Research on the Mechanism of Main Roof Advanced Breaking and Supporting Technology When Long-wall Face Passing Through Abandoned Roadways

: Unlike general long-wall mining, the roof activity is more intense when longwall face passes through the abandoned roadway. Technically, the coal pillar between the abandoned roadway and the long-wall face will suddenly fail with a certain critical value of its width, leading to the roof breaks in advance and other production-restricted problems because of the support loss, which will be a great threat to underground mining activities. In order to guarantee a safe mining condition, therefore, it is greatly necessary to uncover how the roof breaks in advance and how to cope with it. From the stability maintaining of the key block perspective, this paper took for research that the 12404-1 long-wall face of Wulanmulun coal mine, China. The critical value of the coal pillar’s width was determined to be about 5m by theoretical analysis, likely, the appropriate support force of the abandoned roadway’s roof is about 4020KN per meter. Meanwhile, a numerical simulation method was adopted to study the ground pressure when the longwall face passing through the abandoned roadway. Correspondingly, a compound supporting technology involving the roof presplit technique, anchor cable supporting and pumping pillar supporting were proposed for the roof of the abandoned roadway, and it practically worked well.


Introduction
In China, underground mining plays an important role in the exploitation of coal resources, more than 3 billion tons of coal is extracted from underground mines every year, and thousands of kilometers roadway is tunneled to extract such amount of coal. These roadways play a functional role in the coal mine production: a few are used for transportation while others are used for ventilation (FEM-DEM numerical analysis of support design when long-wall face strides across and passes a cross-cut) [1]. Some of these roadways would be abandoned when reached their servicing age. Therefore, it is inevitable to encounter a situation in which these roadways lay in the range of the long-wall face, and the long-wall face must pass through these abandoned roadways for a positive production [2].
However, there are differences in strata behaviors of whether the long-wall face passes through the roadway. In this case, a series of problems will arise which would terribly restrict the production efficiency of the long-wall face. Numerous engineering practices have verified that, the coal pillar located between the long-wall face and the abandoned roadway would suddenly fail when the width of the coal pillar reaches a critical value. Meanwhile, the main roof may break in advance due to the support loss from the coal pillar, resulting in coal rib spalling, support crushing, roof falling, etc. Undoubtedly, these issues could be a great threat to the safety of mining activities. In this area, where stronger ground pressure would occur, many studies have been initiated in the past years [3][4], Li et al [5] and Gu et al [6] studied the relationship between the supports and surrounding rock when the long-wall face passing through the abandoned roadway. Xie et al [7] studied the ground pressure mechanism when the long-wall face passing through the abandoned roadway. Bai et al [8][9] and Zhao et al [10] established a mechanical model of the roof advanced breaking when the longwall face passing through the abandoned roadway. Gong et al [11][12][13][14] analyzed the influencing factors of the roof's advanced breaking when the long-wall face passing through the abandoned roadway. Respectively, AAM (adjusting the angle method) to pass through the abandoned roadway, FRT (filling the roadway technique) to control the roof of abandoned roadway, were proposed in these studies above.
Despite those previous researches and the evidence of the existence of advanced breaking, the advanced breaking mechanism of the main roof is still unclear, which hinders the management of support resistance. Moreover, the key block stability of the broken roof strata and its influence on the long-wall face surrounding rock need further analysis and research. In addition, better control of the surrounding rock also is key for safe mining [15][16][17]. Therefore, it is very important to propose a corresponding supporting technology to the roof of the abandoned roadway.
Considering these issues remained, in this paper, the mechanism of the roof advanced breaking is revealed and a theoretical model is established to analyze the stability of the key block when the long-wall face passes through the abandoned roadway; the ground pressure around the long-wall face and abandoned roadway is further studied by DEM numerical simulation; a new abandoned roadway supporting technology is proposed to control the stability of the long-wall face; and to verify this new support design, the support resistance is monitored when the long-wall face passing through the abandoned roadway.

