Experimental investigation on the shear strength, consolidation and permeability characteristics of cross-linked biopolymer-stabilized soils

Abstract

This study investigates the influence of guar gum (G), xanthan gum (X), and their cross-linked blend on the shear strength and consolidation characteristics of soils with contrasting plasticity and mineralogical characteristics. Direct shear, one-dimensional consolidation, constant head permeability tests, SEM and XRD analyses were conducted on high-plastic clay, medium-plastic clay, and sand-dominant soil treated with standalone (2G, 2X) and combined (1G+1X) biopolymer dosages under curing periods up to 28 days. The cross-linked treatment produced the most consistent overall improvement. In the high-plasticity soil, cohesion increased from 56.87 to 81.39 kPa and the friction angle increased from 7.8° to 10.5°. In the medium-plasticity soil, cohesion increased to 77.47 kPa and the friction angle reached 12.2°. In the sand dominant soil, the friction angle increased from 39.1° to 42.9°, while cohesion increased from 12.74 to 23.53 kPa. At 800 kPa, the cross-linked blend reduced the void ratio to 0.18 and 0.14 in the high- and medium-plasticity soils, respectively. Hydraulic conductivity decreased to 8.45 × 10⁻⁹, 8.21 × 10⁻⁹, and 9.10 × 10⁻⁵ cm/s in the three soils. These improvements are attributed to polymer bridging, hydrogen bonding, pore filling, hydrogel formation, and mineralogy-dependent soil-polymer interactions. The findings demonstrate the potential of cross-linked biopolymers as sustainable stabilizers for geotechnical applications.

Introduction

The stabilization of problematic soils is an important concern in geotechnical engineering due to their adverse effects associated with inadequate shear strength and high susceptibility to excessive settlement¹. Such deficiencies can significantly compromise the stability, serviceability and load-bearing capacity of civil engineering infrastructure, namely embankments, roads, foundations and retaining systems². Conventional stabilizing agents, such as lime and cement are widely used to enhance the soil properties and make it suitable for the required field applications³. However, their production is energy-intensive and associated with higher carbon emissions⁴. This has encouraged growing interest in sustainable alternatives that can improve soil properties while having minimal or zero environmental impact⁵. In this context, techniques such as EICP, MICP and biopolymer treatment have emerged as sustainable alternatives for the conventional soil stabilization techniques. Although EICP and MICP have demonstrated considerable potential in soil stabilization, their effectiveness is strongly influenced by bacterial availability and temperature conditions⁶. In parallel with biological techniques, industrial solid wastes and waste-derived cementitious materials have been widely investigated for soil and sediment stabilization. Materials such as ground granulated blast-furnace slag, phosphogypsum, desulfurization gypsum, electrolytic manganese residue, fly ash and other industrial by-products can partially replace conventional cement while promoting waste reutilization and reducing the environmental burden associated with landfill disposal. Lang⁷ developed nano-modified, low-carbon supersulfated cement systems based on slag and sulfate-rich industrial wastes for dredged-sediment stabilization, demonstrating that waste-derived binders can achieve effective strength development through microstructural densification and the formation of cementitious reaction products. Wang⁸ proposed a solid-waste-based binder for stabilizing dredged sediment with high moisture and organic-matter contents and subsequently reusing the treated material as liquefied backfill for underground infrastructure. As a more practical and sustainable alternative, biopolymers have emerged as promising soil treatment agents owing to their biodegradable nature, low dosage requirement, and remarkable ability to modify soil behaviour through particle binding, gel formation, and pore-structure alteration. Among the available biopolymers, guar gum and xanthan gum have gained particular attention due to their availability, hydrophilic nature and strong interaction with soil particles. These polysaccharide-based additives can enhance interparticle bonding and matrix attraction through a three-dimensional matrix, thereby improving load transfer within the soil skeleton⁵. Prior studies have shown that soils treated using guar and xanthan gum have exhibited commendable improvements in strength and durability, although the efficiency of biopolymer treatment depends on soil mineralogy, fines content, dosages and curing period.

Guar gum treated soils have shown improved engineering behaviour, and Xanthan gum treatment has been reported to increase cohesion, stiffness, and reduce hydraulic conductivity. Xanthan improved soil strength through adsorption onto clay particle surfaces and by forming a rigid biopolymer matrix between relatively uncharged particles. Its strengthening effect was most pronounced in well-graded soils containing fines and was influenced by soil type, moisture condition, xanthan dosage, and mixing procedure⁹. Guar gum imparted cohesion to sand and exhibited a moisture-sensitive strengthening response, demonstrating that a microbial biopolymer can transform a cohesionless granular soil into a bonded matrix¹⁰. Sand treated with guar gum exhibited a hysteretic strength response during repeated wetting–drying cycles, with progressive degradation caused by monomer dissociation during wetting and only partial recomposition upon drying. The strength decreased by approximately 30% after 10 wetting-drying cycles, while nearly 80% of the dry strength was still retained even under severe cyclic exposure¹¹. Xanthan treatment improved the direct shear response of sand, and the shear behaviour varied with xanthan content and gel phase¹². Xanthan treatment enhanced the shear strength parameters (from 32º in the control to 30-36º in polymer-treated samples), whereas cohesion increased markedly with guar gum, showing the highest value of cohesion (447 kN/m²) and xanthan gum reaching a value of 218 kN/m¹³. In the case of loess deposits, the use of both xanthan and guar gum produced a progressive increase in shear strength after a curing time of 14 days. Use of guar gum improved cohesion from 43.0 kPa to 61.9 kPa, and for xanthan, cohesion ranged from 43.0 kPa to 58.0 kPa, with an increase in the friction angle¹⁴. Latifi¹⁵ reported a 6-fold increase in the UCS of peat treated with a 2X dosage after a 28-day curing period. Latifi¹⁶ also evaluated the effect of 2.5X on kaolinite and bentonite soils and noticed a significant improvement in UCS. Direct shear tests conducted on bentonite revealed that cohesion increased by 5.6 times and ϕ increased by 1.23 times at the dosage of 1X. For the exact dosage of 1X on kaolinite, cohesion and ϕ values increased by 9.1 and 1.5 times, respectively.

