## 4 Discussion  
### Microscopic Mechanism of the Mechanical Behavior  
The above phenomena could be explained by the variations of soil fabrics at various remolding water contents. As can be observed from SEM testing results (Figure 12), clay particles at \(w_{\mathrm{opt}}\) and its dry side exhibited a tendency to cluster, forming clay aggregate. For both \(w_{\mathrm{opt}}\) and its dry side, the pore structure could be categorized into two distinct types: macro-pores located between clay aggregates and micro-pores situated within clay aggregate. In contrast, on the wet side, clay particles remained dispersed without aggregation, and micro-pores were relatively uniformly distributed among particles. Figure 11: (**a**) Variation in [PERSON]’s ratio \(v\) with \(w\); (**b**) variation in dilatancy angle \(\psi\) with \(w\). Although the SEM testing offered qualitative observation of the soil morphology, a precise quantification of the pore size distribution was achieved through MIP testing, as shown in Figure 13. During MIP testing, mercury intrusion was performed by incrementally increasing the applied pressure. As the intrusion pressure increased, mercury progressively filled the pores from smaller ones to larger ones within the sample. The cumulative volume of mercury intruded at each pressure step was carefully recorded. The corresponding pore diameter \(d\) was calculated using the following equation:  
\[d=4\sigma\text{cos}\gamma/p \tag{6}\]  
where \(\sigma\) = mercury-solid interfacial tension (0.484 N/m); \(\gamma\) = mercury-solid contact angle (141.3\({}^{\circ}\)); and \(p\) = intrusion pressure. Based on these measurements, the cumulative pore size distribution curve was firstly constructed by plotting the cumulative intruded pore volume against pore diameter on a logarithmic scale. Subsequently, the differential pore size distribution curve was obtained by differentiating the cumulative data with respect to the logarithm of pore diameter (Figure 13). This differential curve illustrates the percentage of pores at a specific pore diameter \(d\). Figure 13 indicated that the clay soils at optimum water content (\(w_{\text{opt}}\)) and its dry side exhibited a bimodal pore structure, consisting of micro-pores (\(d\leq\) 0.4 \(\upmu\)m) and macro-pores (\(d>\) 0.4 \(\upmu\)m). Compared to the dry side, the clay soils at \(w_{\text{opt}}\) contained fewer macro-pores and a higher proportion of micro-pores. However, on the wet side of \(w_{\text{opt}}\), the pore structure became unimodal, predominantly composed of micro-pores. The pore structure variation was consistent with the soil fabric evolution, as illustrated in Figure 14. When on the dry side of \(w_{\text{opt}}\), large clay aggregates were formed due to high matrix suction [36]. Consequently, the soil fabric was defined by an aggregate-aggregate structure with numerous large pores between aggregates (Figure 14a). When reaching \(w_{\text{opt}}\), the aggregation tendency was reduced, and the quantity of clay particles remaining in dispersed state increased, partially occupying the large pores between aggregates. This resulted in an aggregate-dispersed structure (Figure 14b). After \(w_{\text{opt}}\), the formation of clay aggregate was inhibited due to the insufficient matric suction. In this case, the soil fabric was dominated by dispersed clay particles (Figure 14c). Based on the testing results of SEM and MIP, the soil fabric evolution with increasing \(w\) could be inferred [32; 36], as illustrated in Figure 14. On the dry side of \(w_{\text{opt}}\), the soil fabric was controlled by an aggregate-aggregate structure (Figure 14a), with water primarily confined within the micro-pores inside the clay aggregate. At \(w_{\text{opt}}\), part of the clay aggregate disintegrated into dispersed clay particles, leading to a transition in soil fabric towards an aggregate-dispersed structure (Figure 14b). In this case, most of the water was still retained within the aggregate, while a portion was distributed among the dispersed clay particles.