Study on the strength and erosion resistance of soil-amended substrates for rock slopes in Fugu County

Abstract

The Fugu area of Yulin City, Shaanxi Province, China, is a typical Pisha sandstone region that has long faced severe soil erosion issues. While traditional engineering measures provide temporary relief, they struggle to achieve long-term ecological restoration. To address this, this study incorporated xanthan gum and superabsorbent polymer into loess soil at specific ratios for slope soil amendment. Strength and erosion resistance were evaluated through direct shear and 70 mm/h rainfall erosion tests, with reinforcement mechanisms examined via scanning electron microscopy. Results indicate that at a 0.3% XG and 0.15% SAP mixture ratio, the shear strength of samples reached a maximum of 162.75 kPa, representing an increase of approximately 35.6%, while cohesion increased by about 71.7%. Furthermore, under the combination of 0.6% XG and 0.3% SAP, slope soil loss mass decreased by 42.1%, while rainfall infiltration decreased by 32.8%. Combined with SEM results, the incorporation of XG and SAP enhances intergranular bonding by filling soil pore spaces, thereby strengthening particle cohesion and reducing slope erosion. It also increases soil cohesion and improves macroscopic shear performance. In summary, these findings provide valuable references for ecological protection of Pisha sandstone slopes.

Introduction

Fugu County in Yulin City, Shaanxi Province, is a typical Pisha sandstone region. Large-scale engineering projects and mining activities have left extensive exposed rock slopes¹. These slopes undergo wet-dry cycles due to atmospheric forces and precipitation, causing surface rocks to weather, flake off, and collapse easily. This poses threats to surrounding structures and the daily lives of local residents². Simultaneously, these exposed hard rock slopes often feature steep inclines, exposed bedrock, and an inability to support vegetation, leading to severe soil erosion that impacts the surrounding environment. Achieving long-term stability and ecological restoration for such Pisha sandstone rock slopes remains a critical challenge that urgently requires solutions³.

Traditional slope stabilization techniques for Pisha sandstone slopes often employ shotcrete technology to prevent erosion⁴, combined with lattice beam structures to reinforce the slope body. To address vegetation growth challenges on steep Pisha sandstone slopes, hexagonal bricks are embedded within the lattice beams, with loess soil filling the brick voids for planting herbaceous plants. This method enables rapid vegetation restoration on the slope surface⁵. However, the thin planting layer limits root growth space, while summer droughts and winter cold result in low plant survival rates. Compounded by concentrated, intense rainfall in the region, loess within the hexagonal bricks erodes severely, leaving only the lattice beams and brick framework. Prolonged wind erosion and rainwater runoff severely compromise slope stability and may trigger secondary geological hazards. These factors significantly constrain the application of bio-remediation measures on steep Pisha sandstone slopes.

As an innovative technological paradigm integrating engineering measures with ecological management, ecological slope protection technology overcomes the limitations of traditional slope reinforcement—which prioritizes engineering over ecology—and demonstrates significant application advantages and development potential. However, it is worth noting that the practical application of this technology still faces critical challenges that require urgent solutions. During the initial vegetation establishment phase, before the sod forms a continuous and complete cover layer, the slope surface cannot effectively resist the dual effects of raindrop impact and runoff scouring. This creates a crucial “vulnerable period” in ecological protection, known as the “protection gap”^6–8. During this phase, exposed slopes are vulnerable to severe hydraulic erosion. The root systems of young vegetation in the germination stage have weak anchoring capabilities and are easily damaged by torrential rain runoff, ultimately jeopardizing ecological restoration goals⁹.

From the perspective of soil matrix improvement, employing ecological slope protection technology to enhance soil retention and reduce soil erosion can significantly improve slope resistance to erosion^10,11. Currently, organic soil conditioners are widely applied in agriculture and environmental engineering, demonstrating environmentally friendly properties and excellent efficacy^12,13. Some studies have also found that adding appropriate amounts of organic conditioners does not adversely affect soil properties^14–16. Xanthan Gum (XG), a natural polysaccharide conditioner, significantly enhances soil cohesion and erosion resistance by forming stable three-dimensional gel networks between soil particles. Its excellent biocompatibility ensures safe application in ecological slope protection scenarios^17–19. Superabsorbent polymers (SAP) exhibit outstanding water retention and storage capabilities, rapidly absorbing and storing tens to hundreds of times their own weight in water. They release this moisture slowly during drought stress periods, effectively prolonging soil water retention and providing stable water supply for vegetation growth^20–22. Therefore, applying appropriate organic amendments to improve soil not only enhances surface stability but also effectively boosts water retention capacity, thereby promoting vegetation growth^23,24.

Therefore, this study aims to investigate the combined effect of XG and SAP on the stability of imported soil layers in rock slopes. Through erosion and direct shear tests, combined with scanning electron microscopy (SEM) analysis, the following objectives are achieved: - Quantify improvements in the mechanical properties and erosion resistance of modified soils; - Elucidate the synergistic mechanism of XG and SAP in mitigating the early “protection gap” phenomenon. This research aims to provide reference for slope reinforcement using xanthan gum and superabsorbent polymers, and to establish a theoretical basis for ecological restoration of Pisha sandstone rock slopes.

Materials and methods

Test materials

The soil used in the test was silty loam, Soil samples for this study were collected in May 2025 from a natural rock slope west of the Fourth Primary School in Fugu County, Yulin City, Shaanxi Province, China.(111°00′48″ E, 39°01′08″ N; elevation 1200 m). Prior to sampling, we engaged in thorough communication with the project management team, clearly outlining the sampling purpose (pure scientific research, no commercial use) and the minimal sample quantity required (approximately 106 kg). We obtained their informed consent and verbal permission. The sampling activities strictly adhered to the principle of minimal disturbance, causing no damage to the local ecological environment.

