Application of Guar Gum Treatment of Basalt Residual-Soil Shallow Slope in Early Ecological Restoration

Abstract

This paper found that environmentally friendly guar gum biopolymers are helpful for stopping the erosion of basalt residual-soil shallow slopes, while also improving the problems of poor stability, difficult growth of early vegetation, and weak initial resistance to the rainfall scouring of these slopes under extreme climatic conditions. Then, to illustrate the effects of the guar gum treatment, laboratory tests have been conducted, including a soil strength test, water retention and water absorption tests, a disintegration test, and a simulated rainfall erosion test, and the pattern of its effect on vegetation growth has been explored. The results indicate that as the content of guar gum increases, both the cohesion and angle of internal friction exhibit a trend of first increasing and then decreasing; the angle of internal friction varies within a range of 21° to 26°. The evaporation rate, water absorption rate, and disintegration rate of this guar gum-treated soil were significantly reduced, while the cracking of the surface layer was significantly improved. The disintegration rate of the soil is only about 2%, as the guar gum content is greater than 1%. Moreover, there is no sign indicating that vegetation germination was affected by the guar gum, meaning that it maintains a favorable environment for vegetation to grow. Guar gum-cured slopes were significantly protected under heavy rainfall washout conditions, with a 94.85% reduction in total soil loss from the slope surface compared to untreated slopes. Since the pores of soil are filled with guar gum hydrogel, the erosion resistance of soil is greatly enhanced. The results of this study will provide a scientific basis for engineering the protection of shallow slopes of basalt residual soils.

1. Introduction

With the rapid infrastructure development in southwestern China, numerous traffic construction projects are inevitably intersecting with the basalt residual-soil region. However, these soils possess undesirable properties such as a high liquid limit [1], a large pore ratio, and the tendency to be dry–hard and wet–soft [2,3]. Under extreme climatic conditions, the mechanical properties of shallow slopes composed of basalt residual-soil deteriorate, leading to frequent natural disasters like soil and water losses, erosional landslides [4,5], and disintegration collapse [6]. Furthermore, soil erosion causes seeds and nutrients on the shallow slopes to wash away, resulting in severe land degradation and geo-environmental issues. If unchecked, this erosion will undoubtedly have a negative impact on the local natural environment and economic development. Traditionally, non-environmentally friendly protective materials like cement and lime have been used to mitigate soil erosion on shallow slopes [7]. However, these materials contribute to global climate change, disrupt habitats for plants, animals, and microorganisms, and have a negative impact on ecosystems. Therefore, it is crucial to study the erosion mechanisms of shallow slopes and devise ecological management measures. This approach not only addresses the immediate issue of soil erosion but also promotes sustainable environmental practices that are vital for the long-term health of ecosystems and communities.

One of the most important measures is slope vegetation [8], which plays a vital role in preventing long-term erosion. Vegetation protection can effectively reduce runoff rates and erosion on slope surfaces, due the ability of vegetation necks to effectively intercept fast-falling raindrops, and the roots of plants construct root–soil complexes [9]. However, in extreme climatic conditions, vegetation on slopes may be shed before it begins to stabilize, and the poor early effects of vegetation protection mean that good news cannot be expected in the short term in this way [10], not to mention the disturbances of human activities, such as trampling or grazing [11]. Absolutely, enhancing soil cohesion and thus restoring soil strength and erosion resistance before vegetation cover is established on slopes is a vital countermeasure for supporting early vegetation growth. This strategy not only reduces the risk of erosion but also fosters the establishment of resilient vegetation, thereby enhancing the stability and ecological integrity of slopes in challenging environments.

