Improved Shear Strength Performance of Compacted Rubberized Clays Treated with Sodium Alginate Biopolymer

Abstract

This study examines the potential use of sodium alginate (SA) biopolymer as an environmentally sustainable agent for the stabilization of rubberized soil blends prepared using a high plasticity clay soil and tire-derived ground rubber (GR). The experimental program consisted of uniaxial compression and scanning electron microscopy (SEM) tests; the former was performed on three soil-GR blends (with GR-to-soil mass ratios of 0%, 5% and 10%) compacted (and cured for 1, 4, 7 and 14 d) employing distilled water and three SA solutions—prepared at SA-to-water (mass-to-volume) dosage ratios of 5, 10 and 15 g/L—as the compaction liquid. For any given GR content, the greater the SA dosage and/or the longer the curing duration, the higher the uniaxial compressive strength (UCS), with only minor added benefits beyond seven days of curing. This behavior was attributed to the formation and propagation of so-called “cationic bridges” (developed as a result of a “Ca²⁺/Mg²⁺ ⟷ Na⁺ cation exchange/substitution” process among the clay and SA components) between adjacent clay surfaces over time, inducing flocculation of the clay particles. This clay amending mechanism was further verified by means of representative SEM images. Finally, the addition of (and content increase in) GR—which translates to partially replacing the soil clay content with GR particles and hence reducing the number of available attraction sites for the SA molecules to form additional cationic bridges—was found to moderately offset the efficiency of SA treatment.

1. Introduction

The rapid economic development of modern societies has resulted in a dramatic rise in waste generation. Waste tires from the automotive industry, commonly referred to as end-of-life tires (or ELTs), are among the largest and most problematic of these waste streams and hence demand further attention. ELT stockpiles are often associated with harmful environmental impacts, particularly when subjected to improper waste management techniques—that is, occupancy of valuable landfill space (owing to the tires’ low mass-to-volume ratio), water stagnation and air pollution (on ignition) [1,2,3]. To alleviate these burdens on the environment, particularly the need for landfilling, local communities and governmental agencies have been increasingly persuaded to recycle and reuse ELTs, as a construction material, within their infrastructure systems (e.g., pavements, embankments, retaining walls and bridge abutments). Recycled ELT-based derivatives, with synthetic rubber being their main constituent, are resilient, light-weight and skin-resistive; these attributes make them one of the most suitable candidates for developing high-performance geomaterials for a variety of geotechnical engineering applications [4,5,6]. Recent research involving the addition of recycled ELT-based products—mainly in the form of granulates, herein referred to as ground rubber (GR)—to low-grade clay soils has shown that the compacted soil-GR blend demonstrates excellent properties in terms of diminishing the soil’s swell–shrink volume change capacity, as well as its desiccation-induced cracking potential [6,7,8,9,10,11,12,13,14,15]. Moreover, the application of GR-based additives has been reported to greatly enhance the soil’s damping, which adds greatly to its seismic resistance [16,17,18,19,20]. In terms of shear strength and bearing capacity, the addition of GR, at low GR-to-soil mass ratios (mainly less than 10%), has been found to produce relatively small improvements; however, soil-GR composites prepared with higher GR contents have been reported to create serious strength and stiffness concerns, and thus are generally not considered as high-performance geomaterials [6,9,11,13,21,22,23,24,25,26]. Accordingly, for those projects where the strength and stiffness of the composite geomaterial are primary concerns, the compacted soil-GR blend requires stabilization.

Much like natural soils, the stabilization of soil-GR blends can be accomplished using cementitious binders (e.g., Portland cement, hydrated lime, fly ash and silica fume). The introduction of these agents to the soil-GR–water complex results in the formation and propagation of a series of short- and long-term chemical reactions, which encourage flocculation of the soil and soil-GR constituents, thereby leading to major improvements in composite stiffness and shear strength [22,24,27,28,29]. It is well accepted that cementitious binders, even though effective in terms of stabilization, are generally not environmentally sustainable, since their application is often accompanied by significant energy and carbon emissions footprints [30,31,32]. This drawback alone highlights the urgency of minimizing the use of these binders in practice. A common solution in this context involves replacing these traditional binders, even though partially, with low-cost and more environmentally sustainable materials. Promising alternate materials capable of meeting both geotechnical performance and sustainability requirements, as reported in the research literature, include synthetic or natural polymers, resins and sulfonated oils [31,33,34,35,36,37,38,39,40,41].

Much like traditional cementitious binders, the introduction of polymers to the soil–water medium can encourage flocculation of the clay particles through various clay–polymer interaction mechanisms—that is, van der Waals or hydrogen bonding, charge neutralization (by way of electrostatic attraction) and cationic bridging for neutral, cationic and anionic polymers, respectively [42,43,44,45]. Among the multitude of commercially available polymer-based soil stabilizers, those derived from natural resources, commonly referred to as biopolymers, appear to possess a variety of promising soil-amendment features while outperforming synthetic variants in terms of sustainability, hence demanding further attention [32]. Sodium alginate (SA) is a water-soluble, linear polysaccharide derived from brown algae; it consists of two linked anionic monomers—that is, β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues [46,47]. In terms of its polymeric structure, SA is composed of homo-polymeric regions of G-residues (G-blocks) and M-residues (M-blocks), interspersed with regions of mixed monomers or MG-blocks [48]. SA has been successfully employed within a variety of industries, including, most importantly, its widespread application as a thickener, a gelling agent and an emulsifier in the food industry [49,50,51]. In the geotechnical context, reported applications for SA have been limited to natural soils and include increasing soil compaction efficiency, enhancing soil shear strength/stiffness and soil seepage/erosion control [52,53,54,55,56,57,58,59]. Though promising, the results reported by these studies, particularly in the context of clay soil stabilization, are still limited (and somewhat inconsistent) to warrant SA as an ad hoc soil stabilization solution. With waste-based geomaterials, such as soil-GR blends, gaining ground as a viable construction material in practice, the need to re-evaluate the efficacy of traditional and emerging soil stabilization agents (including biopolymers) for these new geomaterials (particularly those containing non-soil components) arises as an inevitable necessity. To the authors’ knowledge, the outcomes of biopolymer treatment, including the use of SA as a compaction liquid, on the mechanical performance of soil-GR blends (as well as other waste-based geomaterials containing non-soil components) have not yet been investigated (nor understood), thus implying the need for further research to fully understand the true stabilization potentials and/or limitations of SA-based agents. In the authors’ view, the lack of such published data, among other factors, has hindered the acceptance of biopolymers, as additives, for widespread usage in ground improvement projects.