The soil is a loamy silt of Pleistocene eolian loess origin near a slope. It exhibits a light yellow to yellowish-brown color, a stiff plastic consistency, and no distinctive odor. After field collection, the soil underwent preliminary crushing. Following natural air drying, it was sieved through a 2 mm mesh to remove impurities and gravel components. Basic physical properties of the test soil were determined using the methods specified in the Standard for Geotechnical Test Methods (GB/T 50123 − 2019)²⁵, including the light compaction test, particle size analysis, and limit moisture content test. The results are presented in Table 1. According to the gradation analysis, the coefficient of uniformity C_u and the coefficient of curvature C_c of the test soil are 7.24 and 1.12, respectively. The particle size distribution is shown in Fig. 1.

Table 1.

Physical properties of test soils

Plastic Limit (%)	Liquid limit (%)	Plasticity index	Relative density	Optimal moisture content	Maximum dry density (g/cm3)
17.6	28.4	9.8	2.71	18.7	1.72

Fig. 1.

Soil particle size distribution curve

The xanthan gum used in the experiment is an anionic polymeric polysaccharide produced by fermentation of carbohydrates by Xanthomonas campestris pv. rapeseed. It is a powdered solid substance with a pale yellow color, as shown in Fig. 2. It exhibits excellent water solubility, hydrophilicity, and stability, with a relative molecular mass of 2 × 10⁶–2 × 10⁷. Upon contact with water, it forms a colorless, viscous aqueous solution.

Fig. 2.

(a) Xanthan Gum Sample; (b) Superabsorbent Polymers Sample

Water Retention Agents, also known as Superabsorbent Polymers (SAP), are white powdery solids, as shown in Fig. 2. Their primary component is sodium polyacrylate, a polymeric compound featuring both hydrophilic and hydrophobic groups. With a molecular weight below 10,000, it exhibits strong water absorption and retention properties. These agents can absorb large amounts of water in soil and release it gradually, thereby improving soil moisture conditions (Table 2).

Table 2.

Basic physical parameters of the modifier

Improvement agent	Heat resistance range (℃)	Acid and Alkali resistance range (viscosity ± 10%)	Solubility	pH (1% solution)
Xanthan gum	− 98 to 90	3–11	Soluble	6–8
Superabsorbent polymers	− 40 to 150	4–10	Insoluble	6.5–7.5

Experimental design

This experiment employed a two-factor, three-level orthogonal experimental design. The optimal ratios of xanthan gum (XG), Superabsorbent Polymers (SAP), and soil were adopted from Wang et al.¹⁰, where SAP was found to be optimal at 0–0.5% of soil mass, and XG at 0–1% of soil mass. The content of xanthan gum (C_XG) and water-retaining agent (C_SAP) in the amended soil are defined as follows:

1

2

In the formula: C_XG represents xanthan gum content (mass fraction), %; m_XG denotes xanthan gum mass, g; m_SAP indicates Superabsorbent Polymers mass, g; m_S signifies soil mass, g.

Based on relevant literature and existing research findings, this study set the addition concentrations of xanthan gum (XG) in the modified soil to 0%, 0.3%, and 0.6%, and the addition concentrations of the Superabsorbent Polymers (SAP) to 0%, 0.15%, and 0.3%. Following an orthogonal experimental design, nine treatments were established, each with three replicate samples. A total of 27 samples were ultimately prepared. The specific experimental design is detailed in Table 3.

Table 3.

Orthogonal test design

Treatment number	XG concentration (% mXG/mS)	SAP concentration (% mSAP/mS)	Number of replicates
1	0	0	3
2	0	0.15	3
3	0	0.3	3
4	0.3	0	3
5	0.3	0.15	3
6	0.3	0.3	3
7	0.6	0	3
8	0.6	0.15	3
9	0.6	0.3	3

Test method

Strength test:

To investigate the effect of xanthan gum and superabsorbent polymer additions on the strength of modified soil, direct shear tests were conducted on modified composite soil specimens at different concentrations designed according to the orthogonal experimental design table (Table 3).

Based on the results of the light compaction test, the dry density of all specimens was set at 1.72 g/cm³ (95% compaction) with a moisture content of 16%. The specimen preparation process is illustrated in Fig. 3. All specimens were formed using a single static compaction method with a jack, producing cylindrical specimens with a diameter of 61.8 mm and a height of 20 mm according to experimental requirements. During specimen preparation, a specific proportion of dry soil was thoroughly mixed with the modifier to ensure uniform dispersion of xanthan gum and water-retaining agents, preventing agglomeration upon contact with large amounts of water. Subsequently, water was added in batches to the soil-modifier mixture and stirred evenly. Finally, the soil material was placed in the mold and statically pressed for 20 min to form the specimen. After demolding, the specimens were wrapped in plastic wrap, sealed, and placed in a moisture-retaining container to achieve equilibrium in internal moisture and stress. Under indoor conditions, the prepared specimens were cured in a constant-temperature, constant-humidity curing chamber at 20 °C for 24 h.

Fig. 3.

Sample Preparation and Shear Process

The direct shear test employed the ZLB-1 triple-cell direct shear creep tester manufactured by Nanjing Soil Instrument Equipment Co., Ltd. In accordance with GB/T 50,123 − 2019 Standard Test Methods for Geotechnical Engineering, the shear rate was set at 0.8 mm/min, with normal stresses set at 50 kPa, 100 kPa, and 200 kPa. To minimize experimental error, three parallel specimens were prepared for each treatment. The average value obtained from the measurements of these three specimens was used as the result for analysis.

Prior to testing, the moisture content of all specimens after curing was measured to be approximately 16.2%. After testing, the moisture content of the specimens showed no significant change (less than 0.1%). When the stress-strain curve exhibited a peak characteristic, the shear stress at the peak point was selected as the shear strength value. If the curve showed no distinct peak, the shear stress at a shear displacement of 4 mm was used to determine the shear strength. Based on the Mohr-Coulomb strength criterion, the corresponding shear strength parameters—cohesion c and internal friction angle φ—were calculated.