Biopolymer treatment with guar and xanthan tends to reduce soil permeability by filling pores and thickening the pore fluid. Falling-head tests conducted on sand and silt showed that the hydraulic conductivity (k) was significantly reduced by the addition of guar and xanthan. For sand, the initial value of 3.4 x 10^-2 cm/s decreased to about 60% of the initial value at 2X, whereas guar gum decreased to 27% of the initial value at 1.5% dosage¹⁷. For collapsible loess, the k of 4.9 x 10^-4 cm/s decreased by 18.6% with 0.5X, while 0.5G reduced the initial k to 88% at a dry unit weight of 14 kN/m³, confirming the biopolymer-induced reduction in hydraulic conductivity¹⁸. The study by Vydehi¹⁹ showed that as the dosage of xanthan or guar gum increased, the soil became noticeably less permeable, with the biopolymers filling pore spaces and slowing water flow. Nugent²⁰ tested kaolinitic clay and found that lower dosages of guar and xanthan reduced the compression index (C_c); however, higher dosages increased C_c value compared to untreated soils. Cabalar²¹ evaluated the effect of 3X on the compression index (C_c) and swelling index (C_s) of plastic clay and found that C_c increased 6-fold and C_s increased 11-fold compared to the untreated case. Low-plasticity clays amended with xanthan generally showed increased compressibility and swelling, whereas high and medium-compressible silty clays treated with xanthan or guar gum showed decreased collapse potential, compression index, and swelling index¹⁷. Expansive clay treated with up to 2.5X reported that the soil’s compressibility decreased by approximately 65% after 28 days of curing, with the compression curves shifting downward and the stiffness improved as the biopolymer gel filled the pores and created a denser structure¹⁸.

Crosslinking of biopolymers has its origins in both polymer chemistry and food science, where mixing different polymers is essential to form a high-strength gel with superior viscosity, which cannot be achieved with single biopolymers²². Research conducted in 1980 and 1990 revealed that crosslinking of galactomannan (G) and anionic polysaccharides (X) resulted in strong hydrogen bonding, entanglement and electrostatic interactions, which are essential in food processing, pharmaceuticals and the petroleum industry^23,24. Crosslinking between guar and xanthan has already been explored in various environmental and industrial applications due to the stable and viscous nature of the resulting hydrogel. Guar and xanthan crosslinking is proven successful in providing the long-term stabilization of iron particles and fly ash deposits^25–28. Xanthan gum is a rigid, anionic polysaccharide, while guar gum is a flexible, neutral polysaccharide. When used together in the presence of moisture, their molecular chains interact and cross-link, forming a continuous gel-like network that surrounds and bridges soil particles²⁹. Muguda³⁰ used crosslinked blends and found that equal amounts of guar gum and xanthan gum can achieve strength enhancement comparable to traditional stabilizing agents such as lime and cement. Experimental evidence on crosslinked biopolymer systems indicates a substantial increase in strength and stiffness, along with marked reductions in hydraulic conductivity, highlighting the potential of crosslinked biopolymer-treated soils for barrier and ground improvement applications.

Despite the encouraging progress in biopolymer-based soil stabilization, the available literature remains weighted toward index properties, compaction response, and unconfined compressive strength. The guar gum and xanthan gum have individually been reported to improve the engineering properties of soils. But, their comparative performance and potential cooperative effect when used as a blend remain insufficiently understood, particularly for soils with contrasting mineralogy and plasticity. Comparatively limited attention has been given to the combined influence of guar gum and xanthan gum when used as a cross-linked blend, particularly in relation to shear strength, consolidation and permeability. The available literature also provides insufficient comparative evidence on how the same biopolymer system performs in soils with markedly different plasticity, particle-size distribution and mineralogical composition. Furthermore, many studies report macroscopic improvements without adequately relating them to soil-specific interaction mechanisms such as hydrogen bonding, polymer bridging, hydrogel formation, pore filling and particle aggregation. A combined assessment of these parameters is therefore necessary to establish the suitability of crosslinked biopolymers for practical geotechnical applications. Against this background, the present study investigates the influence of standalone and crosslinked biopolymers on the shear strength and consolidation behaviour, across soils of different plasticity and mineralogical composition. The study examines the variation in cohesion, angle of internal friction, void ratio, compression index, coefficient of consolidation and hydraulic conductivity, while XRD and SEM observations are used to interpret the influence of soil mineralogy and microstructural modification.

Materials

Soils

Three distinct, naturally available soils with low, medium and high plasticity characteristics were excavated and collected for the study to investigate the effect of biopolymer crosslinking across a broad spectrum of soil behaviour. Soil S1 was collected from Ibrahimpatnam, Ranga Reddy district, Telangana, India (17.10° N 78.62° E). Soil S2 was procured from Singarayakonda, Prakasam district, Andhra Pradesh, India (15°14′32.1″ N 80°01′59.6″ E), and Soil S3 was collected from Sanghi Nagar, Rangareddy district, Hyderabad, Telangana, India (17°16′0.48″ N 78°40′32.88″ E), respectively. The excavated soil samples were air-dried, pulverized and stored in airtight containers to confirm homogeneity. Three soils were tested for consistency limits to classify them according to their plasticity characteristics. XRD tests were conducted to identify the predominant mineral governing their plasticity behaviour.

Biopolymers

G and X obtained from SRL Chemicals Ltd, Mumbai, India, were used for this study. The physical and chemical properties of biopolymers sourced from the manufacturers are provided in Table 1.

Table 1.

Properties of guar and xanthan gum.

Property	Guar gum	Xanthan gum
Color	White to pale yellow	White to off-white
Ash (%)	2	13
Loss on drying (%)	13	15
pH (1% aq. sol)	-	6.0-8.0
Viscosity (1% aq. @ 25°C) (mPa.s)	-	600
Pyruvic acid	-	1.5
Particle size (µ)	< 75	< 75

Criteria for biopolymer mixing and dosage selection

The selected biopolymers (G & X) were incorporated into the soils, standalone and in cross-linked combinations, to examine their independent and synergistic effects. Dosages of biopolymers (both standalone and crosslinked) adopted for the study are presented in Table 2.

Table 2.

Biopolymer dosages adopted for the study.

