Experimental and Mechanical Characteristics of Xanthan Gum and Calcium Lignosulfonate-Cured Gravel Soil

Abstract

The objective of this study was to enhance the mechanical properties of gravelly soil and to consider the binding and filling effects of xanthan gum and calcium lignosulfonate. To this end, gravelly soil samples were prepared with various dosages of xanthan gum and calcium lignosulfonate, and their curing effects were investigated. The mechanical properties and strength parameters of the cured gravelly soil were investigated using unconfined compressive strength (UCS) tests and conventional triaxial compression tests. Furthermore, scanning electron microscopy (SEM) was employed to examine the microstructure and curing mechanisms of the gravelly soil treated with these additives. The findings demonstrate that as the dosage increases, both xanthan gum and calcium lignosulfonate markedly enhance the compressive strength and shear strength of the gravelly soil. The curing effect of xanthan gum was found to be more pronounced with higher dosages, while the optimal curing effect for calcium lignosulfonate was achieved at a dosage of 4%. The gravelly soil treated with xanthan gum exhibited superior performance compared to that treated with calcium lignosulfonate when the same dosage was used. Moreover, at elevated confining pressures, the binding effect of xanthan gum and calcium lignosulfonate on the gravelly soil was less pronounced than the strength effect imparted by the confining pressure. This suggests that the impact of dosage on the shear strength of the gravelly soil is diminished at higher confining pressures. The stabilization of crushed stone soil by xanthan gum is a complex process that involves two main mechanisms: bonding and cementation, and filling and film-forming. The curing mechanism of calcium lignosulfonate-cured gravelly soil can be summarized as follows: ion exchange, adsorption and encapsulation, and pore filling and binding effects. The findings of this research offer significant insights that are pertinent to the construction of high earth–rock dams and related engineering applications.

1. Introduction

In parallel with the acceleration of economic growth, the construction of reservoirs and dams has markedly increased. Of these, earth–rock dams are the most prevalent type of dam, typically utilizing locally available soil–rock mixtures. In comparison to other conventional materials, soil–rock mixtures, which are primarily composed of red sandstone and silty clay, exhibit lower rock strength and are susceptible to disintegration and fragmentation as a result of external environmental factors [1,2]. Furthermore, the combined effects of water leaching and sunlight exposure can cause silty clay to become soft [3,4], which may potentially compromise the stability of earth–rock dams and the safety of personnel.

It is common practice among those utilizing traditional stabilizers to employ inorganic materials, including lime, cement, and fly ash, with the objective of enhancing soil properties. Some researchers have demonstrated that traditional stabilizers can enhance soil strength [5,6,7,8], cohesion and friction angle [9,10,11], as well as water stability [12,13,14] and freeze–thaw durability [15,16,17]. However, these stabilizers may also have an adverse impact on the environment.

To address the issue of environmental pollution, a technique known as microbial-induced carbonate precipitation (MICP) [18,19] has been developed. Tian et al. [20] demonstrated that MICP significantly enhances the shear strength of expansive soils, as well as improves cohesion and friction angle. Teng et al. [21] observed that MICP enhances the strength of silty clay and provides a long-lasting urease reaction. As reported by Li et al. [22], the stabilization of soft soils through the mixing of cement with MICP resulted in notable improvements in cohesion, friction angle, compressive strength, elastic modulus, and toughness. However, current MICP technology is confronted with a number of challenges, including limited microbial activity periods, considerable environmental interference, and inconsistent curing.

The use of xanthan gum and lignosulfonate as novel sustainable stabilizing materials effectively addresses the aforementioned issues. Xanthan gum is a biopolymer with low concentration and high viscosity, which can effectively bind soil particles, improve soil integrity, and control water movement in the soil to improve soil water retention compared with traditional soil improvement methods [23]. Wan et al. [24] found that clay treated with a xanthan gum and guar gum composite had a stronger cohesive force, smaller friction angle, and higher shear strength than that treated with a single colloid. Bozyigit et al. [25], using scanning electron microscopy, found that xanthan gum and guar gum can form filamentous and reticulated structures with clay particles and that this flocculation is the main factor in increasing strength. Banne et al. [26] demonstrated that xanthan gum has a significant impact on the strength and slope stability of laterite. Lignosulfonate, which is composed of negatively charged polycyclic organic compounds, exhibits a strong affinity for high-valent metal ions present in the soil. Furthermore, lignosulfonate offers additional benefits, including high economic value, renewability, and non-pollution [27]. Li et al. [28] demonstrated that lignosulfonate enhances the compressive strength of loess through a chemical reaction with clay minerals via hydrolysis. Zhou et al. [29] demonstrated that lignosulfonate enhances soil water stability without the formation of new minerals.

