Strength Properties and Water-Blocking Stability of Hydrophobically Modified Silty Clay

Abstract

In this study, Qinghai silty clay was hydrophobically modified, and its engineering properties, including water-blocking performance, strength characteristics, and durability, were investigated under varying hydrophobic agent contents and compaction degrees. The findings reveal that: (a) The prepared hydrophobic soil exhibits excellent water repellency, significantly exceeding the threshold for extreme hydrophobicity. When the hydrophobic agent content reaches approximately 13%, the water droplet infiltration time peaks, and the moisture content after immersion remains at a relatively low level. (b) The hydrophobic soil barrier layer effectively blocks the upward migration of groundwater driven by capillary action. In the column test, after 15 days of capillary water action, the water content in the upper soil layer remains nearly unchanged, and the hydrophobic soil layer retains its dryness and excellent water-blocking performance. (c) Under optimal hydrophobic agent content, the unconfined compressive strength (UCS) of hydrophobic soil increases by approximately 48% to 68% compared to ordinary soil. Moreover, the strength improvement becomes more significant with higher compaction degrees. (d) Hydrophobic soil is most sensitive to alkaline environments, while its strength reduction rate in acidic and saline environments is slightly higher than in water environments. (e) It is recommended to maintain the hydrophobic agent content between 13% and 15.5% for Qinghai silty clay to achieve a balance between hydrophobicity, strength performance, and economic feasibility.

1. Introduction

The rapid expansion of transportation infrastructure in China has significantly increased the construction of highways and railways in cold regions and seasonally frozen soil areas. In these regions, moisture migration within the soil raises subgrade moisture content, directly affecting its strength and deformation characteristics [1]. Additionally, the cyclic freezing and thawing of the subgrade intensify issues such as uneven frost heave and thaw settlement [2,3,4], posing significant risks to the performance and durability of the overlying highways and railways. Effectively blocking moisture migration pathways is essential to reducing subgrade moisture content, mitigating frost heave, and maintaining subgrade stability.

In modern subgrade engineering, commonly employed methods for water blocking and isolation include increasing the thickness of subgrade fill, replacing native soils with granular isolation layers, and incorporating impermeable or low-permeability barrier layers to physically obstruct water infiltration [5,6,7]. For example, Henry and Holtz [8] investigated the efficacy of geosynthetic materials and gravel layers in mitigating capillary water migration, concluding that geosynthetics demonstrate superior water-blocking efficiency compared to gravel cushions. Similarly, Wang et al. [9], through a series of laboratory and field experiments, demonstrated that geotextiles effectively disrupt capillary water migration while simultaneously providing drainage and filtration functions, thereby reducing the occurrence of road upheaval during the spring thaw in seasonally frozen regions. Zhu et al. [10] investigated the effectiveness of solidification, geotextile isolation, and buffer layers in controlling capillary water rise in silty subgrades through a combination of model and field experiments. Their findings indicated that combining a 40 cm lime-treated soil layer (cured for 45 days) with a geomembrane yielded optimal performance in suppressing capillary water rise in silty subgrades. However, these impermeable barriers often face challenges such as prolonged curing times and strength-related issues. For example, problems like geomembrane gas bubbles, needle punching, rupturing, and creeping due to degradation [11] significantly reduce the water-blocking and impermeability performance of these isolation layers.

In recent years, advancements in hydrophobic materials and hydrophobic soils have provided innovative solutions to geotechnical challenges associated with inadequate soil moisture control, such as frost heave and subsidence in subgrades. Hydrophobic soil, also referred to as artificially hydrophobic or water-repellent soil, is a novel geotechnical material with exceptionally low water affinity [12,13,14]. This material is increasingly regarded as a promising alternative to traditional materials, effectively mitigating engineering hazards caused by groundwater rise or rainfall infiltration. Numerous studies have been conducted on the modification and preparation of hydrophobic soils in recent years, achieving significant progress.

