## 2 Experimental Descriptions
### Experimental Procedures  
To reveal the influences of loading stress and loading velocity on the compression and particle breakage behaviours of the gauge backfill materials, each test was repeated three times according to the test scheme. The mean value of the three test results was taken to eliminate test errors. Specific experimental procedures are as follows:  
1. Prepare the specimen and place it into the steel barrel  
The specimen preparation methods are described in Section 2.2. After each group of specimens had been thoroughly mixed with different particle sizes, they were each loaded into the steel barrel several times, and the surface of the specimens was then levelled by hand after all of them have been loaded into the steel barrel. Before loading the specimen into the steel drum, lubricant was applied to the inner wall of the steel drum to reduce the error caused by friction. The particle size grades of the specimens are illustrated in Figure 3.  
2. Weigh each specimen and record the initial height in the chamber  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Main Control Factor**} & **Numbered Specim** & \multirow{2}{*}{**Loading Stress**} & \multirow{2}{*}{**Loading Velocity**} \\ \hline \multirow{4}{*}{Loading stress} & i-1 & 5 MPa & \\  & i-2 & 10 MPa & \\  & i-3 & 15 MPa & \\  & i-4 & 20 MPa & \\ \hline \multirow{4}{*}{Loading velocity} & i-5 & & 1 kN/s \\  & i-6 & & 5 kN/s \\ \cline{1-1}  & i-7 & 20 MPa & 10 kN/s \\ \cline{1-1}  & i-8 & & 15 kN/s \\ \hline \hline \end{tabular}
\end{table}
Table 1: Single-factor rotation test scheme.  
Figure 3: Gangue particles with different sizes.  
After the specimen was placed into the steel barrel, it was covered by the loading plate. The height of the inner chamber (\(H_{1}\) = 300 mm) and the thickness of the loading plate (\(H_{2}\) = 40 mm) are given, and the distance (\(H_{3}\)) between the surface of the cover plate and the upper edge of the steel barrel can be measured. The initial height of the particle packing (\(H_{0}\)) can then be calculated by the equation: \(H_{0}\) = \(H_{1}-H_{2}-H_{3}\). The values of \(H_{0}\) should be recorded for calculating the axial strain.
3. Load and compress the specimen  
The steel barrel was placed in the centre of the test platform; hereafter, the test was carried out according to the loading velocity designed in the test scheme. The test was stopped when the set maximum stress was reached. Loads and displacements were recorded until the specimen was unloaded. The compaction process for this test is in accordance with the Energy Industry Standard of the People's Republic of China \"Method of compaction testing of solid backfilling materials (NB/T 51019-2014)\" [40].
4. Remove the specimen and sieve the particles  
After the loading process was completed, the steel barrel was disassembled to remove the specimen. The aggregates, having been compressed, were taken out and screened grade by grade. The mass of each grading particle in the specimen was recorded for subsequent analysis. Similarly, the compressed particles are classified into eight grades, i.e., 0\(\sim\)1.25, 1.25-2.5, 2.5-5, 5-10, 10-16, 16-20, 20-25, 25-30 mm.