Nowadays, biopolymers are widely recognized as sustainable and environmentally friendly materials, garnering significant attention in the geotechnical engineering field due to their ability to improve soil properties. Common biopolymers, including xanthan gum, guar gum, starch, jellied gum, and cellulose, are primarily natural polysaccharides derived from algae, fungi, or bacteria. These biopolymers offer several advantages such as stability, ease of availability, and reduced contamination, as highlighted by Fatehi et al. [12]. Due the biopolymers being of high viscosity and rheological, they have been applied to enhance foundation-bearing capacity, slope stability, desertification control, and erosion control field [13,14,15]. For example, Hamza et al. [16] showed that the stress–strain behavior, the unconfined compressive strength (UCS) value, and the California bearing ratio (CBR) of treated expansive soils tended to increase with the increase in guar gum content and curing time, making it a better-quality material for pavement construction. Sujatha et al. [17] found that the dry density of the treated soil showed an increasing trend, the optimum water content showed a decreasing trend with the increase in guar gum content, and the unconfined compressive strength of the soil increased by 131% after 90 days of conditioning with 2% guar gum content. Chang et al. [13] showed that a 0.5% biopolymer significantly reduced soil erosion while improving soil water-retention capacity and promoting vegetation growth. Wang et al. [18] showed that biopolymers provide water storage space for the specimens, which is conducive to the germination and survival of vegetation. In addition, biopolymers were also used for increasing the soil’s ability to attenuate heavy metals, making them an ideal landfill liner material [19]. Thus, biopolymer-treated soils are superior to cement-treated soils in terms of ecological protection. It is worth mentioning that guar gum is one of the most readily available and economical galactomannans. In an aqueous environment, guar gum is extremely soluble, and the interaction between the galactose unit on the mannose unit and the water molecules leads to intermolecular chain entanglement, which helps in thickening and increasing the viscosity of the solution [20]. Even 1% guar gum would produce a highly viscous gelatinous solution at room temperature. Wang et al. [21] compared the effects of guar gum and xanthan gum through treating expansive soil, and of course, their results showed that guar gum is much better. Fu et al. [22] showed that guar gum can effectively improve the mechanical properties, water-holding properties, water stability, and scour resistance of pre-disintegrated carbonaceous mudstone in the surface layer of charcoal mudstone slopes, as well as reduce their permeability coefficient. This is a new type of environmentally friendly protective material for the surface layer of carbonaceous mudstone slopes. Bao et al. [23] compared the age-related deterioration of guar gum and polypropylene fibers in loess slopes, and the results showed that both materials can produce good protection for loess slopes, and the initial protection of the guar gum mixture was better than the polypropylene fiber-reinforced soil. Yang et al. [24] demonstrated that the application of guar gum significantly reduced cumulative washout by 64.4% during heavy rainfall, compared to unprotected soil. Furthermore, they observed an increase of 55.2% in the average flow rate. These findings suggest that guar gum has the potential to enhance the erosion-resistance characteristics of early shallow slopes, making it a promising material for sustainable slope protection in challenging environments.

In summary, the application of biopolymers has been extensively studied in silty soil, expansive soil, and loess, but basalt residual soil remains an area of necessary in-depth study due to its unique properties, mineral composition, and structural characteristics. Currently, there is a paucity of research focusing on the anti-erosion properties of shallow slopes treated with biopolymers or on measures to enhance vegetation support on early shallow slopes. This paper aims to address this gap by selecting guar gum as an additive to improve the performance of shallow slopes in basalt residual soil. The erosion resistance of soil treated with guar gum will be systematically evaluated through a series of experiments, including direct shear tests, water retention and water absorption tests, disintegration tests, vegetation growth tests, and simulated rainfall erosion tests conducted with different concentrations of guar gum. The findings of this work will provide an important scientific basis for engineering construction and the protection of early shallow slopes in the basalt residual-soil area, contributing to more sustainable and effective slope-stabilization practices.

2. Materials and Methods

2.1. Material

In this work, the test soil samples were taken from a weathered residual-soil erosion area in Kunming, Yunnan, China, as shown in Figure 1, and the soil depth was 0.5~1.0 m. The latitude and longitude coordinates of the sampling points were 103°02′ and 24°57′, respectively. The basic physical parameters of the soil were determined according to GB/T50123-2019 [25], as shown in

Table 1. Basic physical parameters of basalt residual soil.

Specific Gravity ${\mathit{G}}_{\mathit{S}}$	Liquid Limit ${\mathit{W}}_{\mathit{L}}$ (%)	Plastic Limit ${\mathit{W}}_{\mathit{P}}$ (%)	Plasticity Index ${\mathit{I}}_{\mathit{P}}$	Maximum Dry Density (g·cm−3)	Optimum Water Content (%)
2.81	58.67	31.05	27.62	1.53	36.21

; this indicates that the soil is a kind of high liquid-limit clay. The particle size distribution curve of basalt residual soil used in this study is shown in Figure 2.