Most scholars tend to focus only on soil improvement, and relatively few studies have been carried out on improving gravel soil. Pei et al. [30] found that bipolymeric materials can significantly improve the cohesion and friction angle of gravelly soils; Wang et al. [31] found that the addition of lime can increase the strength of gravelly soils, and that the curing effect is most significant at a dosage of 9%. However, there is still a lack of new sustainable curing materials to achieve the curing of gravelly soils, and related studies have failed to explain the micro-mechanism of their action. Accordingly, the present study employs the use of xanthan gum and calcium lignosulfonate as stabilizing agents with the objective of solidifying gravelly soils. The mechanical properties and parameters of the stabilized samples were analyzed through unconfined compressive strength tests and conventional triaxial compression tests. Furthermore, scanning electron microscopy (SEM) was utilized to examine the microscopic mechanisms of xanthan gum and calcium lignosulfonate in stabilizing gravelly soils, with the objective of providing a reference for the engineering practice of using these materials in the improvement of gravelly soil.

2. Materials and Methods

2.1. Experimental Materials

2.1.1. Silty Clay

Soil samples were obtained from a geographical area within Hunan Province. The fundamental physical characteristics are illustrated in

Table 1. Basic physical indicators of powdery clay.

Liquid Limit/%	Plastic Limit/%	Plasticity Index	Maximum Dry Density/(g.cm−3)	Optimum Moisture Content/%	Specific Gravity	PH
32.4	15.4	17	1.87	15.4	2.72	4–5

, and the particle size distribution is depicted in

Table 2. Particle size distribution of powdery clay.

Particle size/mm	<0.005	0.005~0.075	>0.075
Mass percentage/%	10.21	73.77	16.02

. By means of interpolation, the uniformity coefficient was found to be Cu = 3.35, while the curvature coefficient was determined to be Cc = 1.22. Given that Cu is less than 5 and Cc ranges from 1 to 3, the soil is classified as poorly graded. In accordance with the “Code for Design of Building Foundations” (GB50007-2011 [32]), the soil is categorized as silty clay with a plasticity index of 10 < Ip ≤ 17.

2.1.2. Red Sandstone

The red sandstone is sourced from a geographical area within Hunan Province. The basic physical properties of this material are presented in

Table 3. Red sandstone’s basic physical index.

Density/(g·cm−3)	Specific Gravity	Water Absorption Rate/%	Porosity/%
2.17	2.72	2.6	6.12

.

2.1.3. Xanthan Gum

The experiment employed xanthan gum (molecular formula: C₃₅H₄₉O₂₉), obtained from Hefei Qian Sheng Biotechnology Co., Ltd. (Hefei, China). The substance is a white powder with a slight odor and is readily soluble in both cold and hot water. The solution is neutral, freeze–thaw stable, and insoluble in ethanol, which classifies it as a hydrophilic viscous colloid. The fundamental characteristics of xanthan gum are presented in

Table 4. Xanthan gum’s basic properties.

Curing Agent	Appearance	Xanthan Gum Content/%	Viscosity/cP	PH
Xanthan gum	White powder	>99	>1500	7.6

.

2.1.4. Calcium Lignosulfonate

This study employs calcium lignosulfonate (CLS), with the molecular formula C₂₀H₂₄CaO₁₀S₂; this will be referred to later as lignin, procured from Hefei Tissue Culture Biotechnology Co., Ltd. CLS is a yellow–brown powder with a slight aromatic odor. It is non-toxic and harmless, highly soluble in water, hygroscopic, and exhibits good stability. It is classified as an anionic surfactant. The fundamental characteristics of lignin are presented in

Table 5. Lignin’s basic properties.

