Study on the Shear Strength and Durability of Ionic Soil Stabilizer-Modified Soft Soil in Acid Alkali Environments

Abstract

Soft soils, characterized by high compressibility, low shear strength, and high water sensitivity, pose serious challenges to geotechnical engineering in infrastructure projects. Traditional stabilization methods such as lime and cement face limitations, including environmental concerns and poor durability under chemical or cyclic loading. Ionic soil stabilizers (ISSs), which operate through electrochemical mechanisms, offer a promising alternative. However, their long-term performance—particularly under environmental stressors such as acid/alkali exposure and cyclic wetting–drying—remains insufficiently explored. This study evaluates the strength and durability of ISS-modified soil through a comprehensive experimental program, including direct shear tests, permeability tests, and cyclic wetting–drying experiments under neutral, acidic (pH = 4), and alkaline (pH = 10) environments. The results demonstrate that ISS treatment increases soil cohesion by up to 75.24% and internal friction angle by 9.50%, particularly under lower moisture conditions (24%). Permeability decreased by 88.4% following stabilization, resulting in only a 10–15% strength loss after water infiltration, compared to 40–50% in untreated soils. Under three cycles of wetting–drying, ISS-treated soils retained high shear strength, especially under acidic conditions, where degradation was minimal. In contrast, alkaline conditions caused a cohesion reduction of approximately 26.53%. These findings confirm the efficacy of ISSs in significantly improving both the mechanical performance and environmental durability of soft soils, offering a sustainable and effective solution for soil stabilization in chemically aggressive environments.

1. Introduction

Soil improvement has always been a crucial issue in geotechnical engineering [1,2,3,4]. Soft soils, characterized by high compressibility, low shear strength, and excessive moisture sensitivity, pose significant challenges in geotechnical engineering applications such as foundation support and infrastructure development [5,6,7,8]. Traditional soil stabilization techniques, including the use of cement and lime, have been widely employed to improve the mechanical properties of soft soils. However, these methods often suffer from limitations such as high carbon emissions, poor chemical resistance, and decreased performance under dynamic environmental conditions. Moreover, their long-term effectiveness is questionable in chemically aggressive environments, such as those exposed to acid rain, industrial effluents, or alkaline leachates. To overcome these limitations, ionic soil stabilizers (ISSs) have emerged as a sustainable alternative: they employ anionic surfactants to modify the diffuse double layer around clay particles, promoting flocculation and microstructural densification without the environmental drawbacks of cementitious additives [9,10,11].

Recent advancements in soil stabilization highlight the growing interest in non-traditional chemical additives. ISS technology leverages anionic surfactants to reduce the thickness of the double-diffused layer (DDL) around clay particles, thereby diminishing inter-particle repulsion and promoting particle aggregation [9,12]. Studies by Tavakoli et al. (2018) [13] and Gautam et al. (2020) [14] demonstrated ISS efficacy in reducing swell potential and improving unconfined compressive strength of expansive clays. Luo et al. (2016) [12] and Wu et al. (2021) [10] demonstrated that ISS treatments reduce swell potential and enhance unconfined compressive strength by altering particle surface charges and bound-water film thickness. However, critical knowledge gaps persist regarding the long-term durability of ISS-treated soft soils under environmental stressors. Niu et al. (2024) [15] conducted a comprehensive analysis of the shear strength of cement stabilized with different ratios and long-term (70-day) stabilizers, taking into account the durability of wet–dry cycles. However, the impact of acidity and alkalinity on the strength of solidified soil was overlooked. In particular, the combined effects of moisture fluctuations, chemical contamination (such as exposure to acid or alkali), and hydraulic conductivity on the evolution of shear strength have not been thoroughly investigated. Here, environmental durability refers to the ability of stabilized soils to maintain their mechanical integrity when subjected to the simultaneous influence of chemical (acid/alkali), hydrological (wetting-drying cycles), and hydraulic (permeation) stressors.

