## 4 Conclusions  
Smooth rock blocks and rough rock blocks were used here for fabricating artificial FSRMs and ice-rock mixtures, respectively. On the basis of microstructure observations of the frozen soil-rock interfaces, uniaxial compressive and Brazilian testing were applied to the FSRM samples and ice-rock mixture samples under different frozen temperatures to investigate the deformation mechanisms of the FSRMs and the influence of the soil-rock interface on strength. Moreover, the AE and the UPV technologies were used to enhance the understanding of the mechanism of the deformation behavior in the FSRMs under loading. Finally, the microstructure failure modes of the soil-rock interfaces were identified, and the major conclusions are given as follows:  
Figure 14: The characteristics of cracks at the soil–rock interface and ice–rock interface: (**a**) smooth ice–rock interface crack; (**b**) rough ice–rock interface crack; (**c**) smooth soil–rock interface crack; (**d**) rough soil–rock interface crack. Figure 15: Three possible crack propagation types for FSRMs. Under the uniaxial compression test, the main failure mode for FSRMs was characterized by bulging deformation and no obvious surface crack before the peak stress point. Most of the surface cracks appeared at the post-peak stage. With a decreasing frozen temperature, the surface tension cracks gradually evolved into shear cracks. All the ice-rock mixture samples showed obvious axial cracks at the peak point, and the axial stress decreased rapidly after the peak strength point. Under the Brazilian test, the main failure mode was characterized by tortuous cracks propagating adjacent to the center of the disk that were perpendicular to loading platens. The failure cracks appeared near the peak stress point stage. The AE results show that a large degrees of deformation occurred in the stress post-peak stage under the uniaxial compression test; however, the deformation mainly occurred at the peak stress stage with the Brazilian test. This indicates that the obvious motion of rock block and crack propagation occurred in the post-peak stage and the creeping and friction of soil particles and ice granules were amused in the pre-peak stage under uniaxial compression test. The UPV results show that the maximum UPV often appeared in the post-peak stress stage. This indicates that the obvious cracks mainly initiated and grew in the post-peak stage. Combining the results above, we can speculate that the creeping of the frozen soil and ice granules, crack propagation, and rock block activities jointly result in the failure of FSRMs. The soil-rock interface will evolve into a soil-rock contact band at the surface of the rough rock block. The soil particles and water will squeeze into the pores to form the soil-rock contact band. The compressive and tensile strength of the frozen soil-rock contact band was larger than that of the soil-rock interface, and the strength difference increased with decreasing frozen temperatures. The soil-rock contact band tension strength was even larger than with rock block particles at relatively lower negative temperatures; it could cause breakage of rock block. From the failure crack morphology perspective, three crack propagation patterns are shown. In the first pattern, cracks between the soil-rock interface will propagate along the accurate soil-rock interface in FSRMs containing smooth rock blocks and will present a flat crack surface; however, cracks will propagate along the contour envelopes of rock blocks in FSRMs containing rough rock blocks and will resultingly present a rough crack surface, as the second pattern. In the third pattern, the crack will pass through rough rock block. This study provides a point for discussion and further research on cold engineering, especially in foundation engineering inside frozen soil (such as pile foundation). According to the third crack pattern of this study, the crack propagation could pass through the rock block.