Introduction

Expansive soil is a kind of highly plastic clay which contains a considerable quantity of hydrophilic minerals such as montmorillonite and illite, and exhibits the engineering characteristics of “expansion upon water absorption and shrinkage upon water loss”1–2. Its engineering performance is extremely unstable. Under hydraulic action, when montmorillonite comes into contact with water, the soil undergoes swelling and shrinking deformations, crack development, and strength reduction, causing severe damage to pavement structures and some shallow slopes3–4. Consequently, the reinforcement and modification treatment of expansive soil has consistently been a research hotspot5–6. With the progress of ecological and environmental protection construction, scholars have initiated the utilization of polymers as modifiers for expansive soil to compensate for the harm caused by traditional improvement methods to the ecological environment. Currently, there are two types of water-soluble polymers commonly employed to modify expansive soil: one is sodium polyacrylate, and the other is polyurethane, both of which can modify expansive soil. Nevertheless, some scholars have indicated7,8,9 that the application of sodium polyacrylate with high water absorption and high viscosity to modify cement and bentonite yields remarkable results, and sodium polyacrylate exhibits excellent performance in enhancing the mechanical properties of modified expansive soil and resisting soil deformation. In light of the abundant achievements of sodium polyacrylate in soil modification, it is of great significance to analyze the modification effect of sodium polyacrylate on weak expansive soil to enable more rational utilization of sodium polyacrylate for the modification of expansive soil.

Sodium polyacrylate has been extensively utilized in concrete modification and has attained remarkable outcomes7, which is associated with its inherent chemical modifier. As it is a highly absorbent resin and is acknowledged as an environmentally friendly soil stabilizer10, it has an excellent modification effect on soil with significant swelling and shrinkage deformation and high moisture content upon contact with water11. It is a hydrophilic cross-linked polymer containing a considerable number of carboxyl and hydroxyl groups. Its properties are stable and not influenced by temperature, acid, base, and salt, and it is widely employed in food additives and industrial thickeners. It is colorless, odorless, not readily biodegradable, and harmless to the environment, presenting broad application prospects12. Wang Jiaquan et al.8 conducted a study on sodium polyacrylate-modified red clay and demonstrated that the shear strength of the modified red clay initially increased and then decreased, reaching a peak value at 2%, which was 394.21% higher than that of the unmodified red clay. Chen Hongxin et al.9 investigated sodium polyacrylate-modified calcium bentonite and revealed that in comparison with the unmodified soil, the modified soil formed an integrated lamellar structure and the montmorillonite layer spacing became smaller. Generally, sodium polyacrylate has a favourable impact on soil modification, does not cause environmental pollution, and enables the rational utilization of resources and the protection of the ecological environment.

In conclusion, the modified soil with high water absorption and high viscosity using sodium polyacrylate has achieved remarkable results. Due to this characteristic, it is of great significance to modify the mechanical fracture and microscopic mechanism of weak expansive soil. This is extremely crucial for engineering construction and environmental protection in expansive soil areas. Therefore, the mechanical fracture and expansion characteristics of sodium polyacrylate-modified weak expansive soil, as well as the variation of intergranular porosity and the internal mechanism of the modified expansive soil, were investigated through laboratory experiments. Based on the test results, the optimal ratio of cement to soil modified by sodium polyacrylate is proposed, which can provide a numerical basis for the road construction of weak expansive soil.

Materials and methods

Soil sample material

The trial soil was obtained from a road engineering project, extracted at depths ranging from 1 to 1.5 m. The soil displayed a grayish-white coloration and exhibited natural hard plasticity. Following standard procedures13, the naturally air-dried swelling soil was procured through exposure to air. The moisture content of the air-dried soil sample was determined to be 15.64%, rendering it suitable for crushing and screening processes. After passing through a 2 mm sieve, impurities within the soil were removed. The results of the fundamental physical parameters of the soil sample are presented in Table 1, with visual representation provided in Fig. 1. The particle size distribution curve for the swelling soil is illustrated in Fig. 2, indicating that 90% of the particles have diameters less than 0.075 mm, classifying it as fine-grained soil. The coefficient of variation is recorded at 2.98, while the curvature coefficient stands at 0.13, suggesting that the particle size distribution is favorable. Moreover, the liquid limit is measured at 42.54%, which is below 50%. The free swelling rate records at 43.1%, thus enabling us to conclude that this type of swelling soil exhibits weak swelling characteristics.