Soil type	Biopolymer dosage	Guar gum	Xanthan gum
S1	V	---	---
	2G	2%	---
	2X	---	2%
	1G +1X	1%	1%
S2	V	---	---
	2G	2%	---
	2X	---	2%
	1G +1X	1%	1%
S3	V	---	---
	2G	2%	---
	2X	---	2%
	1G +1X	1%	1%

For individual treatment, an optimum dosage of 2% by dry soil weight was adopted, reflecting their optimum performance reported in earlier research¹⁹. Guar gum and xanthan gum were added at 1% dosages individually (2% combined) to ensure intermolecular interactions and superior bonding efficiency without any workability issues³⁰. The biopolymer dosages were incorporated into the soils using the dry-mix method, selected over wet mixing to maintain uniform distribution, consistent hydration, and prevent premature gel formation³¹. For the 1G+1X treatment, equal dry masses of G and X were first blended before their addition to the soil. To obtain a homogeneous mixture, the biopolymer powder was initially blended with a small fraction of soil and subsequently mixed with the remaining soil in successive portions. The dry mixture was manually mixed until no visible biopolymer agglomerates were present. Water was then added gradually to reach the target moisture content, followed by further mixing before compaction or specimen preparation.

Experimental methods

Grain size distribution

The grain size distribution of the three soils was determined using sedimentation analysis (for < 75 µ) as per ASTM D7928-21e1³². For particles greater than 75 µ in S1, S2, and S3, grain-size distribution analysis was performed using the dry-sieve method as per ASTM D6913M-17³³.

Figure 1 shows the grain size analysis of three selected soils. The GSD curve of S1 indicates a predominant fine fraction, with finer percentage values ranging from 53% to 73% across particle sizes of 0.001-0.06 mm. The grain-size distribution of S2 shows a moderate proportion of fine particles, with the percentage finer increasing from 21% to approximately 47% at 0.07 mm.

Fig. 1.

Grain size distribution of selected soils.

Figure 1 also reveals that soil S3 contains a predominant portion of coarse-grained material and only 4-6% of the soil passes through a 75 µ sieve. The shape of the curve shows that the soil is dominated by sand and gravel fractions, characteristic of a coarse-grained soil.

Details of particle size fractions of S1, S2 and S3 determined using both dry sieve and sedimentation analysis are presented in Table 3.

Table 3.

Particle size fractions determined from sieve analysis.

Soil type	Gravel (%)	Sand (%)	Silt (%)	Clay (%)
S1	0	19	13	68
S2	0	23	45	32
S3	7	85	8	---

The soils were tested for their physical properties in accordance with the standard codes, and are presented in Table 4.

Table 4.

Properties of collected soil samples.

Property	S1	S2	S3	Code
Specific gravity	2.24	2.35	2.67	ASTM D854-23 (ASTM 2023)48
Sand (%)	19	23	85	ASTM D6913M-17 (ASTM 2021)33
Silt and clay (%)	81	77	8	ASTM D7928-21E01 (ASTM 2021)32
Free swell index (%)	45	13.5	9.1	ASTM D4546-21 (ASTM 2021)49
Liquid limit (%)	63	46	NA	ASTM D4318-17e1 (ASTM 2018)50
Plastic limit (%)	31	28	NA
Shrinkage limit (%)	22.3	11.3	NA
USCS classification	CH	CI	SP-SM	ASTM D2487-17R25 (ASTM 2025)51
Maximum dry density, MDD (kN/m3)	14.8	19.3	19.2	ASTM D698-12R21 (ASTM 2021)52
Optimum moisture content, OMC (%)	27.3	14.2	12.9

Direct shear test

The direct shear test is one of the most widely used methods for determining soil shear strength parameters, which are essential for evaluating soil stability and bearing capacity. For each test, samples were compacted into a standard shear box with plan dimensions of 60 mm x 60 mm and a height of 25 mm. Soil samples were prepared at their respective OMC and MDD to ensure uniformity between untreated and treated samples. Direct shear tests were conducted as per ASTM D3080/3080M³⁴ on all the selected soils to quantify the effect of G, X and their blend on the shear strength characteristics of soils with varying plasticity characteristics. Direct shear tests were conducted using a constant horizontal displacement rate of 0.125 mm/min for all specimens. The same rate was adopted for S1, S2, and S3 to maintain a consistent basis for comparison of treatment effects. Specimens for direct shear testing were prepared at the respective optimum moisture content of each mixture. The specimens were not subjected to soaking or vacuum saturation before testing. Accordingly, the test results represent the behaviour of compacted, initially unsaturated soil-biopolymer mixtures under unsoaked laboratory conditions. Curing periods of 1, 14, and 28 days were adopted for the study, and the variation in cohesion (c) and the angle of internal friction (ϕ) was evaluated at the end of each curing period.

Constant head permeability test

A constant-head permeability test was performed on S3 to determine the hydraulic conductivity (k) under steady-flow conditions. The test procedure was in accordance with ASTM D2434³⁵, which specifies a remoulded specimen being compacted into the permeameter mould at the desired OMC and MDD. A constant head across the specimen was maintained, and the amount of water flowing through the soil over a fixed time interval was measured and used to compute the hydraulic conductivity. A constant piezometric head difference of 1.0 m was maintained across the 0.127 m high specimen, resulting in a hydraulic gradient of 8.0.

One-D fixed ring consolidation test

A consolidation test is an essential test conducted to evaluate the compressibility and settlement behaviour of soils under sustained loading. The consolidation tests were conducted on soils S1 and S2 to determine the variation of void ratio (e) and hydraulic conductivity (k) post the biopolymer treatment (standalone and crosslinked). The specimens were prepared using a standard oedometer with a 60 mm diameter and 20 mm height, in accordance with ASTM D2435/D2435M³⁶. Soil samples were prepared at their respective OMC and MDD to ensure uniformity between untreated and treated samples. The specimens compacted at their respective optimum moisture contents were placed in the consolidation ring and subjected to a seating load. The cell was subsequently inundated with water under the seating load before application of the incremental vertical loads. Then, incremental vertical loads were applied in stages, and the subsequent dial gauge readings were recorded and analyzed to evaluate variations in e and k.

Prior to specimen preparation, the optimum moisture content of each untreated and biopolymer-treated soil mixture was determined separately using the Standard Proctor compaction test. Direct shear and consolidation specimens were prepared using the respective optimum moisture content of each mixture. The detailed compaction characteristics of these soil-biopolymer mixtures have been reported previously³⁷.