Recent research highlights the vulnerability of stabilized soils to cyclic wetting–drying [16,17,18,19] and chemical degradation [20,21], yet systematic investigations into acid–alkali resistance of ISS-modified soft soils are scarce. Furthermore, permeability reduction is frequently cited as one of the key benefits of ISS treatment, based on its ability to densify the soil matrix, reduce pore connectivity, and limit fluid ingress [22]. However, the correlation between permeability reduction and post-permeation mechanical strength retention has yet to be firmly established through controlled experimentation. In the absence of empirical validation, the assumed protective effect of lower permeability remains speculative. A clearer understanding of how reduced permeability contributes to strength preservation—particularly after exposure to hydraulic permeation—is essential to assess the true long-term benefits of ISS and to support its wider implementation in geotechnical practice.

In this context, our study aims to fill two critical knowledge gaps: (1) quantifying the combined effects of acid (pH 4) and alkali (pH 10) environments on the shear strength and cohesion of ISS-treated soft soils, and (2) elucidating the interrelationship between permeability reduction and post-permeation strength resilience under cyclic wetting–drying. Accordingly, we present a comprehensive experimental program—comprising direct shear, variable-head permeability, and acid/alkali wetting–drying tests—to (i) assess mechanical enhancements and (ii) evaluate the durability performance of ISS-modified soils. By integrating recent advances and addressing these understudied factors, this work provides new insights into the design of environmentally durable stabilization strategies for soft clay engineering.

2. Materials and Methods

2.1. Test Material

The soft soil used in this experiment was sourced from the foundation soil near the engineering site, as shown in Figure 1. The sampling depth was approximately 10 m. The soil samples were air-dried, pulverized, and sieved through a 2 mm mesh for further use. A series of physical property tests were conducted on the samples, measuring key indicators such as natural moisture content, natural density, liquid limit, plastic limit, specific gravity, and grain size distribution. The results are presented in

Table 1. Basic properties of the soil samples tested.

Soil Type	Specific Gravity/Gs	Optimal Moisture Content/%	Liquid Limit/%	Plastic Limit/%	Plasticity Index
Clay	2.69	30.5	67.5	38.2	29.3

. The grain distribution curve of the soil samples is depicted in Figure S1 (see Supplementary Materials).

ISS is a dark brown, water-soluble liquid that is stable for long-term storage. It is a highly concentrated anionic surfactant, non-toxic, non-corrosive, pollution-free, and non-flammable, making it a safe engineering material. The ISS used in this study is a proprietary formulation supplied by Shanghai Youyuan Construction Engineering Co., Ltd., Shanghai, China. The main active components include anionic surfactants, organic polymers, and inorganic salts. Based on manufacturer documentation, the major chemical constituents include C₁₂H₂₅SO₄Na, –CH₂CH(CONH₂)–, and small amounts of Na₂SiO₃, which collectively contribute to the chemical modification of the soil matrix.

2.2. Experimental Content and Methods

2.2.1. Direct Shear Test

The pre-sieved soil samples were air-dried and stored for later use. An ionic soil stabilizer was diluted with water at an optimal mass ratio of 1:100, as recommended by the manufacturer. Based on the dry soil mass, the moisture content and compaction effort were carefully controlled. Cylindrical specimens with a dry density of 1.4 g/cm³ were prepared using a light compaction hammer, with a diameter of 102 mm and a height of 20 mm, as shown in Figure 2. Both untreated and stabilized specimens were prepared at moisture contents of 36%, 30%, and 24%. These specimens were sealed in airtight containers and cured for 72 h before being subjected to direct shear testing [23]. Specimens were cured at 25 ± 2 °C in sealed containers for 72 h—a duration sufficient for ionic exchange equilibrium [12]. This protocol prevents moisture loss, ensuring reaction completion under controlled humidity (>95% RH). It is acknowledged that longer curing durations or field conditions (e.g., temperature fluctuations, humidity) may influence strength development, which will be considered in future studies. After the test, moisture content was measured to ensure deviation remained within ±1%. Quick shear tests were conducted at a shear displacement rate of 12 r/min under vertical stresses of 100, 200, 300, and 400 kPa. Each test was typically completed within 5 min.