In view of the limited literature on employing SA in ground improvement practice, this experimental study investigates the application of an SA-based biopolymer as an environmentally sustainable agent for the stabilization of soil-GR blends (prepared using a high plasticity clay soil). The primary objectives were to examine the outcomes of SA treatment, considering the effects of both SA dosage and short-term curing duration, on the shear strength and microstructure properties of compacted soil-GR blends. The experimental program consisted of uniaxial compression and scanning electron microscopy (SEM) tests; the former was performed on three different soil-GR blends (with GR-to-soil mass ratios of 0%, 5% and 10%) compacted (and cured for 1, 4, 7 and 14 d) using distilled water and three different SA solutions—prepared at three different SA-to-water (mass-to-volume) dosage ratios of 5, 10 and 15 g/L—as the compaction liquid. Finally, the fundamental principles of soil chemistry, along with typical SEM images, were employed to identify and hence discuss the soil–SA and soil-GR–SA amending mechanisms.

2. Test Materials

2.1. Test Soil

The soil selected for this experimental investigation was a high-plasticity clay; its pertinent physical and mechanical properties were determined in accordance with relevant ASTM and Australian standards, and the results are summarized in

Table 1. Physical and mechanical properties of the test soil.

Soil Property	Value	Standard
Particle-size distribution (PSD)
Sand fraction (2–4.75 mm) (%)	1	ASTM D422 [60]
Silt fraction (2–75 μm) (%)	47	ASTM D422 [60]
Clay fraction (<2 μm) (%)	52	ASTM D422 [60]
Consistency/Atterberg limits
Liquid limit (LL) (%)	84.3	AS 1289.3.9.1 [61]
Plastic limit (PL) (%)	32.0	AS 1289.3.2.1 [62]
Plasticity index (PI) (%)	52.3	AS 1289.3.3.1 [63]
USCS soil classification	CH a	ASTM D2487 [64]
Compaction properties for standard Proctor energy (SPE)
Specific gravity of soil solids, $G_{\mathrm{s}}^{\mathrm{S}}$	2.73	ASTM D854 [65]
Optimum moisture content (OMC), wopt (%) b	28.0	ASTM D698 [66]
Maximum dry unit weight (MDUW), γdmax (kN/m3) b	14.6	ASTM D698 [66]
Void ratio at MDUW, e c	0.834	ASTM D698 [66]

^a Clay with high plasticity; ^b Tested under the standard Proctor energy (SPE) level and ^c Calculated as e = $G_{\mathrm{s}}^{\mathrm{S}}$γ_w/γ_dmax − 1 (where γ_w = unit weight of water).

.

Referring to Figure 1, which illustrates the particle-size distribution (PSD) curves for the test materials; the test soil contained 1% fine sand (0.075–0.425 mm), 47% silt (2–75 μm) and 52% clay (<2 μm), dry mass basis as per ASTM D422 [60]. In terms of consistency, the fall-cone liquid limit (LL)—tested by means of a standard 80 g–30° fall-cone device in accordance with AS 1289.3.9.1 [61]—and the rolling-thread plastic limit (PL)—tested in accordance with AS 1289.3.2.1 [62]—were measured as LL = 84.3% and PL = 32.0% (mean value calculated based on three measurements, conducted by a single operator, with a standard deviation of 2.3%), thereby producing a plasticity index value of PI = LL − PL = 52.3%, as per AS 1289.3.3.1 [63]. Following the Unified Soil Classification System (USCS) outlined in ASTM D2487 [64], the test soil can be classified as clay with high plasticity (CH). Furthermore, the specific gravity of the soil solids was measured as $G_{\mathrm{s}}^{\mathrm{S}}$ = 2.73 (ASTM D854 [65]). In terms of compactability, the optimum moisture content (OMC) and maximum dry unit weight (MDUW)—tested under the standard Proctor energy (SPE) level (ASTM D698 [66])—were measured as w_opt = 28.0% and γ_dmax = 14.6 kN/m³, respectively; the latter is equivalent to a relatively high void ratio of e = 0.834.

2.2. Tire-Derived Ground Rubber

Commercially available tire-derived ground rubber (GR), sourced from a local distributor, was used to prepare rubberized clay blends (see Figure 2a). Referring to the PSD curves shown in Figure 1; the particles of the GR material were found to fall into the fine–medium sand (0.075–2 mm) gradational category, with its particle diameters corresponding to 10%, 30%, 50%, 60% and 90% finer being determined as d₁₀ = 0.172 mm, d₃₀ = 0.330 mm, d₅₀ = 0.462 mm, d₆₀ = 0.521 mm and d₉₀ = 0.961 mm, respectively (ASTM D422 [60]). In view of these diameters, along with the calculated uniformity and curvature coefficients (i.e., C_u = d₆₀/d₁₀ = 3.03 and C_c = d₃₀²/d₆₀d₁₀ = 1.22), the GR material can be classified as equivalent to poorly-graded sand (SP) in accordance with the USCS classification framework (ASTM D2487 [64]).