The study employed guar gum, a biopolymer extracted from guar bean seeds [26], as its primary material. Guar gum’s galactomannans are known for their non-toxic, non-hazardous, and environmentally friendly properties [27]. This polymer can be easily dispersed in water, regardless of temperature, to form a viscous liquid exhibiting exceptional functional characteristics like pseudo-plasticity, rheology, and high shear stability, even at low concentrations [28]. For the vegetation component of the study, ryegrass was chosen based on the local climatic conditions. Ryegrass is a common choice for slope protection, often utilized for soil erosion control, flood mitigation, and dust stabilization. Prior to sowing, ryegrass seeds were carefully screened to eliminate any insufficiently mature or damaged seeds, ensuring the best possible germination and growth conditions.

2.2. Test Methods

2.2.1. Direct Shear Test

The shear strength of soil, which represents its maximum bearing capacity to resist shear damage, is crucial for evaluating the risk of slope failures [29]. Type ZJ strain-controlled direct shear apparatus was employed to determine the shear strength of the soil, and the test procedure was carried out according to the GB/T50123-2019 [25] fast shear test. The shear rate was 0.8 mm/min; the vertical positive stress levels were 100 kPa, 200 kPa, 300 kPa, and 400 kPa, respectively; and the maximum shear displacement was 6 mm. In this paper, the mass ratios (mb/ms) of the guar gum with the soil were 0.0%, 0.5%, 1.0%, 1.5%, and 2.0%. Each type of soil with different proportions was loaded into different molds at one time and shaped by the static compaction; the dimensions of the specimens were cylinders with a diameter of 61.8 mm and a height of 20 mm. After that, the samples were cured under standard controlled conditions (temperature 20 ± 2 °C, relative humidity 80%) and the curing times were set at 1, 3, and 7 days. For each test condition, six parallel specimens were prepared, resulting in a total of 42 specimens for the entire study.

2.2.2. Water Retention and Water Absorption Test

To simulate slopes with low-lying terrain or abundant groundwater, uniform capillarity tests were conducted to assess the influence of guar gum on moisture migration rates. The mass ratios of guar gum to soil were set to 0.0%, 0.5%, 1.0%, 1.5% and 2.0%. The mixture of gum and soil was configured as a slurry with a water content of 50%. This slurry was then uniformly shaken for 5 min to eliminate any trapped air bubbles. The slurry was then poured into a mold measuring 61.8 cm in diameter and 40 cm in height and left to stand for 24 h to ensure homogeneous water content throughout the specimen. In order to avoid the potential influence of external environmental factors, the specimens were placed in a drying oven maintained at a constant temperature of 25 ± 1 °C to facilitate dehydration. Once the water content of the specimen reached a stable level, a permeable stone with a thickness of 10 mm was placed beneath it. An appropriate amount of water was then added to a tray placed under the permeable stone, ensuring that the water level did not exceed the thickness of the stone. During the test, the mass of the specimen was measured at regular intervals to monitor moisture migration. Additionally, the development and progression of any cracks on the surface of the specimen were carefully recorded.

2.2.3. Disintegration Test

Soil disintegration is the phenomenon in which a certain volume of soil absorbs water, disperses, and disintegrates into fragments and particles in hydrostatic water [30]. The disintegration test device was referred to as the clayey soil test disintegrator, developed by Li et al. [31] from Chang’an University, as shown in Figure 3. Static compaction was used to shape the disintegration specimen into a cylinder with dimensions of Φ61.8 mm × 40 mm. The prepared soil samples were wrapped in cling film and cured for 1, 3 and 7 days under standard curing conditions. During the test, the iron-wire hanging net containing the soil sample was suspended so that the soil sample was completely immersed in water for disintegration. The water stability characteristics of the soil sample were analyzed by recording the disintegration characteristics of the soil sample during the whole test. At the same time, the morphological characteristics of soil-sample disintegration were captured by camera. When the disintegration was stable, the disintegration residual-soil sample was dried and weighed. The disintegration process was quantitatively analyzed using the disintegration rate—as the rate turns larger, the erosion resistance of the soil becomes weaker. The disintegration rate can be calculated with Equation (1).

$$D={\displaystyle \frac{m_o-m_s}{m_s}}$$

(1)

where $D$ denotes the disintegration rate of the specimen, $m_o$ denotes the dry soil mass of the initial specimen, and $m_s$ denotes the mass of dry soil at the time of disintegration and stabilization of the specimen.