2.2.2. Permeability Test

A variable head permeability test [5,24] was conducted using ring-cut soil samples with an internal diameter of 61.8 mm and a height of 40 mm, as shown in Figure 3. The samples were saturated by vacuum saturation. Each sample was placed inside a saturation device within a vacuum chamber. A thin layer of petroleum jelly was applied between the vacuum chamber and its acrylic lid to ensure an airtight seal. The vacuum pump was turned on, and once the internal pressure reached one standard atmospheric pressure and was maintained for at least one hour, the inlet valve was opened to slowly introduce the pre-prepared saturating solution. During this process, the pressure reading was kept constant. Once the solution fully submerged the specimen, the pump was turned off, and the sample was left undisturbed for 12 h to ensure complete saturation. The test was conducted at a water temperature of 20 °C. Following permeability testing, the stabilized soil samples were air-dried indoors until reaching a moisture content of approximately 30%. They were then sealed in fresh-keeping containers with plastic wrap and equilibrated for 12 h to ensure uniform moisture distribution. From each batch, four cylindrical specimens (diameter 102 mm, height 20 mm) were extracted using a ring cutter. These were subjected to quick shear tests under vertical stresses of 100, 200, 300, and 400 kPa using a direct shear apparatus. The results were used to compare the shear strength properties (cohesion and internal friction angle) of untreated and stabilized soils.

2.2.3. Direct Shear Test Under Acid–Alkali Wetting–Drying Cycles

The soaking solutions used in this test had pH values of 8.06 (water), 4.0 (hydrochloric acid), and 10.0 (sodium hydroxide), as shown in Figure 4. Due to the volatile nature of hydrochloric acid—whose concentration may be diluted by rainwater during field exposure—a moderate concentration (pH = 4) was selected to better simulate actual environmental conditions. To simulate realistic mildly alkaline field conditions (e.g., exposure to leachate or alkaline industrial runoff), a diluted NaOH solution with pH = 10 was prepared, rather than a fully concentrated sodium hydroxide solution. To ensure sample uniformity, pH = 4 and pH = 10 solutions were prepared and placed in a vacuum chamber. The ISS-stabilized soft soil specimens were saturated by vacuum pumping for 6 h, followed by immersion under vacuum for at least 12 h to achieve full saturation and uniform internal moisture distribution. The saturated specimens were then air-dried to reach a moisture content of 30% ± 0.1%. These were sealed in fresh-keeping bags and stored in airtight containers for 24 h to allow for moisture equilibration. The prepared acid- and alkali-contaminated stabilized soft soil samples were subjected to direct shear testing after each of the wetting–drying cycles. The shear strength data collected after each cycle were used to evaluate the effects of repeated wetting–drying on the mechanical properties (cohesion and internal friction angle) of stabilized clay under acid and alkali contamination.

3. Results and Discussion

3.1. Effect of Moisture Content on Strength of Solidified Soft Soil

The moisture content significantly affects the shear strength of soil [25], making it crucial to study the strength characteristics of solidified soft soil under varying moisture conditions. The relationship between shear stress and shear displacement was plotted, and the peak or stable value was selected as the shear strength. The internal friction angle and cohesive force, which influence shear strength, can be described by the Coulomb formula [26].

The internal friction angle and cohesive force of the solidified soft soil were determined through direct shear testing, as shown in Figure 5. At lower moisture content, both the cohesive force and internal friction angle of solidified soft soil increase noticeably, though the rate of increase diminishes. The results indicate that the internal friction angle and cohesive force of the solidified soil are significantly improved compared to those of untreated soil. Specifically, the internal friction angle increases by 9.50%, 4.44%, and 8.10% for moisture contents of 36%, 30%, and 24%, respectively. The cohesive force increases substantially, by 25.15%, 47.52%, and 75.24%, respectively. These findings suggest that the curing effect is most pronounced at lower moisture content, particularly in terms of the considerable increase in cohesive force.

The addition of ISS reduces the thickness of the double electric layer between soil particles, converting the hydrophilicity into hydrophobicity. This results in a reduced distance between particles, an increased particle density, and enhanced cementation between particles, all of which improve the soil’s strength and solidification effect by strengthening its cohesive force. Similar trends were observed by Luo et al. (2020) [9], who noted that ISS-treated clays showed significant strength gains due to double-layer compression and flocculation effects induced by anionic surfactants.

