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Article

Shear Strength and Mechanism Analysis of Sodium Polyacrylate-Modified Soft Soil

by
Minglin Zhou
1,2,
Zhikui Liu
1,*,
Hanying Bai
1 and
Zhendong Zhou
3
1
School of Civil Engineering, Guilin University of Technology, Guilin 541004, China
2
Wuhan Qianceng Geotechnical Engineering Co., Ltd., Wuhan 430223, China
3
China Inner Mongolia Forest Industry Group Co., Ltd., Yakeshi 022150, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2026, 16(4), 1881; https://doi.org/10.3390/app16041881
Submission received: 14 January 2026 / Revised: 6 February 2026 / Accepted: 9 February 2026 / Published: 13 February 2026
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Abstract

To investigate the improvement effect of sodium polyacrylate on the shear strength of silty clay, this study explores the curing treatment of silty clay using sodium polyacrylate. Liquid-plastic limit tests and triaxial shear tests were conducted to examine the impact of sodium polyacrylate on the liquid-plastic limits and shear strength of silty clay, as well as to determine the optimal dosage. Additionally, low-field nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) tests were performed to further reveal the micro-mechanism of sodium polyacrylate’s action. The results show that as the sodium polyacrylate content increases, the liquid-plastic limits of silty clay increase significantly. Compared to untreated samples, when the sodium polyacrylate content is 7%, the liquid limit and plastic limit increase by 132.7% and 167.3%, respectively. Meanwhile, the cohesion of the modified samples increases with the sodium polyacrylate content, while the internal friction angle first increases and then decreases. When the sodium polyacrylate content rises from 0% to 7%, the cohesion and internal friction angles of the modified samples increase by 522.9% and 70.6%, respectively. Through comprehensive analysis of the experimental results, it was determined that the optimal dosage of sodium polyacrylate is 5%. Microstructural analysis indicates that sodium polyacrylate interacts with soil particles through hydrogen bonding and ion bridging, filling the pores between particles and encapsulating their surfaces. This improves the pore structure of the soil and enhances the bonding strength between particles. This study provides a theoretical basis for the application of sodium polyacrylate in soft soil improvement.

1. Introduction

With the growing awareness of environmental protection and the accelerating process of urbanization, there has been an increase in the construction of projects related to roads and municipal engineering, housing and building projects, water conservancy and port engineering, port channel dredging, and land reclamation. These projects are often directly associated with soft soils or soft soil regions. Soft soils, such as silt and silty soils, typically exhibit characteristics such as high moisture content, prolonged self-weight consolidation, high compressibility, and low shear strength. These properties make their direct application in engineering projects challenging [1,2]. For example, when soft soils are used as foundation or subgrade materials, they are prone to causing issues like ground settlement [3]. Therefore, these soft soils require treatment before application to meet the demands of engineering construction. Currently, one of the most effective treatment methods involves curing the silt with cementitious materials. After compaction, the treated soil can be used as filling or foundation material. This method demonstrates significant advantages in both construction efficiency and treatment effectiveness, making it an effective way for resource utilization of soft soils [4,5]. Consequently, improving the treatment technology of soft soils to ensure their compliance with bearing capacity and settlement requirements, while also achieving their resource utilization, has become a core issue in the management and resource utilization of soft soils [6].
In the system of soft soil treatment and comprehensive utilization technologies, vacuum preloading and surcharge preloading are the most widely applied methods. However, their treatment cycles are often relatively long. To shorten the construction period, drainage boards are commonly combined with vacuum preloading techniques in engineering projects to accelerate the expulsion of pore water and the consolidation process, thereby significantly reducing the treatment time for soft soils [7,8]. On the other hand, the application of traditional curing agents, such as lime, cement, and fly ash, in soft soil stabilization has been confirmed by relevant studies, demonstrating their significant strengthening effects [9]. However, chemical curing treatments often cause irreversible long-term changes to the engineering properties of soft soils [10]. For instance, the addition of cement rapidly raises the pH of the soil to 12–13, and the alkaline hydroxide ions released by hydration by-products may have adverse effects on biological organisms, while also increasing the brittleness of the stabilized soil [10,11,12,13]. Furthermore, the extensive use of cement-based materials can lead to a series of urban environmental issues, such as increased surface runoff, exacerbated urban heat island effects, and suppression of vegetation growth [14]. The irreversible hydration reaction of cement makes it difficult for cement-soil mixtures to return to their original state, further highlighting the pressure on resources and the environment posed by traditional cementitious materials [15]. Microbially induced calcium carbonate precipitation (MICP) and urease-induced calcium carbonate precipitation (EICP) technologies have made progress in soil stabilization. However, their applicability remains somewhat limited. These techniques are more suitable for coarse-grained soils like sands, whereas fine-grained soils have small pore sizes that hinder the transport of bacterial strains and nutrients. Additionally, MICP technology may generate high concentrations of ammonium chloride by-products, which require subsequent treatment [16,17,18]. Since the late 20th century, geotechnical engineering has explored the use of synthetic chemical polymers, such as polyacrylamide, acrylic acid, and polyethylene, as soil stabilizers to replace traditional curing agents like lime and cement [10]. Although these synthetic polymers show certain advantages in improving shear strength and erosion resistance, their potential toxicity and water pollution risks have raised widespread concerns, which somewhat limits their broader application [19,20].
Sodium polyacrylate is a highly water-absorbent resin considered an environmentally friendly soil stabilizer [21]. It has shown excellent performance in enhancing the compressive strength, crack resistance, water retention, and wetting-collapse resistance of soils [22,23,24,25]. Although the macro-level improvement effects of sodium polyacrylate on soil have been validated, existing studies predominantly focus on single-component soils, with insufficient understanding of its applicability and variability in natural multi-component soils. Specifically, the dominant mechanisms of interaction between sodium polyacrylate and soil particles have not been thoroughly explored. This limitation has, to some extent, hindered the broader application of sodium polyacrylate in soil stabilization and resource utilization.
Based on the aforementioned research background, this study focuses on soft plastic silty clay as the research object, with sodium polyacrylate selected as the stabilizer. The main innovation of this study lies in the application of the environmentally friendly material sodium polyacrylate for the stabilization of silty clay. Different sodium polyacrylate dosages were used to prepare improved soil samples, and triaxial consolidated undrained shear tests were conducted to systematically explore the effect of sodium polyacrylate on the shear strength of the soil. The study also systematically investigates the effect of curing time on the mechanical and durability properties of the treated soil. Furthermore, combining testing methods such as nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), the study investigates the influence mechanism of sodium polyacrylate on the shear strength of soil from the perspectives of moisture distribution, interaction types between materials, and microstructural evolution. Additionally, by employing these multi-scale characterization techniques, the interaction mechanisms between sodium polyacrylate and soil particles are revealed. The aim is to provide a theoretical basis and new insights for the resource utilization of soft soil stabilization treatment and the enhancement of engineering stability.