Erosion test:

To evaluate the effectiveness of xanthan gum and superabsorbent polymer in enhancing the erosion resistance of soil-reinforced slopes with engineering measures, artificial rainfall erosion tests were conducted. The tests employed a customized rainfall simulation system capable of replicating the typical heavy rainfall conditions in the Yulin region of Shaanxi Province. This system comprised a steel frame structure, an erosion channel, a 0.75-kilowatt centrifugal pump, a flow stabilizing tank, a pressure gauge, and four solid cone spray nozzles (with basic parameters of a standard 60°spray angle, 3.2 mm orifice diameter, and flow rate of 600 L/h to ensure uniform droplet distribution and stable spray patterns). All nozzles are mounted at a height of 2.4 m above the slope surface. The rainfall device configuration is shown in Fig. 4a. The scour channel model measures 75 cm×50 cm×30 cm with freely adjustable slope gradient and is positioned within the test area.

Fig. 4.

(a) Rainfall Device; (b) Rainfall Uniformity Test

Before commencing the test, the rainfall intensity for this trial and the uniformity of the test rainfall must be determined. The test rainfall intensity is calculated using the locally derived rainfall intensity formula for Yulin City. The selected rainfall intensity formula for Yulin City is:

3

In the formula, i: rainfall intensity (mm/min); T: return period (a); t: rainfall duration (min).

This formula employs Pearson’s Type III distribution to conduct frequency analysis on minute-by-minute rainfall data from Yulin City spanning 1951–2015. Parameters obtained through least squares fitting accurately reflect the spatiotemporal characteristics of short-duration heavy rainfall in the semi-arid northern Loess Plateau region. It has been widely applied in local water conservancy, geotechnical engineering, and ecological restoration research.

According to the typical rainfall conditions for slope erosio n in the Pisha sandstone area specified in the Yulin Flood Control Planning (2016–2030), a return period of T = 10 a and a rainfall duration of t = 60 min were adopted. Substituting these parameters into the formula yielded a rainfall intensity of 1.17 mm/min, which was converted to the standard unit as 70 mm/h.Prior to each experiment, a grid consisting of 18 collection cups was used to verify the rainfall uniformity, as shown in Fig. 4b. The rainfall intensity was calibrated to 70 mm/h, and pre-experiments confirmed that the Christiansen uniformity coefficient (C_u) was greater than 85%, ensuring sufficient spatial distribution uniformity of the rainfall.

Following the orthogonal experimental design table, xanthan gum and superabsorbent polymer at corresponding concentrations were thoroughly mixed with the soil. The mixture was then sequentially spread into the scouring channel, with compaction performed after each layer to achieve a density of 1.72 g/cm³. Hexagonal bricks were placed on the surface layer. Modified soil was then filled into the brick voids using the original method and compacted. Water was added at the optimum moisture content of 18.7%. After filling, the scouring channel slope was adjusted to 60°and cured for 24 h. During the test, rainfall intensity was adjusted to 70 mm/h, and the scouring duration was set to 60 min.

This rainfall experiment lasted a total of 60 min, with data collection conducted every 10 min. The experimental data included: soil loss mass and rainfall infiltration volume. soil loss mass was measured by placing the muddy water mixture collected from the slope surface into a collection bucket and allowing it to settle. After 12 h of settling, the mixture was dried and weighed; the measured weight represented the soil loss mass. Rainfall infiltration was determined by collecting the muddy water mixture from the bottom of the soil substrate into a collection box. The weight of the collection bucket was measured, and the difference between the measured mass and the bucket’s mass yielded the rainfall infiltration rate.

After flushing concludes, add a 0.2% polyacrylamide solution to the collected bucket to accelerate sedimentation. After 12 h of settling, separate the sediment-water mixture and place it in an oven maintained at 105 °C. After 12 h of drying, weigh the soil mass to obtain the soil loss mass and rainfall infiltration volume per 10 min resulting from rainfall flushing on the slope.

SEM test:

The analysis was conducted using a Thermo Fisher Quattro s scanning electron microscope. Specimens were prepared containing 0.6% xanthan gum, 0.3% water-retaining agent, and a combination of 0.6% xanthan gum + 0.3% water-retaining agent. A representative soil block approximately 1 cm³ in size was taken from the fresh fracture surface of the specimen. Prior to SEM observation, the sample was placed in a vacuum drying oven and vacuum-dried at 50 °C for 8 h until constant weight was achieved. The samples were then mounted on conductive adhesive and subjected to vacuum gold sputtering under vacuum conditions. The gold coating thickness was approximately 10 nm to enhance surface conductivity. Finally, the microstructure of the samples was observed and photographed using scanning electron microscopy.

Results

Strength test

Characteristics of stress-strain curves:

Through direct shear tests, the effects of xanthan gum (0%, 0.3%, 0.6%) and superabsorbent polymer (0%, 0.15%, 0.3%) on the shear strength of composite soils were systematically evaluated. Tests were conducted under three levels of normal stress: 50, 100, and 200 kPa. The stress-strain curves for each modified soil sample under single-additive and composite-modified conditions are shown in the Fig. 5.

Fig. 5.

Stress-Strain Curve of Specimen with SAP Addition. (a) SAP 0%, (b) SAP 0.15%, (c) SAP 0.3%

Figure 5 shows the stress-strain relationship for specimens modified with a single SAP. During the low-strain stage (0–2% shear strain), the axial stress of the unmodified soil (SAP 0%) specimen increases linearly (Fig. 4a), indicating elastic deformation. Subsequently, the specimens entered the plastic deformation stage, where axial stress continued to increase slowly. Under a normal stress of 50 kPa, they exhibited typical strain softening behavior, with a peak strength of approximately 66 kPa corresponding to a peak strain of 3.1%. Compared to the plain soil specimen, the slope of the elastic deformation curve for the specimen with 0.15% water-retaining agent content exhibited a steeper trend (Fig. 4b), with the elastic strain ranging from 0 to 3%. As the water-retaining agent content increased, the slope of the elastic phase curve continued to rise, though at a gradually decreasing rate. As the specimen underwent further shear, the rate of axial strain increase slowed, transitioning into the plastic deformation phase until reaching peak deviatoric stress. After achieving yield strength, the specimen stabilized structurally, exhibiting no significant decrease in deviatoric stress despite its stabilization.

