Movement Law of Overlying Strata with Loading Effect in the Large Space Structure of Deep Mines

Under the action of high-stress field, overlying strata with load characteristics in large space structure formed by changes of working faces in deep mines intensifies the movement of the overlying strata on the working face, which may easily cause dynamic disasters at the working face. Considering the large space structure of deep mines and taking gob-side entry retaining as a breakthrough point, this study analyzes various types of overlying strata structures and their loading effects. The concept and differentiation method of deep “three layers” are introduced. The movement of the overlying strata with loading effect is analyzed by establishing a large-scale Flac3D numerical calculation model and on-site inspection. The results indicate that the “arch” structure formed by the working face shifts to the middle and rear parts of the working face under the influence of the large space structure, and the overlying strata continuously evolves upward during this process. Finally, the “arch” structures of the working face are integrated within the overlying strata to form a symmetrical structure. In the deep “three layers,” “Mk” provides the force source for normal weighing of the working face. “Mj” shifts the stress relief of the working face to the middle–rear position, which may easily cause strong strata pressure at the working face. The uniformly distributed load of “My” ensures the stress relief position at the working face. Through on-site monitoring, the disturbance range and stress of Working Face 418 with large mining height are greater than those of Working Face 416. The changes in pressure relief positions of the working face are as follows: ventilation roadway > middle > intake roadway.


Located in the middle part of Huanglong Mining Area, Huangling No.2 Coal Mine in Shaanxi
Province is the main producing mine, with a production capacity of 8.0 Mt/a. The preliminary survey and exploration results of the mine indicate that with general dip angles of 1° to 5° within the mining range of Panel 4, 2# coal seam is stable to relatively stable, as shown in Figure 1. Within the panel, the surface elevation ranges from +1157 m to +1364 m, the underground elevation ranges from +711 m to +732 m, and the average burial depth is about 530 m. The occurrence of coal seam is characterized by the overlying strata with interlayered fine sandstone and siltstone. A 183.7 m-thick medium sandstone is found in the upper part of the overlying strata. The characteristics of surrounding rock are shown in Table 1.  Mainly composed of quartz and feldspar, with argillaceous cementation, dotted mica flakes, and dark minerals with wavy bedding, long columnar core, and RQD value of about 56.1%.
Mudsto ne 0 to 4.69 Contains numerous horizontal beddings and small cross beddings, characterized by structural planes such as joints, fissures, and sliding surface. Three working faces of 414, 416, and 418 are stoped in sequence. For each working face, the strike length is 2632 m and the inclined length is about 300 m. The average mining heights of Working Faces 414, 416, and 418 are 4.0, 6.0, and 6.0 m, respectively. The safety coal pillar reserved at the working face is 40 m. The dimensions of the intake and ventilation roadways are 4.6 m × 3.8 m and 5.4 m × 3.6 m, respectively. The retreating longwall mining method with full-seam mining is adopted, and Working Faces 416 and 418 use 175 ZYT12000/28/63D supports in total.

Catastrophe analysis
With one-way mining in the panel, the working faces are generally divided into three types, namely, first mining, GER, and island mining faces ( Figure 4). After the first mining face is mined, the unilateral GER of the next working face is formed. Along with the changes in working face, the overlying strata move within a relatively large range. The large space structure is developed by the overlying strata disturbance ranges of the stope with large mining height and GER. During mining, the stress and energy storage of the mining strata vary with time and mining positions, and the released energy level and transfer range increase, which intensifies the development of surrounding rock fissures and may easily result in large floor heave of ventilation roadway and rib spalling at the working face. The lateral supporting force and surrounding rock movement generated after the mining of the previous working face is stabilized directly affect the static stress and overlying strata movement of the adjacent working face at the first mining stage. Under the influence of depth, the forces acting on the surrounding rock of the working face vary with the burial depth of rock strata. With the aggravation of mining in the large space structure formed under the conditions of large depth and stope with large mining height and GER, the internal fissures of the overlying strata develop with a high degree of fissure evolution. This phenomenon may easily cause sudden roof instability and generate strong strata pressure. Statistics show that four major dynamic disasters occurred from 2017 to 2019, which damaged the mining system and equipment and seriously restricted safe, efficient, and sustainable mining, as shown in Figure 5.