2. Materials and Methods

2.1. Experimental Materials

The silty clay used in this study was sourced from a construction site along the Yangtze River in Wuhan, Hubei Province, China, with a sampling depth of 4–5 m. The soil appears gray-black and is in a soft plastic state. According to the “Standard for Geotechnical Test Methods” (GB/T 50123-2019) [26], the mineral composition and chemical composition of the sample were determined using an X-ray diffractometer (X’ Pert PRO Powder, PANalytical, Almelo, The Netherlands) and an X-ray fluorescence spectrometer (ZSX Primus Π, Rigaku Corporation, Akishima, Japan), respectively. The test results are presented in Figure 1, Table 1 and Table 2. As shown in Table 1, the main mineral components of the silty clay sample include quartz, feldspar, mica, calcite, needle-like goethite, and clay minerals, with kaolinite, illite, and chlorite as the dominant clay minerals. According to Table 2, the oxide composition of the silty clay is primarily composed of SiO2, Fe2O3, and CaO, which together account for approximately 87.86% of the total oxide content. To determine the basic chemical properties of the silty clay in a water environment, the clay was mixed with distilled water at a mass ratio of 1:100 and thoroughly stirred to form a uniform suspension. After standing, the pH value of the supernatant was measured using a pH meter (Shanghai Instrument & Electronics Scientific Instruments Co., Ltd., Shanghai, China), yielding a result of 7.81.
To further characterize the particle size distribution and other physical properties of the silty clay, the sample was first subjected to liquid nitrogen freeze-drying. The particle size distribution was then analyzed using a Malvern laser particle size analyzer (Mastersizer 3000, Malvern Panalytical, Malvern, UK). The results, as shown in Figure 2, indicate that the median particle size (d50) of the silty clay sample is 0.1 mm. The relative density was determined using a pycnometer, while the specific surface area of the solid particles was measured using a specific surface area analyzer (JW-BK200C, JWGB SCI. & TECH., Beijing, China). The cation exchange capacity of the sample was determined using an inductively coupled plasma mass spectrometer (ICP-MS 7800, Agilent, Santa Clara, CA, USA). Additionally, the liquid limit and plastic limit of the silty clay were measured using a combined liquid-plastic limit tester (Nanjing Soil Instrument Factory Co., Ltd., Nanjing, China). The results of these tests are summarized in Table 3. As shown in Table 3, the particle composition of the silty clay is primarily composed of silt and clay fractions. Its specific surface area is 28.7 m2/g, indicating a generally fine particle size. The liquid limit and plastic limit of the silty clay are 41.3% and 25.1%, respectively, and the cation exchange capacity is 20.8 cmol/kg. The silty clay used in this study is classified as high-liquid limit clay (CH).
The sodium polyacrylate used in the experiments was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). It is a white powder and an analytical grade, water-soluble polymer. A schematic diagram of its molecular structure is shown in Figure 3. As seen in Figure 3, sodium polyacrylate molecules contain sodium carboxylate (-COONa) groups. These polymer monomers polymerize to form high-molecular-weight polymers, and interactions between the polymers create a network structure. The molecular weight of the polymer is typically less than 10,000 g/mol [27]. When sodium polyacrylate powder dissolves in water, the Na+ ions from the molecular chains dissolve in water, while the carboxyl (-COO−) ions carry a negative charge, causing repulsive forces between the carboxyl groups. At this point, the sodium polyacrylate molecules interact with more water molecules, exhibiting strong hydrophilicity. It dissolves easily in both cold and hot water, forming a high-viscosity hydrogel. To characterize the viscosity of the sodium polyacrylate hydrogel, distilled water (pH = 6.5) was used as the solvent. The sodium polyacrylate powder was mixed with distilled water at a mass ratio of 1:100 and stirred thoroughly until completely dissolved, resulting in a translucent hydrogel. The viscosity of sodium polyacrylate hydrogel is one of its key physical properties, directly influencing the material’s effectiveness in improving the liquid limit, plastic limit, and other mechanical properties of silty clay. Polymer concentration, temperature, and shear rate are the three main factors affecting its viscosity. To measure the viscosity of the hydrogel, this study used a digital viscometer produced by Shenzhen Dingxin Yi Experimental Equipment Co., Ltd. (Shenzhen, China). Before testing, the prepared hydrogel was placed in a temperature-controlled water bath, with its temperature monitored using a mercury thermometer. Once the hydrogel reached the set temperature and stabilized, viscosity measurements were performed at a fixed speed of 6 r/min. Each condition was tested three times, and the final result was taken as the arithmetic average of the three measurements. Under the conditions of 25 °C and a rotor shear rate of 6 r/min, the viscosity measured was 1957 mPa·s.