As shown in Fig. 6, under 50 kPa normal stress, all specimens exhibit distinct peaks in their stress-strain curves. When xanthan gum content increased from 0.3% to 0.6%, the average peak shear strength rose by 30.5%. Under 100 kPa normal stress, the peak position of each specimen gradually shifted backward with increasing xanthan gum content. The corresponding peak strain increased from 3.1% to 4.2%, indicating a weakening of strain softening behavior. A general trend toward transitioning from strain softening to strain hardening was observed. At 200 kPa normal stress, the stress-strain curves of all specimens continued to rise with increasing strain during shear without exhibiting a distinct peak.

Fig. 6.

Stress-Strain Curve of Specimen with XG Additive. (a) XG 0%, (b) XG 0.3%, (c) XG 0.6%

Figure 7 shows the stress-strain curves of composite modified soils with different ratios of superabsorbent polymer (SAP) and xanthan gum (XG) and unmodified soils under normal stresses of 50 kPa, 100 kPa, and 200 kPa. Test results indicate that the SAP 0.15% + XG 0.3% ratio (Fig. 7a) exhibits the optimal soil modification effect: This sample group exhibited distinct peak characteristics under all normal stresses, with peak strengths significantly superior to both the high XG content group (Fig. 7(b)) and the high SAP content groups (Fig. 7c and d) at the same SAP dosage. Under 100 kPa normal stress, the peak strength reached 106 kPa, which is 29.3% higher than the 82 kPa recorded for the group with the same SAP content and 0.6% XG content, and significantly outperforms the shear strength of the native soil control group. In contrast, for other mixtures, the peak strength of the specimens showed a significant decreasing trend as the XG content increased (a 22.6% decrease for the SAP 0.15% group, with the SAP 0.3% group averaging a 67.8% decrease). Notably, the SAP 0.3% + XG 0.6% group (Fig. 7d) exhibited no distinct peak in the curve, with shear strength lower than that of the native soil and a tendency toward brittle failure. Comparative analysis indicates that the composite ratio of SAP 0.15% and XG 0.3% effectively enhances peak soil strength, with specimens achieving a maximum shear strength of 162.75 kPa—representing an approximate 35.6% increase—demonstrating ideal improvement effects. Conversely, excessively high XG content or improper combinations of SAP and XG dosages degrade soil mechanical properties.

Fig. 7.

. Stress-Strain Curves of Specimens Under SAP-XG Composite Modification. (a) SAP 0.15%+XG 0.3%, (b) SAP 0.15%+XG 0.6%, (c) SAP 0.3%+XG 0.3% (d) SAP 0.3%+XG 0.6%

Shear strength index:

Based on the results of the direct shear test and Coulomb’s law, linear fitting curves were plotted for the shear strength (τ) versus normal stress (σ) of modified composite soils containing either xanthan gum or superabsorbent polymer as a single additive, as shown in Fig. 8.

Fig. 8.

(a) Fitted Segments for superabsorbent polymer in Modified Composite Soil; (b) Fitting segment for xanthan gum-modified composite soil

The effects of superabsorbent polymer and xanthan gum on the shear strength of modified composite soil are shown in Tables 4 and 5. As indicated in the tables, the shear strength of the modified composite soil increases with increasing normal stress, exhibiting a linear relationship. Linear function fitting yielded R² values close to 1, indicating good fitting results consistent with Coulomb’s law.

Table 4.

Effect of superabsorbent polymer on the shear strength of modified composite soil

Test serial number	XG	SAP	Linear fit formula	R 2	Internal friction angle φ/°	Cohesion C/kPa
1	0	0	y = 0.37x + 40.43	0.998	20.8	40.43
2	0	0.15	y = 0.47x + 65.06	0.995	25.4	65.06
3	0	0.3	y = 0.47x + 48.06	0.999	25.6	48.06

Table 5.

Effect of Xanthan gum on shear strength of modified composite soil

Test serial number	XG	SAP	Linear fit formula	R 2	Internal friction angle φ/°	Cohesion C/kPa
1	0	0	y = 0.37x + 40.43	0.998	20.8	40.43
4	0.3	0	y = 0.39x + 52.29	0.992	21.5	52.29
7	0.6	0	y = 0.62x + 35.52	0.997	32.1	35.52

Figure 8a and Table 4 indicate that superabsorbent polymer exert a concentration-dependent effect on soil strength parameters. Compared to the control group (Test 1, c = 40.43 kPa, φ = 20.8°), low-concentration superabsorbent polymer significantly increased cohesion to 65.06 kPa (Test 2, 61.0% increase), while high concentrations reduced cohesion to 48.06 kPa (Test 3, 18.9% increase). The internal friction angle exhibited a monotonically increasing trend with increasing superabsorbent polymer concentration, rising from 20.8° to 25.4° (Experiment 2, 22.1% increase) and 25.6° (Experiment 3, 23.1% increase). Xanthan gum addition similarly significantly altered soil strength parameters (Fig. 7b, Table 5). Low xanthan gum concentrations elevated cohesion to 52.29 kPa (Test 4, 29.3% increase), while high concentrations reduced cohesion to 35.52 kPa (Test 7, 12.1% decrease). Notably, the internal friction angle exhibited greater sensitivity to xanthan gum concentration, increasing from 20.8° in the control to 21.5° (Test 4, 3.4% increase) and 32.1° (Test 7, 54.3% increase).

By analyzing specimens modified with a composite of xanthan gum and superabsorbent polymer, linear fitting curves were plotted for the shear strength versus normal stress of the composite-modified specimens, as shown in Fig. 9.

Fig. 9.