X-ray Diffraction (XRD) test

X-ray diffraction tests were conducted to identify the mineralogical composition of three selected soils following standard procedures. The powdered soil samples were scanned using a Bruker D8 Advance diffractometer. The obtained diffractograms were processed and analyzed using MATCH Phase identification software, to identify experimental peaks with a standard database. The analysis provided valuable insights into the dominant minerals in each soil and helped to characterize them based on their plasticity.

XRPD analysis was conducted on the guar and xanthan gum to identify the crystalline and amorphous structural features. This analysis helped to identify the interaction and bonding potential of the selected biopolymers in soils of different plasticity characteristics.

The XRPD pattern of guar gum shows broad, diffuse peaks rather than sharp, well-defined ones. The analysis revealed that no distinct crystalline peaks are present in guar gum. Figure 2 shows a broad hump around 20°, indicative of amorphous polysaccharide structures³⁸. Guar gum is known for its high amorphous galactomannan structure, and the dataset obtained confirms the same. Guar gum is amorphous, supports rapid hydration and gel formation, exhibits high water absorption, and possesses strong binding ability with soil³⁹.

Fig. 2.

XRPD analysis of guar gum.

Xanthan was also analyzed for the mineral composition using XRPD patterns, and the details of the major peaks are presented in Fig. 3. Despite more visible features than guar gum, the peaks remain broad. Peaks observed in the range of 19-22° indicate semi-ordered amorphous regions, a typical feature of xanthan gum, and no crystalline impurities are observed⁴⁰. Xanthan gum, a semi-amorphous polymer, forms a very strong film and a high-viscosity gel upon hydration.

Fig. 3.

XRPD analysis of xanthan gum.

The XRD analysis of S1 shows predominant peak characteristics of montmorillonite, with strong reflections in the low-angle region 5-10° and high-intensity peaks near 19-20°. The observed peak locations in Fig. 4 (a) are consistent with the basal spacing of smectite-group minerals, confirming that montmorillonite is the dominant clay mineral.

Fig. 4.

XRD pattern of soils (a) S1 (b) S2 (c) S3.

The sharp, intense reflections observed at 12°, 20°, and 24° are indicative of kaolinite fingerprint peaks. As observed in Fig. 4 (b), well-defined and narrow peaks indicate a highly crystalline kaolinite structure. The lack of broad basal reflections in the low-angle region and the dominance of kaolinite peaks confirm the absence of smectite minerals and the limited swelling behaviour of S2.

The XRD pattern of Soil S3 shown in Fig. 4 (c) reveals extreme and sharp reflections at 20-27°, reflecting the diffraction peaks of quartz. Quartz, a predominant mineral and several secondary peaks at higher angles (> 40°), confirm the presence of quartz signatures, indicating that S3 is highly crystalline and rich in silica.

The mineralogical interpretation presented herein is qualitative and based on phase identification from XRD patterns.

Scanning Electron Microscopy (SEM)

SEM was used to investigate the particle arrangement, surface morphology and microstructural features of the selected soils and biopolymers.

Biopolymers and soil samples treated with biopolymers were examined using SEM (Hitachi, Tokyo, Japan; Oxford Instrument, Oxford, UK), and SEM images were acquired. The SEM image of guar gum shows an irregular, amorphous, and agglomerated surface morphology, a characteristic feature of polysaccharide-based biopolymers. Figure 5 (a) confirms a large, folded, and clustered mass with no evidence of crystalline edges, as indicated earlier by XRPD analysis. Mudgil⁴¹ reported a similar behaviour of guar gum in their SEM image analysis.

Fig. 5.

Scanning electron image of a) Guar gum b) Xanthan gum.

The SEM micrograph of xanthan gum reveals a flaky and ribbon-like morphology, a prominent characteristic feature of a highly branched polysaccharide structure and similar observations were reported by a few researchers^42,43. As shown in Fig. 5(b), the particles appear as twisted, elongated fragments, indicating a semi-amorphous nature of xanthan gum.

To ensure reproducibility, same procedures were followed in the soil processing, biopolymer mixing, compaction and curing for all untreated and treated specimens. The guar gum, xanthan gum and cross-linked blend were incorporated through a controlled dry-mixing procedure, and water was added gradually until the respective target moisture content was reached. Each specimen was prepared at the corresponding optimum moisture content and maximum dry density of the mixture. The specimen dimensions, curing duration and testing conditions were kept identical for each treatment category. Direct shear tests were performed at a constant horizontal displacement rate, consolidation tests were conducted using the same load-increment sequence, and permeability tests were carried out under a constant hydraulic gradient. All equipment was operated in accordance with the relevant ASTM procedures, and the same test protocol was followed throughout the experimental program to minimize the preparation related variability.

Results and discussion

The effect of biopolymer treatment in standalone and crosslinked combinations on the targeted geotechnical properties is analyzed and presented in this section.

Evaluation of shear strength parameters

Soil S1

Table 5 shows the variation in cohesion (c) and the angle of internal friction (ϕ) for S1 at the selected biopolymer dosages and different curing periods. The untreated soil has significant cohesion due to natural suction and electrostatic attraction between clay particles. The angle of internal friction (ϕ) is low, as the soil is completely filled with fine clay particles. Upon the addition of biopolymers, the hydrogel forms and interacts effectively with the clay particles. On day 1, the cohesion of the biopolymer-treated samples remained unchanged or decreased slightly, likely due to the biopolymer’s high affinity for water, which creates a temporary lubrication effect at the microcontacts. With the curing period increased to 14 days, cohesion increased significantly across all biopolymer-treated samples; however, the increase was more pronounced in 1G + 1X treated samples. G forms hydrogen bonds with the edges of clay particles, and X forms a stiffer polymer network due to its backbone of β-(1→4) chains.

Table 5.

Variation of Cohesion (c) (kPa) and ϕ (Degrees) for S1.

Biopolymer dosage/ Curing period	Day 1		Day 14		Day 28
Soil S1	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)
V	47.07	5.2	50.99	5.9	56.87	7.8
2G	43.14	4.3	60.80	6.1	80.37	9.8
2X	45.11	4.9	56.87	6.8	79.43	10.2
1G+1X	45.11	5.1	57.85	7.1	81.39	10.5

The hydrogel formed by cross-linking forms a polymeric layer around clay surfaces, leading to their flocculation. A consistent increment in cohesion was observed from 43.14 to 60.80 kPa in 2G samples and from 45.11 to 56.87 kPa in 1G+1X treated samples. By the end of the 28-day curing period, the cohesion of the biopolymer-treated soil reached an optimum. It could be due to dehydration of the hydrogel, which tightly wraps around and anchors the clay clusters through adhesion, as shown in Figure 6. The trend of increasing cohesion in biopolymer-treated samples follows the order 1G+1X > 2X > 2G, indicating the enhanced rigidity of the hydrogel formed upon crosslinking of G and X.