Table 1 Basic physical property indexes of expanded soil.
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Fig. 1
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Experimental soil.

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Fig. 2
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Grain size curve of soil for test.

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Sodium polyacrylate

The modifier utilized in the experiment was industrial-grade sodium polyacrylate, a colorless and odorless white powder. Its structural formula was [-CH2-CH (COONa) -]n. The particle size distribution is depicted in Fig. 2, while the fundamental physical parameters are presented in Table 2. It possesses high water absorption capacity, is non-toxic and environmentally friendly as a water-soluble polymer. Moreover, its viscosity remains unaffected by temperature fluctuations or exposure to acids, alkalis, or salts; however, strong alkalis can lead to an increase in viscosity.

Table 2 Basic physical properties of sodium polyacrylate.
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Methods

In order to investigate the influence of sodium polyacrylate on modified expansive soil, the test was conducted according to the standard13 as shown in Fig. 3, and the mechanical swelling test is shown in Table 3.

Fig. 3
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Diagram of the test process.

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Table 3 Mechanical expansion test.
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Dry and wet cycle test

Place the sample (the same as the shear test sample) in an electric drying oven at 40 °C (simulating the highest air temperature in natural conditions) as shown in Fig. 3e. The drying period is 24 h and the wetting period is 6 h (simulating the maximum rainfall time in the test area). For each group of tests, record the changes in surface cracks on the sample, and measure the length and width of the cracks using a vernier caliper. The test is repeated four times for a total of four dry-wet cycles.

Microscopic tests

A sample of the central soil from the compression test specimen was taken, dried at 50 °C in an oven and ground, then sifted through a 0.075 mm sieve and sealed in bags for use. The powdered sample was applied to conductive adhesive, then vacuum-frozen and gold-plated for improved conductivity. The SEM tests were conducted using a Hitachi field emission scanning electron microscope as shown in Fig. 3m, with a resolution of 1.0 nm (15 kV) / 1.3 nm (1 kV) and an accelerating voltage of 0.1 to 30 kV. The XRD tests were conducted using an Ultima IV X-ray diffractometer as shown in Fig. 3n, with an X-ray diffractometer accuracy of ≤ 0.030, a resolution of ≤ 0.130, a repeatability of ≤ 0.0020, a scanning angle of 10° to 80° (2θ), a scanning step of 0.02, and a scanning speed of 5 °/min.

Results and discussion

Effect of sodium polyacrylate on the physical properties of expansive soils

Figure 4a depicts the liquid limit of the modified expansive soil. As is apparent from the figure, the liquid limit and plasticity index decrease with the increasing content of the modifying agent. The most significant reduction occurs within the range of 0–4%, and subsequently, the rate decelerates and stabilizes after exceeding 4%. The plastic limit gradually rises with the increase in the modifying agent content and remains nearly unchanged after the content reaches 4%. This is comparable to the variations in the liquid limit and plasticity index of the modified soil by cement and industrial alkali slag14. The maximum decrease in the liquid limit and plasticity index amounts to 52.14% and 77.36% respectively, while the maximum increase in the plastic limit is 20.83%. Once the modified agent content reaches 4%, the changes gradually stabilize, indicating that the addition of sodium polyacrylate has a prominent inhibitory effect on the swelling of expansive soil.

Fig. 4
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Physical properties of modified expanded soil.

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As shown in Fig. 4b, certain variations can be observed in the optimal moisture content and maximum dry density of the modified soil. Overall, the curve of the dry density of the modified soil in relation to the moisture content assumes an inverted V shape. The maximum dry density reduces with the increase of the doping amount, while the optimal moisture content increases. When the doping amount rises from 0 to 5%, the maximum dry density decreases from 1.76 g/cm³ to 1.67 g/cm³, representing a reduction of 5.11%. The optimal moisture content increases from 20.3 to 26.08%, indicating an increment of 28.47%. Beyond a doping amount of 4%, the maximum dry density gradually stabilizes. When the doping amount is 4%, the maximum dry density and the optimal moisture content of the modified soil are lower than those of the unmodified soil, and the modification effect is the most favorable at this point.