2.2. Specimen Preparation

The silty clay samples retrieved from the field were oven-dried at 105 °C to a constant weight, then crushed using a hammer and sieved through a 2 mm soil sieve to obtain uniformly sized sample powder. The sieved samples were sealed and stored for later use. The results of the indoor density test indicated that the dry density of the silty clay sample retrieved from the field was approximately 1.58 g/cm3. Therefore, in the laboratory, the dry density was set as the control standard at 1.58 g/cm3, and the initial water content of the samples was set to wt% = 30%. The required amounts of silty clay and distilled water were weighed according to this proportion. The dry mass ratios of sodium polyacrylate to silty clay (mR) were set to 0%, 1%, 3%, 5%, and 7%. The test samples were prepared by mixing silty clay, sodium polyacrylate, and distilled water, with the sample dimensions being 39.1 mm in diameter and d = 80 mm in height. Based on the mass of the silty clay, the corresponding amount of sodium polyacrylate was weighed, While the measured distilled water was placed in a beaker, the sodium polyacrylate powder was slowly added under magnetic stirring, while the beaker was sealed with plastic wrap. Previous studies have shown that ion strength, pH, dosage, temperature, stirring speed, and time are key factors influencing the viscosity and uniformity of sodium polyacrylate solutions [27]. Based on this, in this study, we controlled the ion strength and pH of the hydrogel preparation using distilled water and maintained the experimental temperature at 25 °C. The specific procedure was as follows: sodium polyacrylate was mixed with distilled water and placed on a digital heating magnetic stirrer (MYP19-2, Shanghai Meiyingpu Instrument Manufacturing Co., Ltd., Shanghai, China) with a stirring speed of 30 r/min for 6 h until a uniform hydrogel was formed. Subsequently, the weighed silty clay was placed in a tray, and the prepared hydrogel was added for manual mixing until it was evenly blended without noticeable aggregation. After mixing, the mixture was placed in a sealed bag and kept in a moisture-retaining chamber for 24 h to ensure uniform moisture distribution. The sample required for testing was then prepared using static pressure. After preparation, the sample was subjected to vacuum saturation with distilled water, demolded, and wrapped in plastic film. The wrapped samples were placed in a walk-in constant temperature and humidity chamber (T = 20 °C, relative humidity = 90%) for curing. The samples were taken out at 0 d, 7 d, 14 d, and 21 d for consolidated undrained shear tests.

2.3. Limit Water Content Test

To investigate the effect of sodium polyacrylate content on the liquid-plastic limit of silty clay, a combined liquid-plastic limit tester was used in this study. The dry mass ratio (mR) of sodium polyacrylate to soil was set at 0%, 1%, 3%, 5%, and 7%, respectively, corresponding to five different dosage levels. For each dosage condition, three parallel tests were conducted, resulting in a total of 15 samples. According to the Standard for geotechnical testing methods (GB/T 50123-2019) (China 2019), the liquid limit and plastic limit of silty clay were determined using the combined liquid-plastic limit method. The specific steps are as follows: Approximately 15 samples of air-dried soil, each weighing about 80 g and passing through a 0.5 mm sieve, were each mixed with different amounts of distilled water to form a homogeneous soil paste, which was then sealed in a moisture-preserving container and left to stand for 24 h. Prior to testing, the soil paste was thoroughly mixed again and compactly placed into the sample cup, with the surface smoothed. The filled sample cup was then placed on the base of the testing device, adjusting the cone so that its tip just touched the surface of the soil. The cone was released, allowing it to sink into the soil under its own weight, and the depth of penetration was recorded after 5 s. Three parallel samples were prepared for each mixture, with the cone penetration depths controlled at 3–4 mm, 7–9 mm, and 15–17 mm, respectively. After each test, at least 10 g of the sample near the cone was taken for moisture content determination. A relationship curve was plotted on a double logarithmic coordinate system with moisture content as the x-axis and cone penetration depth as the y-axis. The liquid limit (corresponding to a 17 mm penetration depth) and plastic limit (corresponding to a 2 mm penetration depth) were determined from this curve.

2.4. Low-Field Nuclear Magnetic Resonance Tests

Prior to conducting the consolidated undrained shear tests, a low-field nuclear magnetic resonance (NMR) test was performed to investigate the moisture distribution characteristics within the sample. A low-field NMR analyzer (MacroMR12-110H-l, Suzhou Niumai Analytical Instrument Co., Ltd., Suzhou, China) was used for the testing. During the experiment, the samples, which had been cured to the specified age, were removed from the constant temperature and humidity chamber. The external wrapping film was promptly removed, and after confirming the sample’s integrity, the low-field NMR test was conducted immediately to capture the moisture distribution within the sample’s pore structure.
Low-field Nuclear Magnetic Resonance (NMR) is a powerful and non-invasive technique for moisture characterization, enabling both the quantitative analysis of moisture content and its distribution within a material’s pore structure. The principle behind NMR is as follows: when a sample is exposed to an external static magnetic field, protons (such as hydrogen nuclei) with spin magnetic moments generate net magnetization. A radio frequency pulse is then applied, causing the magnetization vector to tilt and enter a non-equilibrium state. After the pulse ceases, the proton system gradually relaxes back to thermodynamic equilibrium, emitting a decaying signal. By applying Fourier-transform and inversion to this signal, the transverse relaxation time (T2) distribution spectrum is obtained, which reflects the moisture response characteristics at various pore scales. The integral area of the T2 distribution spectrum is generally proportional to the sample’s moisture content [28]. Additionally, for materials with cylindrical pore structures, a theoretical relationship exists between the transverse relaxation time (T2) and the pore radius (R), as given by Equation (1) [29]:
1 T 2 ≈ ρ 2 2 R
where T2 is the transverse relaxation time, R is the pore radius within the specimen, and ρ2 is the transverse surface relaxivity, which depends on the physicochemical properties of the solid matrix.