2.2.4. Vegetation Growth Test

To investigate the impact of guar gum on vegetation growth, an experiment lasting 28 days was meticulously conducted. In order to ensure sufficient water for seed germination and growth, 365 mL of water was poured every day during the planting period, which was comparable to the average daily rainfall reported in the southwest region in 2022. During the experiment, the germination rate of vegetation was recorded, and the growth of vegetation and root development were observed. When this test was completed, the simulated rainfall erosion test was continued.

2.2.5. Simulated Rainfall Erosion Test

Soil erosion resistance is a crucial aspect of soil behavior, especially in areas prone to rainfall and runoff [32]. It represents the soil’s capacity to withstand mechanical damage and erosion caused by water flow. In order to study the interaction between guar gum and soil and the effect of guar gum on soil erosion resistance, a series of indoor simulated rainfall erosion tests were carried out. In this study, a custom rainfall erosion test simulator was used, and the schematic diagram of the simulator is shown in Figure 4.

The main structure of the test slope was filled in a cuboid model box with a size of 54 cm × 27.4 cm × 6 cm (L × W × H), and the slope gradient was set to 45°, as shown in Figure 4. The rainfall device was composed of four nozzles, which were connected to the tap water pipe and placed 30 cm above the sample. This arrangement facilitates uniform simulated rainfall covering the entire sample surface. At the bottom of the model box, epoxy resin was used to paste sandpaper with a particle size of 0.25 mm to simulate the interface roughness between the undisturbed soil. In this study, two types of slopes were tested: an untreated slope and a slope treated with guar gum. The method of layered compaction was used for filling. For the untreated slope model, the plain basalt residual soil was directly used to fill the slope surface model in two layers. The filling thickness of the first layer was controlled to be 2 cm, and the filling thickness of the second layer was controlled to be 4 cm. For the guar gum-treated slope model, it was filled in two parts along the vertical direction of the test box. Firstly, the base of the model was filled with plain basalt residual soil, and the thickness of the bottom filling was controlled to be 2 cm. Then, the protective layer of the model was filled with evenly mixed guar gum-modified basalt residual soil, and the thickness was controlled to be 4 cm. After the first layer was filled, the surface was properly roughened to enhance the adhesion between the layers. When filling the soil layer, the slope was firmly locked on the test bench to reduce the impact of the vibration of the slope on the surrounding compacted soil layer. The dry density of the filling soil of the untreated slope model and the guar gum treated slope model was set to 1.75 g/cm³, and the moisture content was set to 35.41%. The prepared sample was wrapped in a plastic film and cured for 7 days.

Referring to the annual average rainfall of Kunming City, Yunnan Province (which is approximately 900 mm), the experimental rainfall intensity was set at 60 mm/h, as the duration of the rainfall was 1 h. A glass rotameter was used to control the water flow rate, ensuring that the slope was scoured at a consistent rate. During the test, soil loss was collected from a catchment tank at specific time intervals (5 min, 15 min, 30 min, and 60 min). This allowed for a detailed analysis of how soil loss varied over time. After the test, the collected soil was dried, and its mass was calculated to determine the total soil loss.

3. Results and Discussion

3.1. Effect of Guar Gum Content and Curing Time on Shear Strength Parameters

Figure 5 illustrates the relationship between the guar gum content and shear strength parameters of the soil specimens. It is evident from the figure that the addition of guar gum significantly enhances both the cohesion and internal friction angle of the treated specimens. Notably, the improvement in cohesion is more pronounced than that of the internal friction angle. After allowing the specimens to cure for 3 days, it is observed that both the cohesion and internal friction angle exhibit a trend of increasing initially and then decreasing as the guar gum content gradually increases. This trend suggests that there is an optimal concentration of guar gum that maximizes the shear strength of the soil. Specifically, when the content of guar gum reaches 1.5%, the shear strength parameters attain their peak values. However, it is noteworthy that the difference in strength between specimens treated with 1.0% and 1.5% guar gum is relatively minor. Therefore, in practical construction, it is economically advisable to use a guar gum content of 1.0% instead of 1.5%, as it offers a comparable level of strength enhancement while reducing material costs.