Because when no modifier is added, the soil structure is loose and its overall quality is poor, accompanied by a larger dry density. After the addition of polyacrylamide powder, it absorbs water to form a gel, enhancing the adhesion between grains, making the soil compact and structurally superior, thereby reducing the dry density of the soil. After the addition of polyacrylamide powder, it mainly forms a gel by absorbing water, the adhesive force between particles increases, and the overall compaction degree of the soil is enhanced, resulting in a lower dry density of the soil, indicating an increase in strength and good mechanical properties. On the other hand, polyacrylamide has a high water absorption rate, while the soil’s own water content is low, so it cannot fully dissolve the acrylic acid to form a gel. Acrylic acid can only form a gel by absorbing water molecules. As the content of the polyacrylamide modifier increases, the modified soil in the unit volume contains more modifier, leading to a higher water absorption saturation threshold value of polyacrylamide and clay particles than that of ordinary soil, that is, the optimal water content will increase. The change in the compressibility of the modified expansive soil is similar to the results obtained by Li et al.15.

Effect of sodium polyacrylate on the swelling properties of modified expansive soils

Figure 5a shows the free swelling rate of the modified expansive soil at different curing times, and Fig. 5b displays the swelling force of the modified expansive soil at various curing times. It can be seen from Fig. 5a that with the increase of the addition ratio, the swelling rate of the modified expansive soil decreases. After 60 days of modification, when the addition ratio increased from 0 to 5%, the swelling rate of the modified soil dropped by 54.57%. The swelling rate of the modified soil decreased by 50.12%, 50.95%, and 53.01% at other curing periods respectively. This indicates that the addition of sodium polyacrylate significantly reduces the swelling rate of the expansive soil. As the curing time proceeds, the swelling rate of the modified soil decreases slightly, and the inhibitory effect varies for samples with different addition ratios.

Fig. 5
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Swelling index of modified expansive soil.

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It can be discerned from Fig. 5b that the reduction rate of the swelling force after modification ranges from 15 to 57% in contrast to the unmodified 73.58 kPa. For the modified soil, both the swelling rate and the swelling force exhibit a downward trend with the augmentation of the addition ratio. This is comparable to the reduction rate of the swelling force of RHA-CCR stabilized swelling soil described in literature16, which lies between 10.6% and 51.5%.

Effect of sodium polyacrylate on the strength-deformation characteristics of modified expansive soils

Figures 6 and 7 illustrate the axial stress-strain curves of the modified expansive soil. It can be discerned from the figures that the entire axial strain curve can be categorized into three stages: the elastic compaction stage (OA), the non-elastic compaction stage (OB), and the sample failure stage (OC). The overall curve is upward concave and shows strain softening. In the OA stage, the void ratio of the modified soil gradually decreases, and the internal space of the sample gradually densifies, resulting in an increase in strength and deformation. In the OB stage, the stress initially rises to a peak and then drops before entering the failure stage, and the deformation reaches its maximum. In the OC stage, the stress gradually decreases with the increase in strain, and the slope gradually reduces. The strain at the peak of the stress is known as the failure strain. After the sample fails, the stress decreases with the increasing strain, namely the strain softening phenomenon, which is most obvious in Fig. 6 after 0 days of curing.

Fig. 6
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Axial stress-strain curve of modified expansive soil.

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Fig. 7
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Axial stress-strain curve of modified expansive soil.

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Figure 7 shows that the compressive strength of the modified samples has increased to varying degrees, with a growth rate of 16.96 kPa/d from 7 to 14 days, 9.69 kPa/d from 14 to 28 days, and 4.93 kPa/d from 28 to 60 days. This indicates that the chemical reaction between the modifier and the expansive soil is the most rapid from 7 to 14 days, and that sodium polyacrylate and the expansive soil have not completed the reaction by 14 days, so the chemical reaction will continue. For the same curing period, the modification effect of different amounts of the modifier on the expansive soil is highly significant. With the increase of the modifier content, the compressive strength of the modified soil initially rises and then drops. The compressive strength reaches its maximum when the modifier content is 4%.

Take the compressive strength after 60 days of curing as an example, the compressive strength of the sample with a 4% dosage of the modified soil was 291.74 kPa. The compressive strengths of the modified soil with other dosages were lower than this value, and the compressive strength actually declined when the dosage exceeded 4%. This suggests that there is an optimum value of 4% for the modified soil. Moreover, the Na2O in sodium polyacrylate undergoes a chemical reaction with the OH- in the soil to reach a balance. When the dosage of sodium polyacrylate is overly high, the Na2O cannot fully exert its effect, and it has minimal influence on the strength and deformation resistance of the soil.