2.5. Consolidated Undrained Shear Test

To investigate the effect of sodium polyacrylate on the shear strength of silty clay, consolidated undrained shear tests were conducted using a stress–strain controlled triaxial shear testing apparatus (TKA-TTS-1, Nanjing TKA Technology Co., Ltd., Nanjing, China) after the samples had reached the designated curing time. The maximum axial load of this equipment is 4 kN. The tests followed the “Standard for Geotechnical Test Methods” (GB/T 50123-2019), with consolidation pressures set at 100 kPa, 200 kPa, 300 kPa, and 400 kPa. The consolidation was considered complete when the consolidation degree reached 95%. After consolidation, shear was applied at a rate of 0.5 mm/min until the sample’s strain reached 15%, at which point the test was terminated. During the experiment, data on stress, strain, and pore water pressure were recorded, from which the shear strength parameters of the sample were obtained.

2.6. Fourier-Transform Infrared Spectroscopy Measurement

To characterize the main functional groups and analyze the interactions between sodium polyacrylate and silty clay, Fourier-Transform Infrared Spectroscopy (FTIR) tests were conducted on silty clay, sodium polyacrylate, and their modified soil samples. The tests were performed using a Fourier-transform infrared spectrometer (Thermo Nicolet NEXUS670, Thermo Fisher Scientific Inc., Madison, WI, USA), with a scanning range of 4000–350 cm−1 and a resolution of 4 cm−1. To minimize moisture interference and ensure the comparability of the spectra, all samples were freeze-dried using liquid nitrogen and then ground into powder. A suitable amount of sample powder was mixed with spectroscopic-grade potassium bromide at a mass ratio of 1:100, ground uniformly, and pressed into transparent pellets for testing. FTIR analysis is based on the characteristic absorption of infrared radiation at specific wavenumbers by different chemical bonds/functional groups. By identifying shifts in the position and intensity of the absorption peaks, the functional group composition and potential interactions between the materials can be qualitatively determined.

2.7. Scanning Electron Microscopy Observation

To visually compare the microstructure of silty clay and its sodium polyacrylate-modified samples, field emission scanning electron microscopy (FE-SEM) tests (Zeiss GeminiSEM 300, ZEISS, Oberkochen, Germany) were conducted after the consolidated undrained shear tests. For comparative analysis, untreated silty clay was set as the control group. All samples were tested under the same preparation and imaging conditions. Initially, all samples underwent liquid nitrogen freeze-drying, which involved rapid freezing followed by drying. Representative samples were then selected for gold sputtering treatment before being placed into the electron microscope chamber. Under the set acceleration voltage and working distance, the morphology of the samples was observed, and images were captured to analyze the changes in particle contact, cementation, coating features, and pore structure before and after the incorporation of sodium polyacrylate.

3. Results and Analysis

3.1. Liquid and Plastic Limits of Sodium Polyacrylate-Modified Soil

Liquid limit (wL) and plastic limit (wP) tests were conducted on silty clay modified with different amounts of sodium polyacrylate to explore the effect of sodium polyacrylate on the consistency of silty clay. The test results are shown in Figure 4.
As shown in Figure 4, with the increase in sodium polyacrylate content, the liquid limit and plastic limit of the modified silty clay significantly increased. The liquid limit and plastic limit of untreated silty clay were 41.3% and 25.1%, respectively. When the sodium polyacrylate content was increased to 7%, the liquid limit and plastic limit of the modified silty clay reached 96.1% and 67.3%, representing increases of 132.7% and 168.1%, respectively. The plasticity index also increased from 16.2 to 28.8. This indicates that sodium polyacrylate significantly improves the plasticity of silty clay, making it more adaptable and workable for various engineering applications. With the incorporation of sodium polyacrylate, the liquid limit and plastic limit of the soil were substantially increased, reflecting its significant improvement in enhancing soil cohesion and plasticity. This phenomenon is similar to the liquid-plastic limit test results for sodium polyacrylate-modified red clay reported by scholars [21], further confirming the effectiveness of sodium polyacrylate in improving different types of soil. As a polymer, sodium polyacrylate’s hydrophilicity and ability to form hydrogels allow it to significantly alter the hydration and particle interactions in soil, thereby increasing the liquid limit and plastic limit. The incorporation of sodium polyacrylate not only enhances the structural stability of the soil but also improves its flowability and plasticity in moist environments, providing a superior foundation for the soil’s strength and stability.