Curing Agent	Appearance	Lignin Content/%	Insoluble Matter/%	PH
CLS	Yellowish-brown powder	>99	<0.5	4–6.5

.

2.2. Experimental Procedure

In order to compare the effects of xanthan gum and lignin on gravel soil, the natural powdery clay and red sandstone were crushed using an ST-E200 crusher. Subsequently, they were screened through a 2 mm sieve and a 20 mm sieve with the help of a standard vibrating sieve machine. After that, the materials were dried in a 101A-3 electric blast drying oven at 105 °C for 24 h. The vegetarian crushed stone soil (hereinafter referred to as ‘vegetarian soil’) was prepared with a mass fraction of 60% crushed stone, 32% powdered clay, and 8% water content. Meanwhile, the curing soil was made by adding 2%, 4%, and 6% of xanthan gum and 2%, 4%, and 6% of lignin to the total mass of crushed stone, powdered clay, and water. The soil and curing soil were compacted in five layers, and specimens with a standard height of 80 mm and a diameter of 39 mm were prepared using a standard triaxial sampling machine. Subsequently, all prepared specimens were wrapped and sealed with cling film and placed into an HTC-250 incubator for standard maintenance for 7 days. After the end of maintenance, the specimens were put into the saturator for vacuum saturation, and the TSZ-1 automatic triaxial instrument was used to carry out the unconfined compressive strength test and the conventional triaxial compression test without consolidation and drainage, as illustrated in the flowchart in Figure 1. For each condition, 3 specimens were fabricated, resulting in a total of 84 specimens. Subsequently, all the acquired data were averaged and computed.

3. Test Results and Analysis

3.1. Unconfined Compressive Strength

Figure 2 illustrates the unconfined compressive strength ($q_u$) and stress–strain (${\epsilon }_1$) curves for crushed gravel soil that has been stabilized with varying amounts of xanthan gum and lignin. The figure illustrates that the curves for both the crushed gravel soil and the soil stabilized with varying dosages of xanthan gum and lignin initially increase before subsequently decreasing. In light of the observed trend in these curves, the response can be classified into four distinct stages. ① Compaction Stage: At low stress levels, the voids between soil particles and sandstone particles undergo compaction, resulting in a nonlinear increase in the stress–strain curve. ② Linear Elastic Stage: As the axial load continues to increase, no internal damage occurs at this stage, and the deformation remains reversible, with the curve showing a linear increase. ③ Crack Propagation Stage: When the load reaches the crack initiation strength of the specimen, microcracks begin to form on the surface and progressively develop into prominent macrocracks. The curve shows a rapid upward trend during this phase. ④ Post-Peak Stage: Under continuous axial loading, the deformation of the specimen rapidly increases. After the peak strength of the specimen is reached, the load capacity sharply decreases, and the stress–strain curve shows a steep drop. This is because with the incorporation of xanthan gum and lignin, the two in contact with water will form a cement to fill the gaps between the geotechnical particles, thereby increasing the cohesion and friction angle between the specimen micro-particles, and thus increasing the specimen’s load-bearing capacity, so that the crushed rock soil cured by xanthan gum and lignin has a higher peak strength than the natural soil. And as the axial strain continues to increase, the surface of the specimen will form an inclined shear fracture surface. In view of the role of internal cohesion and friction angle of the specimen, the specimen is able to maintain a certain amount of residual strength, and xanthan gum and lignin can effectively increase the specimen’s cohesion and friction angle so that compared with the natural soil, the residual strength of the treated specimen is higher [33,34,35].

