## 2 Laboratory Uniaxial Compression Test
### Test Apparatus and Scheme  
The uniaxial compression test of soil-rock mixture was conducted using a servo-controlled soil triaxial apparatus with a maximum loading force of 2 kN. Axial loading was applied to the specimen using displacement control at a rate of 1.1 mm/min. The particle grading curves indicate that the rock content of the soil-rock mixture in the study area ranged between 50% and 60%, suggesting that the rock content of the specimens prepared for the uniaxial compression test should also be set between 0-60%. However, during the sampling process, it was found that weak cohesion between the soil matrix and rock blocks made it challenging to mold soil-rock mixture specimens with high rock content. Consequently, the maximum rock content selected for this test was 30%.  
Before specimen preparation, the volume of soil matrix and rock blocks was determined according to the test scheme as listed in Table 3. The volumetric rock content was  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Sampling Location** & **Coefficient of Curvature, \(Cc\)** & **Uniformity Coefficient, \(Cu\)** \\ \hline East side of the slope & 0.48 & 56.82 \\ Northeast corner of anchor pit & 0.71 & 67.27 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Grading indexes of soil–rock mixture from different sampling locations.  
Figure 3: Rock blocks for uniaxial compression test with size of (**a**) 2–5 mm and (**b**) 5–10 mm.  
Figure 2: (**a**) Grading curves and (**b**) particle size distribution histograms of soil–rock mixture from different sampling locations.  
selected as the rock content control index. In the specimen preparation process, the reference specimen with 0% rock content (pure soil specimen) was first prepared to obtain the density of the soil. By replacing the soil in the 0% rock content specimen with an equivalent volume of \"rock\", soil-rock mixture specimens with varying rock contents were prepared. To better illustrate the particle distribution characteristics of the experimental material, the constrained grain size (D\({}_{60}\)) and effective grain size (D\({}_{10}\)) were determined based on the provided particle distribution curves. The constrained grain size, D\({}_{60}\), refers to the particle size below which 60% of the total mass of the material is comprised. Similarly, the effective grain size, D\({}_{10}\), represents the particle size below which 10% of the total mass of the material is comprised [39; 40].  
The maximum dry density and optimum water content were utilized to ensure optimal mechanical properties, as well as comparability and high reproducibility. In various construction projects, such as subgrade slopes or dam construction, compacted soils typically approach the state defined by their optimum moisture content and maximum dry density. Therefore, the specimens in this study were prepared under these conditions, resulting in relatively higher strength. Compaction tests for soil-rock mixtures with varying rock contents were conducted using a heavy compaction apparatus, yielding the maximum dry density and optimal moisture content for each rock content condition. The optimal moisture content was then used as the target moisture content to determine the required water mass for the specimen preparation as shown in Table 3.  
The soil and rock blocks were initially placed in a drying oven for drying. Following this, the calculated amount of water was sprayed onto the soil matrix and rock blocks. After thorough mixing, the required soil-rock mixture was placed in a closed container for at least 24 h to ensure uniform water content. The geo-material mass was determined based on the dry density and placed into a mold measuring 125 mm \(\times\) 61.8 mm in five layers. Compaction was performed until each layer reached the required height, and the surface was shaved before adding the second layer. This process continued until the final layer was compacted, resulting in the soil-rock mixture specimen shown as Figure 4.