The solidification mechanism of ISS-reinforced soft soil is illustrated in Figure 6. In soft soil, the internal friction angle and friction strength are primarily influenced by the dislocation and occlusion at the contact points between soil particles, while the cohesive force is governed by the physical and chemical interactions between particles. The curing mechanism of ISS primarily affects the thickness of the bound water film surrounding the soil particles, enhancing the interaction forces through cementation within a certain range. However, it does not significantly increase the interparticle biting force at the contact points. As a result, the internal friction angle of the soil remains relatively unchanged. In summary, while the ISS treatment strengthens the cohesive interactions between particles and enhances the overall cementation, its effect on the internal friction angle is minimal. The primary changes are in the cohesive forces due to the reduction in water film thickness, rather than substantial alterations in the soil’s frictional behavior [27].

3.2. Effect of Permeability on Strength of Solidified Soft Soil

Studying the permeability characteristics of solidified soft soil is crucial as it directly governs water flow [28], affecting both engineering safety and efficiency. Water infiltration can also accelerate damage from chemical erosion, freeze–thaw cycles, and other factors, ultimately reducing durability. Therefore, understanding the permeability coefficient is key to assessing the suitability of engineering projects, predicting long-term behavior, optimizing design, and ensuring both safety and cost-effectiveness. Based on Darcy’s law, a laboratory variable-head permeability test was conducted on consolidated soil [29]. The permeability coefficients of untreated soft soil and consolidated soil were compared, along with changes in the strength of solidified soil before and after permeability testing.

The variable-head permeability coefficient should be calculated using the following Equation (1):

$$k_T=2.3{\displaystyle \frac{aL}{A\left(t_1-t_2\right)}}\log {\displaystyle \frac{H_1}{H_2}}$$

(1)

where $a$ is the area of the variable head pipe, $L$ is the sample height, $t_1,t_2$ is the beginning and ending time of the measured water head, and $H_1,H_2$ is the starting and ending head.

Table 2. Permeability coefficient.

Untreated Soil
Number	Time/s	H1/cm	H2/cm	2.3aLA−1T−1	lgH1/H2	KT	Average Value
1	16,800	129.5	91.7	2.442 × 10−6	1.499 × 10−1	3.660 × 10−7	3.364 × 10−7
2	6600	91.7	81.2	6.215 × 10−6	5.281 × 10−2	3.282 × 10−7
3	56,400	81.2	30.8	7.273 × 10−7	4.210 × 10−1	3.062 × 10−7
4	14,400	30.8	23.3	2.849 × 10−6	1.212 × 10−1	3.452 × 10−7
Solidified Soil
Number	Time/s	H1/cm	H2/cm	2.3aLA−1T−1	lgH1/H2	KT	Average Value
1	90,480	124.5	101.9	4.534 × 10−7	8.700 × 10−2	3.944 × 10−8	3.902 × 10−8
2	87,120	101.9	80	4.708 × 10−7	1.051 × 10−1	4.948 × 10−8
3	87,300	80	67	4.699 × 10−7	7.702 × 10−2	3.619 × 10−8
4	82,920	67	58	4.947 × 10−7	6.265 × 10−2	3.099 × 10−8

presents a comparison of the average permeability coefficients of untreated soil and solidified soil at 20 °C. As shown in the table, the water permeability of solidified soil is significantly reduced, with the permeability coefficient decreasing by 88.4%. Under the same conditions, the permeability of soft soil decreases by an average of 88.4% after treatment with the ionic soil stabilizer (ISS). This indicates that ISS effectively reduces the permeability of soil. Our results also corroborate those of Wu et al. (2021) [10], who found that the microstructural densification induced by ISS results in fewer interconnected pores and reduced hydraulic conductivity. However, unlike most prior studies, we quantitatively link permeability changes to strength outcomes under post-permeation conditions, offering new evidence for the protective role of ISS in hydraulic environments.

As shown in Figure 7, the shear strength of the soil varies with vertical loads (100 kPa, 200 kPa, 300 kPa, and 400 kPa). Under identical loading conditions, the shear strength of the four soil types consistently follows the order impervious solidified soil > permeable solidified soil > impervious untreated soil > permeable untreated soil. For instance, under a 400 kPa load, the impervious solidified soil exhibits the highest shear strength, while the permeable untreated soil shows the lowest, with a strength reduction of approximately 40–50%. Permeation significantly weakens the strength of untreated soil, with the reduction becoming more pronounced at higher loads. In contrast, the solidified soil demonstrates excellent impermeability, with the shear strength of permeable solidified soil only 10–15% lower than that of impervious solidified soil, and on average, about 40% higher than that of permeable untreated soil.