(a) Fitting segment of water retention agent for modified composite soil at low xanthan gum concentration; (b) Fitting segment for superabsorbent polymer in modified composite soil at high xanthan gum concentration

When adding superabsorbent polymer to a low-concentration xanthan gum (0.1%) base, cohesion exhibited a nonlinear response with increasing water-retaining agent concentration (Fig. 9a, Table 6). The cohesive strength of the reference group (Test 4, c = 52.29 kPa) first increased to 69.42 kPa (Test 5, 32.8% increase) under the influence of the water-retaining agent, then decreased to 63.14 kPa (Test 6, 20.7% increase). The internal friction angle exhibited a similar trend, increasing from 21.5° to 29.3° (Test 5, 36.3% increase) before decreasing to 24.1° (Test 6, 12.1% increase). This indicates a synergistic enhancement effect between low-concentration xanthan gum and the water-retaining agent, but excessive water-retaining agent leads to a decline in strength parameters.

Table 6.

Effect of Water Retention Agents at Low Xanthan Gum Concentrations on Shear Strength of Modified Composite Soil

Test serial number	XG	SAP	Linear fit formula	R 2	Internal friction angle φ/°	Cohesion C/kPa
4	0.3	0	y = 0.39x + 52.29	0.992	21.5	52.29
5	0.3	0.15	y = 0.56x + 69.42	0.998	29.3	69.42
6	0.3	0.3	y = 0.44x + 63.14	0.987	24.1	63.14

The interaction between high-concentration xanthan gum (0.3%) and superabsorbent polymer exhibited a significant inhibitory effect (Fig. 9b, Table 7). The cohesion of the reference group (Test 7, c = 35.52 kPa, φ = 32.1°) initially increased to 48.63 kPa (Test 8, 36.9% increase) under the influence of the superabsorbent polymer, but a high concentration of the agent caused it to sharply decrease to 26.57 kPa (Test 9, 25.2% decrease). Changes in the internal friction angle were more pronounced, rising from 32.1° to 36.9° (Test 8, 15.0% increase) before sharply decreasing to 16.7° (Test 9, 48.0% decrease), indicating an antagonistic effect when high concentrations of xanthan gum and superabsorbent polymer were added together.

Table 7.

Effect of superabsorbent polymer at High Xanthan Gum Concentrations on Shear Strength of Modified Composite Soil

Test serial number	XG	SAP	Linear fit formula	R 2	Internal friction angle φ/°	Cohesion C/kPa
7	0.6	0	y = 0.62x + 35.52	0.997	32.1	35.52
8	0.6	0.15	y = 0.75x + 48.63	0.983	36.9	48.63
9	0.6	0.3	y = 0.29x + 26.57	0.973	16.7	26.57

Erosion test

The strong erosion resistance of the surface soil layer on slopes effectively reduces the likelihood of soil erosion occurring on restored slopes under factors such as precipitation, providing sufficient nutrients and a suitable environment for vegetation growth.

Under rainfall intensity conditions of 70 mm/h, the erosion test of the control group showed that within the first 0–6 min, raindrops initially caused minor erosion, gradually penetrating downward from the soil surface. From 12 to 20 min, as the soil reached saturation, slow surface flow commenced. Runoff remained minimal at this stage, with no significant erosion of the topsoil occurring—only fine-grained soil and dissolved substances were washed away. After 28 min of rainfall erosion, distinct rill formation appeared on the slope surface, with some soil being washed down to the toe. This marked the peak erosion stage, as shown in Fig. 10.

Fig. 10.

(a) Before rainfall erosion; (b) During rainfall erosion; (c) After rainfall erosion

As shown in Table 8, the cumulative erosion rate of the composite soil gradually decreased with increasing concentrations of XG and SAP, and the degree of slope damage also progressively reduced. When no amendments were added to the composite soil, the cumulative soil loss of the specimens reached as high as 1749.09 g. As the concentration of the modifier increased, the degree of slope damage also gradually improved. When XG was 0.6% and SAP was 0.15%, the cumulative soil loss of the composite soil was only 471.37 g, and the slope erosion inhibition rate reached 73.1%. This indicates that the modifier can effectively enhance the integrity of the composite soil, resisting external erosion forces and maintaining slope stability.

Table 8.

Cumulative soil loss Under Different Treatments

Test number	XG concentration	SAP concentration	Cumulative soil loss (g)	Reduction compared to control (g)	Erosion suppression rate (%)
1	0%	0%	1749.09	-	0
2	0%	0.15%	1485.5	263.59	15.1
3	0%	0.3%	832.4	916.69	52.4
4	0.3%	0%	1395.97	353.12	20.1
5	0.3%	0.15%	769	980.09	56.3
6	0.3%	0.3%	836.9	912.19	52.2
7	0.6%	0%	933.72	815.37	46.6
8	0.6%	0.15%	471.37	1277.72	73.1
9	0.6%	0.3%	614.98	1134.11	64.8

To further investigate the impact of SAP on slope erosion processes, this study analyzed the effects of different SAP concentrations on rainfall infiltration rates. Results indicate that under identical XG concentrations, SAP concentration exhibits a negative correlation with rainfall infiltration rates. When XG concentration was 0% (Fig. 11a), cumulative infiltration under the 0.3% SAP treatment decreased by 23.6% compared to the CK. When XG concentration increased to 0.6% (Fig. 11c), this inhibitory effect became more pronounced, with infiltration under the 0.3% SAP treatment decreasing by 41.2% relative to the CK. Rainfall infiltration increased with prolonged rainfall duration across all treatments, but the rate of increase varied significantly. The infiltration rate in high-concentration SAP treatments slowed markedly during the latter rainfall period (30–60 min), indicating that their water-holding capacity gradually manifested over time.

Fig. 11.

Trend of superabsorbent polymer Effects on Rainfall Infiltration (a) Xanthan gum concentration: 0%; (b) Xanthan gum concentration: 0.3%; (c) Xanthan gum concentration: 0.6%

Due to SAP’s water-absorption expansion properties, when the underlying soil gradually reaches saturation, high-concentration SAP (0.3%) effectively fills soil pores through enhanced water-absorption expansion, thereby reducing water migration rates and inhibiting rainfall infiltration. Furthermore, as XG concentration increases, the soil matrix pore structure is further optimized, creating a synergistic effect with SAP that significantly reduces total rainfall infiltration.

