## 4 Failure Mechanism of Soil-Rock Mixture at the Meso-Scale  
### Failure Mechanism of Soil-Rock Mixture with Varying Rock Contents  
The state at which the post-peak strength drops to 0.7 times the peak strength is defined as the failure state. In the pure soil model, the displacement direction deviation of particles is evident on both sides of the red dotted line shown in Figure 10(a). Particles in the lower-middle region are observed to separate from the model, while the left region is dominated by lateral expansion, with particle displacement occurring primarily in the horizontal direction. The failure characteristics changes when a certain amount of high-strength rock blocks are present in the soil matrix. In the model with a low rock content (5%), significant contact failure and particle movement are observed in the lower region of the model in contact with the wall. The model can be divided into three zones: zones 1, 2, and 3 (Figure 10(b)). The displacement vectors of the rock blocks distributed on both sides of the long crack (the dividing line between zones 2 and 3) differ significantly from those of the nearby soil particles. The displacement vector diagram shows that rock blocks gradually control the particle displacement direction and the location of local failure. Soil-rock contacts are more prone to fracture, leading to particle separations. The local failure of soil-rock mixture primarily initiates at the soil-rock contact surfaces, with slightly lower strength under uniaxial compressions.  
Figure 10: Axial stress–strain curves of the simulation and experimental results.  
Figure 11c shows that the distribution of displacement direction in the soil-rock mixture model with rock content of 30% is more disordered. As the rock content increases, the likelihood of rock blocks being distributed both inside and on the surface also increases, significantly influencing the failure mechanism of soil-rock mixture. In addition to the two main cracks extending from the upper left to the lower right, vertical tensile cracks are present, and the shape of the cracks is more irregular. Due to the weaker strength at soil-rock contacts, bonding failure tends to occur first at these interfaces. During crack propagation, phenomena such as branching, intersection, and interlocking arise due to obstruction by surrounding rock blocks. The cracks then continue to propagate into the weaker soil matrix, ultimately leading to the formation of multiple shear fractures. Some of the rock blocks and surrounding soil particles become embedded and bound together, forming local soil-rock clusters. These clusters moved as a unit under axial stress, with damage occurring either at the weaker soil-rock interface or within the soil matrix, leading to lateral expansion and local detachment.  
In summary, as rock content increases, the soil-rock interfaces generated within the soil-rock mixture bear external forces alongside the soil matrix and rock blocks, forming an internal \"soil-rock block interface\" system. The relatively weak strength of these soil-rock interfaces makes them the primary path for local failure and crack propagation under uniaxial compression. As a result, the failure pattern gradually shifts to being controlled by the rock blocks, ultimately leading to a transition in the failure mode of the soil-rock mixture from longitudinal fracture to multiple shear fractures.