**1. Introduction**

Close-distance coal seam groups are widely distributed in China [1,2]. Limited by technical and coal seam reserve conditions, wide coal pillars are often reserved when mining the overlying coal seam to maintain the stope mining roadway [3,4]. The many remaining coal pillars after the stope mining of the coal seam alter the stress environment of their floor, causing stress elevations dominated by vertical stresses in the floor rock near the remaining coal pillars [5,6]. Under such high stresses, the surrounding rock of the stope mining roadways in the underlying coal seam inevitably develops cracks [7,8]. In the meantime, the superposition of lateral support pressure stresses during the mining at the working face of the underlying coal seam further aggravates the stope-mining roadway deformation and failure, which seriously affects the safe production of the mine [9–13]. Therefore, studies on the failure mechanism and stability control techniques of roadways under multiple disturbances in close-distance coal seams are urgently needed.

**Citation:** Fu, Q.; Yang, K.; He, X.; Liu, Q.; Wei, Z.; Wang, Y. Destabilization Mechanism and Stability Control of the Surrounding Rock in Stope Mining Roadways below Remaining Coal Pillars: A Case Study in Buertai Coal Mine. *Processes* **2022**, *10*, 2192. https://doi.org/10.3390/pr10112192

Academic Editors: Feng Du, Aitao Zhou and Bo Li

Received: 27 September 2022 Accepted: 23 October 2022 Published: 26 October 2022

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To clarify the effects of remaining coal pillars on the mine pressure behaviors of stope mining roadways in the underlying coal seam, scholars worldwide conducted extensive research based on methods such as mining theory analysis, numerical simulation, and field applications. Research by Zhang et al. [14] concluded that subjected to high vertical stress, part of the roadway under the remaining coal pillar developed large deformations and became difficult to maintain, and the stress concentration and surrounding rock deformation on the side of coal pillars could be effectively reduced by the reasonable roadway layout. Fang et al. [15] demonstrated that roadways were easily destabilized under the effect of mining if the remaining coal pillars were narrow in the gob of the overlying coal seam or if the roadway of the underlying coal seam was directly below while the roadwaysupporting coal pillars near the working face of the current coal seam were small. Numerical simulations have been conducted to analyze the stress transfer of the coal pillar floor in the overlying coal seam [16,17]. To address the challenge of gob-side roadway stability control in close-distance coal seams, Zhang et al. [18] determined the design parameters of the roadway-side filling body based on the structural mechanics model of the gob-side entry retaining surrounding rocks and proposed a support technology combining high-strength anchor rods, anchor cables, and steel beams. Jiang et al. [19] analyzed the coal pillar-induced impact patterns in deep close distance coal seams and medium mining depth far distance coal seams. Through comprehensive research, Liu et al. [20] analyzed the destabilization and failure processes of roadways in the gob of close-distance coal seams and proposed a joint active-passive control method to improve the bearing performance of the surrounding rock. Xia et al. [21] adopted numerical methods to analyze the dynamic mechanical characteristics of the stope mining roadway under small-width coal pillars when subjected to repeated mining disturbances and proposed corresponding roadway surrounding rock control methods. To address the asymmetric plastic failure of the withdraw roadway under the dual action of remaining coal pillars in the overlying coal seam and mining disturbance, He et al. [22] proposed a zoning control strategy and achieved roadway stability. The research results at different engineering geological conditions practically solved the engineering difficulties brought by the remaining coal pillars to the mining of close-distance coal seam groups and enriched the technical application system of rock seam control. However, when the remaining coal pillars in the overlying coal seam are parallel to the advancing direction and spatially overlap with the underlying stope, the disturbance of the high stress in the floor at the remaining coal pillar increases under the dual action of the remaining coal pillar and the mining of the underlying coal seam, which significantly affects the layout and stability of the underlying coal seam roadways [23].

Based on the background of the occurrence conditions of the coal seams in Buertai Coal Mine, methods including theoretical analysis, numerical simulation, and field measurement were adopted to study the stress transfer pattern of the T-shaped remaining coal pillar floor in the overlying coal seam and analyze the destabilization and stability control principle of the roadway surrounding rock during the mining process of the adjacent working face in the underlying coal seam. The reasonable layout scheme of the 42202 mining roadway was put forward, and the surrounding rock dynamic reinforcement control techniques were also proposed. The research results could provide some reference for projects of similar engineering backgrounds.

#### **2. Geological Condition**

The Buertai Coal Mine has a simple geological structure, predominantly monoclines with gentle dip angles and no-fault development. The coal mine contains 9 mineable coal seams, and mainly the 2-2 and 4-2 coal seams are being mined now. The 2-2 coal seam is 3 m thick and 300 m deep on average. The 4-2 coal seam is 6.12 m thick and 368 m deep on average, and comprehensive mechanized top coal caving mining is adopted.

Stope mining at the two working faces of the overlying 2-2 coal seam, 22201 and 22202, has been completed, leaving a 20 m wide sectional coal pillar. Skip mining was carried out when stope mining the 22202 working face, leaving a 70 m skip mining coal pillar. The spatial layout of the two remaining coal pillars resembles a mutually perpendicular T-shape. The 42201 and 42202 working faces are below the 22201 and 22202 working faces, and the stope mining roadways of the adjacent working faces, i.e., the transport roadways of 42201 and 42202, are dug simultaneously, leaving a roadway-supporting coal pillar of certain width in the section. The location of the Buertai coal mine and schematic diagram of the 42202 stope layout is shown in Figure 1.

**Figure 1.** (**a**) The location of the Buertai coal mine. (**b**) Schematic diagram of 42202 stope layout.

The original support design of the 42202 stope mining roadway is as follows. The roof is under the combined support by "left-hand thread rebar bolt without longitudinal bar + welded mesh + anchor cable + π-type steel strip". The bolts are ϕ22 × 2200 mm in dimension and arranged rectangularly with six sets per row with an inter-row distance of 1000 × 1000 mm. The anchor cables are ϕ22 × 8000 mm in dimension and arranged rectangularly 3 sets per row with an inter-row distance of 2100 × 2000 mm. The coal pillar rib is under the combined support of "bolt + diamond-shaped mesh + wooden pallet". The rebar bolts are ϕ22 × 2100 mm in dimension and arranged rectangularly in five sets per row with an inter-row distance of 800 × 1000 mm. The solid coal rib is under the combined support of "FRP bolt + double-layer high-strength plastic mesh + wooden pallet." The bolts are ϕ22 × 2100 mm in dimension and arranged rectangularly in five sets per row with an inter-row distance of 800 × 1000 mm, as shown in Figure 2.

**Figure 2.** Initial support measures of mining roadway in 42202 stope.

During the stope mining at the working faces under the same working conditions, the stope mining roadways showed the following failure characteristics.

(1) When the stope mining roadway is only affected by a single sectional coal pillar in the overlying coal seam, the roof and floor displacements are between 200 and 220 mm, and the displacements of the two ribs are between 190 and 215 mm. When affected by approximately T-shaped remaining coal pillars, the displacements of the roadway roof and floor increase by 54.5–56% to 440–500 mm, and the displacements of the two ribs increase by 51.3–55.2% to 390–480 mm. Therefore, the deformation of the roadway below the T-shaped remaining coal pillar has increased significantly, and the surrounding rock failure is severe.

(2) On-site investigation shows that the force on the bolts in the roadway rib had a sudden stepwise decrease, and the pallet fell off. Thus, the range of surrounding rock plastic failure continuously increased under the effect of multiple disturbances, and the confining pressure of the bolt-supported section and rock mass integrity decreased, which significantly reduced the anchoring capacity of the bolts.