After curing for 3 days, the cohesion values of the untreated specimen and those treated with different concentrations of guar gum were measured. The untreated specimen exhibited a cohesion of 71.35 kPa, while the treated specimens with 0.5%, 1.0%, 1.5%, and 2.0% guar gum content demonstrated cohesion values of 99.85 kPa, 115.00 kPa, 120.50 kPa, and 117.78 kPa, respectively. These results indicate a significant increase in cohesive strength, with increments of 39.94%, 61.18%, 68.89%, and 65.07% compared to the untreated specimen. The enhancement in cohesion can be attributed to the ability of guar gum monomers to absorb water and form a hydrogel. This hydrogel fills the voids and acts as a cementing agent between soil particles [33]. As a result, the soil particles become more densely packed, leading to an increase in shear strength. However, it is noteworthy that when the guar gum content exceeds a certain threshold, it can lead to increased spacing between soil particles. This, in turn, creates a “lubricating effect” that renders the specimen more vulnerable to damage [34,35]. This is reflected in the decrease in cohesion observed for the specimen treated with 2.0% guar gum content, which is slightly lower than that of the 1.5% treatment. The internal friction angle of the samples ranged from 21° to 26°, exhibiting a variation of only 5° across all treatments. As the guar gum content increased, the internal friction angle initially increased and then decreased. Specifically, when the guar gum content exceeded 1.5%, its modifying effect on the internal friction angle diminished. This behavior can be attributed to the formation of thicker gel membranes by higher concentrations of guar gum, which reduced the effective contact points between soil particles. As a result, it is easier for the particles to slide against each other, leading to a decrease in the internal friction angle.

Figure 6 shows the relationship between curing time and shear strength parameters. The cohesion of the specimens increased to varying degrees with increasing curing time, while the change in the internal friction angle was insignificant. At 1.0% guar gum content, the cohesive force of the specimens at different curing times were 95.25 kPa, 115.00 kPa and 128.15 kPa, respectively. After 3 days of curing, the cohesive force of the guar gum treated specimens increased significantly.

Over time, as the water content in the soil continues to drain, the guar gum gradually transitions from a hydrogel state to a biofilm or flocculated gel matrix with high tensile strength. This transformation reinforces the binding forces between soil particles, resulting in a gradual increase in the cohesion of the treated samples. Both the concentration of guar gum and the duration of short-term curing exert significant influence on the cohesion of the soil yet have a relatively minor impact on its internal friction angle. During the short-term curing process, the internal friction angle of the soil is primarily determined by factors such as the degree of occlusion among soil particles, surface roughness, and stress state. Consequently, the content of guar gum and the duration of short-term curing exert less influence on it, explaining the lack of significant changes observed during testing.

3.2. Effect of Guar Gum Content on Water Retention and Water Absorption

Figure 7a shows the variation curves of water content in specimens as a function of drying time at different concentrations of guar gum. Throughout the drying process, both untreated specimens and those treated with guar gum exhibit a decrease in water content as drying time increases. Nevertheless, the extent of this decrease varies with the amount of guar gum. Specifically, higher concentrations of guar gum result in higher water content in the specimens at a given drying time. This suggests that guar gum effectively enhances the soil’s water-holding capacity. This enhancement is attributed to the abundance of hydrophilic groups in guar hydrogels, which facilitate adsorption onto the surface of soil particles [20]. After 18 h of drying, fine cracks began to emerge on the surface of the untreated specimen. As the drying time progressed, these cracks gradually expanded outwards, accompanied by the formation of new cracks (Figure 8a). Conversely, the surface of the specimens treated with guar gum remained crack-free even after 96 h of drying (Figure 8b). Fu et al. [22] used guar gum biopolymers to modify pre-disintegrated carbonaceous mudstones and obtained similar results to those mentioned above.

Figure 7b depicts the variation curves of water content in specimens as a function of infiltration time at different concentrations of guar gum. After 24 h of infiltration, the untreated specimen exhibited a water content of 54.39%, while the specimens treated with 0.5%, 1.0%, 1.5%, and 2.0% guar gum had water contents of 44.17%, 41.78%, 39.32%, and 28.24%, respectively. The addition of guar gum to the soil significantly reduced the water absorption capacity of the specimens. During the infiltration process, cracks appeared on the surface of the untreated soil, accompanied by significant volume expansion and softening (Figure 8c). Conversely, the surface of the guar gum-treated specimens remained free of bright water, exhibited minimal volume expansion, and maintained a hardened state throughout (Figure 8d). The untreated specimen reached water absorption stabilization almost immediately, whereas the specimens treated with higher concentrations of guar gum did not reach their maximum capacity instantaneously. The rapid saturation of the untreated soil led to a significant difference in water content between the interior and exterior of the soil, resulting in uneven force distribution and increased susceptibility to cracking. In contrast, the slow water absorption rate in the guar gum-treated soils minimized the damage caused by moisture to the specimens.