For instance, Zhang et al. [15,16] employed desert sand as a base material to examine the influence of particle surface structure on the hydrophobicity and impermeability of superhydrophobic particles. Their approach involved regulating the content and incorporation methods of coated resin and micro/nanoscale auxiliary materials. Further, experiments using miniature soil column models explored the water-repellent and salt-blocking performance of these particles. Similarly, Li et al. [17] modified recycled aeolian sand aggregates with silane coupling agents and silicone resin, demonstrating that concrete incorporating hydrophobically modified aggregates exhibited superior workability and mechanical properties compared to conventional recycled sand aggregates. Ren et al. [18] successfully synthesized superhydrophobic sand with a static water contact angle of 154° by chemically etching sand with sulfuric acid and hydrogen peroxide, followed by surface modification with octadecyltrimethoxysilane. Ray et al. [19] prepared hydrophobic marine sand by hydrolyzing tetraethoxysilane under alkaline conditions to coat sand particles with silica sol, followed by treatment with perfluorodecyltrichlorosilane. This method yielded a water contact angle of 151° and a rolling angle of 9.5°, demonstrating stable hydrophobicity and excellent liquid resistance. Liu et al. [20] constructed polydopamine and titanium dioxide coatings on sand particles, followed by hydrophobic modification using perfluorodecyltrichlorosilane, producing superhydrophobic sand with excellent water retention and thermal stability. Chen et al. [21] applied superhydrophobic emulsions to modify loess, achieving superhydrophobicity when the emulsion content exceeded 3%. The modified loess demonstrated excellent water stability in immersion tests. Wu et al. [22,23,24] incorporated octadecylamine as a hydrophobic agent into naturally hydrophilic soils, producing modified samples with varying octadecylamine contents and initial moisture levels. Their results indicated that the modified soil achieved extreme water-repellency level and exhibited superior hydrophobic performance.

Li et al. [25] used a novel hydrophobic agent to modify Qinghai silty clay, preparing hydrophobic soil samples with varying degrees of compaction and hydrophobic agent dosages. Contact angle tests and breakthrough pressure experiments revealed the influence of compaction and agent dosage on water repellency and breakthrough pressure. Similarly, You et al. [26] modified clay by adding superhydrophobic polymer (SHOIP) in varying amounts. Their study assessed the impact of hydrophobic agent content on the contact angle, permeability coefficient, and pore distribution of the modified clay.

Overall, while these studies have successfully synthesized hydrophobic soils using chemical methods and investigated their fundamental properties, several limitations persist. The long-term stability of their water-blocking performance is still unclear, and studies on the environmental durability of hydrophobic soils are lacking, making it difficult to provide practical guidance for their application in subgrade engineering.

In this study, Qinghai silty clay, a typical seasonal frozen soil, was hydrophobically modified, and hydrophobic soil samples with varying compaction levels and hydrophobic agent contents were prepared. Then, a series of experiments, including water drop penetration tests, immersion tests, capillary water rise height tests, and unconfined compressive strength tests, were conducted to evaluate the hydrophobicity, water resistance, strength characteristics, and environmental durability of the modified soil. The objectives of the study include elucidating the water-blocking performance of hydrophobic soil as a barrier and assessing its engineering properties under acidic, alkaline, and saline conditions. Furthermore, optimal dosages for engineering applications are proposed, providing theoretical insights to support the application of hydrophobic soils in subgrade engineering for water resistance in cold and seasonally frozen regions.

2. Materials and Methods

2.1. Experimental Materials

This study adopted Qinghai silty clay, a typical frozen soil from the Xiangpi Mountain region of Qinghai Province, China, to explore the hydrophobic modification of fine-grained soils in cold regions. The fundamental physical parameters of Qinghai silty clay are summarized in

Table 1. Basic physical parameters of the Qinghai silty clay.