Figure 12 shows the relationship between XG concentration and soil loss mass at different SAP concentration levels (0%, 0.15%, 0.3%). Experimental data indicate that under identical SAP concentrations, XG concentration exhibits a significant negative correlation with soil loss mass. At a SAP concentration of 0.3% (Fig. 12c), the 0.6% XG treatment reduced soil loss mass by 65.8% compared to the control group, demonstrating outstanding erosion resistance.

Fig. 12.

Trend of Xanthan Gum’s Effect on soil loss mass: (a) SAP concentration 0%; (b) SAP concentration0.15%; (c)SAP concentration0.3%

Runoff erosion rates in all treatment groups showed an increasing trend with prolonged rainfall duration, though the magnitude of increase varied. The high-concentration XG treatment group (0.6%) demonstrated excellent erosion resistance from the onset of rainfall. This was primarily attributed to XG rapidly forming a colloidal substance upon contact with water, creating a relatively stable protective layer on the slope surface. As rainfall duration increased, runoff erosion rates across all treatments gradually stabilized, indicating that the soil surface structure reached a new equilibrium state after rainfall scouring. Overall, SAP concentration also significantly influenced runoff erosion rates. Under 0.6% XG conditions, adding 0.3% SAP further reduced runoff erosion by 28.4%, demonstrating a synergistic effect between the two soil conditioners.

To further analyze the impact of various factors on experimental indicators, including runoff erosion and rainfall infiltration, and to identify the optimal treatment level for combined effects, this study employed range analysis to evaluate the orthogonal experiment results. The range R is calculated as:

4

where is the mean value of runoff erosion amount for the j-th factor at the i-th level, and is the range of the j-th factor. A larger indicates a more significant effect of the factor on the test result.The results are presented in Tables 9 and 10.

Table 9.

Extreme Range Analysis Table for soil loss mass

Factors	Mean K			Range R	The optimal level
	K1	K2	K3
SAP(A)	1359.59	908.62	761.43	598.17	3
XG(B)	1355.66	1000.62	673.36	682.31	3

Table 10.

Extreme value analysis of rainfall infiltration

Factors	Mean K			Range R	The optimal level
	K1	K2	K3
SAP(A)	18.78	14.54	10.00	8.78	3
XG(B)	21.15	13.86	8.31	12.84	3

Analysis results indicate that the order of magnitude for the range R of each factor is RB > RA. The ranking of factor influence on indicators is consistently Factor B > Factor A, i.e., XG > SAP. XG exerted the most significant influence on both soil loss mass and rainfall infiltration. Analysis of the optimal combination revealed that under high XG concentration, slope runoff erosion and rainfall infiltration were minimized, indicating an optimal XG concentration of 0.6%. Subsequent analysis of SAP revealed that higher concentrations of SAP resulted in lower runoff erosion and rainfall infiltration rates. Thus, the optimal SAP concentration is 0.3%.

Comprehensive analysis of the range data indicates that XG addition yields superior soil and water conservation performance on slopes compared to SAP. This finding provides crucial theoretical support for optimizing ecological slope protection materials. In practical applications, priority should be given to determining the optimal XG concentration, followed by optimizing the SAP ratio to achieve the best erosion resistance.

Discussion

Shear strength index

The results of the direct shear test indicate that the addition of XG and SAP significantly enhances the shear strength of the soil to a certain extent. This enhancement is primarily manifested in a substantial increase in cohesion (c), while the effect on the internal friction angle (φ) is relatively limited. Among these, the combination of XG (0.3%) and SAP (0.15%) demonstrated the optimal strength performance, achieving a maximum shear strength of 162.75 kPa, while also significantly enhancing the cohesion of the specimens.

The significant enhancement in cohesion is primarily attributed to the three-dimensional polymer gel network formed by XG within the soil structure. As an anionic polysaccharide, XG’s molecular chains undergo extensive hydration in water, forming a viscoelastic gel structure. This structure interacts with soil particle surfaces through hydrogen bonds, van der Waals forces, and electrostatic attraction, effectively enveloping and bridging particles to enhance interparticle cohesion, thereby significantly improving soil integrity²⁶. Additionally, the polymeric film layer formed by XG within the soil partially fills pores, reducing porosity and further increasing structural density and strength.

The incorporation of SAP played a crucial synergistic role in this process. As a superabsorbent polymer, SAP forms stable “micro-reservoirs” within the soil matrix, regulating and maintaining locally optimal moisture content while delaying water evaporation. This provides a reliable water supply for the hydration and sustained stability of XG’s network structure. This synergistic mechanism of “water retention-film formation-bonding” not only enhances the bonding efficacy of XG but also improves the volumetric stability of the soil in dry environments, reducing crack development caused by water loss shrinkage^27,28.

Compared to the significant increase in cohesion, the change in internal friction angle was relatively minor, remaining largely at its original level. This phenomenon indicates that the improvement effect of XG and SAP primarily influences the bonding behavior between particles rather than altering the surface roughness or interlocking capacity of the particles themselves. As a key parameter reflecting interparticle friction and interlocking, changes in internal friction angle are typically closely related to particle shape, gradation, and surface characteristics. Previous studies employing cellulose-based or biopolymer amendments to soils have shown that polymer modification exerts minimal influence on friction angles, with strength improvements primarily attributable to enhanced cohesion²⁹. Since XG and SAP primarily act at the soil microstructure level without significantly altering particle gradation or surface morphology, their impact on the internal friction angle remains limited³⁰.

The improvement effects of XG and SAP do not increase continuously with higher proportions but instead exhibit an optimal concentration threshold. When the proportion exceeds a certain range (e.g., 0.6% XG + 0.3% SAP), both cohesive strength and internal friction angle show a declining trend. For instance, modifiers like lignin, sodium carboxymethyl cellulose (CMC), and palm fiber all exhibit strength peaks at specific concentration ratios, further validating the importance of controlling additive concentrations^31,32. This primarily stems from two mechanisms: (1) Polymer steric hindrance effect: Excessive XG molecular chains form over-crosslinking within the soil matrix, paradoxically enhancing interparticle repulsion. This disrupts the original structure, leading to “over-bonding” and reducing overall soil strength. (2) SAP over-absorption and expansion: High SAP content undergoes significant volume expansion upon water absorption, potentially causing localized soil structure loosening or even forming weak zones. This weakens inter-particle interlocking and friction, thereby reducing shear strength³³.

