**1. Introduction**

Gob-side entry retaining is one of the most commonly used mining method in nonpillar coal mining. In this method, the former entry is artificially retained as the tailgate for the next mining panel by constructing a filling wall made of concrete blocks, pigsties, high-water packing material, and other fill materials, which can greatly improve resource

**Citation:** Wang, Q.; Guo, Z.; Zhu, C.; Yin, S.; Yin, D. The Deformation Characteristics and Lateral Stress of Roadside Crushed Rocks with Different Particles in Non-Pillar Coal Mining. *Energies* **2021**, *14*, 3762. https://doi.org/10.3390/en14133762

Academic Editor: Manoj Khandelwal

Received: 13 May 2021 Accepted: 22 June 2021 Published: 23 June 2021

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recovery and reduce roadway drivage rate [1,2]. However, in case of complex geological mining engineering, the conventional gob-side entry retaining method encounters inevitable difficulties, due to high stress, high dynamic disturbance and large deformation issues [3–5]. Furthermore, the high cost of filling materials and time-consuming have also severely restricted the wide application of the conventional gob-side entry retaining method.

**Figure 1.** Principle of the GERRF method: (**a**) No roof pre-fracturing; (**b**) With roof pre-fracturing.

The crushed rocks are the maintenance body for the GERRF method, its compression and deformation characteristics appear to be very important on the stability of the retained entry. Much research has been focused on the instantaneous compressible deformation [13–15] and creep deformation [16–18] of crushed rocks by uniaxial compression test. However, in these uniaxial compression tests, the crushed rocks were completely constrained on all sides, the boundary conditions were significantly different from the GERRF method [19,20]. Therefore, an innovative experimental device to simulate the boundary condition of crushed rocks in gob area of GERRF was developed. Using the device, the compression tests of crushed mudstones with different particle sizes were carried out. In the tests, the deformation behavior of crushed mudstones with different particle sizes in GERRF method was studied, together with that of the lateral stress giving rise to lateral deformation of support structure was measured, which expects to provide experimental evidence for deformation prediction and supporting design of the GERRF method.

#### **2. An Innovative Experimental Device**

In the previous research data obtained from uniaxial compression tests, crushed rocks achieved a greater compactness under axial stress. However, the crushed rocks in the GERRF method are difficult to compact tightly, but rather acquire a new equilibrium state with the surrounding rock. Therefore, an innovative experimental device simulated the geometric structure of the GERRF method was developed, as shown in Figure 2. The experimental device comprises of a loading plate, a cubic frame containing and a base plate. The internal dimensions of the device are 400 mm × 400 mm × 400 mm. Three of the vertical sides of cube are fabricated with Q235 solid steel, while the other side is the simulated surface of gangue rib, which comprises of high-strength wire mesh and scaled down support bars. In accordance with the adopted geometrical similarity ratio of 1:10, the dimensions of the scaled down support bar are 440 mm in height, 10 mm in width and 5 mm in thickness. A total of eight scaled down support bars are arranged on the simulation surface of gangue rib. The arrangement spacing is about 55–56 mm.

**Figure 2.** Design principle of the innovative experimental device: (**a**) Schematic diagram of the structure of the GERRF method; (**b**) The structural composition of the experimental device.

The MTS hydraulic servo loading system used in this study is illustrated in Figure 3. The maximum axial load of the triaxial loading system is 2000 kN, and the accuracy of the measurement of axial load is less than 0.01 kN.

**Figure 3.** The MTS hydraulic servo loading system.