Property	Value
Maximum dry density, ρdmax (g·cm−3)	1.75
Liquid limit, wL (%)	27.2
Plastic limit, wP (%)	15.1
Plasticity index, IP	12.1
Optimum moisture content, w (%)	15.5
Specific gravity of soil particles, GS (g·cm−3)	2.71
Sand, >75 μm (%)	24.69
Silt, 5–75 μm (%)	16.11
Clay, <5 μm (%)	59.2

, and its grain size distribution curve is shown in Figure 1. The soil is classified as CL in accordance with ASTM D2487 standard [27].

The CN01C superhydrophobic soil emulsion (CN), produced by Zhejiang Shuke Nanotechnology Hydrophobic Co., Ltd. (Jiaxing, China), was utilized as the hydrophobic modification agent for Qinghai silty clay. This hydrophobic agent is a water-based, odorless, milky-white neutral product, primarily composed of organosilanes and their derivatives. The characteristics of the hydrophobic agent are summarized in

Table 2. Characteristics of the hydrophobic agent.

Property	Characteristics/Value
Physical state	Milky liquid
PH	7 ± 1
Viscosity (Pa · s)	0.5
Volatile organic compounds (%)	≤3
Water absorption per unit area (g/m2)	≤1000
Water contact angle (degrees)	≥100

.

2.2. Specimen Preparation

To prepare hydrophobic soil samples for subsequent experiments, Qinghai silty clay was thoroughly blended with varying amounts of hydrophobic agent, in accordance with the relevant provisions of the “Standard for Geotechnical Testing Methods” (GB/T 50123-2019) [28]. This process facilitated the formation of a dense “hydrophobic film” on the surface of the soil particles, effectively transforming the soil from hydrophilic to hydrophobic. Previous studies have shown that the hydrophobic performance is optimal when the hydrophobic agent’s mass fraction is adjusted to align with the optimal moisture content of Qinghai silty clay.

To evaluate the effects of hydrophobic agent content and soil compaction levels, five hydrophobic agent content levels (i.e., 8%, 10.5%, 13%, 15.5%, 18%) and three compaction degrees (i.e., 0.85, 0.90, 0.95) were selected, corresponding to dry densities of 1.49 g/cm³, 1.58 g/cm³, and 1.66 g/cm³, respectively. The compaction degree used in this study was selected based on the common index values for construction quality control in practical filling projects (e.g., subgrade). The sample preparation process involved the following steps: (1) the test soil was oven-dried at 105 °C for 12 h, then cooled and crushed into fine particles for later use; (2) the hydrophobic agent was added to the soil and mixed thoroughly to ensure uniform distribution and full interaction with the soil particles; (3) the prepared soil samples were left to air-dry in a natural environment for 4 days.

2.3. Hydrophobic Performance Tests

2.3.1. Water Drop Penetration Time (WDPT) Tests

The water drop penetration time (WDPT) method was utilized to evaluate the hydrophobicity of the soil samples [29]. Based on the penetration time (t) of a water droplet into the soil, hydrophobicity can be categorized into five levels: non-hydrophobic (t < 5 s), slightly hydrophobic (5 s ≤ t < 60 s), moderately hydrophobic (60 s ≤ t < 600 s), strongly hydrophobic (600 s ≤ t < 3600 s), and extremely hydrophobic (t ≥ 3600 s) [30]. To investigate the effects of varying hydrophobic agent contents and soil compaction degrees on hydrophobicity, ring-shaped soil samples (diameter: 61.8 mm; height: 20 mm) were prepared with hydrophobic agent contents of 8%, 10.5%, 13%, 15.5%, and 18% and compaction degrees of 0.85, 0.90, and 0.95, yielding a total of 15 groups. To minimize the influence of water droplet evaporation during testing, all samples were placed in a temperature- and humidity-controlled chamber. Observations and recordings of droplet morphology on the sample surfaces were conducted at regular intervals until the droplets were completely absorbed. To reduce the impact of sample heterogeneity and randomness on the experimental results [22], water drop penetration times were measured at five different positions on the surface of each sample, and the average of these measurements was used as the final result.