The shear strength of all soil types increases linearly with vertical load; however, the slopes of the strength–load curves differ. The impervious solidified soil has the steepest slope, indicating the highest rate of strength gain, while the permeable untreated soil has the lowest slope, reflecting the weakest load responsiveness. Notably, solidified soil retains strong load sensitivity even after permeation, with the slope of the permeable solidified soil approximately 35% greater than that of the permeable untreated soil. Overall, the application of the ionic soil stabilizer (ISS) significantly improves both the mechanical strength and impermeability of the soil. After permeation, the solidified soil exhibits minimal strength loss (less than 15%) and maintains substantially higher load-bearing efficiency compared to untreated soil, particularly in permeable conditions, where the strength advantage reaches up to 40%.

The cohesive force and internal friction angle of the soil samples were determined through quick shear tests conducted on both permeable and impervious specimens, as shown in Figure 8. After permeation, the untreated soil samples exhibited a reduction of 34% in cohesive force and 43% in internal friction angle. In contrast, the solidified soil showed only a 1% reduction in cohesive force and a 44% reduction in internal friction angle. These results indicate that the ISS effectively enhances the cohesive strength of the soil and provides improved resistance to seepage.

The ISS primarily functions through an ion exchange mechanism. When the polymeric organic compounds in the stabilizer dissolve in the soil, they weaken the adsorption between soil particles and pore water, disrupting the original soil structure. Simultaneously, physical and chemical reactions occur, leading to the formation of cementitious compounds that create irreversible bonding among particles. As the stabilizer reacts with the moisture in the soil, crystalline structures form and interlock with soil particles, generating a high-strength skeletal framework.

Moreover, the charged ions in the ISS solution alter the surface electrical properties of soil particles, reduce the thickness of the adsorbed water film, and enhance interparticle attraction. These effects collectively increase soil density, reduce permeability, and significantly improve the mechanical stability and impermeability of the treated soil.

3.3. Effect of Acid–Alkali Drying and Wetting Cycles on Shear Strength of Solidified Soft Soil

Soils are subjected not only to repeated wet–dry cycles caused by tides and rainfall but also to pH fluctuations resulting from acid rain, industrial emissions, and seawater intrusion [30]. These cyclic environmental conditions can significantly degrade the strength of solidified soils, exacerbate volume changes (such as cracking and swelling), and even lead to structural failure. Therefore, investigating the deterioration mechanisms of solidified soils under wet–dry cycles combined with acidic and alkaline environments is essential for ensuring the long-term safety, stability, and durability of engineering projects, such as roadbeds, embankments, and land reclamation.

A total of 48 prepared specimens were first subjected to three wet–dry cycles in immersion solutions of water, acid (hydrochloric acid), and alkali (sodium hydroxide). Following this, vertical loads of 100 kPa, 200 kPa, 300 kPa, and 400 kPa were applied for shear testing. The strength characteristics were analyzed based on the type of immersion solution and the number of cycles.

As shown in Figure 9, the shear strength of solidified soil immersed in water gradually decreases with the number of wet–dry cycles. For specimens saturated in hydrochloric acid solution, the strength initially increases slightly before declining, with the overall reduction being relatively minor. In contrast, specimens subjected to wet–dry cycling in sodium hydroxide solution exhibit a more pronounced strength reduction compared to those immersed in water.

Although the shear strength of solidified specimens decreases after wet–dry cycling, the overall decline is limited. The ionic soil stabilizer enhances the bonding forces between clay particles by altering the double electric layer structure through electrochemical mechanisms. This significantly reduces the hydrophilicity of soil particles. As an anionic surfactant, the stabilizer releases a high concentration of negatively charged ions, which attract positively charged metal ions in the adsorbed water layer. These metal ions are displaced into the free water and subsequently expelled or evaporated, reducing the thickness of the bound water layer and facilitating soil compaction. This process is irreversible and significantly improves the soil’s resistance to damage from wet–dry cycling.