The scanning electron microscopy (SEM) technique was employed to observe the GR material’s morphological features (see Figure 2b). From visual inspection, the GR particles can be characterized as non-spherical and somewhat angular in shape, with occasional cavities and micro-cracks distributed along their surfaces, hence showcasing a rough surface texture [39,67]. Furthermore, as stated in the manufacturer’s literature, the GR material possessed a specific gravity of $G_{\mathrm{s}}^{\mathrm{GR}}$ = 1.09 (i.e., 2.5-fold lower than that of the test soil with $G_{\mathrm{s}}^{\mathrm{S}}$ = 2.73), a specific surface area of 0.05 m²/g, and a softening point of 170 °C. In terms of average chemical composition, the GR material consisted of 55% styrene–butadiene copolymer, 25–35% carbon black, 5–20% acetone extract, 2–3% zinc oxide and 1–3% sulfur, mass basis.

2.3. Sodium Alginate Biopolymer

Commercially available SA, sourced from a local distributor, was used as the soil-GR-stabilizing agent. It was supplied in solid form (slightly yellow powder) and, as per the manufacturer’s instructions, is to be diluted with water for application. Referring to Figure 3; SAs consist of two linked anionic monomers, namely, β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues, often structured as homo-polymeric regions of G-residues (G-blocks) and M-residues (M-blocks), interspersed with regions of mixed monomers or MG-blocks [48]. As reported in the manufacturer’s literature, the SA material used in this investigation possessed a molecular weight of 216 g/mol, a pH of 5.5–7.5 for a 1% aqueous solution (at 25 °C) and an average dynamic viscosity of 24.5 cP (1 cP = 10⁻³ Pa.s) for a 1% aqueous solution (at 25 °C).

3. Test Program

3.1. Mix Designs and Sample Preparations

The primary objectives of this experimental study were to investigate the outcomes of SA treatment, considering the effects of both SA dosage and short-term curing duration, on the shear strength and microstructure properties of soil-GR blends. Accordingly, a total of fourteen mix designs—including five untreated soil-GR blends and nine SA-treated soil-GR cases—were examined (see

Table 2. Summary of the investigated soil-GR–SA mix designs and their properties.

Group	Sample	fGR (%) a	DSA (g/L) b	Tc (d) c	wo (%) d	γdo (kN/m3) e	${\mathit{G}}_{\mathbf{s}}^{\mathbf{mix}}{ }^{\mathbf{f}}$	eog
Untreated	R0S0T0	0	0	0	28.0	14.6	2.73	0.834
	R5S0T0	5			26.2	14.3	2.55	0.749
	R10S0T0	10			24.5	13.8	2.40	0.706
	R20S0T0	20			22.2	13.4	2.18	0.596
	R30S0T0	30			20.6	12.9	2.03	0.544
SA-treated	R0S5T1,4,7,14	0	5	1, 4, 7, 14	28.0	14.6	2.73	0.834
	R0S10T1,4,7,14		10
	R0S15T1,4,7,14		15
SA-treated	R5S5T1,4,7,14	5	5	1, 4, 7, 14	26.2	14.3	2.55	0.749
	R5S10T1,4,7,14		10
	R5S15T1,4,7,14		15
SA-treated	R10S5T1,4,7,14	10	5	1, 4, 7, 14	24.5	13.8	2.40	0.706
	R10S10T1,4,7,14		10
	R10S15T1,4,7,14		15

^a GR content, as per Equation (2); ^b SA dosage, as per Equation (3); ^c Curing duration; ^d Molding (as-compacted) MC; ^e Molding DUW; ^f Soil-GR mixture specific gravity, calculated by Equation (4); and ^g Molding void ratio, calculated as e_o = $G_{\mathrm{s}}^{\mathrm{mix}}$γ_w/γ_do − 1 (where γ_w = unit weight of water).

). For ease of presentation and analysis, herein, the following coding system employed to designate the investigated soil-GR–SA mix designs:

$$R_x{S}_y{T}_z$$

(1)

where R_x = x% GR (i.e., GR-to-soil mass ratio); S_y = y g/L SA (i.e., SA-to-water mass-to-volume ratio); and T_z = z days of curing.

It should be mentioned that, for those mix designs containing only GR, the GR material was incorporated into the test soil using four different GR contents of f_GR = 5%, 10%, 20% and 30%, with the GR content being defined by Equation (2). For SA-treated cases, the granular SA material was diluted with distilled water and applied as the mixing liquid employing three different SA dosages of D_SA = 5, 10 and 15 g/L, with the SA dosage being defined as per Equation (3). Moreover, the GR content for SA-treated cases was limited to f_GR ≤ 10%; as will be discussed in Section 4.1, GR contents greater than 10% can lead to serious shear strength concerns (under compression) and hence are unsuitable options for the development of high-performance rubberized soil blends.

$$(\%) f_{\mathrm{GR}}=\frac{M_{\mathrm{GR}}}{M_{\mathrm{DS}}}\times 100$$

(2)

$$\left(\mathrm{g}/\mathrm{L}\right) D_{\mathrm{SA}}=\frac{M_{\mathrm{SA}}}{V_{\mathrm{DW}}}$$

(3)

where f_GR = GR content (in %); D_SA = SA dosage (in g/L); M_DS, M_GR and M_SA = mass of oven-dried soil, GR and granular SA, respectively (in g); and V_DW = volume of distilled water (in L).