Fig. 6.

Interaction between clay and hydrogel in soft and crystallized states.

Angle of internal friction (ϕ) was 5.2° in untreated soil, and it increased to 5.9° by 14 days and 7.8 ° at 28 days. The 2G and 2X treated samples have shown a slight increase in ϕ with increasing curing period, compared with the untreated samples. The 1G+1X treatment resulted in a ϕ of 10.5° at the end of the 28-day curing period, which is slightly higher than the untreated and standalone biopolymer-treated samples. The slight increase in ϕ of biopolymer-treated soils can be attributed to the edge-to-face orientation of soil particles after the flocculation. However, the increase in ϕ remained small relative to C, reflecting limited structural rearrangement in the high-plasticity soil.

Soil S2

The untreated S2 specimens have shown distinct behaviour in the variation of shear strength parameters, due to their relatively lower clay content compared to S1. It has displayed a lower cohesion of 30.4 kPa and a slightly higher ϕ of 8.2° at the initial curing age. Similar to S1, individual treatments with guar and xanthan have reduced ϕ values compared to the untreated, with values of 6.3° and 7.8° at 2G and 2X dosages, respectively. As shown in Table 6, the cohesion remains close to that of untreated, suggesting incomplete hydration of the biopolymeric action.

Table 6.

Variation of cohesion (c) (kPa) and ϕ (Degrees) for S2.

Biopolymer dosage/ Curing period	Day 1		Day 14		Day 28
Soil S2	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)
V	30.40	8.2	35.30	9.3	41.18	9.5
2G	28.43	6.3	41.18	10.1	67.66	12.5
2X	27.45	7.8	44.12	9.5	72.56	11.5
1G+1X	31.38	7.9	47.07	9.2	77.47	12.2

On day 14, cohesion increases across all dosages; 2G rises from 28.43 to 41.18 kPa, and 1G+1X increases by 15.7 kPa to 47.07 kPa. The phenomenon can be linked to the adsorption of polymer coating on clay platelets and the flocculation of fine silty grains. Unlike S1, the S2 specimens have shown a notable increase in ϕ at 2G and 2X dosages, with values of 10.1° and 9.5°, respectively. It shows the effective interparticle enhancement resulting from polymer-induced bonding and from contact points between clay-coated silt grains. As depicted in Fig. 6, biopolymer treatment results in larger soil aggregates and improved particle interlocking, indicating enhanced interparticle bonding. Table 6 shows that cohesion increases substantially at the end of 28-day curing, with values of 77.47 kPa at 1G+1X and 72.56 kPa at 2X, respectively, consistent with the hydrogel’s long-term dehydration. The ϕ values are found to rise significantly, up to 12.5° for 2G and 12.2° for 1G+1X, possibly due to the enhanced interlocking and reduced lubrication between the hardened hydrogel and soil particles.

Soil S3

The large, coarse fraction in S3 has shown a distinct response to the biopolymer treatment compared with the clay-rich soils S1 and S2. A negligible cohesion of 7.84 kPa and a high ϕ value of 35^o were obtained for S3, a soil composed of granular fractions and high quartz content. Table 7 shows the variation in shear strength parameters with increasing curing period for S3.

Table 7.

Variation of cohesion (c) (kPa) and ϕ (Degrees) for S3.

Biopolymer dosage/Curing period	Day 1		Day 14		Day 28
Soil S3	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)	c (kPa)	ϕ (o)
V	7.84	35.5	13.72	37.2	12.74	39.1
2G	14.70	31.9	18.63	36.8	20.59	41.8
2X	11.76	30.2	16.67	35.1	19.61	42.2
1G+1X	13.72	29.4	20.59	36.2	23.53	42.9

At day 1, the ϕ values decreased across all treatment dosages, owing to the rich polymeric lubrication at the grain contacts. Interestingly, and contrary to both S1 and S2, cohesion was found to be increased in the biopolymer-treated soil samples of S3. It may be due to the early-stage glueing effect of the hydrogel, which coats the soil particles, a faster process than the electrostatic reaction with clay particles. The ϕ values of biopolymer-treated samples were found to increase to 35-37° at day 14, suggesting a transition in hydrogel behaviour from lubrication to stiff interlocking. However, the observed ϕ values for biopolymer-treated soils are slightly lower than those of the untreated samples.

However, after the 28-day curing period, a dramatic improvement in ϕ was observed in all biopolymer-treated soils, with ϕ increasing to 42.9° for 1G+1X-treated samples and 42.2° for 2X-treated samples. This is due to the complete hardening of the hydrogel and increased roughness at the particle contacts, as shown in Fig. 7.

Fig. 7.

Interaction between sand and hydrogel in soft and crystallized states.

The proposed soil-biopolymer interaction mechanism is clearly reflected in the SEM observations of biopolymer-treated soils shown in Fig. 8. The untreated sample exhibits a rough, highly porous fabric with weak interparticle contacts, whereas the addition of biopolymers leads to hydrated gel formation that progressively fills pores and coats sand grains. Upon drying, the crystallized hydrogel forms roughened contact surfaces and interparticle bridges, resulting in reduced visible pores and fewer microcracks. These microstructural features directly explain the improvement in shear strength observed in the test results.

Fig. 8.

SEM images of a) S1 b) S1+1G+1X c) S2 d) S2+1G+1X e) S3 f) S3+1G+1X.

The high specific surface area and negative charge of the clay particles in S1 promote hydrogen bonding with the polymer chains of the biopolymers upon addition. G and X can adsorb onto clay particles and form strong bonds among them, further densifying upon curing. Thus, a high enhancement in cohesion and nominal improvement of ϕ were observed in S1. A hybrid interaction between the clay-silt soil mixture S2 and the polymer chains led to improved friction and moderate cohesion as the curing progressed. The physical anchoring of soil grains in soil S3 through the wrapping-around mechanism of hydrogel resulted in a significant improvement in ϕ and a slight increase in cohesion due to the weak cementing action offered by the hydrogel.