The compressive failure mode of the sample after 60 days of curing is illustrated in Fig. 8. The failure mode of the sample changes from brittle flaking to ductile fracture. The cracks in the unmodified sample are mainly in the form of “Y” and inverted “Y”. The 1% addition rate shows ductile flaking, and the 2–4% addition rate is mainly in the transitional state. This indicates that sodium polyacrylate restrains the cracks and vertical deformation of the modified soil, and the 4% addition rate has the least deformation and no obvious cracks. Once the addition rate exceeds 4%, the sample becomes drier and shows larger deformation and cracks in the form of fracture. It can be noted from Fig. 8 that the compressive displacement of the unmodified sample varies significantly, and a bulge emerges in the middle of the sample. The failure of the modified sample is more remarkable at both lower and higher addition rates (1% and 5%, respectively), suggesting that a certain amount of sodium polyacrylate has a significant effect on suppressing the vertical deformation of expansive soil.

Fig. 8
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The 60d compressive damage pattern.

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Hatibu18 categorizes the soil deformation modes ranging from brittle failure to ductile failure into seven types, as depicted in Fig. 9. If the final state of the sample exhibits obvious characteristics of brittle columnar structure, brittle spalling, and brittle fracture, it is respectively designated as Type A, Type B, and Type C, all of which fall into the category of brittle failures. If the sample presents obvious fragmentation or shearing traits, it is in the transitional phase from brittle failure to ductile failure and is labeled as Type D. Ductile failure of the sample refers to the conspicuous phenomenon of ductile fracture or ductile flow, which is classified into Types E, F, and G in accordance with the degree of failure. By integrating the compression failure diagram of the unconfined sample in Fig. 8, it can be observed that the compression failure of the modified expansive soil lies within the 2 − 4% dosage range and pertains to ductile failure. As the content of the modifier rises, the failure type shifts from Type E to Type C.

Fig. 9
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Damage patterns of modified soils.

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The strain value corresponding to the peak compressive strength on the stress-strain relationship curve is an important indicator for evaluating the brittleness or ductility of a material. The greater the brittleness of the material, the weaker its ability to resist deformation, and consequently, the lower its strength. Conversely, if the material possesses good resistance to deformation, it exhibits good ductility and high strength. Figure 10 presents the variation in the destructive strain of the modified expansive soil. It can be observed from the figure that after the addition of a modifier, the destructive strain of the modified soil differs from that of the unmodified soil and decreases to a certain extent. As the curing period increases, the destructive strain decreases for the same dosage. For instance, when the modifier dosages are 0%, 1%, 2%, 3%, 4%, and 5% respectively, the corresponding destructive strains of the samples are 5.7%, 5.3%, 5%, 4.54%, and 4.67%. This indicates that as the dosage increases from 0 to 4%, the destructive strain of the samples continuously decreases. However, once the dosage exceeds 4%, the destructive strain increases. Compared with the unmodified sample, the destructive strains of the samples with more or less modifier decrease to varying degrees. By comparing the compressive strength, it can be found that the greater the destructive strain, the lower the strength of the sample and the greater its brittleness. On the contrary, the smaller the destructive strain, the higher the strength of the sample and the better its ductility.

Fig. 10
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Damage strain of modified expansive soil.

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Figure 11 presents the shear stress-displacement curve of the modified soil without curing. It can be observed from the figure that when the content of the modifier remains unchanged and the overburden pressure increases, the disparity between the shear strength and the residual shear stress gradually diminishes. Because during shearing, the modified soil incorporates particles of higher strength. At a specific displacement, the modified particles are capable of enhancing the shear strength of the soil. As the displacement gradually escalates, the modified soil particles gradually fracture or roll over, and the strength attains its peak and subsequently declines. The greater the overburden pressure, the more readily the modified soil particles break.

Fig. 11
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Shear stress-displacement curves for modified expansive soils maintained for 0d.

