## 3 Results and Analysis
### Compaction Characteristics and Pore Size Distribution
#### 3.1.2 Pore-Size Distributions of Soils with Different Compaction Degrees  
The pore size distributions of soils with different moisture contents are important for elucidating the characteristics of compacted soil. According to the experimental theory of nuclear magnetic resonance, and considering the reduction in CBR caused by harsh environments, the soil used for studying the pore size distribution by NMR should be saturated. However, expansive soil contains considerable hydrophilic clay minerals (e.g., montmorillonite and illite), which expand when absorbing water. Since the soil with the maximum dry density should be compacted at the optimal moisture content (the initial moisture content of compacted soil with different compaction degrees is 16.4%), the samples should be saturated before NMR testing; the volume will expand, causing the void ratio to become larger than sample in its the initial state. Thus, it is necessary to modify the void ratio. Correction was performed by using a vernier caliper to measure the diameter and height of the saturated sample, making multiple measurements until the error fell within the allowable range. Subsequently, the void ratio was calculated and defined as the corrected void ratio. The void ratios before and after correction are illustrated in Figure 6. The void ratio of the saturated samples is obviously greater than the void ratio of the unsaturated sample, suggesting that soil swelled. The void ratio decreased with increasing compaction degree, suggesting the compaction degree of the saturated soil sample decreased. Note that the compaction degree used in this study was the compaction degree before saturation. The \(T_{2}\) value reflects the pore sizes in the sample, i.e., water in large pores has a larger \(T_{2}\) value and the water in small pores has a smaller \(T_{2}\). The pore size distribution curve can directly and quantitatively represent the pore composition and distribution. The \(T_{2}\) time distribution curves of samples with different compaction degrees are plotted in Figure 7, and the \(T_{2}\) time distribution curve shows one to three peaks with an increase in compaction degree. To simplify the analysis, the pores of the Jiamusi expansive soil were classified according to \(T_{2}\) value: the main peak corresponding to micropores, the Sub Peak 1 corresponding to mesopores, and the Sub Peak 2 belonging to macropores. It can be seen from Figure 7 that the main peak is prominent, and the subpeaks are small. The pore size distribution curves with different compaction degrees according to Equations (1)-(4) were calculated (Figure 8). The diameters of the micropores were reduced from 0.28-39.43 \(\upmu\)m to 0.0047-0.25 \(\upmu\)m, and the mesopores were decreased from 39.43-413.02 \(\upmu\)m to 1.75-11.3 \(\upmu\)m, suggesting that compaction significantly impacted the pore structure. The macropores of the soil with a compaction degree of 80% could reach 1 mm, which might be attributed to fractures produced by damage in the test after saturation. The proportion of pores with different sizes is represented by the ratio of the area corresponding to the peak to the total area of the \(T_{2}\) curve. The proportion of each peak for the samples with different compaction degrees is shown in Figure 9. The main peak of the samples with compaction degrees of 80%, 90%, 95% and 100% take up proportions of 0.77, 0.92, 0.96 and 1, respectively. The difference for soils with different compaction degrees was indicated in the  
Figure 6: The relation of void ratio e with compaction degree before and after sample saturation. proportions of pores exhibiting different sizes. As the compaction degree increased, the proportions of mesopores and macropores decreased from 0.12 and 0.11 to 0, and the decrease rate of Sub Peak 2 is higher than that of Sub Peak 1, indicating that compaction significantly affected mesopores and macropores, performing better on macropores.