The synergistic modification by XG and SAP not only enhances soil mechanical properties but also confers excellent water retention and crack resistance. The polymer film formed by XG on the soil surface effectively prevents direct water evaporation, while SAP regulates soil moisture through its water absorption-release cycle, slowing drying rates and reducing the formation of drying shrinkage cracks. This moisture retention and crack-resistant effect enhances the engineering adaptability of the modified soil in arid or semi-arid regions, making it particularly suitable for applications such as slope protection, roadbed filling, and ecological restoration projects.

Improved soil erosion resistance

The erosion test results indicate that the combined use of XG and SAP significantly enhances the soil’s erosion resistance. Among the treatments, Treatment 8 (0.6% XG + 0.15% SAP) demonstrated the best performance in cumulative soil loss over 60 min, with only 471.37 g lost and an erosion inhibition rate as high as 73.1%. Although Treatment 9 (0.6% XG + 0.3% SAP) exhibited higher initial loss (217.49 g at 10 min) but demonstrated markedly enhanced later erosion resistance, with cumulative loss at 60 min reduced to 39.80 g, indicating excellent long-term erosion resistance. This performance evolution stems from the synergistic effects of the amendments on soil structure, water migration, and surface stability^34,35.

XG, as a biopolymer, rapidly hydrates upon contact with water. Its extended molecular chains adsorb onto soil particle surfaces through hydrogen bonding, van der Waals forces, and physical entanglement, forming a dense and resilient biocementation membrane on the slope surface³⁶. This membrane structure effectively seals surface soil pores, reducing initial rainfall infiltration rates. This, in turn, mitigates the development of internal pore water pressure and diminishes the shear erosion kinetic energy of runoff on the soil. SAP further enhances this process: its particles swell dramatically upon water absorption, effectively filling inter-particle voids and compacting soil structure. Additional range analysis reveals that XG exhibits a higher range value than SAP. This indicates XG as the dominant factor, closely linked to its ability to rapidly form surface sealing layers.

The improvement in erosion resistance relies not only on surface protection but also on enhanced stability of soil aggregates within the soil matrix. XG’s long molecular chains effectively encapsulate fine particles and bridge coarse particles, connecting dispersed soil particles into stable large aggregates through powerful cementation. SAP, meanwhile, regulates and maintains the optimal moisture content within the soil matrix, providing an optimal microenvironment for XG’s hydration and the long-term stability of the soil’s internal structure. This synergistic effect significantly enhances the water stability and mechanical strength of the aggregates, enabling them to effectively resist shear failure and transport by runoff. This mechanism shares similarities with fiber-reinforced soils (e.g., coconut fiber)³⁷. Coconut fiber provides rapid “physical reinforcement” by creating a surface erosion-resistant skeleton. Through fiber interweaving, it forms a three-dimensional spatial structure that aggregates with soil particles, preventing dispersion under water impact and enhancing erosion resistance. This makes it suitable for short-term rapid protection, particularly for controlling cracks in clay slopes. In contrast, the synergistic effect of XG-SAP primarily involves chemical bonding and water retention enhancement. This enables deep soil consolidation and self-healing, making it more suitable for the long-term stability requirements of ecological restoration projects.

XG-SAP composite modification mechanism

The results from the aforementioned series of experiments demonstrate that xanthan gum-sodium carboxymethylcellulose composite modification can enhance the mechanical strength and toughness of soil by increasing cohesive forces between soil particles. The mechanical behavior exhibited by the modified soil is an external manifestation of changes in the soil’s intrinsic microstructure. To further elucidate the effects of xanthan gum and superabsorbent polymer on soil microstructure, Fig. 13 presents the scanning electron micrograph of XG-SAP-modified clayey soil.

Fig. 13.

SEM micrographs of XG-SAP treated clay. (a) XG- improved soil (×2000), (b) SAP-improved soil (×1000), (c) XG‑SAP composite treated soil (×1000)

As shown in Fig. 14a, xanthan gum adheres to the surfaces of soil particles and fills the pores between them. The colloidal film enveloping clay particles transforms their contact mode from point-to-point to face-to-face. Soil particles are bridged and bonded together through the membrane structure formed by xanthan gum, creating a stable, integrated structure. The three-dimensional network formed within the soil confines soil particles and restricts their displacement. The elasticity of the colloidal film counteracts soil deformation through its own deformation, thereby enhancing soil cohesion and shear strength. On one hand, the carboxylate (-COO-) groups on the xanthan gum polymer chains attract cations from clay minerals, forming cation bridges with negatively charged clay minerals. This alters the surface charge properties of clay particles, promoting their aggregation. On the other hand, xanthan gum forms electrostatic bonds or hydrogen bonds with clay minerals through its inherent carboxyl (-COOH) and hydroxyl (-OH) groups, constructing a stable spatial network structure. Therefore, the clay modification efficacy of xanthan gum progressively increases with higher polymer content. This enhancement stems from the strengthened bonding and cementation, filling and film-forming, bridging and flocculation effects between xanthan gum and clay particles as the gum concentration rises.

Fig. 14.

Schematic diagram of the improvement mechanism of XG‑SAP treated soil

As observed in Fig. 14b, the addition of superabsorbent polymers increases the contact area and interfacial friction with soil particles. This enables the water-retaining agent to tightly interlock with soil particles, effectively restricting their movement. Furthermore, the superabsorbent polymers facilitate the adhesion or embedding of soil particles, creating an “anchoring” effect that further enhances the cohesive force between the water-retaining agent and soil particles. At optimal concentrations, the water-absorbed and expanded HAP fills the voids between soil particles, tightly binding them together. This increases the complexity and stability of the soil’s internal structure, enabling more uniform load distribution under stress and reducing the risk of localized failure.