Following the mix designs outlined in Table 2, the oven-dried test soil and GR material were first blended in dry form. On achieving visible homogeneity of the two ingredients, the required volume of liquid—either distilled water (for R_xS₀T₀, where x = {0, 5, 10, 20, 30}) or SA solution (for R_xS_yT_z, where x = {0, 5, 10}, y = {5, 10, 15} and z = {1, 4, 7, 14}) corresponding to the standard Proctor OMC value of various soil-GR blends (see w_o values in Table 2)—was added to each of the rubberized soil blends and thoroughly mixed by hand. Extensive care was taken (visually) to pulverize any clumped particles, particularly for higher SA dosages, thereby targeting homogeneity of the various test mixtures. As per common practice [6,13,39], samples for the uniaxial compression tests (see Section 3.2) were formed in a series of 50-mm diameter and 100-mm high stainless-steel molds in three equal-height layers by means of the static compaction technique, such that each layer was able to accomplish the standard Proctor MDUW value of the respective soil-GR blend (see γ_do values in Table 2). To assure adequate bonding between adjacent layers of the compacted samples, the surface of the first and second compacted layers was scarified. Furthermore, for those mix designs containing SA, four short-term curing durations of T_c = 1, 4, 7 and 14 d were considered, whereby the compacted samples were sealed (using multiple layers of cling wrap) and maintained at ambient laboratory conditions prior to uniaxial compression testing. Another point worth noting is that the specific gravity for the various soil-GR blends was estimated by the following theoretical relationship [8,68]:

$$G_{\mathrm{s}}^{\mathrm{mix}}=\frac{G_{\mathrm{s}}^{\mathrm{S}}{G}_{\mathrm{s}}^{\mathrm{GR}}\left(M_{\mathrm{DS}}+M_{\mathrm{GR}}\right)}{G_{\mathrm{s}}^{\mathrm{S}}{M}_{\mathrm{GR}}+G_{\mathrm{s}}^{\mathrm{GR}}{M}_{\mathrm{DS}}}$$

(4)

where $G_{\mathrm{s}}^{\mathrm{mix}}$ = soil-GR mixture specific gravity (values presented in Table 2); M_DS and M_GR = mass of oven-dried soil and GR, respectively (in g); and $G_{\mathrm{s}}^{\mathrm{S}}$ and $G_{\mathrm{s}}^{\mathrm{GR}}$ = specific gravity of soil solids and GR particles (= 2.73 and 1.09), respectively.

To ensure that the compacted samples were uniform in fabric (i.e., blending homogeneity) and hence repeatable in behavior, on completion of static compaction, the variations of both the DUW and MC parameters were measured along the height of representative samples, namely, R₀S₀T₀, R₁₀S₀T₀, R₀S₁₅T₀ and R₁₀S₁₅T₀. As per common practice [69,70,71], this was achieved by slicing each of the aforementioned compacted samples into five equal-height segments and testing each segment for DUW and MC, as illustrated in Figure 4. The variations of both the DUW and MC parameters were found to be marginal, as supported by the low standard deviation (SD) values (e.g., 0.2 kN/m³ ≤ SD ≤ 0.3 kN/m³ for DUW), thus confirming the suitability and repeatability of the employed sample preparation technique (in terms of mixing and compaction).

3.2. Uniaxial Compression Test

Uniaxial compression tests were performed following ASTM D2166 [72]. The prepared SPE-compacted samples (prepared as per Section 3.1) were axially compressed using a constant displacement rate of 1 mm/min (equivalent to an axial strain rate of 1%/min), as commonly employed for testing GR mixed fine-grained soils [6,10,67]. For each sample, the axial strain and corresponding mobilized axial stress were measured (using a closed-loop servo-controlled hydraulic compressive machine with a maximum load capacity of 50 kN) at predefined time intervals until such time that the ultimate/peak axial stress (required for sample failure)—defined as the uniaxial compressive strength (UCS)—was fully mobilized.

On account of the four curing durations (i.e., T_c = 1, 4, 7 and 14 d) adopted for SA-treated samples, a total of 41 uniaxial compression tests—that is, 5 for the untreated soil-GR blends and 36 for the SA-treated soil-GR mixtures—were conducted to address the fourteen mix designs outlined in Table 2. To assure sufficient accuracy, triplicate samples were tested for representative mix designs, namely, R₀S₀T₀, R₁₀S₀T₀, R₀S₁₅T₇ and R₁₀S₁₅T₇, with the coefficient of variation (CV) for the UCS parameter being calculated as CV = 3.4%, 5.7%, 3.8% and 4.9%, respectively. These low CV values corroborate the repeatability of the adopted sample preparation technique, as well as the implemented uniaxial compression testing procedure.

3.3. Scanning Electron Microscopy Test

Representative samples, including R₀S₀T₀, R₁₀S₀T₀, R₀S₁₀T₇ and R₁₀S₁₀T₇, were investigated using the SEM technique to observe the evolution of soil fabric due to the addition of GR and/or SA. On completion of curing (if applicable), the aforementioned SPE-compacted samples (prepared as per Section 3.1) were first carefully fractured into small cubic-shaped specimens (measuring approximately 1 cm³ in volume). These specimens were allowed to air-dry for approximately fourteen days; the air-dried specimens were then scanned at various magnification ratios ranging between 150× and 20,000×. It should be mentioned that the Philips XL20 (Koninklijke Philips N.V., Amsterdam, The Netherlands) scanning electron microscope was employed for SEM imaging. Its main specifications, as reported in the manufacturer’s literature, included a resolution of 4 μm and a maximum magnification ratio of 50,000×.