2.3.2. Immersion Tests

To investigate the hydrophobic performance and long-term stability of hydrophobic soil under extended water immersion conditions, cylindrical specimens (diameter 39.1 mm, height 80 mm) with an optimal hydrophobic agent content of 13% and compaction degrees of 0.85, 0.90, and 0.95 were selected for immersion tests. Moisture content changes in the specimens were monitored over time to identify trends and calculate the degree of saturation.

During the tests, the prepared specimens were placed in a temperature- and humidity-controlled chamber filled with water, supported by a mesh frame to ensure full submersion. At predetermined intervals (e.g., 6 h, 12 h, 24 h, 48 h, 72 h, 120 h, 168 h, 240 h, 360 h, 480 h, and 720 h), the specimens were removed from the chamber, and their mass was measured using an electronic balance with a precision of 0.01 g. The corresponding moisture content of each specimen was then calculated. To minimize water evaporation during weighing, surface water droplets were carefully wiped off with a damp towel, and the weighing process was conducted promptly.

2.4. Capillary Water Rise Tests

The capillary water rise test was conducted following the procedures outlined in the Standard for Soil Test Methods (GB/T 50123-2019) [28]. Utilizing the principles of the vertical tube method, a custom-designed acrylic tube was fabricated to perform the soil column model test. The dimensions and schematic of the tube are presented in Figure 2. The acrylic tube features an inner diameter of 10 cm, a wall thickness of 0.5 cm, and a length of 85 cm. The tube’s bottom was left open to allow water to enter from below. Starting 15 cm above the base, four evenly spaced horizontal holes (each 1 cm in diameter) were drilled at 10 cm intervals along the vertical axis. These holes facilitated moisture content measurements at various heights. To prevent moisture loss during testing, each hole was sealed with a rubber stopper [31].

Two sets of capillary water rise tests were conducted: a control group and an experimental group. In the experimental group, a 3 cm-thick hydrophobic soil layer, prepared with a hydrophobic agent content of 13%, served as the water barrier. In the control group, the tube was filled entirely with untreated Qinghai silty clay. A schematic of the setup is presented in Figure 3. The Qinghai silty clay used in the tests has an initial moisture content of approximately 3%, representing the stable condition achieved after oven-drying, crushing, and re-absorbing moisture from the air under indoor laboratory conditions.

The experimental procedure was as follows: (1) The hydrophobic soil was air-dried, crushed, and stored for use as previously described. (2) A thin layer of petroleum jelly was applied to the inner walls of the lower section of the acrylic tube in the experimental group to prevent gaps between the hydrophobic soil and the tube wall, which could affect measurement accuracy. (3) The bottom of the acrylic tube was sealed with a permeable membrane and filter paper to prevent soil leakage while allowing free water entry from below. (4) Soil samples were placed in the tube using a layered compaction method. The amount of soil for each layer was calculated to achieve a compaction degree of 0.90. Each layer was compacted and scarified before adding the next. (5) The prepared soil columns were immersed in a water tank, and the water level was adjusted to maintain an identical water head for both groups. Water was replenished periodically to ensure a constant water head. (6) At various heights along the soil columns, moisture content was measured by extracting soil samples through four evenly spaced holes and applying the oven-drying method. The average of the four measurements at each height was recorded. After sampling, the soil was promptly returned to the column at the corresponding moisture content. (7) Moisture content changes at different heights were recorded at capillary rise intervals of 1 day, 2 days, 3 days, 5 days, 7 days, 10 days, and 15 days. The blocking efficiency of the hydrophobic soil layer was evaluated based on these measurements.