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As depicted in Fig. 11, the shear stress-displacement curve of the unmodified sample is of a strain-hardening type, and it manifests a strain-softening type when the dosage exceeds 2%. The modification with sodium polyacrylate elevates the brittleness of the sample, and the strength declines after attaining the peak stress. The strain-softening phenomenon is most prominent at a 4% dosage, and the phenomenon of decreasing strength and increasing softening becomes more conspicuous as the dosage exceeds 4%. Additionally, the strain-softening failure phenomenon becomes more pronounced at high pressure (400 kPa). After 0-day curing with different modified agents, the maximum increase in shear strength after modification was 94.38%, 95.2%, 94.92%, 95.25%, 94.78%, and 94.38%, with an average increase rate of 94.7%. The increase rate ranged between 94% and 96%.

Figure 12 presents the shear stress-displacement curve of the modified soil after 60 days of curing. As can be inferred from the figure, the curve of the unmodified soil undergoes a transition from a stiff type to a softening one. The modified soil exhibits hardening behavior at low pressure and strain-softening behavior at 400 kPa. Due to the fact that at high pressure, the interior of the sample is crowded, the modifier enhances the toughness of the sample, and the strength gradually decreases after reaching the peak stress. After 60 days of curing, the shear resistance strength was increased by 95.75%, 95.65%, 96.03%, 96.38%, 96.89%, and 96.67%, respectively, with an average increase of 95.75%. The increase rate mainly ranges between 95% and 97%, which is 1% higher than that at 0 days.

Fig. 12
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Shear stress-displacement curve of modified expansive soil maintained for 60d.

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After 60 days of curing and being subjected to shear (at 400 kPa), the failure morphology is illustrated in Fig. 13. It can be noted from the figure that the unmodified specimens lost a considerable amount of moisture during a prolonged period of curing, presenting surface dryness and several small holes. The modified specimens have a more compact structure. After the shear failure, the displacement and vertical deformation of the specimen with a 4% addition were relatively smaller. Once the addition exceeded 4%, the failure morphology showed a jumping platform and cracks were generated, and the vertical displacement initially decreased and then increased.

Fig. 13
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Shear damage pattern of modified expansive soil maintained for 60d.

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Table 4 shows the axial deformation rate of shear specimens. As is clear from the table, with the increase of the content of sodium polyacrylate, the axial deformation rate of the shear specimens decreases. With the rise of the overburden pressure, the axial deformation rate approaches zero, and a negative value emerges at high pressure, indicating that the sample is in a contracted state. Additionally, the resistance of the soil to vertical deformation strengthens under the influence of sodium polyacrylate, and the vertical load exceeds 300 kPa when the content of sodium polyacrylate is increased to 4%. When the axial deformation of the sample has reduced, there is unreacted sodium polyacrylate in the soil, which worsens the internal structure of the soil and reduces its cohesion. Therefore, after the addition of 0–4% sodium polyacrylate, it has a positive effect on the shear stability of swelling soil; however, the shear stability of the soil reduces when the content exceeds 4%.

Table 4 Axial deformation rate under overlying pressure (60d).
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Effect of sodium polyacrylate on the fracture properties of modified expansive soils

Figure 14 presents the fissure images of modified swelling clay under dry-wet cycling. After 1, 2, and 3 times of dry-wet cycling, it was discovered that with the increase in the number of dry-wet cycling times, the fissure development of the modified soil was sluggish, mainly evolving from secondary fissures to main fissures. The intermediate main fissures were more numerous in the unmodified sample, and there were some larger cracks on the edges with greater deformation. The fissure development of the modified soil was slow within the range of 1 − 4% of the additive, and the cracking was similar to that of the unmodified sample when the additive was less. The optimal modification effect was obtained at a 4% additive, with only a few secondary fissures emerging on the edge of the sample. After the additive exceeded 4%, the cracking was extensive, mainly consisting of main fissures.

Fig. 14
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Modified expansive soils under wet and dry cycles.

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The width of the cracks in the samples was measured using a vernier caliper, and the results are presented in Table 5. It can be observed from Table 5 that the average crack width in modified soil is narrower than that in unmodified soil, indicating a sluggish development of cracks. This finding demonstrates that the binding effect of sodium polyacrylate effectively restrains crack formation in expansive soil. This observation aligns with Lee et al.‘s discovery19 that soil cohesion remains more stable under polymer influence following dry-wet cycling. After four cycles, the average crack widths at different dosages decreased by 9.19%, 25.98%, 43.83%, 61.68%, and 55.91%, respectively.

