## 1 Introduction  
As a special geo-material widely distributed in Southwest China, soil-rock mixture exhibits complex structural characteristics and strength response. Its composition of rock blocks with varying sizes and a soil matrix leads to an extremely heterogeneous loose geotechnical medium system [1; 2]. The difference in the mechanical properties between the components means that the rock content becomes a crucial parameter in influencing the mechanical properties of soil-rock mixture by controlling its structure [3; 4]. When the rock content is low, the rock blocks remain in a suspended state in the soil matrix and the influence of the varying rock content on strength response and failure mechanism is not obvious. The mechanical properties of soil-rock mixture with a suspended structure are similar to those of the soil matrix. Conversely, when the rock content is high, a rock skeleton structure is formed and the pores between rock blocks become evident. The contact friction and occlusion between the rock blocks provides the strength for the soil-rock mixture. Previous studies have demonstrated that according to the evolution law of the structural and mechanical characteristics of soil-rock mixture caused by the varying rock content, 25% and 70% were identified as rock content thresholds [5; 6; 7]. Unlike homogeneous soil and jointed rock, soil-rock mixture with 25%-70% rock content can be considered a cooperative system of'soil-rock interface' [3; 8]. The appearance of the soil-rock interface due to the varying rock content increases the difficulty of using experimental methods to analyze the evolution law of mechanical characteristics of soil-rock mixture. On the one hand, the influence of the soil-rock interface on the whole material is related to the mechanical properties of the soil matrix and the rock blocks; on the other hand, the quality of the soil-rock interface relevant to the rock content determines the proportion and distribution of weak-strength vulnerable interfaces. Thus, both the influence of the rock content and the soil-rock interface on the strength response and failure mechanism of soil-rock mixture should be analyzed comprehensively from the macro and meso perspectives. Currently, studies on the influence factors of mechanical characteristics of soil-rock mixture are conducted using in situ tests, laboratory tests, and numerical simulations [9; 10; 11; 12; 13]. The mechanical parameters of soil-rock mixture under different loading conditions, such as shear strength, compressive strength, elastic modulus, and so on, have been analyzed. However, the experimental apparatus (large triaxial compression apparatus or direct shear apparatus) have limitations in observing the failure mechanism of the soil-rock mixture specimen. To observe the process more intuitively, the uniaxial compression test has been commonly utilized to obtain both the failure mechanisms and strength response of soil-rock mixture. [PERSON] et al. [14] and [PERSON] et al. [15] utilized a uniaxial compression test with the help of Computer Tomography (CT) scanning to investigate the evolutionary process of the structural characteristics of soil-rock mixture at the mesoscopic scale. The results indicate that the elastic incompatibility between the soil matrix and rock blocks results in differential sliding on both sides of the interface, which is the fundamental cause of specimen failure. [PERSON] et al. [16] compared the uniaxial compression curves and failure patterns of pure ice, ice-rock mixtures, frozen soil, and frozen soil-rock mixture specimens, concluding that the presence of rock blocks altered the failure pattern from a penetrating tensile crack to a diagonal crack and induced lateral expansion deformation. Compared to the pure soil specimen, the rock blocks inside the soil matrix can provides resistance to deformation and failure for the whole material.