*2.1. Principle of GERRCP*

As shown in Figure 1, the GERRCP adopts energy-gathered blasting technology to carry out advanced pre-splitting on the roof. The roof is cut off along the pre-splitting damaged structural surface through periodic weighting of the stope, and the fractured roof collapses naturally with the help of underground pressure. The side of the roadway is formed by the broken expanded characteristic of the collapsed gangue, and the flexible support body in a roadway is formed by the sliding and yielding gangue support structure and constant pressure retractable support equipment, which separates the goaf. At the same time, the high strength support of a roadway roof is formed by the constant-resistance large-deformation anchor cable (CRLDAC) with structural characteristics of negative Poisson's ratio, thus realizing the non-pillar mining of a single working face and single roadway [25,26]. This technology realizes the transformation of a roadway roof structure from a long-wall beam to short-wall beam by roof cutting, which ensures the stability of the roadway, can weaken the concentrated stress on the upper part of the coal body adjacent to working face, and can also avoid the roof collapse, rock burst and gas outburst caused by remaining pillars.

**Figure 1.** Section diagram of roof structure of gob-side entry retaining by roof cutting without pillar (GERRCP).

The mechanical model of the roof structure of GERRCP is introduced below (as shown in Figure 2). Under the action of periodic pressure, the immediate roof and main roof fracture and rotate. The main roof is fractured to form rock blocks A, B and C, and the interaction between the rock blocks forms a hinge structure. Rock block A is still supported by the immediate roof, which is relatively stable. Rock block C is supported by the gangue on the side of the goaf, and its stability is poor. Both ends

of rock block B are supported by the immediate roof and gangue in the goaf, respectively, and rotate towards the goaf around the elastic-plastic boundary of solid coal. The following assumptions are made for the mechanical model of surrounding rock: (1) there is no interaction between rock block B and C and the gangue at the side of the goaf; (2) the shear force between rock strata such as immediate roof and main roof is ignored; (3) the supporting capacity of coal body in the lateral plastic zone of the retaining roadway is not considered; (4) the supporting force at the side of the roadway is neglected. The structural mechanical model established according to the assumed conditions is shown in Figure 2.

**Figure 2.** Mechanical model of roof structure of GERRCP.

The above picture is explained as follows. The key parameters of the system are as follows. After the rock strata fracture, the fracture length of key block B formed is [27]:

$$L = L\_s \left( \sqrt{\frac{L\_s^2}{L\_f^2} + \frac{3}{2}} - \frac{L\_s}{L\_f} \right) \tag{1}$$

where, *L*: length of rock block; *Ls*: weighting interval of the immediate roof; *Lf*: working face length. The horizontal force on rock block B is

$$H\_b = \frac{qL}{2(h - S\_b)}\tag{2}$$

where, *Hb*: horizontal force on rock block B; *q*: uniform load acting on the main roof; *h*: the thickness of basic roof; and *Sb*: the subsidence of rock block B.

The research on the mechanism of arch effect and its boundary conditions was studied, and the calculation of the plastic zone was referenced, and the width of the stress limit equilibrium zone in coal body was obtained [27,28].

$$\mathbf{x}\_0 = \frac{h\_r A}{2t m \eta \rho\_0} \ln \left( \frac{k \gamma H + \frac{c\_0}{t m \eta \rho\_0}}{\frac{c\_0}{t m \eta \rho\_0} + \frac{p\_x}{A}} \right) \tag{3}$$

where, *x*0: width of stress limit equilibrium zone in coal body; *hr*: roadway height; *A*: lateral pressure coefficient; *C*0: cohesion of the interface between coal and rock; ϕ0: internal friction angle; *K*: stress concentration factor; γ: average bulk density of overburden; *H*: roadway burial depth; *Px*: support resistance of coal sides.

After mining, the main roof structure breaks under periodic pressure, forming a hinge structure, and the structure is in equilibrium. Through static analysis of hinge structure under this equilibrium condition, the static equilibrium equations of rock block C and B are established, such as Formulas 4 and 5. Among them, the support force provided by the support body in the roadway is simplified

*Processes* **2019**, *7*, 552

as the load collection degree of support, and the support load in the roadway is solved based on the above analysis.

(1) Rock block C:

$$
\Sigma X = 0,\\
H\_b - H\_c = 0 \tag{4}$$

$$
\Sigma Y = 0,\\
qL + V\_b - V\_c = 0\\
\Sigma M\_{B'} = 0,\\

where, *Hc*: horizontal force on rock block C; *Vc*: the vertical force on rock block C; *Vb*: the vertical force on rock block B; *M*2: moment of rock block C at section B; and *Sc*: the subsidence of rock block C.

(2) Rock block B:

$$\begin{aligned} \Sigma M\_{A'} &= 0, -M\_1 - M\_3 + q^\nu L\_R(\mathbf{x}\_0 + L\_R/2) + H\_b(h/2 - \mathbf{S}\_b) - qL^2/2 + M\_2 + V\_b L + q'(\mathbf{x}\_0 + L\_R + h'\tan a) \\ \left[ \left(L\_R + \mathbf{x}\_0 \right)^2 + \left(\mathbf{x}\_0 + L\_R + h'\tan a \right)^2 + \left(L\_R + \mathbf{x}\_0 \right)(\mathbf{x}\_0 + L\_R + h'\tan a) \right]/3 \left(2\mathbf{x}\_0 + 2L\_R + h'\tan a \right) = 0 \end{aligned} \tag{5}$$

where, *M*1: moment of rock block B at section A; *M*3: moment of immediate roof to basic roof; *q*": load collection degree of support in the roadway; *LR*: roadway width; *q* : the uniform load acting on the immediate roof; *h* : the thickness of immediate roof; and α: pre-cracking roof cutting angle.

