*Article* **Selection and Optimization Mechanism of the Lower Return Roadway Layout in the near Residual Coal Pillar Area**

**Xiao-He Wang 1,\*, Hao-Hao Zhang 2, Zheng Wu 1, Xiao-Long Li 1, Yi Sui <sup>3</sup> and Ruo-Qi Gao <sup>4</sup>**


**Abstract:** Background: To optimize the layout position of the residual coal pillar return roadway when mining a close coal seam group and to clarify the optimization mechanism, a roadway optimization layout analysis was conducted on the Tashan coal mine. Methods: Surface displacement monitoring was conducted using field tests, and the main stress magnitude, plastic zone morphology, deformation variables, and connectivity between the plastic zone of the roadway and the plastic zone of the residual coal pillar were analyzed at different locations with the help of FLAC3D numerical simulation software. Results: It was found that, in the process of close coal seam group mining, the residual coal pillar of the overlying coal seam seriously affects the stress state and plastic zone distribution of the lower coal seam roadway. The roadway is arranged in a position that is relatively far away from the residual coal pillar, which could reduce the stress influence of the residual coal pillar on the roadway and guarantee the stability of the roadway. Conclusion: Since the Tashan Mine uses the top release method for mining, the stability of the roadway can be better ensured by placing the roadway in the middle and lower regions of the coal seam and using the layout method to retain small coal pillars.

**Keywords:** residual coal pillar; lower return roadway; optimization mechanism; stress state; plastic zone

## **1. Introduction**

The distribution of coal seam clusters in close proximity to one another is a major characteristic of coal distribution in China. Mining a proximity coal seam group is different from single-seam mining, and it is influenced by the adjacent seam, which can lead to mine pressure and potentially disastrous accidents [1–5]. Mining an upper coal seam of a proximity coal seam group can not only trigger the loosening of the roof of the lower coal seam but also affect the stress state of the surrounding rocks of the lower coal seam roadway, causing roadway deformation damage [6–8].

The coal seam under original rock stress is redistributed after the excavation of the roadway, and once the mining disturbance triggers the local stress concentration in the roadway, it can lead to the occurrence of roadway disasters [9,10]. In order to determine the causes of roadway hazards and reduce the possibility of roadway hazards, some scholars have proposed theoretical approaches for stabilizing surrounding rocks. The classical circular plastic zone determines the range of the plastic zone of the roadway enveloped by rock, which is considered to be a circular area [11–13]. The theory of natural caving supplies states that under the action of mine pressure, the roof of the roadway's surrounding rock will appear loose, causing deformation, damage, and bubble falls. The roof of the roadway forms an arch of support after the roof pressure balances out, and the roof surrounding the rock no longer causes bubble falls [14]. The theory of axial variations

**Citation:** Wang, X.-H.; Zhang, H.-H.; Wu, Z.; Li, X.-L.; Sui, Y.; Gao, R.-Q. Selection and Optimization Mechanism of the Lower Return Roadway Layout in the near Residual Coal Pillar Area. *Processes* **2022**, *10*, 2471. https://doi.org/10.3390/ pr10122471

Academic Editor: Guining Lu

Received: 31 October 2022 Accepted: 17 November 2022 Published: 22 November 2022

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considers that the damage law of the surrounding rock is the result of the joint influence of the surrounding rock stress and the mechanical properties of the rock mass, as well as the deformation characteristics of the surrounding rock [15,16]. The theory of maximum horizontal stress [17] comprises the analysis of the damage characteristics of rocks and the evolution law of pores and fissures inside the rocks from structural defects inside the rocks, which play a significant role in supporting the rock surrounding the roadway [18–21]. Małkowski et al. conducted a long-term study of three different roadways over a period of six years and found that different support schemes can have significant effects on roof deformations [22]. Khalymendyk et al. analyzed the deformation mechanisms of deep laminated rock roadways and determined that the deformation of the roadway rock was a result of the mutual extrusion of the laminated surrounding rocks, as well as the linear relationship of the deformation and damage between the top and bottom slabs [23]; these observations have been verified by other research studies in the field.

Usually, the excavated coal seam is a non-uniform stress field, and the presence of the non-uniform stress field also has an impact on the distribution of the plastic zone [24]. Fenner's formula and Castanet's formula can determine the radius of the plastic zone around the circular hole under hydrostatic pressure conditions [25] to solve the asymmetric damage phenomenon generated by the roadway in the actual production process. Ma et al. proposed a model for the distribution of the plastic zone of the roadway under a nonuniform stress field. They established the roadway butterfly damage theory and derived the boundary equation of the plastic zone of the roadway butterfly [26–30]. Based on this formula, Guo et al. [31] predicted potential hazard zones in circular roadways and proposed evaluation guidelines for dynamic hazard critical points in circular roadways. Reducing the difficulties of roadway support is a key concern for roadways. In order to optimize the design of the transportation roadway in the Beskempir field, Abdrakhman et al. used numerical simulations to analyze the stress characteristics and deformation patterns of the surrounding transportation roadway's rock, and the stability of the surrounding rock was discussed to determine a support scheme for the transportation roadway [32]. Gan et al. [33], Yang et al. [34], and Li et al. [35], as well as a large number of other scholars, analyzed the state of the roadway and used experiments and numerical simulations to demonstrate a reasonable arrangement form for the roadway, which reduces the difficulty of implementing roadway supports.

The lower back mining roadway's close residual coal pillar areas easily triggers the occurrence of roadway disasters due to the complex surrounding rock stress. In this paper, we measured the short-term deformation variables of the return roadway of the 30503 working faces in Tashan coal mine by using the method of roadway surface displacement monitoring, and the measurement results showed that the original roadway arrangement would lead to substantial deformations over a short period of time. By determining the asymmetrical damage inflicted upon the roadway, we established a numerical model for the multi-point arrangement of the lower return roadway in the close residual coal pillar area and simulated the main stress magnitude, plastic zone morphology, and deformation variables of the roadway in the horizontal and vertical directions, respectively. The connectivity between the plastic zone and the plastic zone of the residual coal pillar was analyzed at different positions on the roadway, and the mechanical characteristics and deformation features of the roadway at different positions were determined. Based on the coal mining process of the 30503 working face of the Tashan mine, it was established that the roadway should be arranged in the middle and lower positions of the coal seam as a method for retaining small coal pillars (away from the residual coal pillars), which could ensure the stability of the roadway. This provides the theoretical basis for the reasonable arrangement of the lower return roadway of the residual coal pillars in the mining process of the close coal seam group.

#### **2. Materials and Methods**

#### *2.1. Engineering Background*

The Tashan coal mine is located in the middle and east edge of Datong coalfield, China. The main coal seam comprises 3–5# coal, with an average thickness of 18.0 m. The coal is recovered by releasing the top coal. The upper coal seam is 2# coal; in order to ensure safe production operations, after mining 2# coal, the 2# coal emptying area is filled. According to the mining succession plan, the 30503 working face is the successive mining face. The spatial positions of the 3–5# coal seam and 2# coal seam and the original layout plan of the tunnel are shown in Figure 1; as we are ignoring the influence of other factors, the diagram only depicts the 30503 working face return roadway. After the roadway was excavated at 860 m in depth, serious deformations occurred in the roadway's roof, and the local area had to be completely closed; thus, the 30503 return roadway was repaired, but during the repair process, the roadway experienced continuous deformations, and there was a local large deformation. To analyze the degree of damage, surface displacement observations were made on the roof of the 30503 return roadway, and the degree of damage was analyzed visually in the form of data.

**Figure 1.** Diagram of the layout of the roadway between coal seams #3–5 and #2.
