## 3 Results and Discussion
### Acid- and Alkali-Contaminated Soils Treated with XG-eat Combination
#### 3.2.2 Direct Shear Test  
Figure 14 presents the results of the direct shear tests conducted on the acid- and alkali-contaminated soils treated with the XG-oat soil reinforcing method. It is obvious that the combination of XG and oat significantly improved the soil strength, i.e., the soils treated with the XG-oat combination (i.e., represented by the dotted line) had higher shear strength values than the untreated soils (i.e., represented by the solid line) at any pH values. Moreover, the addition of XG improved the shear strength of the vegetated soils, especially for those exposed to acidic solutions. For example, the maximum shear strength values at pH = 2 were 55.8, 98.7, and 125.2 kPa under the overburden stresses of 50, 100, and 150 kPa, respectively, corresponding to the 0.50% XG-oat combination, while the vegetated soil without XG had much lowered strength values of 53.7, 73.6, and 103.9 kPa under the three overburden stresses, respectively (Figure 14a). With the increase in pH values, the positive effect of XG on improving the soil shear strength gradually diminished. For example, at pH = 2, incorporating XG with any concentration (i.e., from 0.25% XG to 1.00% XG) led to a higher strength compared with the case without XG (Figure 14a). When the pH increased to 9 or 12, only the addition of 0.25% XG and 0.50% XG led to a higher strength compared with the case without XG (Figure 14d,e). Overall, for acidic and neutral pH values (i.e., pH = 2, 5 and 7), the direct shear strength of the soil treated with the 0.5% XG-oat combination was the highest, while for the alkaline pH values (i.e., pH = 9 and 12), the 0.25% XG-oat combination was the most efficient in soil strengthening. Excessive XG contents, which hindered the oat growth, were less effective in improving the soil strength, due to insufficient interaction between the soil particles and oat roots. Figure 15 shows the values of the cohesion and internal friction angle under different pH values or different XG contents. It was observed that for the three treatment conditions (oat-only, 0.25% XG-oat, and 0.5% XG-oat), the cohesion first decreased and then increased, while the internal friction angle showed an opposite trend with the increasing pH value (Figure 15a,b). When a higher XG content (i.e., 0.75% and 1.00%) was used, the cohesion and internal friction angle did not change with a distinct trend (Figure 15a,b). In addition, the cohesion of the acid- and alkali-contaminated soil first decreased and then increased with the increasing XG content, while the internal friction angle showed an opposite trend (Figure 15c,d). However, for the uncontaminated soil, the cohesion first increased and then decreased, and the internal friction angle did not change significantly with the increase in XG content (Figure 15c,d). Figure 13: Formation of crusts on top and accumulation of water on the soil surface (1.00% XG–oat). Figure 14: Shear strength, cohesion \(c\), and internal friction angle \(\varphi\) with increasing XG contents under (a) pH = 2; (b) pH = 5; (c) pH = 7; (d) pH = 9; (e) pH = 12. Figure 16a presents the variation in direct shear strength (at overburden stress = 150 kPa) with the increasing XG content under different pH values. Obviously, adding an appropriate amount of XG (i.e., 0.25% XG and 0.50% XG) helped to increase the soil shear strength. Generally, the shear strength of the soil was positively related to the gemination ratio, i.e., the higher shear strength was accompanied by the higher gemination ratio due to the more sufficient interaction between the oat roots and soil particles (Figure 16b-f). Exceptions were observed in Figure 16c,e, where the 0.50% XG-oat combination led to a smaller gemination ratio but a higher shear strength compared with the 0.25%XG-oat combination. This might be because the shear strength of the soil was not only affected by the root mass ratio but by the remaining XG in the soil as well.