Engineering background
Wulanmulun coal mine is located in the east of Ordos Plateau, inner Mongolia province, China. Because of the production needs, a 12404-1 long-wall face must be set between the 12404-1 tailgate and the 12404-1 headgate, on the west panel of the 12# coal seam. The average thickness of #12 coal seam is 2.51 m, the coal seam dip is 1°to 3° and the average burial depth of #12 is 115.24m. The width of the 12404-1 long-wall face is 113m, the mining height is 2.8m, and the advancing length is 542.7m. As shown in Fig. 1. The new long-wall face must pass through eight abandoned roadways to complete the production.  According to the statistics, all these abandoned rectangular roadways were dug with a width of 5m, and the height is 3m. The surrounding rock conditions of the 12404-1long-wall face are specifically shown in Table 1.

Determination of the critical value of coal pillar
The vertical stress ( y  , MPa) of the coal pillar is the overlap of the front abutment pressure and the pressure on one side of the roadway when there were abandoned roadways ahead. the pressure peaks at both sides of the coal pillar will overlap with the advancement of the long-wall face, which means that y  of the pillar reached the largest. The distribution of abutment pressure is shown in Fig. 2.

Fig. 2 Abutment pressure distribution
The distribution of abutment pressure of the long-wall face can be calculated with the following equation [18]: where 1 y  is the vertical stress from the long-wall face side, MPa; f is the interlayer friction factor; M is the mining height, m; x is the distance from a certain point in front of the long-wall face to the long-wall face, m;  is the internal friction angle, °.
Simplifying the roadway as circular, the tangential stress is the vertical stress around the roadway, that is [19]:  is the vertical stress from the roadway side, MPa;  is the bulk density, where L is the critical value of the coal pillar width, m.
Base on the Mohr-Coulomb curve, c  can be calculated with the following equation under unidirectional force: According to equation (4), y  is about 8.3 MPa. Therefore, it can be solved that the critical failure width of the pillar is about 5 m.

Stability analysis of KBs
In order to prevent a series of problems that restrict production and threaten safety, such as coal rib spalling, support crushing, roadway roof falling caused by the advanced breaking of the roof. From the maintenance of the stability of the KBs perspective, the behavioral law of the overlying strata is analyzed, which could provide a theoretical basis for the roof support.
The hinged rock block hypothesis proposed by a scholar of the former Soviet union Г.Н.κузнецов was referred to explain the mechanism [21]. Taking the key block B in Figure 4 as the analysis object, the mechanical balance model where KBs are hinged is established as shown in  Based on the "S-R" stability theory of masonry beam structure, the key block mainly has two modes of sliding instability and rotation instability [22]. To facilitate the analysis of the stability of the key block, we must make the following assumptions: 1) suppose P2 acts on c/2, P3 acts on d/2, RAB=RCB; 2) Key block B breaks at the limit equilibrium in the coal body. Referring to the balance equation on the force and torque, equations following are set: Approximates that the length of key block B is the sum of the abandoned roadway's width, the failed pillar's width and the length of the general key block [23], which is: Where T R is the tensile strength of the main roof, MPa.
Define x1=f+b+c, which can be calculated by the following equation [24], then f can be solved:  (11) To ensure that the key block B does not occur slipping instability and rotational instability, Shear force AB R and CB R need to meet inequality (13). Stress at the key block hinged joint  and the compressive strength of the rock block c  needs to meet inequality (12).
Now there are three unknowns (T, R, P4), but two effective equations (equation 5 and 7), base on the previous support experiences and the dichotomy mathematic method, we could try to validly give P4, when P4 was given to 4020 KN per meter, the TAB, RAB just satisfy both the inequality (12) and (13), which means key block B does not occur slipping instability and rotational instability.
To further grasp the stress distribution and deformation law of the rock mass around the stope and the roadway during the 12404-1 long-wall face passing through the abandoned roadway, the FLAC3D numerical simulation software is used to assist this study, which could provide a basis for the support design of the roadway.

Model establishment
According to the geological conditions of 12404-1 long-wall face, a numerical model which is divided into 116160 grids and 124821 nodes is established, as shown in Fig. 6. The size of the numerical model is 200 m×165 m×41 m. The numerical model adopts displacement boundary conditions on its surfaces (X, Y direction) and bottom surface (Z direction) to fix the horizontal direction (X, Y direction) and vertical direction (Z direction) displacement of the node at the boundary. And the upper surface of the model is the stress boundary condition. The value of stress equals to the gravity of the overlying strata, which is 2.5MPa, downward. The model adopts the Mohr-Coulomb failure criterion. The physico-mechanical parameters of coal and strata layers are shown in Table 2.