2.5. Unconfined Compressive Strength and Durability Tests

Unconfined compressive strength (UCS) was utilized as the primary indicator for assessing the strength of hydrophobic soil and investigates the influence of environmental factors, such as acidic, alkaline, and saline conditions, on soil strength. The UCS tests are conducted in three stages:

Stage 1: UCS tests were performed on 15 groups of soil specimens prepared with three compaction degrees (0.85, 0.90, 0.95) and five hydrophobic emulsion contents (8%, 10.5%, 13%, 15.5%, and 18%). The relationships between hydrophobic soil strength, hydrophobic agent content, and compaction degree were investigated. Stage 2: UCS tests were conducted on untreated soil specimens with the same compaction degrees to compare strength differences between hydrophobic and ordinary soil. Stage 3: Durability tests were performed under four environmental conditions: pure water, acidic solution (pH = 4), alkaline solution (pH = 10), and saline solution (0.6% NaCl by mass). Hydrophobic soil specimens, prepared using the optimal strength ratio identified in Stage 1, were immersed in these solutions. The UCS of each group was measured at immersion durations of 12 h, 24 h, 3 days, 7 days, 15 days, and 30 days. Strength reduction due to immersion was quantitatively analyzed, and the effects of acidic, alkaline, and saline environments on hydrophobic soil strength were summarized.

The unconfined compressive strength (UCS) tests were performed using a YYW-II unconfined pressure tester. During testing, the bottom plate was raised at a constant rate of 0.065 mm/s, while force gauge readings and axial deformation data were recorded simultaneously. The test was terminated when the force gauge reading reached its peak value and the axial strain either increased by an additional 3–5% or reached a total strain of 20%. Additionally, the parallel testing method was employed in this study, wherein two specimens were tested per trial, and the final strength value was determined by averaging the results.

3. Results and Discussion

3.1. Hydrophobic Performance of Soil Samples

3.1.1. WDPT Tests

Figure 4 shows the variation in water drop penetration time t with hydrophobic agent content w at different compaction degrees. In the figure, K represents the compaction degree.

The experimental results show that the water drop penetration times for all 15 test groups exceeded 3600 s, with most values surpassing 430 min (25,800 s). Based on the classification by Nyman et al. [30], all samples fall within the “extremely hydrophobic” level. These findings demonstrate that variations in hydrophobic agent content and compaction degrees significantly enhance soil hydrophobicity.

At a constant compaction degree, water drop penetration time initially increases with rising hydrophobic agent content, reaching a peak at approximately 13%. Beyond this point, penetration time gradually decreases as the hydrophobic agent content continues to increase. Furthermore, when the hydrophobic agent content is held constant, water drop penetration time increases with higher compaction degrees, indicating that soil density markedly influences hydrophobicity.

3.1.2. Immersion Tests

Figure 5 shows the results of the immersion tests for hydrophobic soil with a hydrophobic agent content of 13% and varying compaction degrees. In the figure, the horizontal axis represents time, the vertical axis represents moisture content, and K denotes the compaction degree.

As shown in Figure 5, the moisture content of hydrophobic soil decreases with increasing compaction degrees during prolonged water immersion. In the initial immersion phase (0–30 h), the moisture content of all soil specimens rises rapidly, reaching the highest rate of change. This behavior is likely due to surface disturbances caused during sample preparation, which lead to increased water absorption in the outer soil layers. In the subsequent phase, the rate of moisture content change gradually declines, and the moisture content stabilizes, indicating minimal to no further water infiltration. At a hydrophobic agent content of 13%, the long-term moisture content trends for hydrophobic soil at different compaction degrees are generally consistent. After 30 days of immersion, the final moisture content of all hydrophobic soil specimens stabilizes between 3% and 5%, corresponding to a degree of saturation ranging from 0.15 to 0.25. When combined with the water drop penetration time (WDPT) results presented earlier, these findings indicate that a hydrophobic agent content of 13% imparts optimal water repellency and water-blocking performance to the soil.