4. Results and Discussion

4.1. Effects of Ground Rubber on Soil Compactability and Compressive Strength

The complete results of the standard Proctor compaction and uniaxial compression tests for the untreated soil-GR blends are summarized in

Table 3. Results of the SPE compaction and uniaxial compression tests for the untreated soil-GR blends—that is, R₀S₀T₀, R₅S₀T₀, R₁₀S₀T₀, R₂₀S₀T₀ and R₃₀S₀T₀.

Sample	OMC, wopt (%)	MDUW, γdmax (kN/m3)	UCS, qu (kPa)	Improvement (%) a
R0S0T0	28.0	14.6	127.6	—
R5S0T0	26.2	14.3	146.4	+15
R10S0T0	24.5	13.8	120.5	−6
R20S0T0	22.2	13.4	115.4	−10
R30S0T0	20.6	12.9	81.3	−36

^a Percent change in the UCS (or q_u) in relation to the test soil (R₀S₀T₀).

. The untreated soil-GR mixtures—that is, R₅S₀T₀, R₁₀S₀T₀, R₂₀S₀T₀ and R₃₀S₀T₀—were all found to produce lower OMC and MDUW values compared with those obtained for the test soil (R₀S₀T₀). In this regard, the greater the GR content, the lower the OMC and MDUW parameters, both following monotonically decreasing trends. The OMC and MDUW parameters for the test soil (R₀S₀T₀) were measured as w_opt = 28.0% and γ_dmax = 14.6 kN/m³, respectively. With the addition of 5%, 10%, 20% and 30% GR (R₅S₀T₀, R₁₀S₀T₀, R₂₀S₀T₀ and R₃₀S₀T₀), the aforementioned values decreased to w_opt = 26.2%, 24.5%, 22.2% and 20.6%, and γ_dmax = 14.3, 13.8, 13.4 and 12.9 kN/m³, respectively.

The observed reductions in the OMC parameter can be attributed to the GR material’s hydrophobic character and hence lower water adsorption–retention capacity compared with that of the clay soil particles [10,73]. As for the MDUW parameter, the GR material’s lower specific gravity (of $G_{\mathrm{s}}^{\mathrm{GR}}$ = 1.09) compared to that of the soil solids (with $G_{\mathrm{s}}^{\mathrm{S}}$ = 2.73) elucidates the lower MDUW values observed for the various soil-GR mixtures [22]. Moreover, because of their high energy absorption capacity, the compacted GR particles may progressively recover their initial (or uncompacted) shapes by way of a so-called “elastic rebound” mechanism, thereby reducing the efficiency of the imposed compaction energy and hence yielding lower MDUW values [67,68]. With this in mind, one can postulate that, the greater the GR content, the higher the energy absorption capacity of the soil-GR mixture and hence the more pronounced the mixture’s overall elastic-rebound recovery.

Referring to Table 3; in terms of strength development, the variations of the UCS parameter with respect to GR content (i.e., the q_u–f_GR relationship, where 0% ≤ f_GR ≤ 30%) demonstrated a rise–fall trend, peaking at f_GR = 5% and then decreasing for higher GR contents, such that the samples containing 20% and 30% GR exhibited noticeably lower UCS values compared with that obtained for the test soil. The test soil (R₀S₀T₀) and its various GR-blended samples containing 5%, 10%, 20% and 30% GR (R₅S₀T₀, R₁₀S₀T₀, R₂₀S₀T₀ and R₃₀S₀T₀) produced UCS values of q_u = 127.6, 146.4, 120.5, 115.4 and 81.3 kPa, respectively. These results can be discussed at the soil-GR agglomeration-scale level as follows.

At the soil-GR agglomeration-scale level, where the GR particles can be portrayed as embedded coarse-grained isolated solid inclusions, the UCS capacity of compacted soil-GR blends is provided by the combination of two resistive forces—namely, the cohesion resistance of its soil solids matrix (soil–soil interface), and an adhesion resistance generated at the surfaces of the isolated embedded GR particles (soil-GR interface) [6]. On account of the adhesion resistance being noticeably lower than the cohesion resistance, compared to the overall cohesion resistance of an individual soil agglomeration containing no GR, the combined cohesion–adhesion resistances for the soil-GR agglomeration would be lower overall and are expected to progressively decrease with increasing the GR content—in other words, the UCS should normally decrease with the addition of (and content increase in) GR, hence explaining the lower UCS values of the samples containing f_GR = 10%, 20% and 30% (R₁₀S₀T₀, R₂₀S₀T₀ and R₃₀S₀T₀) compared with that mobilized for the test soil (R₀S₀T₀). For the sample containing 5% GR (R₅S₀T₀), where the mobilized UCS was found to be greater than that of the test soil, arching between the GR inclusions within the soil-GR agglomerations can be considered as the governing mechanism [6]. In general, the greater the GR content, the more pronounced the positive effects of arching and hence the higher the developed UCS. With this in mind, one can postulate that, for the compacted sample containing f_GR = 5%, the positive effects of arching were still dominant compared to the negative effects of the cohesion–adhesion mechanism described above and, as such, the UCS was able to increase beyond the test soil. Accordingly, for GR contents greater than 5%, where the mobilized UCS dropped below that of the test soil, the negative effects of the cohesion–adhesion mechanism, particularly for f_GR = 20% and 30%, were able to prevail against arching, even though the positive effects of arching were greater compared with 5% GR. It is also worth mentioning that the GR particles themselves possess greater deformability (or lower rigidity) compared with that of the soil solids—that is, the soil-GR agglomerations possess substantially lower stiffness compared with that of the individual soil agglomerations containing no GR [5,67,74]. This notable mismatch in relative stiffness, particularly for GR contents greater than 10%, gives the soil-GR agglomerations a so-called “friable” nature, such that in unconfined compression, the GR-blended samples undergo greater radial expansion and hence mobilize lower UCS values [6].