(3) The load collection degree of support in the roadway can be obtained simultaneously.

$$q'' = \left\{ \begin{array}{c} M\_1 + M\_3 - 2M\_2 + qL^2 - qL(h - 2\mathbf{S}\_b)/4(h - \mathbf{S}\_b) - q'(\mathbf{x}\_0 + L\_R + h'\tan a) \\ \left[ \left(2\mathbf{x}\_0 + 2L\_R + h'\tan a\right)^2 - (\mathbf{x}\_0 + L\_R)(\mathbf{x}\_0 + L\_R + h'\tan a) \right]/3(2\mathbf{x}\_0 + 2L\_R + h'\tan a) \end{array} \right\} / L\_R(\mathbf{x}\_0 + L\_R/2) \tag{6}$$

Based on the new technology of GERRCP, the mechanical model of the roof structure is established. Through the mechanical analysis of the model, the key parameters such as fracture length of key blocks in upper strata and horizontal force acting on it are introduced, and the extension depth of the plastic zone in the solid coal side of the roadway under this technical condition is obtained, which provides certain theoretical support for the support design of the solid coal side of the mining roadway. In addition, through the static analysis of the balanced hinge structure under the condition of GERRCP, the corresponding static equilibrium equation is established, and the support load in roadway under this condition is obtained, which provides corresponding theoretical support for the support problem of mining roadway.

#### *2.2. Technical Process of GERRCP*

The mining mode layout of the traditional "121" construction method is shown in Figure 3a. When the mining system is used for coal mining, pillars are left, which belongs to the mining method of "one working face and two roadways". Different from the traditional mining method, the GERRCP is shown in Figure 3b, which is a typical single working face and single roadway without a pillar mining method. That is to say, the up and down drifts on the first face should be excavated firstly, and then at the same time in the working face of the mining, the retaining roadway, as the transport roadway on the next working face, is formed through the reinforcement of the advance anchor cable, pre-cracking roof cutting and gangue support at the side of the goaf. Therefore, the mining ratio is reduced and non-pillar mining is realized.

The technological process of GERRCP is shown in Figure 4. Its core can be summarized as four steps: strengthening, cutting, protecting and closing, that is: (1) adopt the CRLDAC to actively strengthen the supporting roadway roof according to the designed supporting parameters (Figure 4a); (2) the energy-gathered pre-cracking blasting hole shall be constructed at a certain distance in advance of the working face, and the bidirectional energy-gathered pre-cracking blasting shall be carried out according to the parameters determined by the blasting test to form a slit on the roof at the side of the goaf (Figure 4b); (3) after mining at the working face, the sliding and yielding gangue support structure and the constant pressure retractable support equipment are adopted behind the working face to strengthen the support in time. Under the action of the underground pressure, the roof at the side of the goaf collapses along the structural surface of the roof cutting slit to form the side of a new roadway (Figure 4c); (4) the caving roof is gradually compacted as the working face advances, and the side of the roadway formed by caving is shotcreted to close the goaf. After the roadway is stabilized, the temporary support equipment is removed to realize the retaining new roadway (Figure 4d).

**Figure 3.** Layout of mining mode. (**a**) Layout of traditional mining mode; (**b**) Layout of non-pillar mining mode.

**Figure 4.** *Cont.*

**Figure 4.** Technological process of GERRCP. (**a**) Strengthening roadway roof by constant-resistance large-deformation anchor cable (CRLDAC); (**b**) Pre-cracking roof cutting by energy-gathered blasting; (**c**) Gangue support; (**d**) Closing the goaf by shotcreting.

### **3. Engineering Background**

The coal seam in the 9101 working face of Xiashanmao coal mine is located in the lower part of Taiyuan Formation. The thickness of the coal seam is 1.55–3.5 m, and the designed mining height is 3 m, which belongs to the medium-thick coal seam mining face. The dip angle of coal seam is 2–8◦ and the buried depth is 180–260 m. The distance between 8 # coal seam and 9 # coal seam is 11.20–19.60 m, with an average of 15.24 m. The immediate roof is mudstone, with an average thickness of 4.1 m; the basic roof is mainly sandy mudstone with an average thickness of 8.0 m, according to the drilling measurement on the roof and floor of the working face. The immediate and basic bottoms are mudstone and fine sandstone, respectively, as shown in Table 1.



The test face is the first working face of the first mining area of 9 # coal seams, with a strike length of 480 m and an inclination of 150 m (as showed in Figure 5). The roof is managed by all caving method, which adopts full-seam, comprehensive mechanized, and retreating mining methods. The average 12 m above the 9101 working face is the 8102 working face, which serves as the mining protective layer of 9101 working face. Among them, the 8102 working face adopts the "121" construction method of longwall mining, and the 9101 working face adopts the self-formed roadway without a pillar-mining system, and the mining direction is vertical intersection. The adjacent face is the 9102 working face, which is located at the south of the 9101 working face. The test roadway is the ventilation roadway of the 9101 working face. The roadway section is rectangular, with a width of 4000 mm and a height of 3200 mm.

**Figure 5.** Layout of test working face of GERRCP in Xiashanmao coal mine.