3.2. Capillary Water Rise Height Tests

Figure 6 compares capillary water rise between the control and experimental groups. Figure 6a depicts the control group’s soil column, which lacks a hydrophobic soil barrier, while Figure 6b shows the experimental group’s soil column with a 3 cm-thick hydrophobic soil layer. After 3 days of capillary action, moisture in the control group migrated to a height of approximately 65 cm. In contrast, in the experimental group, only the unmodified Qinghai silty clay beneath the hydrophobic barrier exhibited wetting, while the soil above the barrier remained dry. After 15 days of capillary action, moisture migration in the experimental group remained minimal, with only slight moisture movement detected in the lower section (below 15 cm). Conversely, in the control group, moisture had reached the top, fully saturating the entire soil column.

These findings indicate that the hydrophobic soil layer effectively inhibits upward capillary water rise. Furthermore, even under prolonged immersion, the hydrophobic soil layer retained its high water-blocking efficiency.

Figure 7 illustrates the variation in moisture content of soil columns over different durations of capillary water action (i.e., 1 day, 2 days, 3 days, 5 days, 7 days, 10 days, and 15 days). As shown in Figure 7a, after 3 days of capillary water action, moisture in the control group’s soil column reached the top layer. By approximately the 7th day, the moisture content stabilized at around 30% throughout the column. Although the stabilized moisture content decreased slightly with increasing column height, the overall moisture level remained high. In Figure 7b, results for the experimental group’s soil column show a slight increase in moisture content only at the 15 cm cross-section. The moisture content at this location increased from an initial 3.0% to approximately 5.0%, corresponding to a change rate of about 0.15% per day. At other locations within the experimental group’s soil column, no significant changes in moisture content were observed, resulting in overlapping data.

Figure 7c presents a schematic of the hydrophobic soil layer following the capillary water rise height test. After 15 days of capillary water action, the 3 cm-thick hydrophobic soil layer continued to exhibit excellent water-repellent performance, remaining dry with water droplets rolling freely on its surface. This observation suggests that the slight increase in moisture content at the bottom cross-section of the experimental group’s soil column may be due to minor water migration along the contact interface between the soil and the vertical tube.

3.3. Unconfined Compressive Strength and Durability

3.3.1. Influence of Compaction Degree and Hydrophobic Agent Content

Figure 8 displays the Stress–strain curves of hydrophobic soil under varying hydrophobic agent contents, where K represents the compaction degree and w denotes the hydrophobic agent content. The experimental results show that the Stress–strain curves for all 15 soil specimens exhibit an initial increase, followed by a rapid decline, with a distinct peak in each curve. Beyond this peak, the stress drops sharply, indicating brittle failure in the dried hydrophobic soil specimens. Consequently, the strength value of the hydrophobic soil can be identified as the stress corresponding to the peak point on the Stress–strain curve.

The strength values of all samples are summarized in Figure 9. The results indicate that the unconfined compressive strength (UCS) generally increases with hydrophobic agent content. This increase is most pronounced within the 8–13% range, where UCS shows the highest growth and a strong positive correlation with hydrophobic agent content. However, when the hydrophobic agent content exceeds 13%, the rate of UCS growth begins to slow. Additionally, UCS values are significantly higher under high compaction conditions compared to low compaction conditions. At a compaction degree of 0.85, specimens with a hydrophobic agent content of 18% exhibit a noticeable decrease in UCS. This may be because soil aggregates are more likely to be generated during sample preparation when the hydrophobic agent dosage is relatively high (e.g., 18%). Under low compaction conditions (e.g., 0.85), the prepared soil samples are relatively loose, and the micropores between aggregates cannot be effectively compacted, resulting in weaker inter-particle connections compared to high compaction levels.

Therefore, for strength optimization, a hydrophobic agent content within the range of 13–15.5% is recommended as the optimal dosage.