3.2. Effect of Sodium Polyacrylate Content on Shear Strength

Figure 5 shows the relationship between net confining pressure and failure strength for silty clay samples modified with different amounts of sodium polyacrylate (mR = 0%, 1%, 3%, 5%, and 7%) after a curing period of 0 days. As shown in Figure 4, the incorporation of sodium polyacrylate significantly increased the peak strength of the silty clay compared to untreated silty clay (mR = 0%). This indicates that the addition of sodium polyacrylate enhances the shear strength of the soil, likely due to the cementing effect of sodium polyacrylate between soil particles, improving the contact and cohesion between the particles. At all sodium polyacrylate dosages, the peak strength of the modified silty clay increased with the net confining pressure, reflecting a stronger shear resistance under higher confining pressures. Notably, for the same net confining pressure, as the sodium polyacrylate content increased from 0% to 5%, the peak strength of the modified silty clay gradually increased, indicating that sodium polyacrylate can effectively improve the shear strength of the soil within this dosage range. However, when the sodium polyacrylate content further increased from 5% to 7%, the peak strength of the modified silty clay began to decrease. Compared to untreated silty clay, when the sodium polyacrylate content was 7%, the liquid limit and plastic limit increased by 132.7% and 167.3%, respectively. This suggests that at higher dosages, sodium polyacrylate may have a negative impact on the soil’s strength, possibly due to excessive incorporation altering the internal structure of the soil or limiting the interactions between polymer molecules, which affects the bonding between soil particles. These experimental results demonstrate that the sodium polyacrylate dosage has a significant effect on the shear strength of silty clay and that an optimal dosage range exists, within which sodium polyacrylate can effectively enhance the mechanical properties of the soil. This experimental outcome is consistent with previous studies on sodium polyacrylate-modified red clay [21], further confirming the effectiveness and dosage-dependence of sodium polyacrylate in improving soil mechanical properties.
Based on the Mohr–Coulomb failure criterion, the shear strength on the failure surface of the soil can be expressed as follows:
τ f = c + σ tan φ
where τf is defined as the shear stress on the failure surface, representing the soil’s shear strength; σ denotes the normal stress acting on that surface; c and φ represent the soil’s cohesion and internal friction angle, respectively.
Based on the data obtained from the consolidated undrained tests, we plot the total stress circle for failure, using the normal stress σ as the x-axis and the shear stress τ as the y-axis. The center of the circle is located at (σ1f + σ3f)/2, and the radius is (σ1f − σ3f)/2, where the “f” footnote denotes the values at failure. The failure envelope is then drawn, with the angle of the envelope representing the internal friction angle and the intercept of the envelope on the y-axis representing the cohesion. Based on the stress–strain relationship of silty clay samples modified with different amounts of sodium polyacrylate (mR = 0%, 1%, 3%, 5%, and 7%, respectively), data analysis was performed to obtain the relationship curve between the consolidated undrained shear strength parameters of the samples and the sodium polyacrylate content, as shown in Figure 6. From Figure 6, it can be observed that the cohesion of the silty clay samples increased with the sodium polyacrylate content, while the internal friction angle first increased and then decreased. When the sodium polyacrylate content increased from 0% to 5%, the cohesion of the samples increased from 19.6 kPa to 115.7 kPa, showing an increase of 490%, while the internal friction angle increased from 12.2° to 37.5°, showing an increase of 207%. However, when the sodium polyacrylate content increased from 5% to 7%, the cohesion of the samples slightly increased from 115.7 kPa to 122.2 kPa, and the internal friction angle decreased from 37.5° to 20.8°, a reduction of 44.5%. This indicates that sodium polyacrylate can significantly affect the shear strength parameters of silty clay. Combining the relationship between net confining pressure and peak strength at different sodium polyacrylate dosages, the optimal dosage of sodium polyacrylate for the silty clay samples in this study is 5%. This result is similar to previous studies on sodium polyacrylate-modified red clay, where the optimal dosage was 3%. The difference between the two studies primarily arises from differences in the particle size distribution and mineral composition of silty clay and red clay.

3.3. Effect of Curing Age on Shear Strength of Soil

Figure 7 shows the relationship between net confining pressure and peak strength for silty clay samples modified with different amounts of sodium polyacrylate (mR = 1%, 3%, 5%, and 7%) after curing at constant temperature and humidity for 0, 7, 14, and 21 days. The tests were conducted under different confining pressures (100 kPa, 200 kPa, 300 kPa, and 400 kPa) following consolidated undrained shear testing. As shown in the figure, all sodium polyacrylate-modified silty clay samples exhibit a consistent pattern: under the same confining pressure, the peak strength of the samples increases with the curing time. This suggests that, over time, the interaction between sodium polyacrylate and soil particles gradually strengthens, further improving the shear strength of the soil. Additionally, under the same curing time, the peak strength of the samples increases with the confining pressure, indicating that higher confining pressures help compact the soil structure, further enhancing its shear performance. Further analysis shows that under the same curing time and confining pressure, the samples with 5% sodium polyacrylate content exhibit the highest peak strength, outperforming the other dosages. This indicates that, at a 5% dosage, sodium polyacrylate most effectively enhances the shear strength of the silty clay. As the sodium polyacrylate content increases, the shear strength of the soil gradually improves. However, when the dosage increases further to 7%, the rate of strength improvement slows, indicating that there is a limitation to the optimal dosage range of sodium polyacrylate. Excessive amounts of sodium polyacrylate may have a detrimental effect on the soil structure. These experimental results demonstrate the significant role of sodium polyacrylate in improving the mechanical properties of soil. Its effectiveness is not only influenced by the dosage but also closely related to factors such as curing time and confining pressure. Therefore, the application of sodium polyacrylate in soil improvement is time-sensitive and dosage-dependent, requiring reasonable dosage selection based on actual field conditions to maximize the enhancement of soil shear strength.
Figure 8 shows the curves of cohesion and internal friction angle of silty clay samples modified with different amounts of sodium polyacrylate (mR = 0%, 1%, 3%, 5%, and 7%, respectively.) as a function of curing time, based on the stress–strain data of the modified samples. From Figure 8, it can be observed that the cohesion and internal friction angle of the samples exhibit different trends with increasing curing time. Specifically, under different sodium polyacrylate dosages, the cohesion of the samples continuously increases with the curing time, indicating that the effect of sodium polyacrylate in the soil gradually strengthens. This is likely due to the gradual establishment and enhancement of the bonding force between the polymer and soil particles.
In contrast, the internal friction angle of the samples shows a trend of first increasing and then decreasing with increasing curing time. When the sodium polyacrylate content increases from 0% to 5%, the internal friction angle increases with the extension of the curing time. This may be because the distribution and effect of sodium polyacrylate in the soil gradually stabilize, enhancing the inter-particle friction. However, when the sodium polyacrylate content increases from 5% to 7%, the internal friction angle gradually decreases with increasing curing time. This phenomenon may be due to the higher sodium polyacrylate content causing changes in the internal structure of the soil, where the interactions between polymer molecules may begin to be limited, affecting the frictional resistance between particles. These results suggest that both the sodium polyacrylate dosage and curing time significantly influence the mechanical properties of the soil, particularly the cohesion and internal friction angle. Within a certain range, increasing the sodium polyacrylate content can improve the cohesion and internal friction angle of the soil, but excessive dosages may lead to a reduction in the soil’s frictional strength.

