## 3 Large-Scale Triaxial Test on the Shear Failure Characteristics of a Soil-Rock Mixture  
### Experimental Procedures  
The soil-rock mixture material is extracted from the waste slag dam filling material generated during the construction of a pumped-storage power station in Northwest China, as depicted in Figure 3. The garbage disposal area utilizes this material. The slag dump site is a perennial ditch with a discharge of 0.5 m\({}^{3}\)/s. The width of the ditch bottom is generally 8\(\sim\)13 m. The lithology on both sides is granodiorite and the width of the valley bottom is generally more than 20 m. The lithology on both sides is Sinian Dengying Formation striated marble. Due to the limitation of topographic conditions, the construction slag forms several small-scale spoil yards, the maximum height difference is less than 100 m, and the actual confining pressure is less than 1 MPa. In order to study the accuracy of its mechanical properties, the sample is analyzed with large size and small confining pressure in this triaxial test. Figure 4 shows the test materials. The sample has a stone composition of 52%, a measured natural water content of 10%, and an average specific gravity (\(G_{\text{S}}\)) of 2.83. Representative samples were obtained using the quartering method, and the grading curve was plotted. The obtained sample exhibits discontinuous gradation. Due to the limitation of the test size, the original gradation is adjusted using the equivalent replacement approach. Figure 3: Construction site drawing of the slag dump. The equivalent replacement approach involves substituting large particles with soil particles that are smaller than the maximum allowable particle size of the instrument but larger than 5 mm in proportion and with equal mass. The objective is to closely preserve the mechanical characteristics of the scaled material to the original grading constituents by controlling the amount of fine material present. The equipment can accommodate particles up to 60 mm in size. The scaled grading curve is shown in Figure 5.  
The minimum dry density of the sample was determined using the fixed-volume method to fill the sample cylinder. The sample height was measured using a shaking table, and the sample volume was calculated to obtain the maximum dry density. The design code points out that \"the filling standard of gravel and sand should take the relative density as the design control index\". Referring to the actual design standard of the project, this paper chooses the relative density of 0.6 as the sample preparation control. Different sample dry densities are generated based on the maximum dry density and the minimum dry density. The initial void ratio \(e_{0}\) of the sample was determined through conversion. The test and calculation results are displayed in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test Number**} & **Minimum Dry** & **Maximum Dry** & **Sample Dry** & **Initial Void** \\  & **Density g/cm\({}^{3}\)** & **Density g/cm\({}^{3}\)** & **Density g/cm\({}^{3}\)** & **Ratio \(e_{0}\)** \\ \hline
1 & 1.65 & 1.85 & 1.76 & 0.61 \\
2 & 1.71 & 1.91 & 1.82 & 0.55 \\
3 & 1.82 & 2.15 & 2.00 & 0.42 \\
4 & 1.98 & 2.28 & 2.15 & 0.32 \\
5 & 2.16 & 2.39 & 2.30 & 0.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative density test results. Figure 4: Test material. Figure 5: Grading curve of the S-RM before shearing. The triaxial test employs a large triaxial compression tester with a sample size of \(\phi\)300 \(\times\) H600 mm, a maximum confining pressure of 3.0 MPa, a maximum axial stress of 21 MPa, a maximum stroke of 300 mm, and a shear rate of 0.4 mm/min, as illustrated in Figure 6.  
Five groups of unsaturated isotropic consolidated drainage (CD) shear tests with different dry densities (\(\rho_{\rm d}\) = 1.76, 1.82, 2.00, 2.15, and 2.30 g/cm\({}^{3}\)) were conducted, and the confining pressures were 100, 200, 400, and 800 kPa, respectively.