The improvements in the soil properties are attributed to possible soil-biopolymer interactions, including hydrogen bonding, polymer coating, particle bridging, and possible cation-mediated interactions. FTIR observations reported previously for the same soil-biopolymer systems showed changes in characteristic functional-group bands, supporting the occurrence of soil-polymer interactions consistent with hydrogen bonding³⁵. Nevertheless, the present study does not directly establish the molecular-scale interaction mechanism. Therefore, cation bridging and polymer-network formation are interpreted as conceptual mechanisms based on soil mineralogy, biopolymer functional groups, and published literature.

Variation in pressure -void ratio

Changes in void ratio and mineralogy govern the compressibility behaviour of S1 and S2. One-dimensional consolidation tests were performed on soils treated with the selected biopolymer dosages, and the void ratio was evaluated at various loading and unloading phases.

Soil S1

The void ratio of untreated soil decreased steadily from 0.46 at 12.5 kPa to 0.32 at 400 kPa, and then to 0.27 at 800 kPa. The curve for untreated soil in Fig. 9 indicates its transition from recompression to virgin compression as the structure gradually loses water from the interlayers. The reduced rate of void ratio decrease observed in the stress range of 100-400 kPa is primarily attributed to the transition from initial fabric rearrangement and macro-void collapse to a denser, more stable load-bearing soil skeleton.

Fig. 9.

Variation of void ratio vs consolidation pressure for S1.

The 2G-treated soil sample had a higher void ratio (0.53) than the untreated samples. It can be attributed to the absorption of water by the G strands and clay particles. The higher the G dosage, the greater the affinity, leading to a further increase in the void ratio¹⁷. Conversely, when the pressure reaches 400 kPa, then 800 kPa, expulsion of pore water from the soil reduces the void ratio. The void ratio settled at 0.22 at 800 kPa, lower than that of untreated soil, confirming that guar gum effectively densifies the soil under high loads. Similar to the 2G treatment, samples treated with 2X dosages also started with a e of 0.52, higher than that of the untreated case. In the 50-200 kPa range, xanthan gum showed reduced flocculation and prevented abrupt failure. The void ratio at 800 kPa reduced further to 0.24, indicating a densified soil structure.

The 1G+1X-treated samples showed greater variation in void ratio. During the load increments, it showed the lowest void ratio among all dosages, reaching 0.18 at 800 kPa. The curve for 1G+1X-treated samples showed an early reduction at 100 kPa, attributed to the collapse of the initial flocculation. Improved performance of crosslinked biopolymer-treated samples can be attributed to the synergistic effect of the flexible G hydrogel and the rigid backbone of X.

Soil S2

During the loading phase, the untreated soil begins at a void ratio of 0.54, reflecting the soil’s intermediate plasticity. Under the consolidation pressures of 400 kPa and 800 kPa, void ratios of 0.35 and 0.28 were identified, indicating less structural collapse than in S1. The reduction in e under the load increment can be attributed to the rearrangement of clay particles. The consolidation of S2 is dominated by the closure of micro-pores rather than by the collapse of the expansive clay layer observed in S1.

As shown in Fig. 10, the 2G-treated S2 samples exhibited a higher void ratio of 0.65 during the initial phase, highlighting the flocculating ability of G even in S1. The curve of the 2G sample shows a steeper drop than that of the virgin soil in the range of 50-200 kPa, indicating floc collapse under increasing load and drawing the soil particles closer together. At 800 kPa, the void ratio was further reduced to 0.19, suggesting the higher densification at large stresses. Similar to S1, 2X-treated S2 samples had a high void ratio, indicating a higher affinity of biopolymers for water. However, the curve of the 2X sample is more uniform than that of the 2G sample, indicating that xanthan gum can form a stable, cohesive gel around the clay particles. The e of 2X-treated sample was 0.17 at 800 kPa, slightly lower than that of S1 at the same dosage. For 1G+1X treated samples, the lowest void ratio of 0.14 was recorded, the lowest among all the biopolymer dosages.

Fig. 10.

Variation of void ratio vs consolidation pressure for S2.

During unloading, the soil showed a rebound in void ratio, attributed to the release of pore water pressure after load removal. The standalone biopolymer-treated samples have shown greater rebound than the untreated samples, whereas the crosslinked biopolymer-treated sample has shown a controlled rebound.

Variation of compression index (C_c)

The compression index (C_c) obtained from the e-log σ compression curve quantifies the volume change in soil at the load increments. In this study, the compression index was evaluated over successive stress intervals to capture the stress-dependent compressibility behaviour of untreated and biopolymer-treated soils, as the virgin compression curve was nonlinear. Generally, an increase in C_c indicates a reduction in void ratio during the load increments. Interpretation of the variation of C_c at different load increments and the probable interaction of biopolymers with S1 and S2 is presented below.

Soil S1

The C_c of untreated soil is 0.0032 at 50 kPa and increases to 0.1329 at 100 kPa, suggesting the initiation of virgin compression and pore filling. When the pressure exceeds 200 kPa, a steady rise in C_c was observed in Table 8, which can be attributed to the squeezing of diffuse double layers at higher stress.

Table 8.

Variation of compression index (C_c) for S1 at the selected biopolymer dosages.

Biopolymer dosage	Consolidation pressure (kPa) a
	50	100	200	400	800
V	0.0332	0.1329	0.1664	0.1993	0.1661
2G	0.0664	0.0334	0.1329	0.3986	0.3322
2X	0.0662	0.0667	0.1993	0.2990	0.1993
1G+1X	0.0997	0.0330	0.1661	0.3966	0.3654

The inclusion of guar gum altered the pattern observed in untreated samples, with C_c increasing to 0.0664 at 50 kPa, indicating a more deformable particle arrangement. After the 200 kPa stress, the C_c continued to increase steeply, reaching 0.3322 at 800 kPa. It can be linked to the flocculation and collapse mechanism of guar gum at lower and higher stresses, respectively. A high C_c value for biopolymer-treated soils indicates their compressible nature, even though the void ratio at higher stress is lower than that of untreated soils. The C_c values of 2X-treated samples were 0.2990 and 0.1993 at 400 and 800 kPa, respectively. The anionic backbone and carboxylate groups of xanthan gum form firm electrostatic interactions with the positively charged edges of clay particles and fill the pore spaces⁴⁴.