The unconfined compressive strength of gravel soil stabilized with different amounts of xanthan gum and lignin is shown in Figure 3. The unconfined compressive strength of gravel soil stabilized with xanthan gum and lignin is higher than that of natural soil. As the xanthan gum content increases, the unconfined compressive strength of the sample shows a linear upward trend with an increase of 97.31%. This is attributed to the xanthan gum filling the voids between the soil and rock particles, thereby increasing the load-bearing capacity of the sample. The unconfined compressive strength of lignin shows the law of first increase and then decrease; at a dosage of 4%, the unconfined compressive strength is the largest, increased by 60.05%; as the dosage continues to increase, the unconfined compressive strength begins to decay. This is mainly because the lignin solidification bonded in the soil particles on the surface and in the pore space to increase the strength of the specimen, and the lignin dosing reached 6%; its content is relatively too much, resulting in the compression of the specimen appearing in the local softening phenomenon [36]. These research results can provide a reference basis for the optimal dosage of xanthan gum and lignin to improve gravel soil in actual projects, which can help to save natural resources and reduce the engineering loss, and then provide a strong feasible support for the research of mixing technology.

3.2. Conventional Triaxial Compression Strength

3.2.1. Stress–Strain Curve Characteristics

The conventional triaxial compression stress–strain curves for gravel soil stabilized with different amounts of xanthan gum and lignin at various confining pressures are shown in Figure 4. In this context, $({\sigma }_1-{\sigma }_3)$ represents the principal stress difference. Similar to the unconfined compression tests, the inherent micropores of the specimen are compressed at initial low loading levels. As the load continues to increase, the specimen enters the linear elastic phase, with the curve approximately linearly rising. As the load continues to increase, the specimen reaches its yield strength and enters the plastic phase, where the stress–strain curve becomes nonlinear and the specimen fails in shear after reaching peak strength. In contrast to the unconfined compressive strength, the residual strength of the specimen remains relatively high under confining pressure (${\sigma }_3$). The natural soil shows strain softening, whereas the strain in the xanthan gum and lignin-enhanced specimens shows a significant transition from a weak softening curve to a weak hardening curve. In addition, the failure mode shifts from brittle to plastic. The stress–strain curve in the elastic phase has a steeper slope for the improved soil. It is also observed that the higher the confining pressure, the less pronounced this phenomenon becomes, and the strain differences between natural and improved soils become smaller.

3.2.2. Strength Parameter Analysis

The peak deviatoric stresses for gravel soil samples stabilized with different amounts of xanthan gum and lignin are shown in Figure 5. It can be seen from the figure that the peak deviatoric stress generally increases with the addition of xanthan gum and lignin, with lignin reaching a maximum value at 4%. As the xanthan gum content increases to 6%, the peak deviatoric stress shows a linear increase of approximately 70.13%. The presence of xanthan gum increases the bonding strength between soil and rock particles, thereby improving soil strength. Conversely, the peak deviatoric stress shows a tendency to first increase and then decrease with increasing lignin content, reaching a maximum of 66.70% at 4%. This behavior is similar to that observed in unconfined compressive tests, where excessive lignin can locally soften the soil, reducing the overall strength. The strength of the xanthan gum-stabilized soil is higher than that of the lignin-stabilized soil at 2% and 6% xanthan gum content, whereas at 4% lignin content, the strength of the lignin-stabilized soil exceeds that of the xanthan gum-stabilized soil. In addition, the difference in strength between the two types of stabilized soil decreases as the confining pressure increases.

As the amounts of xanthan and lignin increase, the strength characteristics of the samples show significant changes. Therefore, the effectiveness of xanthan gum and lignin in improving gravel soil is evaluated by calculating the rate of increase in peak deviatoric stress D using Equation (1). The results are shown in Figure 6.

$$D={\displaystyle \frac{p_i}{p_0}}-1$$

(1)

where $p_0$ represents the peak deviatoric stress of the untreated gravel soil; $p_i$ is the maximum deviatoric stress of the stabilized gravel soil.