4.2. Combined Effects of Ground Rubber and Sodium Alginate on Soil Compressive Strength

Figure 5a–c illustrate the variations of the UCS parameter against SA dosage for the samples R₀S_yT_z, R₅S_yT_z and R₁₀S_yT_z, where y = {5, 10, 15} and z = {1, 4, 7, 14}, respectively. For any given GR content and curing duration, the greater the employed SA dosage, the higher the mobilized UCS, following a monotonically increasing trend. For instance, the untreated sample containing 5% GR (R₅S₀T₀) produced a UCS value of 146.4 kPa; meanwhile, the same sample treated with D_SA = 5, 10 and 15 g/L and cured for T_c = 7 d (R₅S₅T₇, R₅S₁₀T₇ and R₅S₁₅T₇) resulted in higher UCS values of q_u = 260.7, 311.5 and 358.3 kPa (i.e., equivalent to improvements of 78%, 113% and 145% in relation to R₅S₀T₀), respectively. Moreover, for any given GR content and SA dosage, an increase in curing duration led to a notable increase in the UCS parameter up to T_c = 7 d, albeit often with a noticeably higher incremental rate compared with that achieved by increasing the SA dosage (for the investigated ranges of 5 g/L ≤ D_SA ≤ 15 g/L and 1 d ≤ T_c ≤ 14 d). Beyond seven days of curing, the positive effects of curing became less pronounced and, in most cases, rather marginal. As typical cases highlighting the effects of curing duration (for a constant GR content and SA dosage), the samples R₅S₁₀T₁, R₅S₁₀T₄, R₅S₁₀T₇ and R₅S₁₀T₁₄ resulted in q_u = 178.8, 234.1, 311.5 and 336.2 kPa (i.e., equivalent to improvements of 22%, 60%, 113% and 130% in relation to R₅S₀T₀), respectively.

It is also worth mentioning that, for any given SA dosage and curing duration, the UCS–GR content (or q_u–f_GR) relationship (where 0% ≤ f_GR ≤ 10%) exhibited a rise–fall trend (similar to that discussed in Section 4.1 for the untreated soil-GR samples), peaking at f_GR = 5% and then falling below its 0%-GR counterpart for f_GR = 10%. For instance, the samples treated with 10 g/L SA and cured for seven days produced UCS values of 296.1, 311.5 and 230.6 kPa for f_GR = 0%, 5% and 10%, respectively.

Further highlighting the effects of curing duration; Figure 6a–c illustrate the variations of the UCS parameter against curing duration for the samples containing f_GR = 0%, 5% and 10%, respectively. For any given GR content and SA dosage, the mobilized UCS was found to follow an exponentially increasing trend with increasing the curing duration; only minor added improvements were noted beyond seven days of curing. Interestingly, the overall positive contribution to the UCS parameter provided by increasing the SA dosage (for the investigated range of 5 g/L ≤ D_SA ≤ 15 g/L) was found to be somewhat less prominent compared with that offered by extending the curing duration (particularly for T_c < 7 d)—that is, for a given increase in the duration of curing, the magnitude of improvement in the mobilized UCS was often greater than that achieved for a given increase in the SA dosage; as such, any reduction in the UCS imposed by a decrease in the SA dosage may be effectively moderated by increasing the curing duration. For instance, for the sample R₅S₁₀T₄ (see point “A” in Figure 6b), a 5 g/L increase in SA dosage (i.e., R₅S₁₀T₄ ⟶ R₅S₁₅T₄) produced a 12% improvement in the mobilized UCS (follow the path “A ⟶ S” in Figure 6b); meanwhile, a three-day extension in curing for the same sample (i.e., R₅S₁₀T₄ ⟶ R₅S₁₀T₇) resulted in a noticeably higher improvement of 33% (follow the path “A ⟶ T” in Figure 6b).

To investigate the effects of GR content on the efficiency of SA treatment, a new parameter, referred to as the strength-gain-factor (SGF), was defined as follows:

$$\mathrm{SGF}=\frac{q_{\mathrm{u}}^{\mathrm{T}}}{q_{\mathrm{u}}^{\mathrm{U}}}$$

(5)

where SGF = strength-gain-factor; $q_{\mathrm{u}}^{\mathrm{U}}$ = UCS of untreated soil-GR samples—that is, R₀S₀T₀, R₅S₀T₀ or R₁₀S₀T₀; and $q_{\mathrm{u}}^{\mathrm{T}}$ = UCS of SA-treated soil-GR samples—that is, R₀S_yT_z, R₅S_yT_z or R₁₀S_yT_z, where y = {5, 10, 15} and z = {1, 4, 7, 14}.

Figure 7 illustrates the variations of the SGF parameter—that is, the UCS ratio of an SA-treated sample to its non-treated counterpart, as defined in Equation (5)—against GR content for the tested samples. As is evident from this figure, for any given SA dosage and curing duration, an increase in GR content was accompanied by a somewhat notable reduction in the SGF parameter, particularly for f_GR = 10%—for further reference, follow the three trendlines labeled as “D_SA = 5 g/L, D_SA = 10 g/L and D_SA = 15 g/L” for T_c = 4 d. For instance, the samples R₀S₁₀T₄, R₅S₁₀T₄ and R₁₀S₁₀T₄ produced SGF values of 1.63, 1.60 and 1.47, respectively. Moreover, the rate of reduction in the SGF parameter (with respect to GR content) was found to be more pronounced for higher SA dosages and/or longer curing durations—for instance, compare the three trendlines labeled as “D_SA = 15 g/L” for T_c = 1, 4, 7 and 14 d; these trendlines produced reduction rates (i.e., trendline slopes) of ΔSGF/Δf_GR = −0.10, −0.12, −0.27 and −0.29, respectively.

