## 2 Materials and Methods
### Compression Tests  
The test system comprises a WAW-1000D electric servo-motor testing machine and self-made compressors of backfill materials, as shown in Figure 6. The test machine could provide axial force from 0-1000 kN and had a stroke in the range of 0-780 mm. Furthermore, the data acquisition system of the testing machine could obtain mechanical parameters including load and displacement. To meet the test requirements, compressors with different internal diameters (50, 70, 100, 150, and 250 mm) were designed (Figure 7). The compressors consist of a removable steel cylinder, a dowel bar, and a sleeve. The removable steel cylinder was formed by fixing two steel slots on a base using screws. The dowel bar was composed of a base and a steel column, and the size of the base was slightly smaller than the inner diameter of the removable steel cylinder. The dowel bar was used to ensure uniform stress on the WRBM during the loading process. The sleeve was placed on the base of the dowel bar in the loading process to avoid deformation of the base due to stress concentration under load. Figure 5: SEM images of waste rock specimens. Figure 6: Compression test system. The specific test steps are described as follows:  
Step 1: Loading the waste rock specimen in the steel cylinder  
After assembling the removable steel cylinder and placing waste rock with the corresponding particle size into it, the specimen was flattened, and the height of the specimen was back-calculated by measuring the remaining height of the removable steel cylinder using a steel ruler. The loading was stopped after the specimen height reached 150 mm. The particle size distributions and the sizes of removable steel cylinders in each group of specimens are summarized in Table 1.  
Step 2: Assembling the compressor  
The dowel bar was placed on the specimen and the sleeve was put on the dowel bar, during which it should ensure that the dowel bar and sleeve were placed horizontally at the center of the specimen. After assembly, the compressor was placed on the WAW-1000D electric servo-motor universal test system. Step 3: Pre-compression of the specimen  
The computer was turned on to run data acquisition and detect the software (16.5.10.2016). The system was stopped automatically after applying an axial pre-pressure of 2 MPa [29]. To reduce the void ratio of, and strain in, the backfill materials, a certain lateral pressure was applied during field backfilling. To conform to prevailing engineering conditions, the backfill materials were also pre-compressed. Step 4: Axial loading of the specimen  
After running the data and detecting the software (16.5.10.2016), the corresponding program was set and run. The tester utilized a displacement transducer and a pressure transducer to obtain displacement and stress changes in the waste rock specimen. When the axial stress reached 20 MPa, the system stopped automatically. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{2}{c}{**Particle Size**} & \multirow{2}{*}{**Inner Diameter (mm)**} & **Axial Compressive** \\  & **Distribution (mm)** & & **Stress (MPa)** \\ \hline y-1 & 0–5 & & \\ y-2 & 5–10 & & \\ y-3 & 0–10 & & \\ y-4 & 10–15 & & \\ y-5 & 15–20 & & \\ y-6 & 0–20 & & \\ y-7 & & 50 & \\ y-8 & & 70 & \\ y-9 & 0–10 & 100 & 20 \\ y-3 & & 150 & \\ y-10 & & 250 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specific test schemes. Figure 7: Removable steel cylinders with different sizes. The axial strain \(\varepsilon\) of the specimen is the ratio of the displacement during axial loading to the height of the specimen after pre-compression [30] as given by  
\[\varepsilon=\frac{\Delta h}{h-h_{1}} \tag{1}\]  
where \(h\) is the loading height (150 mm); \(h_{1}\) is the displacement of the specimen during pre-compression; and \(\Delta h\) is the displacement of the specimen during axial loading. The void ratio of the waste rock specimen changes constantly throughout the loading process [31].