Figure 6 shows that a higher xanthan content results in a higher rate of increase in peak deviatoric stress, indicating a better stabilizing effect. At 4% lignin content, the rate of increase in peak deviatoric stress reaches a threshold, indicating that the stabilizing effect is optimal at this concentration. The increase in the growth rate of bias stress with increasing perimeter pressure indicates that the effect of xanthan gum and lignin on the strength of gravelly soils is also enhanced under conditions of increasing perimeter pressure. The weakening of the growth rate of bias stress suggests that at higher perimeter pressures, the interstitial space of the rock and soil particles decreases, resulting in a reduction in the amount of xanthan gum and lignin filling, leading to a weakening of the curing effect. Therefore, higher xanthan gum content results in better stabilization of gravel soils, while 4% lignin content provides the best performance. In addition, at higher confining pressures, the cementing effect of xanthan gum and lignin becomes weaker relative to the strength provided by the confining pressure, demonstrating a reduced influence of additive content on the shear strength of gravel soil.

3.2.3. The Nonlinear Characteristics of the Shear Strength of Gravel Soil

Typically, the stress path of gravel soil under different confining pressures is represented by the changes in stress on the maximum shear stress plane ($p^{\prime },q$). The maximum shear stress plane stress is determined according to Equations (2) and (3).

$$p^{\prime }=({\sigma }_1+{\sigma }_3)/2$$

(2)

$$q=({\sigma }_1-{\sigma }_3)/2$$

(3)

where ${\sigma }_1$ and ${\sigma }_3$ represent the axial stress and confining pressure, respectively.

Due to the inability of conventional triaxial compression tests to determine the shear stress and normal stress in the shear plane for gravel soils, the relationship between the stress ($p^{\prime },q$) in the maximum shear stress plane at different confining pressures is described by a power function (Equation (4)) as proposed in reference [23].

$${\displaystyle \frac{q}{p_{\mathrm{a}}}}=A{\left({\displaystyle \frac{p^{\prime }}{p_{\mathrm{a}}}}\right)}^B$$

(4)

where $p_{\mathrm{a}}$ is the atmospheric pressure, set at 101.3 kPa; A and B are constants of the function.

However, the actual shear strength of gravel soil exhibits nonlinearity, making the Mohr–Coulomb criterion inappropriate. Therefore, the strength envelope is represented by the tangent to the appropriate points on the power function curve, as shown in Figure 7.

Using Equation (4), with $p^{\prime }/p_{\mathrm{a}}$ as the horizontal coordinate and $q/p_{\mathrm{a}}$ as the vertical coordinate, the values of ($p^{\prime }/p_{\mathrm{a}},q/p_{\mathrm{a}}$) at different confining pressures can be obtained from the stress–strain curve of the gravel soil, as shown in Figure 8.

From Figure 8, the tangent slope and intercept of a given fitted curve can be determined. The actual shear strength parameters can then be calculated using Equations (5) and (6).

$$k^{\prime }=\sin {\phi }_{\mathrm{c}}$$

(5)

$$p_{\mathrm{a}}{a}^{\prime }=C_{\mathrm{c}}\cos {\phi }_{\mathrm{c}}$$

(6)

where $k^{\prime }$ and $a^{\prime }$ are the tangent slope and intercept of the power function fit curve at a given confining pressure, while $C_{\mathrm{c}}$ and ${\phi }_{\mathrm{c}}$ are the true cohesion and internal friction angles of the gravel soil, respectively.

The results of the calculations using Equation (2) to Equation (6) are shown in

Table 6. Parameters of shear strength of gravelly soils.

Sample	Confining Pressure/(kPa)	${\mathit{k}}^{\prime }$	${\mathit{a}}^{\prime }$	${\mathit{R}}^2$	${\mathit{C}}_{\mathbf{c}}$/(kPa)	${\mathit{\phi }}_{\mathbf{c}}$/°
Natural soil	100	0.495	0.300	0.999	34.97	29.68
	200	0.500	0.402		47.05	30.07
	300	0.520	0.541		64.14	31.32
2% Xanthan gum	100	0.503	0.551	0.980	64.57	28.29
	200	0.519	0.630		74.68	31.30
	300	0.540	0.727		87.53	32.73
4% Xanthan gum	100	0.529	0.577	0.992	68.90	31.97
	200	0.540	0.660		79.49	32.76
	300	0.557	0.770		93.95	33.90
6% Xanthan gum	100	0.541	0.730	0.997	87.92	32.77
	200	0.557	0.801		97.70	33.87
	300	0.576	0.850		105.37	35.21
2% Lignin	100	0.502	0.529	0.999	62.00	30.21
	200	0.518	0.553		65.50	31.23
	300	0.523	0.615		73.12	31.58
4% Lignin	100	0.527	0.545	0.999	64.97	31.85
	200	0.549	0.625		75.75	33.32
	300	0.562	0.690		84.52	34.23
6% Lignin	100	0.519	0.539	0.997	63.88	31.28
	200	0.527	0.599		71.40	31.82
	300	0.541	0.656		79.01	32.77