The observed improvements in the UCS parameter, as achieved by SA treatment for the investigated compacted samples, can be discussed in the context of clay–SA interactions. In the presence of water, the divalent cations present in the vicinity of the negatively charged clay surfaces, including calcium (Ca²⁺) and magnesium (Mg²⁺), tend to substitute the lower-valance sodium cations (Na⁺) of the SA molecules, with Ca²⁺ initiating the substitution process and first replacing Na⁺, owing to its larger ionic radius compared to that of Mg²⁺ [54,57,58,75]. Since these divalent cations would still remain electrostatically attracted to the negatively charged clay surfaces, the so-called “cation exchange/substitution” process described above allows the SA molecules to be attracted (and hence adsorbed) to the negatively charged clay surfaces—in other words, the exchanged divalent cations function as so-called “attraction bridges” between the clay particles and SA molecules [42,44,54]. The formation and propagation (over time) of these strong cationic bridges between adjacent clay surfaces—which bring and hold the clay particles together—induce flocculation of the clay particles, thus increasing the soil’s overall shear resistance (and hence its UCS capacity). Provided that the number of attraction sites—that is, the number of clay particles (or the soil clay content) and/or the amount of exchangeable divalent cations (i.e., Ca²⁺ and Mg²⁺)—available for the SA molecules is not exhausted, one can postulate that, the greater the SA dosage, the higher the propensity for clay particle flocculation and hence the higher the mobilized UCS [39,41,76]; this mechanism explains the monotonically increasing trend observed for the UCS with increasing the SA dosage (see Figure 5). With this in mind, for any given SA dosage and curing duration, the addition of (and content increase in) GR—which translates to partially replacing the soil clay content with GR particles and hence reducing the number of available attraction sites for the SA molecules—is expected to decrease the magnitude of improvement in the UCS compared with that achieved for the test soil containing no GR (see Figure 7). An important by-product of the cation substitution process described above is the formation of hydrogel materials—that is, a network of cross-linked polymer chains which are hydrophilic in nature [57,58,77,78]. The formation, diffusion and hardening (due to hydration) of these hydrogel materials over time can be visualized as a three-dimensional reinforcement network acting in favor of weaving/interlocking the soil–soil and soil-GR agglomerations into a coherent matrix of improved shear strength performance, thus explaining the positive effects of curing on increasing the UCS (see Figure 6). It is worth mentioning that the possible exchange of hydrated inorganic cations in the interlayer for some types of organic cations is also important; such modified materials, however, possess a hydrophobic character and are suitable as components in clay–polymer nanocomposites [79].

4.3. Microstructure Analysis

The microstructure analysis was performed employing an SEM characterization scheme developed by Soltani et al. [80]. Figure 8a–d illustrate SEM micrographs for the samples R₀S₀T₀, R₁₀S₀T₀, R₀S₁₀T₇ and R₁₀S₁₀T₇, respectively. Referring to Figure 8a; the microstructure of the compacted test soil containing no additives (R₀S₀T₀) exhibited a highly porous structure, accompanied by a notable number of large inter- and intra-assemblage pore-spaces, respectively, formed between and within the soil agglomerations (or clay flocs). Such morphological features confirm the existence of a partly flocculated/dense fabric. The microstructure of the sample containing only 10% GR (R₁₀S₀T₀) inherited the same highly porous and irregular structure of the test soil (R₀S₀T₀), with the randomly distributed GR particles—visualized as embedded coarse-grained “isolated solid inclusions”—further contributing towards fabric deflocculation (see Figure 8b).

As a result of SA treatment (see the sample R₀S₁₀T₇ in Figure 8c), the inter- and intra-assemblage pore-spaces exhibited a notable reduction in both number and size, allowing the development of relatively larger and more uniform soil agglomerations compared with those of the test soil (R₀S₀T₀). Such attributes predicate the existence of a fully flocculated/dense fabric, thereby confirming the postulated “flocculation–reinforcement” clay–SA stabilization mechanisms discussed in Section 4.2. Similarly, the concurrent use of GR and SA (see the sample R₁₀S₁₀T₇ in Figure 8d) produced a more flocculated fabric compared with the same GR inclusion containing no SA treatment (R₁₀S₀T₀). The soil-GR connection/bonding interfaces also appear to be improved, potentially enhancing the stability of the soil-GR agglomerations. Another interesting observation with regard to Figure 8d is the appearance of a relatively large GR cluster in the fabric; compared with the soil-GR agglomerations, the GR clusters are more friable in nature. With this in mind, one can postulate that, with the addition of (and, more importantly, content increase in) GR, the UCS response at some points within the GR-blended sample (either with or without SA) may be governed by a dominant, undesirable GR-to-GR particles’ interaction mechanism [4,22,67]. This additional factor may also explain the observed reductions in the samples’ UCS and SGF parameters with increasing the GR content (as discussed in Table 3 and Figure 7, respectively).