At a stress of 50 kPa, the 1G+1X-treated sample showed an increment of C_c about three times that of the untreated soil. Although the cross-linked specimen exhibited a higher incremental compressibility at 400 kPa, C_c decreased slightly at 800 kPa. This behaviour reflects progressive densification of the soil-biopolymer matrix: after substantial pore compression and particle rearrangement at intermediate stresses, fewer compressible voids remained at higher stress levels. Thus, the reduced C_c at 800 kPa indicates stiffening of the compacted soil-polymer structure.

Soil S2

The C_c values of untreated S2 at the different load increments are presented in Table 9. Compared to soil S1, the variation of C_c at the moderate stresses is slightly lower, and it can be attributed to the lesser clay content in S2. In the 2G-treated samples, C_c increases to 0.4651 at 400 kPa, which is 3 times that of the untreated soil. It is also clear that C_c reduced to 0.3986 at 800 kPa, approximately 1.7 times that of the untreated ones. High C_c across all load increments may be due to the highly compressible gel fabric formed upon interaction of G with the clay minerals present in S2. X has not altered the C_c value at the lower stress, which is 0.0864 at 50 kPa. At higher stresses, C_c values rose sharply to 0.3255 and 0.5382 at 400 and 800 kPa, respectively.

Table 9.

Variation of compression index (C_c) for S2 at the selected biopolymer dosages

Biopolymer dosage	Consolidation pressure (kPa) a
	50	100	200	400	800
V	0.0664	0.1661	0.1993	0.1329	0.2325
2G	0.1329	0.1993	0.2325	0.4651	0.3986
2X	0.0864	0.1893	0.2525	0.3255	0.5382
1G+1X	0.0997	0.1329	0.3322	0.3654	0.4651

The crosslinked sample exhibited C_c values of 0.0997 and 0.1329 at 50 and 100 kPa, respectively, which are higher than those of the untreated sample but lower than those of the 2G treatment. At a stress of 800 kPa, C_c is twice that of the virgin soil, indicating the soil’s high compressibility after the biopolymer treatment.

Variation of coefficient of consolidation (c_v)

The coefficient of consolidation (c_v) controls the rate of pore water pressure dissipation during the compression of saturated soils. The c_v significantly controls k, settlement, and the overall stability of geotechnical structures. By determining the change in void ratio during the consolidation test, c_v was evaluated for all the untreated and biopolymer-treated samples.

Table 10 shows the variation in c_v for S1 and S2 at the selected treatment dosages. The inclusion of the biopolymer caused a clear reduction in c_v in both soils, demonstrating its efficiency in lowering hydraulic conductivity. In S1, reductions of 70% and 66% in c_v were observed for the 2G and 2X treatments, compared to the untreated soil, which is 7.68 x 10^-3 cm²/s. The 1G+1X mix showed a 96% reduction in c_v compared to the untreated, which can be attributed to the combined hydrophilic action of guar and xanthan gums. At lower dosages (1% optimum for each during crosslinking), hydrogen bonding and cation bridging between guar and xanthan gum reduce pore-water expulsion, thereby slowing consolidation. In contrast, higher dosages of individual biopolymers may lead to higher consolidation rates, resulting from pore water expulsion driven by increased G-X soil aggregation⁴⁵.

Table 10.

Variation of coefficient of consolidation (c_v) for S1 and S2.

Biopolymer dosage	cv (cm2/s)
	S1	S2
V	7.68 x 10-3	8.68 x 10-4
2G	2.29 x 10-3	4.61 x 10-5
2X	2.63 x 10-3	1.34 x 10-6
1G+1X	2.61 x 10-4	2.96 x 10-6

An identical, but even stronger, variation is observed in S2, c_v reduces from 8.68 x 10^-3 cm²/s for an untreated sample to 4.61 x 10^-5 cm²/s for the 2G dosage and 1.34 x 10^-6 cm²/s for the 2X dosage. The 1G +1X treatment further reduced the c_v to 2.96 x 10^-6 cm²/s, a very low value compared to the untreated soil. The cross-linked blend (1G+1X) produced the lowest c_v in Soil S1, indicating the slowest _consolidation response. However, in Soil S2, the lowest c_v was obtained with 2X, while the cross-linked blend still showed a substantial reduction compared with untreated soil. Therefore, the effect of biopolymer treatment on consolidation rate depends on soil mineralogy and polymer composition.

Variation of hydraulic conductivity (k)

The k values for S1 and S2 were calculated from consolidation test data rather than measured directly in permeability tests. The oedometer tests were conducted on the soils by adding the designated biopolymer dosages and compacting to the desired OMC and MDD values. The constant-head permeability method was conducted on S3 according to the standard procedure. Soil samples were treated with the selected biopolymer dosages of 2G, 2X, and 1G+1X, and allowed to mellow for 24 hours to ensure uniform distribution of the biopolymer throughout the soil. Table 11 presents the k values for all biopolymer-treated soils.

Table 11.

Variation of hydraulic conductivity for soils at the selected biopolymer dosages.

Biopolymer dosage/ Soil type	S1		S2		S3
	k (cm/s)	E	k (cm/s)	E	k (cm/s)
V	1.24 x 10-7	0.32	9.08 x 10-7	0.35	4.20 x 10-3
2G	7.08 x 10-8	0.32	1.32 x 10-8	0.32	8.32 x 10-4
2X	6.42 x 10-8	0.30	3.5 x 10-8	0.32	4.30 x 10-4
1G+1X	8.45 x 10-9	0.29	8.21 x 10-9	0.28	9.10 x 10-5

The k of S1 was found to be 1.24 × 10^-7 cm/s, consistent with that of fine, compressible clays, in which particle spacing and the diffuse double layer control water flow. Following the 2G treatment, the k value decreased by one order of magnitude (7.08 × 10^-8 cm/s), representing a dramatic improvement. It can be attributed to the filling of microvoids between clay particles by a flexible, viscous gel formed upon the addition of guar gum. A k value of 6.42 × 10^-8 cm/s was observed at the 2X dosage, indicating a less pronounced effect than with the 2G treatment. Xanthan forms a rigid hydrogel, incapable of penetrating uniformly like flexible polymer chains of G. The value of k for the crosslinked blend was 8.45 x 10^-9 cm/s, reflecting an integrated hydrogel network combining the flexibility and rigidity offered by guar and xanthan gums, respectively.