Division of deep "three layers"
The large space structure formed by the stope with large mining height and GER is a key factor contributing to dynamic disasters at the working face. Due to large depth, different types of overlying strata have load characteristics, directly causing dynamic disasters at the working face. The combined action of the large space structure and overlying strata with loading effect has a comprehensive time effect, which is the fundamental cause of dynamic disasters at the working face. The different loading effects of the overlying strata on the working face are referred to as the deep "three layers," as shown in Figure 6. The mining coal seam consists of the caving layer Mk(Ⅰ), the middle loading layer Mj(Ⅱ), and the far-field layer My(Ⅲ) from bottom to top. Figure 6. Overview of evolution of overlying strata with loading effect in the large space structure (1) Caving layer (Mk): With the advancement of the working face, the roof in the deep well is characterized by periodic movement of "deflection-rupture-rotation-caving" and fills the entire goaf. Along the direction of the working face, the whole caving layer acts on the coal seam and the support. The disturbance stress formed acts on the support and wall, causing rib spalling at the working face.
(2) Middle loading layer (Mj): The original rock stress, disturbance stress, and disturbance range in the deep well are relatively large. As the movement range of the caving layer increases and under the action of its own load, the strata become "tough." The fissures initially developed in the rock strata give rise to the bending and subsidence of the rock strata along with advancing at the working face and, finally, the closing of fissures. Under the action of external load and dead weight, the bending and subsidence of the rock strata cause development and penetration of internal fissures. Therefore, the middle loading layer has the movement feature of "cracking-bending-subsidence-closing." The formed "arch" structure continuously evolves upward. Affected by the large space structure, the "arch point" moves toward the rear part of the working face and reaches stability. The middle loading layer acts on the caving layer through movement, and a subsidence curve forms on its top, as shown in Figure 5.
(3) Far-field layer (My): Located far from the working face in the far field, this layer controls the overall strata movement after the movement of other strata. This layer has good continuity toward the surface strata basically under static load. It mainly inhibits surface movement.

Determination of heights of deep "three layers"
The factors that affect the distribution of the deep "three layers" are mainly related to mining height, burial depth, working face length, and strata properties. The load action of each layer is also different.
(1) Height of caving layer (Mk) After the working face is stoped, a certain space forms in the goaf, and the roof collapses and fills the goaf. The formula for calculation of "caving zone" by Qian Minggao is where M is the height of the caving zone, h is the mining height, and K is the rock bulking factor, generally with a value ranging from 1.1 to 1.3. The deep static stress (original rock stress) is relatively large, which increases the stability of the surrounding rock near the working face. After deflection, the roof breaks, rotates, and collapses within a short period of time to form stability within the "caving zone," where the value of K is 1.2. The height of the caving layer is calculated as shown in Formula 2.

Bending and Subsidence Curve
(2) Height of far-field layer (My) According to the distribution characteristics of strata, the key layer in the far field is determined. The height of the far field layer My is equal to the height from the far-field key layer to the surface.
(3) Height of middle loading layer (Mj) Under the action of its own load, its internal evolution is complicated. According to the definition of the middle loading layer, with the temporal and spatial changes of mining, it reaches the fracture limit under load, giving rise to delamination and fracture. Its height can be calculated as follows: The height of the middle loading layer (Mj) is determined by the buried depth (H) and the height of the caving layer and far-field key layer.  . Numerical calculation model The coal and rock samples drilled from the mine underwent rock mass mechanical parameter measurement using a WE-2T universal testing machine, a WE-10T hydraulic universal testing machine, and a WE-60T universal material testing machine. Table 2 lists the mechanical parameters of the surrounding rock.

Mining characteristics of Working Face 414
Working Face A is the first mining face, and the disturbance of the entire overlying strata basically presents a symmetrical structure, as shown inFigure 8.

Mining characteristics of Working Face 416
After the stoping of B, the first GER working face reaches stability, and the overall plastic failure of the overlying strata is shown in Figure 9. According to the division of the overlying strata with loading effect, the disturbance characteristics of each loading layer are as follows: (1) Disturbance characteristic of Mk Along with the stoping of the working face, the fine sandstone near Working Face 418 collapsed. Due to its high strength and certain bearing capacity, "Mk" siltstone forms roof overhang. The stress distribution characteristics of siltstone at the layer are extracted, as shown in Figure 10.   zones of A and B working faces are about 60 and 120 m, respectively. At 185 m, the stress value of the fine sandstone pressure relief zone is 0 MPa with a small deviation. Along with advancing, energy is stored at the end of the working face. The variation characteristics of the disturbance stress of the Mj layer in the overlying strata with loading effect in the deep large space structure are shown in Table 3. The fine sandstones at 95 m are affected by caving of the Mk layer, and its movement is apparently intense. After two groups of fine sandstones are disturbed, the overlying strata form superposition of the disturbance stresses from top to bottom, acting on the Mk layer. Therefore, the load from the Mj layer controls the movement of the Mk layer, and the energy release gives rise to "vibration" in the overlying strata. Strong strata pressure is generated in the middle and rear parts of the working face.  The results indicate that the disturbed My layer exerts its dead weight on the Mj layer evenly and reaches stability, thereby effectively restraining stress transmission, preventing surface movement, and ensuring that the load from the Mj layer is applied to the middle and rear parts of the working face.