. It can be seen that $R^2$ is greater than 0.9, indicating a high degree of curve fitting. The true cohesion and angle of friction of the stabilized gravel soil were calculated using Equations (5) and (6) as shown in Figure 9.

Figure 9 illustrates that as the xanthan content is increased to 6%, both the cohesion ($C_c$) and the internal friction angle (${\phi }_c$) demonstrate a linear increase, with increases of approximately 179.39% and 14.12%, respectively. The addition of xanthan gum to gravel soil results in enhanced cohesion, with the internal friction angle exhibiting a comparatively lesser degree of improvement. This is attributed to the gum’s pronounced adhesive characteristics when in contact with water. This results in a discernible cementing effect between soil particles, with gel products forming a network structure that fills voids between coarse and fine particles, thereby binding them together. Conversely, as the lignin content increases to 6%, both the cohesion and the angle of internal friction initially increase and then decrease. The optimum shear parameters are achieved at a 4% content, where the cohesion and angle of internal friction increase by 85.79% and 12.26%, respectively. Therefore, it can be seen that the effect of xanthan gum on the increase in the cohesion of gravel soil is better than that of lignin on cohesion, while for the effect on the angle of internal friction, the two are not much different. The higher the perimeter pressure, the greater the cohesion and angle of internal friction of the two improved soils, while the difference between the cohesion $C_{\mathrm{c}}$ and angle of internal friction ${\phi }_{\mathrm{c}}$ of xanthan gum and lignin dosing improved soils under a perimeter pressure of 300 kPa narrowed down, which suggests that, at higher perimeter pressures, the interstitial space of the rock and soil particles decreases, making the amount of xanthan gum and lignin filling decrease, which leads to a decrease in the amount of increase in the cohesion and the angle of friction of the crushed stone soil, and, thus, that, under high perimeter pressures, the xanthan gum and lignin dosing has a weakening effect on increasing the shear strength of the soil.

4. Microscopic Structural Features and Mechanistic Analysis

4.1. Analysis of Microscopic Structural Features

The study of the microstructure of gravelly soils is an important factor in understanding the changes in strength and structural properties of these soils [37]. To this end, an analysis of the microscopic mechanisms of xanthan gum and lignin-cured gravelly soils was carried out using scanning electron microscopy, the results of which are presented in Figure 10.

Figure 10a,b illustrate that the incorporation of xanthan gum into gravelly soil leads to the development of a binder that interacts with the minerals present in the soil and rock, thereby filling the micropores between the soil particles. Furthermore, xanthan gum forms a three-dimensional polymer reinforcement network, which effectively increases the cohesion between soil particles. This results in an increase in the bearing capacity and shear strength of the soil [38]. Figure 10c,d demonstrate that the addition of lignin to gravelly soils results in the formation of a binder that establishes bridging bonds between soil particles, adsorbs to their surfaces, and fills the micropores. Furthermore, lignin–calcium complexes combine with soil particles to form agglomerates, thereby enhancing the overall cohesion and density of the soil and consequently improving soil structure and shear strength [39].

