# Pores Evolution of Soft Clay under Loading/Unloading Process
## 3 Experimental Results
### Fractal Analysis  
The surface and fragmentation fractal dimension of the pores of specimens \(L_{i}\) and \(U_{i}\) are expressed as \(D_{s}^{L_{i}}\), \(D_{s}^{U_{i}}\), \(D_{f}^{L_{i}}\), and \(D_{f}^{U_{i}}\), respectively, and the \(D_{f}\) of small pores of specimens \(L_{i}\) and \(U_{i}\) as \(D_{fs}^{L_{i}}\) and \(D_{fs}^{U_{i}}\). The change of fractal characteristics under each unloading process was reflected by the variations in fractal dimensions of specimens \(L_{i}\) and \(U_{i}\), which were defined as \(\Delta D_{s}\) and \(\Delta D_{f}\):  
\[\Delta D_{f}=D_{f}^{L_{i}}-D_{f}^{U_{i}} \tag{4}\]  
\[\Delta D_{s}=D_{s}^{L_{i}}-D_{s}^{U_{i}} \tag{5}\]  
If the value of \(\Delta D_{s}\) or \(\Delta D_{f}\) is positive, the fractal dimensions of the pores increase under unloading, otherwise they decrease. \(D_{s}^{L_{i}}\), \(D_{f}^{L_{i}}\), \(D_{fs}^{L_{i}}\), and \(D_{fs}^{U_{i}}\) are shown in Figure 10, and \(\Delta D_{s}\) and \(\Delta D_{f}\) are shown in Figure 11. The \(D_{f}\) of small pores ranged from 1.120 to 1.160, and changed by a small degree. The distribution and size difference of small pores basically was unchanged with pressure, which can be seen as the intrinsic pore characteristics of soft clay. So that the properties of soft clay are mainly influenced by the large pores.  
Figure 10: Fractal dimensions of specimens \(L_{i}\) and \(U_{i}\).  
Below 100 kPa, \(D_{s}^{Li}\) increased by 0.017 under loading, and \(D_{f}^{Li}\) increased from 1.442 to 1.477. The shapes of pores became complicated and the size difference of pores decreased under loading. The value of \(\Delta D_{f}\) increased slightly while \(\Delta D_{s}\) was negative and decreased by approximately 0.080 as the step load increased to 100 kPa. The shape complexity and the size difference of pores slightly increased under unloading.  
As the pressure increased from 100 kPa to 400 kPa, \(D_{s}^{Li}\) decreased by 0.128 under the loading process. Moreover, \(\Delta D_{s}\) increased to a positive value and then reached the maximum of 0.041 at 400 kPa. The pore shapes became less complicated under the unloading process, and the decrease of shape complexity under each unloading was further increased with the increase of the step load of \(L_{i}\). The value of \(D_{f}^{Li}\) changed from 1.477 to 1.522 under loading, while \(\Delta D_{f}\) increased from 0.081 to 0.223 under each unloading process. The size difference of pores reduced under the loading process, while it increased under the unloading process, and the increase of pore size difference was enlarged with the increase of the step load of \(L_{i}\).  
Above 400 kPa, the value of the fractal dimensions changed to a small degree. \(D_{s}^{Li}\) gradually increased to 1.460, while \(D_{f}^{Li}\) slightly decreased to 1.443. \(\Delta D_{s}\) changed from 0.004 to 0.027, and \(\Delta D_{f}\) decreased from 0.223 to 0.069. The complexity of pore shapes increased as the step load of \(L_{i}\) increased under the loading/unloading process. The size difference slightly increased under the loading process and decreased under the unloading process.  
According to the analysis, small pores had intrinsic characteristics, so that the distribution and \(D_{f}\) of the small pores were basically unchanged during loading/unloading. Based on the evolution of the \(D_{s}\) and \(D_{f}\) of pores, the loading/unloading process could be divided into three stages: the natural structural stage (<100 kPa), the structural adjustment stage (from 100 kPa to 400 kPa), and the new equilibrium stage (>400 kPa). The shape complexity and size difference of pores decreased under loading at the structural adjustment stage, while they increased when the soft clay was in the natural state or in a new balance. The shape complexity of pores under unloading changed in a similar trend with the loading process, while the size difference of pores increased under each unloading process.