3.4.5. Discussion on Strength Mechanisms

According to the mercury intrusion test result, the pore size distribution of the silt specimen is bimodal. Therefore, in the shear process, the internal stress of the silt specimen is not uniform. By comparing Figures 15a and 16a, it is obvious that the pore structure in the silt greatly affects the shear stress of the soil and thus further affects the strength and stability of the soil in the range of low water content. According to SEM pictures and mercury intrusion test results, with the increase in XG content, the pores of silt are gradually filled with XG cement. Compared with Figures 15 and 16, it can be concluded that the stress-hardening phenomenon of the silt treated by XG is more obvious than silt when the water content is 2%, 4%, and 8% because the pores of the silt treated by XG are supported by XG, which expands after absorbing moisture. Therefore, the water content of stress softening into stress hardening increased from 8% to 16%. The results indicate that XG can improve the strength and stability of soil after improving the micropore structure of the soil.


**Table 3.** Strength properties of the XG-silt specimens under different conditions (*mbp*/*ms*).

The mercury intrusion test results indicate that the pore size distribution of the silt specimen is bimodal. The dispersed pore distribution of the specimens results in an unstable structure. The silt specimen is easily disturbed by external loads; thus, the strength of the silt specimen is small. After adding XG, the silt pore size decreased, the pore size distribution changed into a unimodal pore size distribution, and the pore structure stability improved. The water retention capacity and shear strength of the silt, therefore, significantly increased. The SEM test results showed that the cohesion and internal friction angle of the silt specimens under different water content was significantly higher because XG blocks the pores of soil and produces a hydrogel that can bond silt particles together [39,55–57].
