# Influence of Soil Particle Size on the Engineering Properties and Microstructure of a Red Clay
## 3 Results and Analysis
### Results of UCS Test  
Take the red clay with a dry density of 1.4 g/cm\({}^{3}\) as an example. The failure pattern of the soil samples after the UCS test is provided in Figure 7. The failure patterns of red clay samples with different particle sizes appear as split and shear failure after the UCS test. It is worth noting that when the particle size is 0.5 mm, although the soil sample shows apparent shear failure, there are also evident cracks on the surface of the soil sample and split failure on the other side. Furthermore, when the particle size is between 0.25 and 2 mm, the bottom of the sample fails first, and the damaged surface expands from the bottom upwards as the pressure increases until the sample is destroyed. The main reason is that the top of the unconfined compression instrument is fixed, and the pressure is loaded onto the soil sample when the bottom rises. When the soil particles are small, the internal structure of the soil sample is dense, and the pores are tiny. As the loading pressure  
Figure 5: BJH adsorption cumulative pore volume. Figure 6: BJH desorption cumulative pore volume. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Grain** & **Micropore** & **BJH Adsorption** & **BJH Desorption** & **BJH Adsorption Average** & **BJH Desorption Average** \\
**Size** & **Volume** & **Cumulative Volume** & **Cumulative Volume** & **Pore Width (4 V/A)** & **Pore Width (4 V/A)** \\
**(mm)** & **(cm\({}^{3}\)/g)** & **(cm\({}^{3}\)/g)** & **(cm\({}^{3}\)/g)** & **(nm)** & **(nm)** \\ \hline
0.25 & 0.001210 & 0.065612 & 0.075129 & 5.5900 & 6.2015 \\
0.50 & 0.000810 & 0.061253 & 0.072472 & 5.5348 & 6.3463 \\
2.00 & 0.001235 & 0.065473 & 0.079685 & 5.5552 & 6.5203 \\
5.00 & 0.001255 & 0.062868 & 0.077363 & 5.5028 & 6.5070 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The pore volume of red clay with different particle sizes. increases, relative slippage between soil particles is arduous, causing the soil sample to split or undergo shear failure from the bottom first. When the particle size is 5 mm, as the pressure increases, no fracture cracks appear at the upper and lower ends of the soil sample; on the contrary, shear bands are generated in the middle, resulting in shear failure. The main reason is that when the soil particles are significant, there are many pores between them, which causes relative slippage. As the loading pressure grows, the pores gradually connect from point to line and then to the surface, forming a shear zone and causing shear failure in the soil sample. As shown in Figure 8, the UCS tests of red clay under different dry density conditions have obtained stress-strain curves of different particle diameters, which all reveal strain-softening with evident peak strength, and the peak point is on the left. As axial strain increases on the left side of the peak point, the axial stress increases rapidly. The soil mass is undergoing compaction, and the internal pores are gradually reduced. To the right side of the peak point, the axial stress rapidly drops and then slowly decreases. Comparing these with the curves of the red clay of different dry densities, as the dry density increases, the peak point gradually moves to the right along the horizontal axis, indicating that the soil mass has a more extraordinary ability to withstand deformation before failure with the increase in dry density. Furthermore, by comparing the angle formed by the peak point and the curve on both sides, it is easy to see that the greater the dry density, the greater the angle; this also indicates that the ability of the soil to withstand deformation before failure enhances with the increase in dry density. In addition, by comparing the residual strength values of damaged soil samples with the exact particle sizes but different dry densities (when the axial strain is 20%), a similar result is obtained that the dry density is proportional to the residual strength.