Figure 9a illustrates the variation curves of water loss rate with drying time for specimens containing different concentrations of guar gum. Initially, as the drying time increases, the rate of water loss decreases for both untreated and guar gum-treated specimens, ultimately stabilizing at zero. However, within the first 40 h of drying, the water loss rate decreases with higher guar gum content. Subsequently, as the drying process continues, the water loss rate actually increases with increasing guar gum content. This trend is primarily attributed to the water-retention properties of guar gum-treated soils. During the initial stages of drying, the water content of the treated soil declines slowly due to the water-binding capabilities of guar gum. As the drying time extends, the remaining water content in the guar gum-treated soil is relatively higher, leading to a faster decrease in water content as the drying process continues.

Figure 9b depicts the variation curves of water absorption rate with infiltration time for specimens treated with different concentrations of guar gum. Initially, the untreated specimen exhibits a water absorption rate of 115.18 g/h, while the treated specimens with 0.5%, 1.0%, 1.5%, and 2.0% guar gum content have absorption rates of 44.20 g/h, 20.57 g/h, 10.25 g/h, and 7.68 g/h, respectively. The addition of guar gum significantly reduces the rate of water movement into and out of the specimens. After 3 h of infiltration, the water absorption rates of both untreated and guar gum-treated specimens gradually converge to near zero. However, the treated specimens continue to absorb water at a slower rate. This behavior is attributed to the absorption and expansion of guar gum into a hydrogel, which blocks pores and creates partially closed pores. This process slows down the movement of water from the external environment into and out of the specimen, enhancing the water stability of the soil.

3.3. Effect of Guar Gum Content on Disintegration Properties

Figure 10 shows the relationship between guar gum content and disintegration rate under varying curing-time conditions. A clear trend is observed: as the guar gum content increases, the disintegration rate of the specimens gradually decreases when the curing time is held constant. Notably, a sharp decline in disintegration rate is seen when the guar gum content rises from 0.0% to 1.0%. Subsequently, the disintegration rate almost stabilized near zero as the guar content continued to increase. Specifically, under a curing time of 3 days, the disintegration rate of the specimens decreases significantly, from 82.86% to 2.34%, as the guar gum content increases from 0.0% to 1.0%. This remarkable reduction demonstrates the significant role of guar gum in enhancing the soil’s resistance to disintegration. The reason for this improvement can be attributed to the formation of a gel protective layer on the surface of soil particles by guar gum. This layer slows down the rate of water movement within and out of the specimen. Consequently, it effectively reduces the pore air pressure when water intrudes into the soil [36]. Sun et al. [30] used cement, kaolinite and lime to improve the disintegration characteristics of granite residual soil. They compared these with traditional curing agents, and the disintegration effect of guar gum-modified residual soil was more significant. Furthermore, it is noteworthy that the disintegration rate of the specimen decreases with increasing curing time and tends to stabilize after 3 days of curing. This stabilization is likely related to the gradual and complete hydration reaction of guar gum, which reaches a point of equilibrium after a certain period.

Figure 11 illustrates the disintegration process of untreated and guar gum-treated specimens following a curing period of 3 days. In the disintegration test, both specimens exhibited the formation of numerous air bubbles on their surfaces upon immersion in water. However, a closer inspection revealed that the untreated specimen displayed a significantly higher quantity and larger size of air bubbles compared to the guar gum-treated specimen. Upon immersion, the untreated specimen underwent complete saturation through water absorption. This saturation was accompanied by the emergence of microcracks, leading to the gradual peeling off of the surface layer composed of small particles. As these cracks continued to propagate, the disintegration rate accelerated rapidly, ultimately leading to the complete disintegration of the specimen. The entire process occurred in just 12 min. Contrastingly, the guar gum-treated specimens demonstrated remarkable resistance to disintegration throughout the entire process. Even after 3 days of testing, there was no significant swelling observed in the volume of the treated specimens, and they were able to maintain their original shape without any signs of damage.

