## 4 Pore Evolution Characteristics of Unloaded Samples under Dry-Wet Cycle
### T\({}_{2}\) Spectrum Area  
The change of the integral area of T\({}_{2}\) spectrum distribution reflects the change of pore volume of the sample after different dry-wet cycles. The spectral area of each peak of the sample after different dry-wet cycles is shown in Table 3. The variation curves of the average spectral area of the sample with dry-wet cycles is shown in Figure 10, where the cycle is 0, which represents the state after unloading.  
The NMR information and pore distribution of unloaded samples under different confining pressures are basically the same, and the distribution of the spectrum area of each sample is also very similar. Combined with the data in Table 3, it can be seen that the distribution of pore structures in the sample is uneven; the micropores account for the vast majority of all pore types and exceed 90%. When the dry-wet cycle is 0 (after unloading and before the dry-wet cycle), the total peak area of the sample is quite different. It can be seen that different initial confining pressures have a great influence on the internal pore structure of the sample. Under the same unloading amount, as the initial confining pressure increases, the total area of the spectrum decreases from 4389.35 to 2231.34, and the reduction ratio reaches 49.16%.  
It can be seen from Figure 10 and Table 3 that under the same confining pressure and unloading amount, the total spectrum area of unloaded samples increases with the dry-wet  
Figure 10: Variation curves of the total spectrum area with dry–wet cycles.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Number of Cycles**} & **Initial** & **Confining** & **Total** & **The First** & **The First** & **The Second** & **The Third** & **The Third** & **Peak** \\  & **Confining** & **Pressure** & **Unloading** & **Area** & **Peak Area** & **Proportion** & **Peak Area** & **Proportion** \\ \hline
0 & & & 4389.35 & 4194.54 & 95.58\% & 194.80 & 4.42\% & & \\
5 & & & 4539.15 & 4411.48 & 97.19\% & 1  
cycle, and the pore volume of samples increases. The maximum increase ratio of the total area of the spectrum is 21.96% when the dry-wet cycle is 0-5, the maximum is 19.98% when the cycle is 5-10, and it is only 2.49% when the cycle is 10-15. It can be seen that in the early stage of dry-wet cycles, the internal structure of unloaded samples was significantly affected by dry-wet cycles. In a certain range, the change trend of spectrum area tends to be gentle--that is, the internal structure of the sample is gradually accumulated by the dry-wet cycle, but the influence sensitivity is reduced. During 15-20 cycles, the maximum increase of total spectrum area is 9.00%, which is larger than that during 10-15 cycles. It shows that during this period, the porosity of the sample increased significantly, and the influence of the dry-wet cycle was close to the structural damage limit of unloaded samples.  
In summary, the damage of the dry-wet cycle to unloaded samples is a cumulative process. On the one hand, the damage cracks caused by the early unloading provide more crack channels for the dry-wet cycle. Water easily infiltrates along crack channels, and the pore water pressure increases, thus forming inter-layer expansion and inter-particle expansion and increasing the cracks. At the same time, water molecules slowly penetrate into the sample along the cracks, causing the dissolution of soluble minerals and cements and also causing the closed pores formed by the cements to gradually transform into open pores. The micropores continue to expand and penetrate, the number of pores gradually increases, and the pore size and the volume increases.