It should be mentioned that the original (or as-compacted) shape and extent of the inter- and intra-assemblage pore-spaces may have changed during the SEM specimen fabrication process (see Section 3.3), owing to sample fracturing and, more importantly, the development of tensile stresses (and hence micro-cracks) within the soil fabric during the fourteen-day desiccation process [80]. However, given that the SEM specimen fabrication process for all of the investigated mix designs was identical in terms of drying conditions (e.g., temperature, humidity and duration), the SEM micrographs presented in Figure 8, which are essentially relative in nature, still provide a rather reliable basis for the comparison of the four different mix designs. Additional tests employing the environmental SEM technique, which enables wet specimens to be observed through the use of partial water vapor pressure in the microscope specimen chamber, should be performed to further improve upon the authors’ postulated microstructure analysis.

5. Conclusions

This study examined the potential use of an SA-based biopolymer as an environmentally sustainable agent for the stabilization of soil-GR blends (prepared using a high plasticity clay soil). The experimental program consisted of uniaxial compression and SEM tests; the former was performed on three different soil-GR blends (with GR-to-soil mass ratios of f_GR = 0%, 5% and 10%) compacted (and cured for T_c = 1, 4, 7 and 14 d) using distilled water and three different SA solutions—prepared at three different SA-to-water (mass-to-volume) dosage ratios of D_SA = 5, 10 and 15 g/L—as the compaction liquid. In view of the experimental results and their interpretation, the following conclusions can be drawn from this study:

• For any given GR content and curing duration, the greater the employed SA dosage, the higher the mobilized UCS, following a monotonically increasing trend. This behavior was attributed to the formation and propagation of so-called strong “cationic bridges” (developed as a result of a “Ca²⁺/Mg²⁺ ⟷ Na⁺ cation exchange/substitution” process among the clay and SA components) between adjacent clay surfaces over time, which induce flocculation of the clay particles and hence increase the sample’s overall shear resistance (i.e., UCS capacity). This flocculation mechanism, and hence the cationic bridging assertion, was further discussed and validated by means of typical SEM images.
• Further, for any given GR content and SA dosage, the mobilized UCS was found to follow an exponentially increasing trend with an increasing curing duration; beyond seven days of curing, however, the positive effects of curing became less pronounced and, in most cases, rather marginal. Interestingly, the overall positive contribution to the UCS parameter provided by increasing the SA dosage (for the investigated SA dosage range) was found to be somewhat less prominent compared with that offered by extending the curing duration (particularly for T_c < 7 d).
• Finally, for any given SA dosage and curing duration, the variations of the UCS parameter with respect to GR content demonstrated a rise–fall trend, peaking at f_GR = 5% and then falling below its 0%-GR counterpart for 10% GR. An increase in GR content for any given SA dosage and curing duration was accompanied by a somewhat notable reduction in the samples’ SGF—that is, the UCS ratio of an SA-treated sample to its non-treated counterpart. This behavior was attributed to the partial replacement of the soil clay content with GR particles, reducing the number of available attraction sites (or clay surfaces) for the SA molecules to form additional cationic bridges.

6. Recommendations

As specified in ASTM D4609 [81], an effective soil stabilization scheme can be characterized as one that is able to produce a minimum improvement of 345 kPa in the mobilized UCS (in relation to the unstabilized scenario) [70,82,83,84]. With this in mind, none of the investigated SA-based mix designs were able to satisfy this requirement—that is, the maximum improvement for the three soil-GR blends containing 0%, 5% and 10% GR can be calculated as 230.8, 226.9 and 148.0 kPa (for the samples R₀S₁₅T₁₄, R₅S₁₅T₁₄ and R₁₀S₁₅T₁₄), respectively. If the addition of GR is also considered as a physical soil-stabilizing factor, the maximum improvement can be calculated as 245.7 kPa (for the sample R₅S₁₅T₁₄ in relation to R₀S₀T₀), which is still less than the 345-kPa requirement. Assuming that the number of attraction sites—that is, the number of clay particles (or the soil clay content) and/or the amount of exchangeable divalent cations (i.e., Ca²⁺ and Mg²⁺)—available for the SA molecules was not exhausted with the employed SA dosages, higher SA dosages (greater than 15 g/L) may produce further improvements in the UCS. Nevertheless, beyond a certain SA (or polymer) dosage for which the available attraction sites are exhausted (or saturated), the clay particle flocculation process is expected to cease—beyond this critical dosage, the excess SA (or polymer) molecules will likely function as a lubricant rather than a flocculant, hence potentially reducing the mobilized UCS [39,41,54]. In view of the UCS results presented in this study, the maximum flocculation capacity (for the SA agent) may not have been achieved. As such, additional uniaxial compression tests employing higher SA dosages are recommended (particularly for GR-blended cases) to identify SA’s maximum stabilization capacity (hence re-examining the postulated clay–GR–SA amending mechanisms), and to recheck whether the 345-kPa requirement can be satisfied.

Furthermore, a systematically controlled test program should be carried out to examine the effects of other critical parameters representing real-life field conditions (or environmental fluctuations)—that is, mellowing time and compaction delay, curing temperature, relative humidity during curing, long-term curing (up to 90 days) performance and cyclic wetting–drying resistance—on the mechanical performance of compacted soil-GR–SA blends. Finally, it is worth mentioning that the leaching of heavy metals from the GR particles into the soil mass and hence water bodies (over time) could potentially raise some environmental concerns. Luckily, the majority of research studies in this context have shown that the degree of soil and water contamination both remain within the allowable limits suggested by various health and environmental agencies—a comprehensive review on this topic is given in Yadav and Tiwari [2]. More importantly, it can be argued that the potential risks involved with recycling and reusing discarded tires within our infrastructure system outweigh the long-term costs and hazards associated with traditional tire disposal techniques. In this context, additional permeability and leaching tests are recommended to further examine and better understand the potential risks involved with employing GR-based geomaterials in practice (as well as the potential role of SA in mitigating these risks).