4.2. Microscopic Structural Mechanism Analysis

Mechanism analysis of xanthan gum curing of gravelly soils: (1) Bonding and cementation mechanism: Xanthan gum, mainly composed of D-glucuronic acid, contains a large number of carboxyl groups (-COOH) and hydroxyl groups (-OH), which form a stable network space structure with the minerals in the gravel soil through electrostatic bonding synthesis or hydrogen bonding, resulting in strong tensile properties, which improve the bonding force of the geotechnical particles and the cohesion and strength of the gravel soil. In addition, the carboxyl group (-COOH) adsorbs cations such as Na⁺, K⁺, and Al³⁺ in the gravel soil and forms ionic bonds with the anions in the minerals in the gravel soil, changing the electrically charged nature of their surfaces, which leads to complex aggregation of geotechnical particles and the formation of agglomerates, and strengthens the integrity of the soil body. Through SEM, it is found that xanthan gum has a strong adhesive force when it meets water, and it can bridge the particles in the soil body [40]. (2) Filling and film formation mechanisms: When in a hydrated state, xanthan gum forms a gel-like substance that fills the micropores between soil and rock particles, thereby enhancing the internal pore structure of the soil. This action serves to reduce stress concentration at the tips of microcracks and diminish the tensile stress field between soil particles. Furthermore, xanthan gum adsorbs onto the surfaces of soil and rock particles, thereby creating a polymeric film that isolates moisture from direct contact with the minerals and reduces the thickness of the hydration film [41]. A schematic diagram illustrating the mechanism of xanthan gum in stabilizing gravelly soil is provided in Figure 11.

The mechanism of lignin-cured gravelly soil: (1) Ion exchange: Lignin dissolves in water, releasing more Ca²⁺; the high valence Ca²⁺ replaces the low valence Na⁺ in the binding water film, and the cations adsorbed by the soil particles undergo an ion exchange reaction, weakening the thickness of the binding water layer adsorbed on the surface of the particles, leading to an increase in the inter-particle force, which improves the particles’ cohesion [42]. (2) Adsorption and encapsulation effect: Lignin mixed into the gravel soil will undergo a protonation reaction to form positively charged lignin sulfate compounds, which will be adsorbed on the surface of the negatively charged soil, wrapping and covering the geotechnical particles to form agglomerates, forming a denser and more stable structure of the soil, and thus improving the compaction and stability of the gravel soil when bearing loads [43]. (3) Pore filling and cementation effects: The incorporation of lignin into gravelly soil results in the filling of larger voids between soil particles, thereby enhancing the soil’s overall compaction. The cementing agents, formed by the calcium bridge of lignin, connect the soil particles. When the concentration of lignin in the soil is excessive, the lack of sufficient soil particles as a framework causes the surplus lignin to accumulate, resulting in localized softening of the soil and reduced compaction. This, in turn, results in a reduction in the soil’s shear strength. A schematic representation of the lignin stabilization mechanism in gravelly soil is provided in Figure 12.

5. Conclusions

This study employs xanthan gum and calcium lignosulfonate as stabilizers to enhance gravelly soil. The mechanical properties and strength parameters of the stabilized gravelly soil were analyzed through the conduct of unconfined compressive strength tests and conventional triaxial compression tests. Additionally, scanning electron microscopy (SEM) was used to investigate the stabilization mechanisms of both additives. The following principal conclusions are drawn:

(1)

As the dosage of xanthan gum is increased, the unconfined compressive strength, peak deviator stress, and the cohesion and internal friction angle of the soil exhibit a nearly linear increase. In contrast, for calcium lignosulfonate-stabilized soil, these parameters initially increase and then decrease. The optimal compressive and shear strength effects are observed at a dosage of 4%.

(2)

At equivalent dosages, xanthan gum-stabilized gravelly soil displays enhanced performance in comparison to calcium lignosulfonate-stabilized soil. Moreover, at elevated confining pressures, the cementing impact of xanthan gum and calcium lignosulfonate on gravelly soil is comparatively diminished in relation to the strength imparted by the confining pressure, suggesting that the influence of dosage on the shear strength of gravelly soil is attenuated.

(3)

The formation of a binder in the gravelly soil by the hydration of xanthan gum can result in the creation of a “bridging” structure with soil particles, thereby enhancing the tensile strength of the soil. Furthermore, the binder fills the micropores between the soil particles, thereby reducing stress concentration and increasing the overall strength of the gravelly soil matrix.

(4)

Calcium lignosulfonate enhances the structural stability of gravelly soil and increases its strength through mechanisms such as ion exchange, adsorption and encapsulation of soil particles, filling of micropores between particles, and cementation.