3.3.2. Strength Comparison Between Hydrophobic Soil and Ordinary Soil

Based on the results of the first stage of unconfined compressive strength (UCS) tests, the optimal hydrophobic agent content for hydrophobic soil was identified as 15.5%. For comparative analysis, ordinary soil specimens with the same moisture content (15.5%) were prepared. The results are presented in

Table 3. UCS values of ordinary and hydrophobic soil specimens at different compaction degrees.

Degree of Compaction	Unconfined Compressive Strength/kPa		Strength Improvement Rate (SIR)/%
	Ordinary Soil Specimen (σP)	Hydrophobic Soil Specimen (σS)
0.85	775.37	1147.37	47.98
0.90	1093.59	1743.47	59.43
0.95	1299.76	2187.18	68.28

Note(s): SIR = [(σ_S − σ_P)/σ_P] × 100% [32].

.

As shown in Table 3, at the optimal hydrophobic agent content, the unconfined compressive strength (UCS) of hydrophobic soil is significantly higher than that of ordinary soil across all three compaction degrees. Specifically, at compaction degrees of 0.85, 0.90, and 0.95, the UCS of hydrophobic soil increases by approximately 48%, 59%, and 68%, respectively, compared to ordinary soil. The results indicate that higher compaction degrees lead to more pronounced improvements in the UCS of hydrophobic soil specimens.

As shown in Figure 10, the strength of ordinary soil specimens increases progressively with higher compaction degrees. At compaction degrees of 0.85 and 0.90, the peak strength occurs at axial strains of 1.33% and 1.56%, respectively. At a compaction degree of 0.95, the axial strain corresponding to the peak strength increases to 1.88%.

Similarly, the strength of hydrophobic soil specimens also improves with higher compaction degrees. At a compaction degree of 0.95, the hydrophobic soil achieves a maximum peak stress of 2187.18 kPa (Figure 10c) and a strength improvement rate of 68.28% (Table 3).

3.3.3. Durability Analysis in Acidic, Alkaline, and Saline Environments

Hydrophobic soil specimens were prepared using the optimal strength ratio identified in the first stage (compaction degree of 0.95 and 15.5% hydrophobic agent content) and exposed to four different environments: water, acidic solution, alkaline solution, and saline solution. The specimens’ strength was tested at various immersion durations (i.e., 12 h, 24 h, 3 days, 7 days, 15 days, and 30 days). Additionally, control specimens, prepared with the same ratio but not subjected to immersion (0 days), were also tested for comparison. The results are summarized in Figure 11.

As shown in Figure 11, the unconfined compressive strength (UCS) of hydrophobic soil specimens decreases to varying extents with increasing immersion time across different solution environments. Consistent with the conclusions from the previous immersion tests, the moisture content of the hydrophobic soil specimens increases rapidly during the first 12 h of immersion, resulting in a significant reduction in strength. After 12 h, the rate of moisture content change gradually slows with extended immersion time. Consequently, the rate of strength reduction also exhibits a similar decline. However, specimens immersed in the alkaline environment experienced significantly greater strength reduction compared to those in acidic, saline, or water environments. This finding indicates that alkaline conditions have the most pronounced detrimental effect on the hydrophobic soil.

The rate of strength reduction for hydrophobic soil specimens immersed in different environments, relative to the strength of the unsoaked control group, is defined as the strength reduction rate (Z, %), as given in Equation (1).

$$Z={\displaystyle \frac{{\sigma }_0-\sigma }{{\sigma }_0}}\times 100$$

(1)

where σ represents the strength of the hydrophobic soil specimen at a specific immersion duration in a given solution (MPa), σ₀ denotes the initial strength of the hydrophobic soil specimen (MPa).