3.4. Effect of Guar Gum on Vegetation Growth

Figure 12 illustrates the germination rate of vegetation in the experiment. For the test, identical quantities of grass seeds were evenly distributed on the soil surface to monitor the germination pattern over time. The results indicate that vegetation grown in soil treated with guar gum exhibited healthy growth, with germination primarily occurring between the 7th and 14th days of cultivation. During the first seven days, germination rates in untreated soil were higher than those in guar gum-treated soil. However, this trend reversed after 14 days of cultivation. Initially, the untreated soil pores facilitated good airflow and water circulation, creating optimal conditions for aeration and thus promoting seed germination [37]. However, in treated soil, the presence of guar gum particles reacted with the soil particles, forming aggregated clumps that filled the pores and disrupted the balance of the air–water cycle. This led to a delay in seed germination [18]. Furthermore, a thin, hard layer formed on the surface of the guar gum-treated soil, hampering the early growth of buds as they struggled to penetrate the soil matrix. After 28 days of cultivation, the total germination rate in untreated soil was 67%, while in the guar gum-treated soil, it was 82%. This represents a 15% increase in total germination in the treated soil compared to the untreated soil. This finding suggests that, despite the initial delay in germination, the long-term benefits of using guar gum-treated soil outweigh the initial challenges.

Figure 13 illustrates the actual growth of vegetation at 7, 14, and 28 days. Initially, after 7 days of cultivation, it is evident that germination in the untreated soil occurred earlier and was more evenly distributed. Conversely, germination in the guar gum-treated soil was sporadic and primarily concentrated around the side slopes. After 14 days, germination in the untreated soil began to stabilize, while germination in the guar gum-treated soil continued to occur sporadically, resulting in uneven vegetative growth. However, by the end of 28 days, the vegetation in the untreated soil appeared sparse, whereas it became lusher and more abundant in the guar gum-treated soil. This observation extends to the root development as well. When examining the root system at the bottom of the soil, it was noted that the untreated soil had fewer roots at the base, most of which were clustered around the periphery. In contrast, the root system in the guar gum-treated soil was well-developed, with dense and intertwined roots. These findings suggest that guar gum does not have any negative impact on vegetation growth. In fact, grass seeds grew better and exhibited a higher germination rate in the treated soil. Additionally, the structural integrity of the soil surface in the guar gum-treated area was maintained, with no visible cracks or fissures. This positive effect can be attributed to the ability of guar gum to modify the soil structure, enhancing its water- and nutrient-retention capabilities, thereby creating a favorable environment for vegetation growth [38,39]. Furthermore, guar gum may contain chemical components that are beneficial for plant metabolism, providing additional nutrients and water to support vegetation growth [9].

3.5. Effect of Guar Gum on the Erosion Resistance of Slopes

Figure 14 illustrates the relationship between total soil loss and rainfall time for untreated and guar gum-treated slopes. It becomes evident that there is a notable difference in the soil loss patterns of the two types of slopes as the rainfall time varies. During the initial 5 min, both the untreated and guar gum-treated slopes exhibited minimal soil loss. However, as the rainfall time progressed, the total soil loss from the untreated slope escalated significantly. Conversely, the soil loss from the guar gum-treated slope increased to a much lesser extent. After 60 min, the untreated slope experienced a total soil loss of 1340 g, whereas the guar gum-treated slope lost only 69 g. This represents a remarkable reduction of 94.85% in soil loss for the guar gum-treated slope compared to the untreated one. The untreated soil, under the combined action of rainwater leaching and splashing, as well as runoff erosion, absorbs a significant amount of free water. This reduction in friction and cementation between soil particles weakens the soil’s resistance to erosion. The pores among the soil particles were filled up with guar solution, and the particles can hardly move, which means that little water could escape by seepage through the surface soil layer of the slop. Also, the treated soil’s resistance to splash erosion by raindrops, softening by seepage and scouring by runoff were greatly enhanced [40,41]. In conclusion, the use of the guar gum biopolymer in this study has proven to be effective in enhancing the erosion resistance of basalt residual-soil slopes, significantly reducing soil loss compared to untreated slopes.