Table 5 Average width of fissures (mm).
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Study on the microscopic properties of modified expansive soils

The mechanism of modified expansive soil by sodium polyacrylate

By employing SEM, the microstructures of both modified and unmodified expansive soils were investigated, along with the impact of the modification reaction on the pore structure. The microstructures of unmodified and modified (with a 4% dosage) soils after 60 days of modification are depicted in Fig. 15. As can be observed from Fig. 15a, the unmodified expansive soil is primarily composed of agglomerates, with some loose particles scattered around them, featuring various sizes and shapes. Additionally, some lamellar montmorillonite is present in the unmodified expansive soil. The soil particles are relatively dispersed, accompanied by large pores, resulting in poor inter-particle connectivity and thereby leading to low strength and significant deformation in the macroscopic perspective.

Fig. 15
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Magnified 11,000 microstructure: (a) unmodified expanded soil, (b) modified expanded soil.

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From Fig. 15b, it is evident that a considerable amount of amorphous hydration products and a small quantity of unreacted modifier are adhered to the surface of soil particles. After the reaction of the modifier, a large number of hydration products are generated and randomly distributed, filling the pores between soil particles and forming a hydration gel film, which renders the spatial structure of the soil more compact. The hydration gel film exerts its tensile and wrapping effects on the soil particles, making the boundaries of the soil particles indiscernible, resulting in enhanced strength and reduced deformation in the macroscopic sense.

Figure 16 presents the XRD pattern of the solidified soil. As indicated by the figure, the XRD pattern of the solidified soil exhibits negligible variation in peak shape, and no new peaks emerge, suggesting that no new minerals are generated during the solidification process of expansive soil. The clay minerals mainly consist of quartz, montmorillonite, and illite, but their respective contents undergo changes. The hydrophilic minerals of montmorillonite and illite decreased by 43.14% after 60 days of curing of the solidified expansive soil. Since expansive soil contains a substantial amount of SiO2, silicate particles will be formed upon contact with water, resulting in K+ and Na+ between the layers of the montmorillonite crystal structure20. Meanwhile, Sodium polyacrylate ionizes a large quantity of OH- and CHOO- groups in the aqueous solution and conducts ion adsorption and exchange with K+ and Na+ between the layers of the montmorillonite crystal structure15, generating some hydration compounds such as NaHCO3.

Fig. 16
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XRD pattern of consolidated expansive soil.

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The solidification mechanism of expansive soil depicted in Fig. 17, the diagram of the modified soil, is presented as follows. When water and sodium polyacrylate are introduced to the soil, the hydroxyl and carboxyl groups within the acrylic acid react with the soil particles and water to form hydrogel. This leads to a decrease in the moisture content of the soil. The hydrogel21 fills the interparticle voids through wrapping and stretching, making the soil structure more compact, as illustrated in Fig. 17b. Macroscopically, this gives rise to a reduction in swelling and minor deformation. The formation of the hydrogel is associated with the curing period. In the early curing stage (7 days or less), the reaction between sodium polyacrylate and the soil is incomplete, resulting in a thin and sparse hydrogel film, which manifests a significant change in early swelling. After prolonged curing (28 days or more), the film thickens as depicted in Fig. 17c, enveloping the soil particles and making the entire soil structure more compact, demonstrating stable swelling and strong resistance to deformation.

Fig. 17
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Mechanism of modified expansive soil.

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Changes in pore size of expansive soil under sodium polyacrylate action

Relying on the high-resolution image fracture analysis software (PCAS)22, the quantitative analysis of micro-porosity of the SEM images was conducted at multiple scales, and the SEM images were processed using the threshold segmentation method. After repeated adjustments, a threshold value of 40 yielded a superior segmentation effect. The pore throat radius was set at 2, and the minimum area was fixed at 50, as depicted in Fig. 18. Figure 18 represents the processed image of the 4% modified sample after 60 days of curing. The pore parameters of different modified samples (at 60 days) were statistically analyzed based on the SEM images, and the statistical results are presented in Table 6.

Fig. 18
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PCAS pore processing image: (a) binarized image, (b) fracture statistics chart.