Based on Equation (1), the strength reduction rates of hydrophobic soil specimens under different solutions and immersion durations were analyzed. As shown in Figure 12, during the initial stages of immersion, the strength reduction rates in acidic, saline, and water solutions are nearly identical. In contrast, the strength reduction rate in the alkaline solution is consistently higher than in the other three solutions at all time points. Over the 15-day period, the strength reduction rates in acidic and water solutions remain similar. After 30 days of immersion, specimens in acidic and saline solutions exhibit slightly higher strength reduction rates compared to those immersed in water.

These results indicate that the influence of different environments on hydrophobic soil strength follows the order: alkaline > acidic ≈ saline > water. Additionally, the degradation of hydrophobic soil in an alkaline environment is significantly higher than in other environments. In fact, many hydrophobic materials (or agents), especially organic polymers like polyesters, polyurethanes, and silanes, may undergo reactions such as hydrolysis under strong alkaline conditions. In such environments, hydroxide ions (OH⁻) attack ester, ether, or amide bonds within the hydrophobic materials, resulting in molecular chain cleavage or the conversion of these bonds into polar functional groups, such as hydroxyl or carboxyl groups. The presence of these polar groups markedly enhances the material’s hydrophilicity, transforming the initially hydrophobic surface into a hydrophilic one and thereby compromising its hydrophobic properties. For hydrophobic soil, prolonged immersion in a strong alkaline environment may lead to a decline in the hydrophobic performance. As a result, water can penetrate the soil more easily, leading to a reduction in its unconfined compressive strength.

Additionally, a comparison was conducted between the Stress–strain curves of ordinary soil specimens immersed in water for 6 h and hydrophobic soil specimens immersed in water for 30 days. As shown in Figure 13, the unconfined compressive strength (UCS) of the ordinary soil specimens was almost completely lost after 6 h of immersion. In contrast, the hydrophobic soil specimens retained a strength of approximately 1400 kPa even after 30 days of immersion, demonstrating superior strength performance. These experimental results indicate that the addition of hydrophobic agents significantly alters the soil’s properties, substantially enhancing its mechanical performance.

4. Conclusions

This study systematically investigated the water repellency, water-blocking performance, strength characteristics, and environmental durability of a modified hydrophobic fine-grained soil through water droplet infiltration tests, immersion tests, capillary rise tests, and unconfined compressive strength tests. The main conclusions are as follows:

(1)

The hydrophobic soil prepared in this study exhibits exceptional water repellency, far surpassing the “extremely hydrophobic” threshold. At a hydrophobic agent content of approximately 13%, the water drop penetration time peaks, and the soil maintains a low moisture content even after 30 days of immersion. Additionally, compaction degree has a significant impact on hydrophobic performance. Higher compaction degrees result in longer water drop penetration times and lower moisture content after immersion.

(2)

As a barrier layer, the hydrophobic soil effectively blocks the upward migration of groundwater driven by capillary action. In ordinary soil columns, moisture reaches the top within three days under capillary forces. In contrast, soil columns containing a hydrophobic soil layer exhibit only a slight increase in moisture content at the bottom even after 15 days of capillary action, while the rest of the column shows no noticeable changes. The hydrophobic soil layer remains dry, maintaining its excellent water-blocking performance.

(3)

At a hydrophobic agent content of 15%, the unconfined compressive strength (UCS) of hydrophobic soil increases by approximately 48–68% compared to ordinary soil, with greater improvements observed at higher compaction degrees. In contrast, while ordinary soil nearly loses its UCS after 6 h of immersion, hydrophobic soil maintains a strength of approximately 1400 kPa even after 30 days of water immersion. Additionally, hydrophobic soil exhibits the highest sensitivity to alkaline environments. The strength reduction rate in acidic and saline conditions is slightly greater than that observed under water immersion. The impact of different environments on hydrophobic soil strength, from most to least severe, follows the order: alkaline environment > acidic environment ≈ saline environment > water.

(4)

Based on the experimental results, the optimal hydrophobic agent content range for Qinghai silty clay is determined to be 13–15.5%, balancing water repellency, strength performance, and economic efficiency.