The S2 has shown a k value of 9.08 × 10^-7 cm/s, in the same order as S1 but slightly higher due to weak interparticle contact and visible micropores. At 2G dosage, the value of k was 1.32 × 10^-8 cm/s, indicating that G binds soils with a coarse-grained fraction strongly compared to very fine-grained soils (S1). When X was used at a 2X dosage, the k value was reduced by an order of one magnitude to 3.5 × 10^-8 cm/s. Crosslinking between G and X produces a continuous biopolymer matrix capable of filling fine pores (G) and larger voids (X) and reduces the k value to a possible extent of 8.21×10^-9 cm/s.

The k value of untreated S3 is 4.2 × 10^-3 cm/s, reflecting its coarse-grained quartz-particle structure. Being non-expansive and very low in surface activity, quartz particles do not retain water to a great extent. Post-biopolymer treatment, a significant reduction in k values was observed at 2G and 2X dosages. A k value of 8.32 × 10⁻⁴ cm/s and 4.3 × 10⁻⁴ cm/s were obtained at the dosages of 2G and 2X, respectively. The greater reduction in k value is achieved with the 1G+1X dosage, at 9.1×10⁻⁵ cm/s, confirming the superior efficiency of crosslinked biopolymers in densifying the soil structure and closing seepage pathways.

The microstructural modification observed in Figure 8 also explains the consolidation and permeability responses of biopolymer treated soils. Polymer coating and pore filling reduce the effective size and continuity of the water-flow channels, while the cross-linked gel network increases the tortuosity of the seepage paths and retains water within the polymer matrix. Under increasing consolidation pressure, the initially flocculated gel-soil structure undergoes rearrangement and pore collapse, resulting in lower final void ratios^46,47. At the same time, obstruction of the interconnected pores delays pore-water dissipation, producing lower coefficients of consolidation and hydraulic conductivity. Thus, the observed changes in shear strength, compressibility, and water-flow behaviour originate from the combined effects of particle coating, aggregation, pore filling, hydrogel hardening, and soil-polymer interactions.

Conclusions

This study evaluated the influence of guar gum, xanthan gum, and their crosslinked blend on the shear strength, consolidation, and permeability behaviour of soils with different plasticity characteristics. Based on the experimental results, the following conclusions are drawn.

• Biopolymer treatment improved the shear strength response of all the selected soils, although the governing mechanism varied with soil type.

• In S1, a strong increase in cohesion is observed, and the increase in friction angle is comparatively small. At 28 days, 1G+1X reached a cohesion of 81.39 kPa and a friction angle of 10.5°, compared with untreated values of 56.87 kPa and 7.8°.

• In S2, the response is more balanced, with both cohesion and friction angle improving. At 28 days, 1G+1X reached 77.47 kPa cohesion and 12.2° friction angle, while 2G gave the highest friction angle of 12.5°.

• In S3, the most important effect is the increase in friction angle, from 39.1° in untreated soil to 42.9° with 1G+1X, along with cohesion increasing from 12.74 kPa to 23.53 kPa.

• The crosslinked blend reduced the void ratio to 0.18 in S1 and 0.14 in S2 at 800 kPa, the lowest among all treatments.

• The c_v values were reduced for 1G+1X dosage in both soils, 2.61 × 10⁻⁴ cm²/s for S1 and 2.96 × 10⁻⁶ cm²/s for S2, when compared to the untreated samples, indicating slower consolidation and reduced water movement.

• The lowest k values were also obtained for 1G+1X with values of 8.45 × 10⁻⁹ cm/s for S1, 8.21 × 10⁻⁹ cm/s for S2, and 9.10 × 10⁻⁵ cm/s for S3.

• The findings confirm that crosslinked biopolymers are promising sustainable stabilizers for geotechnical applications where increased shear strength and reduced hydraulic conductivity are required, including slope protection, and low-permeability barrier systems.

Practical importance of the study

This study demonstrates the practical viability of using cross-linked biopolymers as sustainable soil stabilizers for soils with varying plasticity characteristics. The improvement in shear strength and reduction in hydraulic conductivity highlight the potential use of treated clayey soils in liners and containment systems. The wetting-drying durability and erosion behaviour of the same soil-biopolymer systems were assessed in our previous study³⁷ where the treated specimens exhibited better integrity and resistance to degradation than the untreated soils under controlled laboratory cycles. Crosslinking offers performance comparable to conventional stabilizers while significantly reducing carbon footprint and environmental impact. These findings support the short-term durability of the treatments. However, they should not be interpreted as evidence of permanent field stability. Therefore, the proposed biopolymer treatments may be considered promising for applications with controlled exposure conditions, while long-term field trials and biodegradation assessments remain necessary before broad implementation.

Limitations and scope for future research

Although the present study demonstrates the effectiveness of guar gum, xanthan gum and their cross-linked blend in improving the shear strength and reducing the hydraulic conductivity of soils with contrasting plasticity and mineralogy, certain limitations should be acknowledged. The investigation was conducted under controlled laboratory conditions using selected biopolymer dosages, curing periods and specimen preparation procedures. Therefore, the observed behaviour may vary under field conditions where temperature, moisture fluctuations, microbial activity, groundwater chemistry and repeated loading cycles can influence the long-term stability of the biopolymer network. In addition, the mineralogical interpretation was based on XRD phase identification, while quantitative chemical or oxide composition through XRF analysis was not determined. Future research should therefore focus on long-term field validation of cross-linked biopolymer-treated soils under realistic environmental and mechanical loading conditions. Studies incorporating repeated freeze-thaw, cyclic loading, leaching, biodegradation and microbial exposure are required to assess durability and performance retention. Field-scale mixing, cost analysis, life-cycle assessment and carbon-footprint evaluation should also be examined before the proposed treatment can be recommended for large-scale geotechnical applications.

Author contributions

MAK and AABM contributed to the conceptual development of the study and experimental investigation. MAK designed the methodology, and AUK provided the financial support for the research. MAK and AABM carried out the visualization. BKD carried out the data curation, and AABM supervised all the research work. The original manuscript was prepared by MAK, and subsequent review and editing were carried out by AABM, BKD, and AUR. AABM and AUR handled the resource management, formal analysis, and project administration.

Funding

This work was supported and funded by the King Saud University through “Ongoing Research Funding Program (ORF-2026-701), King Saud University, Riyadh, Saudi Arabia”.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interest

The authors declare no competing interests.