# Influence of Soil Particle Size on the Engineering Properties and Microstructure of a Red Clay
## 3 Results and Analysis
### Results of Macro Cracks  
#### 3.3.1 The Influence of WD Cycles on the Crack of Red Clay with Different Particle Sizes  
Red clay samples with different particle sizes and dry densities were subjected to 12 WD cycle tests, and three-dimensional crack extraction was performed on a ring knife sample of each. The fracture extraction process of the twelfth cycle of the soil sample with a dry density of 1.3 g/cm\({}^{3}\) and particle size of 0.25 mm is illustrated in Figure 11.  
The three-dimensional cracks and pores in the soil sample are filled and extracted, shown in Figure 11a-c, indicating that the method is feasible. The crack changes of red clay samples with different particle sizes and a dry density of 1.3 g/cm\({}^{3}\) after 1, 4, 8, and 12 WD cycles are displayed in Figures 12-15. Figure 11: Fracture extraction process: (**a**) original image; (**b**) crack filling; (**c**) crack extraction. Figure 10: The fitting curve of particle size and UCS at different dry densities. After one WD cycle, it is found that the red clay soil sample with different particle sizes is mainly composed of tiny independent pores, and the larger the particle size, the greater the number of pores in the sample. The soil sample with a particle size of 0.25 mm generated evident cracks after four WD cycles (Figure 13). The particle size was 0.50-5.00 mm, with mainly independent pores. The changes between 0.50 mm and 2.00 mm were small. When the particle size was 5.00 mm, large pores increased, and the number of connected pores in local areas increased. Figure 14 demonstrates that after eight WD cycles, the local cracks of the soil sample with a particle size of 0.25 mm become connected, with a long crack appearing that almost penetrates itself. In the red clay with a particle size of 0.50 mm, small cracks appeared. There was still tiny independent pores and obvious water-loss and shrinkage cracks in the boundary of the soil sample with a size of 2.00 mm. In samples with a particle size of 5.00 mm, pierced cracks also appeared. There were many independent pores, and the soil sample boundary also displayed shrinkage cracks due to water loss. Figure 15 shows that during the 12 th WD cycle, the cracks in the soil sample with a particle size of 0.25 mm were developed intensely; meanwhile, there were almost no independent pores remaining. There were evident pierced cracks at 0.50 mm, and the cracks were wider than 0.25 mm. The range in size of cracks' boundaries increased when the particle size was 2.00 mm, and the number of tiny pores inside the soil sample decreased. When the size was 5.00 mm, the primary cracks became broader and more branched, and the  
Figure 14: Cracks in soil sample after 8 WD cycles: (a) 0.25 mm; (b) 0.50 mm; (c) 2.00 mm; (d) 5.00 mm. Figure 13: Cracks in soil sample after 4 WD cycles: (a) 0.25 mm; (b) 0.50 mm; (c) 2.00 mm; (d) 5.00 mm. Figure 15: Cracks in soil sample after 12 WD cycles: (a) 0.25 mm; (b) 0.50 mm; (c) 2.00 mm; (d) 5.00 mm. independent pores also showed a decreasing tendency. The comparison results infer that as the number of wet and dry cycles increases, cracks on the red clay sample become more pronounced and the first crack appeared when the particle size was 0.25 mm. When the particle size reaches 2.00 mm, the occurrence of boundary cracks is mainly due to water loss and shrinkage. When the particle size reaches 5.00 mm, the most significant number of cracks appears. The pores and cracks were quantified, as were the fractal dimension and total porosity of each soil sample, as shown in Table 6. With the same number of WD cycles, the red clay's total porosity and fractal dimension increased with the growth in particle size, consistent with the results shown in Figure 15. With the same particle size, as the number of wet and dry cycles increased, the total porosity increased first and then decreased.