Under XG-SAP composite modification conditions, xanthan gum bonds to the surfaces of soil particles and water-retaining agents, further enhancing the adhesive force between them through its own strength (Fig. 14c). The interfacial interaction between superabsorbent polymers and soil largely determines the effectiveness of water-retaining agents in improving soil mechanical properties. Xanthan gum acts as a “bond” connecting different material phases, stabilizing the internal structure of the soil. Through the synergistic effect of these two factors, the mechanical properties of the soil are significantly enhanced.

Environmental impact of modifiers

The environmental compatibility of soil amendments is an important consideration for ecological slope protection. Xanthan gum (XG), a natural biopolymer produced by microbial fermentation, is widely recognized for its biodegradability and environmental friendliness. Previous studies have shown that XG can gradually degrade in soil through microbial activity without producing toxic byproducts, and its application has been widely explored in sustainable soil stabilization and agricultural soil conditioning. In addition to its strengthening effect, XG may also improve soil aggregation and water retention, which can benefit vegetation establishment during ecological restoration.

In contrast, the superabsorbent polymer (SAP) used in this study is primarily composed of sodium polyacrylate, which is commonly applied in agricultural water-retention materials and soil conditioners. When applied at relatively low concentrations (generally below 0.5% of soil mass), SAP has been reported to exhibit good environmental compatibility and does not significantly alter soil physicochemical properties or inhibit plant growth. In natural soil environments, SAP particles may gradually undergo physical fragmentation and partial degradation through environmental processes such as ultraviolet exposure and microbial activity.

In the present study, the maximum dosage of XG and SAP was 0.6% and 0.3%, respectively, which falls within the commonly reported safe application range for soil improvement materials. Therefore, the combined use of XG and SAP in this study is unlikely to cause significant adverse environmental impacts. Nevertheless, long-term field investigations are still necessary to further evaluate the ecological effects of polymer-based soil amendments under different environmental conditions.

Engineering applicability and cost considerations

From an engineering perspective, the results of this study suggest that XG–SAP modification has promising potential for improving the stability of highly erodible soils such as Pisha sandstone. The enhanced shear strength and erosion resistance observed in the experiments indicate that this method may be applicable to ecological slope protection, soil conservation, and restoration of degraded landscapes in erosion-prone regions.

Economic feasibility is also an important factor for large-scale applications. Based on current market prices, xanthan gum typically costs approximately 26–32 yuan per kilogram, while superabsorbent polymer (SAP) costs approximately 12–17 yuan per kilogram. According to the optimal mixture ratio determined in this study (0.3% xanthan gum + 0.15% SAP), treating 1 ton of soil requires approximately 3 kg of xanthan gum and 1.5 kg of superabsorbent polymer, resulting in a material cost of approximately 96–122 yuan per ton of soil. Considering the significant improvements in soil shear strength, erosion resistance, and water retention observed in this study, this improvement method offers good economic viability for ecological slope protection projects. However, in practical engineering applications, additional cost factors such as transportation, mixing, and construction must also be taken into account.

Limitations and future research

Although the laboratory experiments provide valuable insights into the stabilization performance of XG–SAP modified soil, several limitations should be acknowledged. First, the experiments were conducted under controlled laboratory conditions with a fixed slope angle and rainfall intensity, which may differ from complex field environments. Second, the curing period of the modified soil in this study was limited to 24 h, and the long-term durability of the polymer-treated soil under natural weathering conditions requires further investigation.

In addition, field-scale experiments are necessary to evaluate the performance of the proposed stabilization method under real environmental conditions, including variations in rainfall patterns, temperature, and vegetation growth. Future research should also explore the long-term ecological effects and durability of polymer-amended soils to further support the practical application of this technique in large-scale ecological restoration projects.

Conclusions

This study investigated the effects of xanthan gum (XG) and superabsorbent polymer (SAP) on the mechanical properties and erosion resistance of Pisha sandstone soil through direct shear tests, rainfall simulation experiments, and microstructural analysis. The objective was to evaluate the feasibility of using XG–SAP composite amendments to improve the stability of highly erodible soils.

The results demonstrate that the addition of XG and SAP significantly enhances both the shear strength and erosion resistance of Pisha sandstone soil. The incorporation of polymer additives increases soil cohesion and internal friction angle, leading to improved resistance to shear deformation. In addition, the modified soils exhibit substantially reduced soil loss under simulated rainfall conditions, indicating improved resistance to rainfall-induced erosion.

Microstructural observations and experimental results suggest that the stabilization effect of the XG–SAP system is mainly governed by a synergistic mechanism involving polymer bonding, hydrogel pore filling, and aggregate stabilization. Xanthan gum forms a biopolymer network that enhances particle cohesion, while SAP regulates soil moisture and improves pore structure through hydrogel expansion. These combined effects contribute to the formation of a more stable soil structure.

Overall, the XG–SAP composite amendment demonstrates promising potential for improving the stability of Pisha sandstone soils in erosion-prone regions. Considering its environmental compatibility and relatively moderate material cost, this approach may provide a sustainable and effective solution for ecological slope protection and soil conservation. Future studies should focus on long-term field validation and large-scale engineering applications to further evaluate the durability and environmental performance of polymer-modified soils.

Author contributions

C.X analyzed the data and drafted the manuscript; ND.D and ZX.M revised the manuscript and guided the experimental implementation; SL.Z and XJ.B were responsible for experimental operations and statistical analysis of the results; C.X participated in the preparation of figures and tables. All authors reviewed the manuscript.

Funding

This research was funded by “Natural Science Foundation of Science and Technology Department of Shaanxi Province” (S2024-JC-YB-2574) and “Xi’an Science Technology Bureau Fund” (23NYGG0050).

Data availability

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

The authors confirm that informed consent for field soil sampling activities in this study was obtained from relevant local authorities. We recognize that formal written permission should be secured in advance for future research and will implement this as standard operating procedure. All samples collected in this study were used exclusively for this non-commercial scientific research.