Figure 15 depicts the erosion process of untreated and guar gum-treated slopes. Initially, the untreated slope surface exhibited uneven holes, with surface soil particles peeling off and the formation of gullies. On the contrary, the guar gum-treated slope displayed only minor cracks. After 60 min of rainfall erosion, the untreated slope’s washout gullies were prominent, leading to landslides and the complete destruction of the slope. Conversely, the overall structure of the guar gum-treated slope remained intact, exhibiting only minor traces of scour erosion with insignificant erosion characteristics. This significant difference indicates that the guar gum-treated slopes possess superior erosion resistance. Under conditions of rainstorm erosion, these slopes can effectively protect the surface, thereby minimizing the risk of slope landslides caused by the deepening of erosion. In essence, the application of guar gum enhances the durability and stability of slopes, making them more resilient to the damaging effects of erosion.

Figure 16 illustrates the relationship between total soil loss and rainfall time on a slope treated with a combination of guar gum and vegetation. After 60 min of rainfall erosion, the untreated slope experienced a significant soil loss of 30.4 g. The vegetation on this slope was exposed at the roots and flattened on the slope surface. In contrast, the slopes treated with guar gum exhibited a much lower soil loss of 10.1 g. These slopes were well-preserved, with only a minor displacement of vegetation. The primary reason for this difference is the dense growth of vegetation on the guar gum-treated slopes, which features broad branches and leaves. The vegetation’s stems effectively intercept fast-falling raindrops, while the roots reinforce the surface soil. This combination effectively mitigates the erosive impact of rainfall or runoff on the slope surface. The synergistic effect of guar gum and vegetation roots creates a protective layer on the slope, defending against the scouring action of rainfall. This protective layer not only reduces soil loss but also preserves the integrity of the vegetation, ensuring its continued role in stabilizing the slope.

4. Conclusions

In this paper, the application of guar gum biopolymers for the treatment of early shallow slopes in basalt residual soils has been thoroughly analyzed. The objective was to assess the impact of guar gum on various soil properties, including strength, water retention and absorption, disintegration, and erosion resistance, as well as its effect on vegetation growth. The results of indoor tests and observations were compiled, leading to several key conclusions. We have drawn several conclusions, as follows:

(1)

The incorporation of guar gum into the soil effectively enhances its shear strength. As the concentration of guar gum increases and the curing time extends, the cohesion of the soil specimens exhibits a trend of initial increase followed by a decrease or gradual increase. Meanwhile, the internal friction angle remains within a range of 21° to 26°, with the magnitude of change being less than 5°. Notably, after a curing period of 3 days, the cohesion and internal friction angle of the soil treated with 1.0% guar gum increased by 61.18% and 16.59%, respectively, compared to untreated soil.

(2)

The presence of guar gum effectively fills the voids between soil particles, thereby reducing the speed of water flow. This, in turn, prevents water evaporation and infiltration, significantly improving the cracking phenomenon on the surface layer of the soil specimens. The disintegration rate of the guar gum-treated soil is significantly lower. After 3 days of testing, the volume of the specimen remains stable with minimal expansion, maintaining its original shape without damage. The disintegration rate is only approximately 2%.

(3)

During the early stages of vegetation growth, the application of guar gum significantly improves the water retention and stability of the soil, creating a favorable environment for plant growth. Under the influence of rainwater leaching, splashing, and runoff scouring, the synergistic action of guar gum and the vegetation root system forms a protective layer on the slope surface. This protective layer enhances the stability of the slope and effectively shields the roots of the vegetation, ensuring their healthy growth.

(4)

Under the action of rainfall erosion, the slopes treated with guar gum exhibit a significant reduction in the amount of soil particles taken away by the erosion process. This reduction is due to the unique properties of guar gum, which effectively binds the soil particles together, enhancing the overall stability of the slope. As a result, the overall structure of the slope is able to remain intact, and the erosion characteristics are not as prominent as on untreated slopes.

In conclusion, guar gum, as an environmentally friendly modifier, demonstrates significant potential and ecological benefits in enhancing the early performance of shallow slope engineering. However, in practical engineering applications, it is imperative to acknowledge that the effectiveness of soil treatment may vary according to the type of biopolymer, soil type, and extreme environmental conditions. Moreover, it is very necessary to explore the effects of biopolymers on soil microbial activity and evaluate soil health levels in the future work.