4. Mechanism Analysis

4.1. Interactions Between Sodium Polyacrylate and Silty Clay

To further analyze the possible types of interactions between sodium polyacrylate and silty clay, Fourier-Transform Infrared Spectroscopy (FTIR) tests were conducted on silty clay, sodium polyacrylate, and their modified silty clay. The test results are shown in Figure 9.
As shown in Figure 9, the FTIR spectrum of silty clay exhibits a strong absorption peak at 3624.4 cm−1, corresponding to O-H stretching vibrations (associated with moisture or hydrogen bonding). This indicates the presence of significant moisture or hydrogen bonds in the silty clay, which may result from water adsorption by clay minerals, or the presence of crystalline water and exposed silanol (Si-OH) and aluminol (Al-OH) groups on the particle surfaces. The absorption peak at 1631.1 cm−1 corresponds to O-H bending vibrations, further confirming the presence of silanol (Si-OH) and aluminol (Al-OH) groups on the surface of the silty clay particles. According to the mineral composition analysis of the silty clay (Table 1), the main mineral components include quartz, feldspar, mica, calcite, goethite, and clay minerals, with kaolinite, illite, and chlorite being the dominant minerals. Therefore, the particle surfaces of the silty clay contain exposed silanol (Si-OH) and aluminol (Al-OH) groups, which provide favorable conditions for the interaction between silty clay and sodium polyacrylate.
The FTIR spectrum of the sodium polyacrylate shows a distinct broad peak around 3410.2 cm−1, corresponding to O-H stretching vibrations, representing the hydroxyl functional groups on the surface of sodium polyacrylate molecules [30]. Additionally, the absorption peak at 1634.6 cm−1 in sodium polyacrylate-modified silty clay corresponds to C-H symmetric or asymmetric stretching vibrations, suggesting the presence of alkyl groups in sodium polyacrylate, particularly in the side chains or heterocycles of the polymer chain. The absorption peak around 1600 cm−1 is typically attributed to C=O stretching vibrations of carboxyl groups, indicating the presence of carboxyl (-COOH) or carboxylate (-COONa) groups in the sodium polyacrylate molecules [30,31]. However, the absorption band in this region at 1600 cm−1 can also be associated with the deformation vibrations of water. The absorption peak around 1633.9 cm−1 corresponds to O-H stretching vibrations, primarily due to the presence of carboxyl (-COOH) groups. The absorption peak at 1132.5 cm−1 is related to C-O stretching vibrations [32], indicating the presence of C-O bonds in the polymer chain of sodium polyacrylate.
After the modification of silty clay with sodium polyacrylate, sharp absorption peaks appeared at 3621.3 cm−1 and 3435.5 cm−1, corresponding to O-H stretching vibrations. This indicates that after sodium polyacrylate adsorbed onto the surface of silty clay particles, some carboxyl groups formed hydrogen bonds with the hydroxyl groups on the surface of the soil particles. Additionally, a distinct absorption peak at 1634.6 cm−1 in sodium polyacrylate-modified silty clay suggests that sodium polyacrylate may form M-O (M = Fe, Al) coordination or ion bridging interactions with Fe3+/Al3+ ions in the system, thereby enhancing the binding strength at the particle surfaces. The absorption peak at 1132.5 cm−1 for silty clay shifted after modification (approximately 1093.9 cm−1), which is consistent with the characteristic of complexation or coordination between the carboxyl groups of sodium polyacrylate and the cations such as Fe3+, Mg2+, and Ca2+ in silty clay. Based on the comprehensive FTIR analysis, it can be inferred that sodium polyacrylate primarily adsorbs onto the surface of silty clay particles through a combination of hydrogen bonding and ion bridging or coordination interactions. This promotes particle–particle bonding, forming a more stable agglomerate structure, which is beneficial for improving the shear strength of silty clay.

