## 2 Uniaxial Compression Test on Earth Materials
### Experimental Stress-Strain Curve  
The uniaxial compressive stress-strain curves of the earth material for different working conditions are given in Figure 10. The stress and strain results for the whole process of loading are obtained according to the load and displacement data collected during the test and processed using the following equations:  
\[\sigma=\frac{F}{A} \tag{2}\]  
\[\varepsilon=\frac{\Delta l}{l} \tag{3}\]  
where \(\sigma\) is the axial stress corresponding to each level of loading during the uniaxial compression test of the earth material, Mpa; \(F\) is the axial force applied at each level of loading, kN; \(A\) is the initial cross-sectional area of the specimen, mm\({}^{2}\); \(\varepsilon\) is the axial strain corresponding to each level of loading during the uniaxial compression test of the earth material; \(\Delta l\) is the displacement deformation under compression at each level of loading, mm; \(l\) is the initial height of the specimen, mm.  
In Figure 10, it can be seen that the stress-strain curves of the eight groups of specimens basically follow the same trend. It consists of two parts: the rising section and the descending section. In the ascending section, there is an initial concave section, i.e., the slope of the curve in the ascending section first increases and then decreases. Due to the gentle curve transition, it can be excluded that the formation of the initial under-concave  
Figure 10: Average stress-strain curves of eight groups of specimens.  
Figure 9: Coefficient of variation of compressive strength of specimens under different working conditions.  
section was caused by the poor contact between the specimen and the loaded end. This feature is also shown in [35]. It can be seen that the initial depression of the stress-strain curve is typical for earth materials. The formation is due to the production process and natural maintenance method of the earth specimen, which will make more initial pores and fine cracks exist inside the specimen. Under the action of a small vertical load, the fine cracks and pores will be first closed and filled by squeezing. That is, the test block is rapidly compressed and deformed under a smaller load. This is characterized in the stress-strain curve as an initial underconcave section.  
In conclusion, we take the ratio of the axial stress \(\sigma\) to the peak stress \(\sigma_{\rm pr}\) (\(y=\sigma/\sigma_{\rm pr}\)) in uniaxial compression of the earth material as the vertical coordinate, and the ratio of the axial compressive strain \(\varepsilon\) to the peak compressive strain \(\varepsilon_{\rm pr}\) (\(\rm x=\varepsilon/\varepsilon_{\rm pr}\)) as the horizontal coordinate. The typical uniaxially compressed dimensionless stress-strain curve of the earth material is finally obtained, as shown in Figure 11. The curve can be divided into four distinct phases, which are: the initial under-concave section (\(0\leq\rm x\leq x_{a}\)), the elastic growth section (\(\rm x_{a}\leq\rm x\leq x_{b}\)), the yielding stage (\(\rm x_{b}\leq\rm x\leq x_{c}\)), and the damage stage (\(\rm x_{c}\leq\rm x\leq x_{d}\)). The four characteristic points \(\rm a\), \(\rm b\), \(\rm c\), and \(\rm d\) on the curve are defined as: point \(\rm a\) is the specimen micro-crack and pore pressure density point; point \(\rm b\) is the specimen yield point; point \(\rm c\) is the specimen peak strength point; and point \(\rm d\) is the specimen failure point.