Vertical stress and plastic zone distribution of coal pillar
With the advancement of the long-wall face, the abutment pressure gradually moves forward, and the width of the coal pillar between the long-wall face and the abandoned roadway is continuously reduced. In Fig. 7(a), when the long-wall face is 7.5 m away from the roadway, the left side of the roadway begins to appear obvious stress concentration, reaching 6.7 MPa. In Fig.  7(b) and (c), when the width of the coal pillar goes 5 m, the vertical stress of the pillar begins to decrease, indicating that the coal pillar begins to break and loses its bearing capacity when the width reached 5m. Continually, when the long-wall face is 2.5 m away from the roadway, the vertical stress on the left side of the roadway will drop to 6 MPa -7 MPa. The vertical stress of the long-wall face increased rapidly.
With the continuous increase of the vertical stress of the coal pillar, the coal body is changed from the elastic to plastic and eventually lost its bearing capacity. Fig. 7(d)(e)(f) shown the distribution of the plastic zone of the rock mass around the roadway when the distance between the long-wall face and the roadway is 10 m, 7.5 m and 5 m respectively.

roof supporting technology of abandoned roadway
In order to ensure the safe production of the long-wall face during passing through the abandoned roadway, the roof presplit technique is used to cope with the advanced breaking, reducing the ground pressure at the long-wall face. under the initial support of the abandoned roadway and the theoretical calculation results, a compound support technology -the 22.4×8000mm anchor cable supporting and pumping pillar supporting -are added to control the roof of the roadway. As shown in Table 3 and Fig. 8. In Fig.8, the left one is the situation before long-wall face passing through the abandoned roadway, and the right one is after. The unobvious failure of the pumping pillar indicates that this kind of new support design well achieved the engineering purpose.

measurement of supporting effectiveness
The 12404-1 long-wall face adopted the Uroica ground pressure observation system for the monitoring. Fig. 9 shows the support resistance curve of 34#, 36# and 38# supports when the longwall face passing through abandoned roadway 2 and 3. When the long-wall face advances 154 m and passes through abandoned roadway 2, the roof breaks in advance, compared with no passing through abandoned roadway 2, the resistance of the support does not increase significantly. When the long-wall face advances 177 m and begins to pass through the abandoned roadway 3, the roof on the middle of the long-wall face breaks in advance again, except the 34# support which was set at the middle of the long-wall face, the highest of its resistance reached 470 bar, others were basically constant in the process. It shows that the pumping pillar has a better support effect on the roof of the abandoned roadway, it did not cause roof instability. (1) This paper analyzed the distribution characteristics of the vertical stress of the pillar, determined the critical width of the pillar is about 5m. The mechanism of the roof advanced breaking is revealed and a more refined theoretical model is established to analyze the stability of the key block when long-wall face passing through the abandoned roadway. Accordingly determined the support force for the roof of the abandoned roadway for 4020KN per meter.
(2) This paper adopted the mathematical method of controlling variables and dichotomy: that is, in the case of three unknowns (T, R, P4) and two equations. Firstly, validly assume the support force P4 according to the previous experience. Secondly, solve the horizontal force T and the shear force R for key block B, checking the effectiveness of T and R. Finally, verify whether T and R satisfy the sliding instability and rotation instability conditions, determining the support force of the roadway roof to maintain the stability of key block B.
(3) According to the geological conditions of Wulanmulun coal Mine, a model was established to simulate the vertical stress of the coal pillar and the plastic zone distribution of the surrounding rock when the long-wall face passing through the roadway. It is concluded that when the width of the coal pillar is about 5 meters, the bearing capacity of the pillar will drop dramatically, which is in good agreement with the theoretical analysis.
(4) According to the results of theoretical analysis and numerical simulation, a compound surrounding rock control technology is proposed during the long-wall face passing through the roadway, that is, pumping pillar supporting and anchor cable supporting. The monitoring analysis of ground pressure and support effectiveness have achieved the expectation.