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As is evident from Table 6, the pore probability entropy approaches 1, and no distinct orientation exists. The average length of each sample decreased progressively, suggesting that the microstructure complexity of the modified sample was comparable. The fractal dimension initially decreases and subsequently increases, indicating that the average aperture is the smallest at 4%. The porosity initially decreases and then increases, reaching the minimum value at 4%, signifying that a 4% content has the most optimal modification effect on expansive soil and the lowest porosity.

Table 6 Statistical parameters of pore space of modified expansive soil.
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In summary, based on the SEM images and PCAS results, it can be inferred that the unmodified sample has larger internal porosity and larger average pores, along with smaller contact forces between particles, thus resulting in low strength and significant deformation. The modified sample has undergone varying degrees of changes in internal porosity, presenting a reduced total porosity and shorter average pore length. Sodium polyacrylate is strongly bound to the soil particles, forming agglomerates and amorphous hydrates, which cause the soil to be tightly bound together, with greater contact forces. As a result, the strength is significantly enhanced. When the modifier exceeds 4%, the pores begin to expand, and the strength decreases and the deformation increases.

Conclusion

To address the problems of low strength, poor water stability, prone to expansion and roadbed subsidence caused by expansive soil in road engineering, a novel modification method is proposed. The expansive soil is modified with sodium polyacrylate, and the effects of sodium polyacrylate on the strength, deformation and fracture characteristics of expansive soil are analyzed through in-situ tests. The modification mechanism of sodium polyacrylate on expansive soil is elaborated from the microscopic aspect. The following conclusions are drawn:

  1. (1)

    Sodium polyacrylate is applicable for modifying expansive soil, and the modification effect is remarkable at a 4% addition ratio. It is suggested that the optimal value be 4%. The incorporation of sodium polyacrylate leads to a reduction in the liquid limit and plasticity index of the modified expansive soil, an increase in the plastic limit, an improvement in the optimum water content, and a decrease in the maximum dry density.

  2. (2)

    Sodium polyacrylate is able to enhance the swelling rate of expansive soil and reduce the swelling force. After the incorporation of sodium polyacrylate, the free swelling rate of the modified soil is decreased to below 40%, and the swelling performance is significantly alleviated, at which point it transforms into non-expanding soil. The maximum reduction reaches 54.57%. Compared with the unmodified soil, the reduction rate of the swelling force of the modified soil ranges from 15 to 57%, and for the modified soil, the swelling rate and swelling force show an overall downward trend as the addition rate increases.

  3. (3)

    After modification by sodium polyacrylate, the compressive and shear strengths can be enhanced. As the dosage increases, the strength initially rises and then declines, with the optimal value occurring at 4%. The longer the curing time, the more beneficial the modification effect. The reaction effect of the modifier is superior after 60 days of curing, achieving the maximum increase in strength. The rate of increase in compressive strength from 7 to 14 days is the fastest at 16.96 kPa/d. After curing, the modified samples show a strain softening type. The addition of sodium polyacrylate can significantly improve the deformation resistance of expansive soil and restrict the cracking and vertical deformation of the modified soil. The minimum damage strain and shear displacement occur at a 4% dosage, and the axial strain rate is close to zero.

  4. (4)

    As the number of dry-wet cycles increased, the cracks in the modified soil developed slowly, evolving from secondary cracks to main cracks. In the unmodified sample, there were a larger number of intermediate main cracks and more considerable deformation. In the modified sample with a 4% additive, only a few secondary cracks emerged at the edge. After adding more than 4%, the cracks became larger, with the main cracks being the predominant form. After four cycles, the average crack widths of the different additive amounts decreased by 9.19%, 25.98%, 43.83%, 61.68%, and 55.91%, respectively.

  5. (5)

    After modification with sodium polyacrylate, the quantity and size of pores in the soil decrease, and water-based gels and fibrous gel films adhere to the surface of soil particles, enhancing the adhesive properties of soil particles and reducing fluidity. This effectively enhances the compactness of the soil. After solidification, no new minerals are generated, and the hydrophilic minerals are reduced by 43.14%. After the interaction between the modifier and the aqueous solution, it induces hydrogen bonds among water molecules through the hydroxyl groups in the molecular chain, promoting the formation of viscous gel films with filling, wrapping, and adhesive properties, which reduce the aqueous solution and pores in the soil and make the structure of the soil more compact.