Among the three immersion environments, the durability of solidified soil follows the order acid > water > alkali. This indicates that the ionic soil stabilizer imparts superior durability and strength retention to treated soils, especially under acidic conditions.

The solidified soil samples underwent three cycles of recycling. As shown in Figure 10, the shear strength of the samples soaked in acidic solution was highest, followed by those in aqueous solution, and lowest in alkaline solution. The solidified soft soil contains a variety of anions and cations, which interact with acidic and alkaline environments. This interaction induces a bidirectional mechanical effect due to the influence of pH. The ionic composition of the solidified soil is complex, and with increasing acid exposure cycles, the shear strength of the soil initially increases and subsequently decreases. Acidic solutions enhance the cementation of ferrous ions, thereby strengthening the soft soil and generating a positive mechanical effect. Conversely, alkaline environments weaken the cementation in iron-bearing soils, leading to negative mechanical effects [31]. The behavior of solidified soil in alkaline solution mirrors that in aqueous solution, as the latter is weakly alkaline. With an increasing number of cycles, shear strength decreases, and this decline becomes more pronounced with higher alkalinity.

As shown in Figure 11, the mechanical behavior of soft soil treated with an ionic soil stabilizer (ISS) under cyclic wetting–drying conditions exhibits a distinct dependence on the environmental medium. These cycles primarily influence cohesion (c), while the internal friction angle (φ) remains relatively stable. In neutral (water) conditions, cohesion decreases progressively with each cycle, with a notable 27.14% drop during the second cycle. This reduction is mainly due to microcrack formation and propagation caused by repeated desiccation, which disrupts the cementitious matrix; water’s lubricating effect contributes to a slight decrease in φ. Under acidic conditions, an initial increase in cohesion (−36.41%) is observed, likely due to H⁺-induced ion exchange that promotes particle aggregation and the temporary formation of acid-resistant aluminosilicate phases. However, with continued cycling, cohesion declines due to acid-induced dissolution of silicate gels and mineral corrosion. In alkaline environments, cohesion consistently decreases with each cycle, with a significant 26.53% reduction in the second cycle. This is primarily attributed to OH⁻-induced decalcification and the breakdown of cementitious compounds (e.g., C–S–H transforming into H₂SiO₄²⁻) [32], alongside increased electrostatic repulsion that disperses soil particles. Early-stage microcracks also allow deeper penetration of OH⁻ ions, intensifying chemical degradation [33]. Across all environments, φ remains largely unaffected by the cycles, likely because it depends on the intrinsic hardness and mechanical interlocking of soil particles, which are less susceptible to short-term chemical alterations.

4. Conclusions

This study aimed to evaluate the mechanical behavior and environmental durability of soft soil stabilized with an ionic soil stabilizer (ISS) under acidic and alkaline conditions. Through a series of direct shear, permeability, and cyclic wetting–drying tests, the research addressed three key objectives: (1) to assess strength enhancement under different moisture contents, (2) to examine the effect of permeability on strength retention, and (3) to evaluate long-term durability under acid–alkali cycling. The main conclusions are as follows:

• After ISS treatment, the soil’s cohesive strength increased by up to 75.24% and the internal friction angle improved by up to 9.50%, particularly when the moisture content was reduced to 24%. This indicates the treatment is most effective in relatively drier conditions.
• The permeability of soft soil was reduced by 88.4% after ISS treatment. This densification of the soil matrix effectively inhibited water penetration and resulted in a strength retention of over 85% post-saturation. In contrast, untreated soils suffered a 40–50% loss in shear strength, highlighting the crucial role of permeability control in long-term mechanical performance.
• Under repeated wet–dry cycles, ISS-modified soil maintained good mechanical stability. The cohesion dropped only by ~26.53% in alkaline conditions, and even increased initially under acid exposure due to ion interactions. In comparison, untreated soils exhibited more severe degradation. The order of durability was acid > neutral > alkaline.
• Although this study focused primarily on the macroscopic strength and durability characteristics, a key limitation is the absence of microstructural and compositional analyses (e.g., SEM and EDX), which will be addressed in future work to further elucidate the underlying stabilization mechanisms.