the reserved roadway is located in an extremely complex superimposed stress field composed of original rock stress, abutment pressure, mining dynamic load, and expansion pressure in the plastic zone [28]. When the roof periodically intensifies, the position of the fracture line will affect the manifestation of strata behaviors. If the fracture line is located in the goaf, a cantilever beam is formed at the edge of the pillar. The pressure from the insufficient span of the overlying strata in the goaf is transmitted to the coal pillar through a cantilever beam. If the fracture line is located inside the coal pillar, the rotational deformation of the basic roof on the coal pillar will transmit several times the initial stress to the coal pillar. All these are not conducive to the stability and maintenance of the roadway.

When 42106AHR (reserved roadway) is subjected to secondary mining, it is 3~5 times of original rock stress under the influence of advanced abutment pressure, abutment pressure of side mined-out area, and concentrated stress of residual coal pillars in coal seam 2-2. Under the disturbance of multiple stress superposition, it will cause rapid deformation and failure of the mining roadway, which can easily lead to the occurrence of strong strata behaviors' manifestation [29,30].

(3) The impact of mining spatial relationship (coal pillar in overlying goaf)

The goaf of coal seam 2-2 is located 75~80 m above panel 42106 of coal seam 4-2. The siltstone between the two coal seams is thick and hard. The horizontal projection distance between 42106AHR and the overlying coal pillar (the coal pillar between panel 22104 and panel 22105) is 43.5 m. The fault protection pillar of coal seam 2-2 also has an important influence on the stress of the lower strata [5]. Stress concentration occurs in a certain range below the coal pillar, and the difference within stress distribution is greater as it gets closer to the edge of the pillar. The spatial location relationship of mining is shown in Figure 3.

**Figure 3.** The spatial position relationship between coal seam 4-2 and coal seam 2-2.

#### *3.2. Technical Principle of Disaster Prevention and Control of Strong Strata Behaviors*

The integrated hydraulic fracturing of the coal seam roof is similar to the artificial creation of a "relief layer" in advance. After fracturing above the working face, the overlying strata produce fractures within the fracturing range. These fractures destroy the previously intact overburden structure within the fracture range. When the overburden load acts on the fractured area, the fractured rock mass is compressed to fill the fracture, which reduces the elastic deformation of the fractured rock mass. Therefore, after fracturing, the support of the upper roof is reduced due to the deformation of rock mass within the fracturing range, as shown in Figure 4. Pan et al. have carried out a detailed theoretical derivation according to the ground prefracturing coal seam roof, and the research results can be a certain reference value for the paper [31].

**Figure 4.** Stress boundary conditions of coal seam in working face after fracturing, where σ<sup>0</sup> is the stress of primary rock, σ<sup>1</sup> is the stress in the unloading zone of the fracturing range, and σ<sup>2</sup> is the maximum stress in the pressurized zone at the fracturing boundary.

In order to obtain the stress control effect of hydraulic fracturing, a numerical mining model (FLAC 3D) was established, and the calculation results are shown in Figure 5. When hydraulic fracturing technology is adopted, the goaf fully collapses, and the caving line overlaps with the pressure relief line, which is distributed along the hydraulic fracturing hole. The maximum vertical stress in the area affected by mining is 56.15 MPa. When hydraulic fracturing is not implemented, we set a 20 m overburden overhanging area at the goaf boundary. The maximum vertical stress in the area affected by mining is 64.09 MPa. After hydraulic fracturing, the peak vertical stress was reduced by 7.94 Pa. The vertical stress in the coal pillar decreases obviously.

Compared with before fracturing, artificial cracking caused by a thick hard roof on the coal seam of the working face roof can make long beams become short beams and large pieces become smaller. Thus, the hard roof no longer has the cantilever function. The impact risk of the dynamic load caused by roof fracture with an increased overhanging area is reduced. At the same time, the hard roof can collapse and fill the goaf in time after fracturing, which reduces the high stress concentration in the coal pillar and also reduces the concentrated dynamic load caused by the roof pressure. It can be seen that hydraulic fracturing can significantly weaken the risk of a dynamic disaster of strong strata behaviors.