4.2. Microstructural of Sodium Polyacrylate-Modified Soil

Figure 10 shows the NMR T2 distribution curves of the samples under different sodium polyacrylate dosages and curing times. As seen in the figure, all the samples exhibit a distinct bimodal feature in their T2 spectra. It is generally believed that the smaller T2 component is associated with bound water or capillary water in small pore diameter pores, while the size of the T2 distribution peaks reflects the presence of free water in larger pores. The variation in peak areas can be used to characterize the relative moisture content distribution of the samples.
As shown in Figure 10a, when the sodium polyacrylate content increases from 0% to 7%, the T2 distribution curve shifts to the right, and the T2 peak area increases. This indicates that after modification with sodium polyacrylate, both small and large pores in the silty clay increase in proportion compared to the untreated silty clay, and as the sodium polyacrylate content increases, the pore volume also increases. This phenomenon is attributed to sodium polyacrylate, a hydrophilic polymer, where the carboxylate groups (-COONa) in its molecules can dissolve in water, forming negatively charged carboxylate ions (-COO−). The repulsive interactions between these ions allow sodium polyacrylate molecules to interact fully with water molecules, forming a hydrogel that fills the pores between soil particles and adsorbs onto their surfaces. As the sodium polyacrylate content increases, the water molecules in the hydrogel significantly increase the moisture content of the modified soil. The incorporation of sodium polyacrylate not only increases the moisture content of the soil but also enhances its porosity, thereby improving the soil’s liquid limit and plastic limit. This means that the addition of sodium polyacrylate not only improves the soil’s hydration but also promotes effective connections between soil particles, enhancing the soil’s plasticity and cohesion. Particularly at higher sodium polyacrylate dosages, the formation of hydrogel and water adsorption becomes more pronounced, directly impacting the engineering properties of the soil, especially in terms of shear strength, rheology, and other performance aspects.
Sodium polyacrylate interacts with water to form a hydrogel network with certain viscoelastic properties, which facilitates particle bridging and aggregation, thereby altering the pore size distribution characteristics of the soil. At lower dosages, the gel primarily results in particle coating and the formation of flocs, which may lead to an increase in the proportion of larger pores between the flocs. As the dosage increases, the gel’s filling and cementing effects on the pores become more pronounced, causing larger pores to be divided or partially filled, thus promoting the conversion of pores to smaller diameters. This microstructural change is consistent with the macroscopic shear behavior, contributing to the improvement in the shear strength of the silty clay. It indicates that pore refinement and enhanced particle bonding are beneficial for improving shear resistance.
As shown in Figure 10b–d, for samples with sodium polyacrylate dosages of 1%, 5%, and 7%, the T2 distribution curves shift to the left with increasing curing time, and the T2 peak area increases. Over time, the T2 peak area gradually decreases, indicating that the moisture content within the samples decreases as the curing time extends. Although the samples were cured in a walk-in constant temperature and humidity chamber (T = 20 °C, relative humidity = 90%), moisture in the soil gradually reaches equilibrium, and evaporation occurs over time. Additionally, the sodium polyacrylate hydrogel in the soil continues to harden and shrink as time progresses. Due to the high hydrophilicity of sodium polyacrylate, the hydrogel can adsorb a large amount of water; however, as the curing time increases, the evaporation of moisture and the hardening of the hydrogel lead to a gradual reduction in water content. At longer curing times, the hardening effect of the sodium polyacrylate hydrogel becomes more significant, further reducing the moisture content and making the network structure of the hydrogel tighter. As the moisture in the soil decreases, the bonding between particles is further enhanced, thereby improving the soil’s mechanical properties, particularly its shear strength. Therefore, the shear strength of the modified soil increases with the curing time, indicating that the improvement effect of sodium polyacrylate gradually manifests during the curing process, with the hardening of its hydrogel playing a key role in enhancing the soil’s shear resistance.
To evaluate the improvement effect of sodium polyacrylate on silty clay, modified silty clay samples with sodium polyacrylate dosages of 1%, 3%, 5%, and 7% were selected for scanning electron microscopy (SEM) testing, as shown in Figure 11 and Figure 12. From Figure 11 and Figure 12, Observations of the microstructure reveal that sodium polyacrylate forms a coating on the surface of soil particles, exhibiting a clear coating effect. Additionally, the presence of particle aggregation or flocculation induced by the sodium polyacrylate hydrogel is confirmed, as soil particles exhibit clustering behavior. The aggregation of soil particles within pores or voids is evident, with the sodium polyacrylate hydrogel filling the interparticle spaces. The addition of sodium polyacrylate leads to the formation of a three-dimensional network or elongated structure, which is distinctly different from the morphology of the soil particles, thereby facilitating the identification of hydrogel formation. The sodium polyacrylate interacts with water to form a hydrogel, which fills the pores between soil particles, leading to a reduction in pore size in the modified soil. This suggests that the addition of sodium polyacrylate improves the pore structure of the soil, promoting cohesion and compactness. At the same time, sodium polyacrylate, as a hydrophilic polymer, can quickly dissolve in water. The carboxylate ions (-COO−) on its molecules can interact with the hydroxyl groups (Si-OH and Al-OH) on the surface of soil particles through hydrogen bonding, adsorbing onto the soil particle surfaces. Additionally, sodium polyacrylate can further strengthen the bonding between sodium polyacrylate and soil particles by forming M-O (M = Fe, Al) coordination or ion bridging interactions with ions such as Fe3+, Al3+, and Ca2+, thereby promoting flocculation of soil particles, reducing the distance between them, and enhancing inter-particle bonding, ultimately forming larger aggregate structures. This process significantly improves the structural strength of the soil. Moreover, the sodium polyacrylate hydrogel itself has certain tensile strength, which further increases the soil’s shear resistance. The combined effects of these factors lead to a significant improvement in the shear strength of the modified silty clay. However, when the sodium polyacrylate content reaches 7%, the internal friction angle of the modified silty clay decreases with increasing sodium polyacrylate content. This is due to the thickening and water-retaining properties of sodium polyacrylate, a hydrophilic polymer. When the dosage increases to 7%, excessive sodium polyacrylate forms too much hydrogel in the soil. These hydrogels fill the pores between particles and envelop the soil particles, leading to separation and disruption of particle connections. Excessive hydrogel formation may cause a decrease in the uniformity and integrity of the soil, weakening the bonding force between soil particles.

5. Conclusions

In this study, shear strength tests were conducted on sodium polyacrylate-modified silty clay, and the micro-mechanisms of the modified silty clay were explored using microstructural analysis techniques such as low-field nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Through experimental analysis, the following main conclusions were drawn:
  • The addition of sodium polyacrylate significantly enhances the shear strength of silty clay. As the sodium polyacrylate content increases, the peak strength of the modified soil significantly rises. Notably, at a dosage of 5%, the shear strength of the modified silty clay reaches its maximum. This phenomenon indicates that sodium polyacrylate improves the shear performance of the soil by forming cementation between soil particles, enhancing the bonding strength between the particles. The hydrophilic groups in the molecular structure of sodium polyacrylate interact with the surface of the soil particles, strengthening the inter-particle bonding and thereby improving the mechanical properties of the soil.
  • Sodium polyacrylate has a significant effect on the shear strength of silty clay under different dosages. The experimental results show that when the sodium polyacrylate content is 5%, the peak strength of the modified silty clay is most significantly improved, exhibiting the best modification effect. However, when the sodium polyacrylate content increases to 7%, the shear strength does not continue to increase, but instead shows a slight decrease. This suggests that at excessively high dosages, the interactions between the excess polymer molecules limit the improvement effect on the soil, leading to an adverse impact on the soil’s structure. Therefore, the optimal dosage of sodium polyacrylate is around 5%, within which the mechanical properties of the soil can be effectively enhanced.
  • When sodium polyacrylate interacts with water, it forms a gel structure that enhances the plasticity of the soil. The gelation process increases the distance between soil particles, but the functional groups in the sodium polyacrylate molecules, such as carboxyl (-COOH) and hydroxyl (-OH), form hydrogen bonds with the silanol (Si-OH) and aluminol (Al-OH) groups on the surface of the soil particles. This interaction promotes the connection between soil particles, leading to the formation of particle aggregates. Furthermore, sodium polyacrylate hydrogel possesses a certain tensile strength, which enables the soil to exhibit good mechanical strength in a hydrated state. The combined effects of these interactions not only improve the structural stability of the soil but also significantly enhance its shear strength, thereby improving its engineering performance.
  • The shear strength of sodium polyacrylate-modified silty clay is closely related to the curing time. As the curing time increases, the shear strength of the soil gradually increases, indicating that the improvement effect of sodium polyacrylate becomes more apparent over time. Specifically, during the early curing stages, the bonding force between sodium polyacrylate molecules and soil particles gradually develops, and the hardening of the hydrogel progressively enhances the soil’s shear strength. Over time, the hydrogel structure becomes more stable and compact, further improving the mechanical properties of the soil. Particularly at longer curing times, as moisture gradually evaporates and the hydrogel hardens, the soil structure becomes more compact, the bonding between particles strengthens, and the shear strength of the soil is effectively increased.

