## 5 Conclusions  
The stress conversion coefficient of soil samples in the process of failure was measured using the uniaxial compression and equivalent tensile method, and the influencing factors of the stress conversion coefficient were studied by using this method. From a microscopic point of view, by carrying out quantitative analyses after microscopic imaging, the generation mechanism and distribution law of the stress conversion coefficient of compressed soil were proposed and verified. The main conclusions of this paper are listed as follows:  
1. The tensile stress generated in the soil under compression is affected by two factors: axial stress and the stress conversion coefficient. The stress conversion coefficient of soil under pressure is not constant, which is why the strength envelope of soil is a nonlinear envelope, such as Mohr's envelope, rather than a straight Coulomb envelope. For the same type of soil, the value of the stress conversion coefficient changes with a change in stress states and soil properties. Among them, an increase in loading rate increases the elastic energy ratio of the soil mass and decreases the dissipative energy ratio, and the stress characteristics of soil mass are brittle, which results in a reduction in the stress conversion coefficient of the soil mass during the compression failure process; moreover, the soil mass changes from the tension failure (bulging failure) mode to the shear failure mode under a uniaxial state. However, an increase in soil water content causes a decrease in the elastic energy ratio of the soil and an increase in the dissipative energy ratio; the stress characteristics of soil exhibits  
Figure 17: Schematic diagram of the transport mode of soil particles in compressed soil.  
more plastic behavior; the stress conversion coefficient increases; the soil changes from the shear failure mode to the tension failure (bulging failure) mode.
2. With an increase in loading rate, the peak stress increases gradually, the peak strain decreases gradually when the peak stress is reached, and the elastic modulus decreases gradually. However, with an increase in water content, only the elastic modulus of soil decreases monotonically, and the peak stress and strain of soil increase first and then decrease.
3. The tensile stress in the compressed soil is caused by the invasion and extrusion of soil particles. When the soil is under compression deformation, the soil particles in the upper layer of the internal soil particles invade the lower soil particles via insertion or shear force, and the lower soil particles are extruded, resulting in local tensile failure or local shear failure. In the compressed soil body, the distribution of tensile stress is not uniform, and tensile stress forms stress concentrations on the soil's surface such that the soil on the surface of the sample is subjected to increased tensile stress than the soil inside the sample.  
Conceptualization, Y.-X.W. and L.-S.T.; methodology, L.-S.T.; software, Y.-X.W.; validation, Y.-X.W. and L.-S.T.; formal analysis, Y.-X.W.; investigation, Y.-X.W.; resources, L.-S.T.; data curation, Y.-X.W.; writing--original draft preparation, Y.-X.W.; writing--review and editing, Y.-X.W.; visualization, L.-S.T.; supervision, L.-S.T.; project administration, L.-S.T.; funding acquisition, L.-S.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant numbers 42277142, 42102303, 41877228). Guangdong Basic and Applied Basic Research Foundation (2018B030311066).  
Not applicable.  
The data presented in this study are available upon request from the corresponding author.  
The authors are grateful to the anonymous reviewers for their helpful comments on the manuscript.  
The authors declare no conflict of interest.