Mining characteristics of Working Face 418
With the gradual increase in large space, the movement of the overlying strata is further intensified. The plastic failure of the entire overlying strata after the stoping of Working Face 418 has reached stability is shown in Figure 13, which is similar to the failure type of Working Face 416. The changes in working faces expand the movement space of the overlying strata, and the overlying strata disturbance of Working Face 418 is more severe than those of the other work faces, with more intense movement of the overlying strata. Shear failure mostly occurs in the current working face and at both ends of the mined coal seam. After Working Face 418 remains stable during mining, the resulting "arch" structures generated by the overlying strata of 414, 416, and 418 working faces move toward the alternation direction, middle part, and end of each working face, respectively, and the overlying strata generally presents symmetrical "arch" plastic failure. The evolution boundary of overlying strata at both ends of the working face is about 60°. After the above three working faces remain stable during mining, the disturbance stress values of observation points of the deep "three layers" are extracted, with disturbance characteristics as follows: (1) Disturbance characteristics of Mk The mining area of the panel is enlarged, and the "Mk" disturbance stress is shown in Figure  14. The Mk layer collapses and acts directly on the support of the working face. The stress is relatively concentrated within 10 m from the end of each working face, and the accumulated energy cannot be released in a timely manner, which may easily cause ultra-long roof overhang and bending and subsidence of roof of the working face. The ventilation roadway in this study is prone to caving and large-area floor heave.
(2) Disturbance characteristics of Mj After the mining of Working Face 418, the stress disturbance of fine sandstones at 95 and 185 m above the working face is shown in Figure 14.  The other is that as the Mj layer is higher, the distribution of disturbance stresses is symmetrical, with small relative stress relief. Therefore, the different stress relief conditions of the Mj layer control the deflection degree of the Mk layer and the distribution of disturbed stresses at the working face and widely act on the plastic zone concentrated area of the Mk layer, which may easily cause "shock bump" and dynamic disasters at the working face.
(3) Disturbance characteristics of My The stress distribution of the far-field layer presents a "concave" type after Working Face 418 remains stable during mining. Compared with the lateral stresses of the other two layers, it has the smallest range of about 15 m, as shown in Figure 16. After disturbance, the stress of each monitoring point remains stable, with a value greater than 1 MPa. The load of the layer is applied evenly on the Mj layer to provide the desirable stress relief characteristics of the Mj layer and restrain upward evolution of strata movement and prevent surface movement. The My layer controls strata movement and ensures the stress relief positions at the working face.  Figure 16(a). Small separation layers and through fissures appear at 15 and 23 m, one through fissure and small separation layers exist at 15 m, and the strata fissure continues to evolve upward at 30 m, as shown in Figure 16(b). Under the disturbance of coal seam mining, the Mk layer moves violently with a height of about 30 m, which is the direct source of weighting for the working face. Under the action of overlying layer with loading effect (i.e., "Mj"), the upward evolution of roof fissures is intensified, which acts on the Mk layer, thus easily causing dynamic disasters at the working face.

Pressure detection of support
The support adopts the SAC electro-hydraulic control system to automatically monitor the working condition of the support during advancing at the working face. The setting load of the support is set to 27.5 MPa. Statistics are made on the support working resistance of Working Face 416 from January to February and Working Face 418 in January. Affected by Working Face 414, the pressure distribution of Working Face 416 from January to February is shown in Figure 18.   Figure 19. Overview of pressure distribution at Working Face 418 The overall pressure of the working face is relatively high, which mainly exists between 55# and 155#, of which 110#-155# (Zone 1) and 55#-100# (Zone 2) have relatively concentrated weighting. Number of pressure distribution positions of the working face: ventilation roadway > middle > intake roadway. The pressure at the tail is large and relatively continuous. The weighting of 7#-12# lasts for a long time, with the pressure value range of 45.3-55.7 MPa. The pressure of Working Face 418 in the large space structure shifts to the tail, with large pressure value and large disturbance stress range. Therefore, the "Mk" layer is the source of normal weighting for the working face. "Mj" controls the deflection degree of the "Mk" layer of the working face and the stress distribution, resulting in strong strata pressure of the working face.

Conclusion
(1) On the basis of the analysis of the occurrence conditions of the overlying strata at the working face, the concept of deep "three layers" is proposed according to the spatial position, thickness, and load characteristics of each layer. The discrimination method and heights of deep "three layers" are also confirmed. The heights of deep "three layers" (Mk, Mj, and My) of the large mining height working face in Panel 4 are about 30, 260, and 210 m, respectively.
(2) The large space structure generates an "arch" structure at the working face, which shifts to the middle and rear parts of the working face. During this process, the overlying strata continuously evolves upward and merges with the "arch" structures developed at other working faces, finally forming a symmetrical "arch" structure.
(3) Shear failure is mostly seen in the overlying strata under disturbance. Among the deep "three layers," "Mk" is the source of weighting for the working face. "Mj" controls the deflection degree of "Mk" and shifts the stress relief of the working face changes to ventilation roadway position. The uniformly distributed load of "My" controls the movement of the rock strata and ensures the stress relief positions at the working face. (4) Through on-site monitoring, the disturbance range and stress value of Working Face 418 are greater than those of Working Face 416 in the large space structure. The changes in pressure relief positions of the entire working face are ventilation roadway > middle > intake roadway.