Author Contributions

M.Z.: Conceptualization, Methodology, Investigation, Formal analysis, Writing—Original Draft, Writing—review and editing. Z.L.: Supervision, Funding acquisition, Writing—Review and Editing. H.B. and Z.Z.: experiment, data processing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China titled as “Disintegration mechanism of red clay in karst area under stress–chemical–temperature coupling” under Grants No. 41867039, and the Guangxi Key Laboratory of Geomechanics and Geotechnical Engineering (No. 20-Y-XT-03).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All datasets generated and/or analyzed during this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

Author Minglin Zhou was employed by Wuhan Qianceng Geotechnical Engineering Co., Ltd. Author Zhendong Zhou was employed by China Inner Mongolia Forest Industry Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. X-ray diffraction spectrogram of silty clay particles.
Figure 1. X-ray diffraction spectrogram of silty clay particles.
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Figure 2. Particle size distribution of silty clay.
Figure 2. Particle size distribution of silty clay.
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Figure 3. Molecular structure schematic of sodium polyacrylate (modified from [27]).
Figure 3. Molecular structure schematic of sodium polyacrylate (modified from [27]).
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Figure 4. The effect of sodium polyacrylate on Atterberg limits of silty clay.
Figure 4. The effect of sodium polyacrylate on Atterberg limits of silty clay.
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Figure 5. Relationship between peak strength and net confining pressure.
Figure 5. Relationship between peak strength and net confining pressure.
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Figure 6. Relationship curves of c and φ versus sodium polyacrylate content.
Figure 6. Relationship curves of c and φ versus sodium polyacrylate content.
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Figure 7. Relationship between peak strength and net confining pressure under different sodium polyacrylate contents; (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
Figure 7. Relationship between peak strength and net confining pressure under different sodium polyacrylate contents; (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
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Figure 8. Relationship curves of c and φ versus curing time under different sodium polyacrylate contents (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
Figure 8. Relationship curves of c and φ versus curing time under different sodium polyacrylate contents (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
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Figure 9. Infrared spectra of silty clay, and sodium polyacrylate and sodium polyacrylate-modified soil.
Figure 9. Infrared spectra of silty clay, and sodium polyacrylate and sodium polyacrylate-modified soil.
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Figure 10. T2 distribution curves of sodium polyacrylate-modified soil (a) curing time = 0 d; (b) mR = 1%; (c) mR = 5%, and (d) mR = 7%.
Figure 10. T2 distribution curves of sodium polyacrylate-modified soil (a) curing time = 0 d; (b) mR = 1%; (c) mR = 5%, and (d) mR = 7%.
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Figure 11. SEM images of polyacrylate-modified soil under different sodium polyacrylate contents (5000×) (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
Figure 11. SEM images of polyacrylate-modified soil under different sodium polyacrylate contents (5000×) (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
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Figure 12. SEM images of polyacrylate-modified soil under different sodium polyacrylate contents (20,000×) (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
Figure 12. SEM images of polyacrylate-modified soil under different sodium polyacrylate contents (20,000×) (a) mR = 1%; (b) mR = 3%; (c) mR = 5%, and (d) mR = 7%.
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Table 1. Mineral composition of silty clay particles.
Table 1. Mineral composition of silty clay particles.
MaterialWhole-Rock Composition Percentage/%Clay Mineral Content/%
QuartzFeldsparMicaCalciteHematiteClay mineralKaoliniteIlliteChlorite
Silty Clay34.215.812.75.68.323.470.524.74.8
Table 2. The oxide contents of silty clay particles.
Table 2. The oxide contents of silty clay particles.
MaterialChemical Composition and Mass Fraction of Each Component/%
Al2O3SiO2Fe2O3CaOK2ONa2OMnOP2O5MgOTiO2Loss on Ignition
Silty Clay21.4151.329.131.853.710.530.140.162.211.158.39
Table 3. Other physical properties of silty clay particles.
Table 3. Other physical properties of silty clay particles.
Physical Properties IndicatorsValueAs per
Surface area (SSA), m2/g28.7the Standard for
Geotechnical Testing Methods [26]
Cation exchange capacity (CEC), cmol+/kg20.8
Specific gravity (Gs)2.65
Median particle size d50, mm0.10
Liquid limit (wL), %41.3
Plastic limit (wp), %25.1
Plasticity index (Ip),16.2
Soil classificationClay of high plasticity
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Zhou, M.; Liu, Z.; Bai, H.; Zhou, Z. Shear Strength and Mechanism Analysis of Sodium Polyacrylate-Modified Soft Soil. Appl. Sci. 2026, 16, 1881. https://doi.org/10.3390/app16041881

AMA Style

Zhou M, Liu Z, Bai H, Zhou Z. Shear Strength and Mechanism Analysis of Sodium Polyacrylate-Modified Soft Soil. Applied Sciences. 2026; 16(4):1881. https://doi.org/10.3390/app16041881

Chicago/Turabian Style

Zhou, Minglin, Zhikui Liu, Hanying Bai, and Zhendong Zhou. 2026. "Shear Strength and Mechanism Analysis of Sodium Polyacrylate-Modified Soft Soil" Applied Sciences 16, no. 4: 1881. https://doi.org/10.3390/app16041881

APA Style

Zhou, M., Liu, Z., Bai, H., & Zhou, Z. (2026). Shear Strength and Mechanism Analysis of Sodium Polyacrylate-Modified Soft Soil. Applied Sciences, 16(4), 1881. https://doi.org/10.3390/app16041881

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