The soil utilized in this experiment was collected from a site approximately 4 meters below the ground near Guilin University of Electronic Science and Technology. Subsequently, the collected soil samples underwent air-drying and were sieved through a 2 mm sieve, as shown in Figure 1. Their fundamental physical properties were determined in accordance with the “Highway Geotechnical Test Code” [13] . According to the provisions of literature [13] , Soil with plasticity index greater than 10 is cohesive soil. When the plasticity index Ip is greater than 17, it is clay. When the plasticity index Ip is greater than 10, but not greater than 17, it is silty clay. The results are presented in Table 1. Notably, the plasticity index of the soil is 20.7, surpassing the threshold of 17, thereby categorizing it as clayey soil.

Seven distinct pH solutions (pH values of 4, 5, 6, 7, 8, 9, and 10) were meticulously prepared using distilled water with a neutral pH of 7 as the base. Hydrochloric acid solution and sodium hydroxide powder were judiciously employed to fine-tune the pH to the desired levels, with each solution being securely sealed using plastic wrap. Subsequently, a uniform application of each of the seven pH solutions was administered onto 500g of soil via a spray bottle. Following a 24-hour period of sealed storage, the treated soils were adjusted to attain a moisture content of 27.5%. The test samples were concocted by combining red clay with varying proportions of clay minerals, yielding four distinct particle size distributions: −10%, 0%, 10%, and 20% clay content. The low-clay content soil was crafted by incorporating clay minerals into soil from which clay had been extracted.

The experimental apparatus employed in this study was a strain-controlled direct shear apparatus, meticulously manufactured by Nanjing Soil Instrument

Factory Co., Ltd. According to the fast shear test of consolidation of middle soil in literature [13] , the shear rate is set at 0.8 mm/min, and the shear loss is required within 3 - 5 min. However, in this test, the sample was always shear loss within 3min. In order to prolong the shear time and observe the shear failure of the test more clearly, the shear rate was set at 0.6 mm/min. And the completion of each test was determined upon the commencement of shear stress reduction. Four discrete normal stresses (100 kPa, 200 kPa, 300 kPa, and 400 kPa) were applied during the shear tests.

To account for the variables of pH and clay content, two distinct sets of experimental protocols were meticulously devised, as outlined in Table 2.

In the compaction process, a total of 105.61 g of moist soil was employed to fabricate ring samples with a dry density of 1.39 g/cm³. Subsequently, the prepared specimens were placed within a humidity chamber, primed for use in direct shear tests.

The experimental data obtained from the direct shear tests underwent linear regression analysis to establish the relationship between the shear strength of red clay and normal stress under varying pH conditions. The results are depicted in Figure 2.

Figure 2 reveals that, under diverse normal stresses, the shear strength of red clay gradually diminishes as the pH shifts towards either acidity or alkalinity. The peak shear strength is attained when the pH of the red clay is neutral. Notably, red clay particles comprise a substantial quantity of oxygen-containing compounds, including Al₂O₃, Fe₂O₃, SiO₂, and others. When these free oxygen-containing compounds engage in chemical reactions with acidic or alkaline solutions, they interact with H⁺ or OH⁻ ions, resulting in varying degrees of dissolution and leaching of the cementing material that binds the clay particles together. This reduction ultimately leads to a degradation of the original soil structure, with the effect becoming more pronounced as the acidity or alkalinity intensifies.

Exposure to strong acids or alkalis leads to an elevation in the concentration of H⁺ or OH⁻ ions, culminating in the erosion of the interparticle cementing material. This erosion results in an enlargement of pore size, dispersion of particles,

and a weakening of the soil structure. Strong acidic solutions have the capacity to dismantle the structural connections that exist between particle clusters and the internal mineral clays within particles. Conversely, when red clay interacts with weak acids or weak alkalis, only the outermost layer of particles experiences erosion, and solely a portion of the particle cementing material is eroded. Interparticle bonding remains partially intact, consequently yielding a higher shear strength when compared to red clay specimens infiltrated by strong acids or strong alkalis. As the pH value continues to increase, the oxygen-containing compounds undergo successive decomposition, causing the cementing effect to gradually wane.

The presence of cementing material within red clay bolsters the structural integrity of the soil. Nonetheless, alterations in the solution medium conditions can lead to modifications in the physical properties of the cementing material, resulting in the structural disruption of the soil. Upon juxtaposing the four graphs, it becomes evident that with an elevation in clay content, the shear strength of the soil progressively ascends. Such as, as shown in Figure 3, under the same pH = 7, shear displacements of shear samples with different clay content gradually decrease with the increase of clay content. This phenomenon arises due to the increased presence of clay content, which correlates with an augmentation in oxygen-containing compounds within the soil. Consequently, more compounds engage in reactions with H⁺ or OH⁻ ions within the acidic or alkaline medium. While only a fraction of the clay minerals participate in these reactions, the majority remains interspersed among soil particles, thereby preserving a degree of interparticle bonding. Consequently, soils characterized by higher clay content manifest superior shear strength in comparison to their counterparts with lower clay content.

Figure 4 presents curves illustrating how shear strength parameters of the soil change with varying pH values for different clay content levels. From the graph, it is evident that as the pH of the infiltrating solution increases, both the cohesion and internal friction angle of the soil gradually decrease for a constant clay

content. Under the same pH conditions, the cohesion of the soil increases with higher clay content, while the internal friction angle decreases with increased clay content. When acidity or alkalinity becomes stronger, and acidity and alkalinity are relatively weak, the reduction in both cohesion and the internal friction angle is approximately the same. However, under strong acidic or alkaline conditions, the reduction in cohesion is significantly less than the reduction in the internal friction angle. This may be attributed to the fact that the internal friction angle is more sensitive to changes in the content of the cementing material affected by chemical reactions with acidity or alkalinity.

The cohesion (c) of the soil is determined by the degree of bonding between soil particles. An increase in the amount of cementing material between soil particles strengthens the bonding, resulting in increased cohesion. When red clay is contaminated by acidic or alkaline solutions, the cementing material between soil particles dissolves, reducing the bonding between soil particles and disrupting the soil structure. Therefore, as the acidity or alkalinity of the solution increases, the cohesion (c) of red clay gradually decreases. In Figure 4, under higher clay content, the decrease in cohesion is less pronounced compared to lower clay content, suggesting that as clay content increases, the interparticle bonding among red clay particles causes relatively minor chemical and physical damage to the soil, resulting in a smoother change in cohesion.

The internal friction angle (φ) of the soil depends on the internal friction between soil particles. A larger specific surface area of soil particles leads to greater internal friction between particles. However, when soil is exposed to acidic or alkaline solutions, the reduction in oxygen-containing compounds between particles decreases the specific surface area, leading to a decrease in internal friction between particles and a gradual reduction in the internal friction angle.