# Pores Evolution of Soft Clay under Loading/Unloading Process
## 3 Experimental Results
### FESEM Images and Pore Size Distribution  
FESEM images are shown in Figures 5 and 6. When the loading pressure was lower than 100 kPa, the microstructure of the soft clay had a flocculent structure. The interaction between soil particles was characterized by face-to-face connection, while the shapes of the pores, between or inside the soil particles, were complicated. However, the microstructure of soft clay turned into a looser flocculent structure under the unloading process. The interaction between soil particles was displayed by face-to-face and face-to-edge types. The area of the large pores decreased, and the shapes of the pores became complicated. When the load ranged from 200 kPa to 400 kPa, soil particles broke into small aggregates. Large pores were collapsed into smaller ones, and were filled with dispersed soil particles. While the loose soil particles were gathered into massive ones under the unloading process, and edge-to-edge and edge-to-point connection types appeared. Pores were connected to develop larger ones, and the boundaries of pores were more apparent than those under the loading process. The microstructure of clay underwent a significant change under the unloading process. At 800 kPa, the interaction between soil particles was represented by face-to-face and face-to-edge types. The loose soil particles were squeezed into massive soil particles, and the boundaries of pores were more apparent than those under loading process. When it was unloaded, the microstructure of the clay changed into a flocculent one, that was much denser than that under 50 kPa. When it was 1600 kPa, the massive soil particles were squeezed into a much denser condition, and formed into a layered state. The area of large pores decreased, and the shapes of the pores became complicated. However, a flocculent and honeycomb structure was developed under the unloading process. The boundaries of the pores became ambiguous again. The observation of FESEM images shows that it was the trend that the smaller pores were developed as the pressure increased under the loading process, and the complexity of pores shapes decreased and then increased. Under the unloading process, the microstructure of the soft clay changed significantly, comparing each loading step. It was necessary to analyze the micro parameters to describe the change of the microstructure. Figure 4: The compression and swelling indices. Figure 5: FESEM images of specimens \(L_{i}\) with \(\times\)5000 magnifications. (**a**) \(L_{1}\) (50 kPa); (**b**) \(L_{2}\) (100 kPa); (**c**) \(L_{3}\) (200 kPa); (**d**) \(L_{4}\) (400 kPa); (**e**) \(L_{5}\) (800 kPa); (**f**) \(L_{6}\) (1600 kPa). Figure 6: _Cont._  
The method of obtaining the cumulative percentage of pore-size distribution is shown in Figure 7. The distribution of pores whose diameters were smaller than 0.2 \(\upmu\)m or larger than 0.6 \(\upmu\)m was linear to pore diameters. The change rate of the curves between 0.2 \(\upmu\)m and 0.6 \(\upmu\)m gradually slowed down. The diameters of 0.2 \(\upmu\)m and 0.6 \(\upmu\)m were regarded as the turning pore diameters. Pores smaller than 0.2 \(\upmu\)m were defined as small pores, while pores ranged from 0.2 \(\upmu\)m to 0.6 \(\upmu\)m as medium ones, and bigger than 0.6 \(\upmu\)m as large ones. The size distribution of small pores and the slope, \(k\), of the fitting line below 0.2 \(\upmu\)m are shown in Table 3, and was basically unchanged under the loading/unloading process. The correlation coefficient, \(R^{2}\), was above 0.950. The change trend of the medium and large pore-size distribution is shown in Figure 8.  
Figure 6: FESEM images of specimens \(U_{i}\) with \(\times\)5000 magnifications. (**a**) \(U_{1}\) (50 kPa); (**b**) \(U_{2}\) (100 kPa); (**c**) \(U_{3}\) (200 kPa); (**d**) \(U_{4}\) (400 kPa); (**e**) \(U_{5}\) (800 kPa); (**f**) \(U_{6}\) (1600 kPa). Figure 7: Pore-size distribution (200 kPa). (**a**) Pore-size distribution of all pores; (**b**) pore-size distribution of small pores (\(<\)0.2 \(\upmu\)m).