## 3 Results and Discussion
### Acid- and Alkali-Contaminated Soils
#### 3.1.2 Direct Shear Test  
Figure 6 presents the relationship between shear stress and shear displacement of the acid- and alkali-contaminated soils at different overburden stresses (i.e., 50, 100, and 150 kPa). The results indicate that the acid-contaminated soils had lower shear stress-displacement curves than the uncontaminated soil (i.e., pH = 7) and alkali-contaminated soils. As mentioned previously, the clayey soil contained Al\({}_{2}\)O\({}_{3}\) with a concentration of 13.51% and Fe\({}_{2}\)O\({}_{3}\) with a concentration of 7.44%. When the clayey soil was polluted by the acidic solution, a reaction between the acid (i.e., H\({}_{2}\)SO\({}_{4}\)) and the oxides in the soil (i.e., Fe\({}_{2}\)O\({}_{3}\) and Al\({}_{2}\)O\({}_{3}\)) occurred, leading to the generation of Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) and Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\). The newly generated substances could dissolve in the solution and flow out of the soil [27,62], resulting in the reduction in the solid phase composition and the formation of a loosened soil structure [59,60]. Figure 7 shows the SEM images of the uncontaminated soil (i.e., pH = 7) and the soil contaminated with the acidic solution (i.e., pH = 2). It is confirmed that the acid-contaminated soil became more porous (Figure 7b,d) compared with the uncontaminated soil (Figure 7a,c), due to the corrosion of oxides and cemented minerals in the soil with the progress of the chemical reaction [27].  
Compared with the acid-contaminated soil, the shear strength values of the alkali-contaminated soils did not reduce so much (e.g., at overburden stress = 150 kPa) and even increased (e.g., at overburden stress = 50 and 100 kPa). This might be attributed to the pH-dependent nature of the charges on the exposed edges of clay particles. According to [33], when the pH value is low, particle edge charges become increasingly positive due to the adsorption of H\({}^{+}\) ions and, conversely, more negative at high pH values due to the adsorption of OH\({}^{-}\) ions [33]. As a result, edge-to-face (E-F) associations tend to dominate interparticle binding in the acidic pH environment [32], while the face-to-face (F-F) associations prevail in the alkaline pH environment [58]. Therefore, the increased  
Figure 5: (**a**) Liquid limit; (**b**) plastic limit of the acid- and alkali-contaminated soils.  
shear strength of the alkali-contaminated soil might be due to the formation of the face-to-face clay particle associations [26].  
Figure 6: Relationship between shear stress and shear displacement of the acid- and alkali-contaminated soils under overburden stresses of **(a)** 50 kPa; **(b)** 100 kPa; **(c)** 150 kPa.  
Figure 7: SEM images of uncontaminated and acid-contaminated soils for **(a)** pH = 7, 2 \(\upmu\)m; **(b)** pH = 2, 2 \(\upmu\)m; **(c)** pH = 7, 5 \(\upmu\)m; **(d)** pH = 2, 5 \(\upmu\)m.  
Figure 8 shows the relationship between the overburden stress and direct shear strength under different pH values, along with the cohesion and internal friction angle for each condition. When the soil was exposed to the alkaline solution, the cohesion increased from 2.5 kPa at pH = 7 to 9.1 kPa at pH = 12, while the internal friction angle decreased from 36.5\({}^{\circ}\) at pH = 7 to 32.5\({}^{\circ}\) at pH = 12, which is consistent with the report of [PERSON] et al. [63]. For the acid-contaminated soil, the cohesion increased from 2.5 kPa to 11.6 kPa and the internal friction angle decreased from 36.5\({}^{\circ}\) to 28\({}^{\circ}\) when the pH value decreased from 7 to 5. When the pH value further decreased to 2, the cohesion decreased sharply from 11.6 kPa to 3.3 kPa and the internal friction angle increased from 28\({}^{\circ}\) to 33.2\({}^{\circ}\).