The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil

The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil Adesola Habeeb Adegoke, Sinenkosi Nxumalo, Okonta Felix This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6845001/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract The use of fibre for the improvement of residual clay soils for road backfill and infrastructure rehabilitation applications was investigated through a series of laboratory model tests. A series of standard tests to evaluate the California Bearing Ratio (CBR) and Indirect Tensile Strength (ITS) of residual clayey soil that was reinforced with strand and discrete fibres (0.6-3.0%) was conducted. The results show improvement in CBR of 25.05% and 19.13% for discrete and strand fibres, respectively, under unsoaked conditions, as well as an increase in ITS with a maximum tensile strength of 37.45 kPa at 1.8% for strand fibres and 30.8 kPa at 2.4% for discrete fibres. A series of laboratory model testing was conducted on fibre reinforced samples that were prepared in a steel reinforced box model (460 mm x 410 mm x 1.0 m). The result revealed that the static bearing capacity of the soil, associated with settlement of 25mm – 50mm, can be improved by the incorporation of 1.8–2.4% strand and 3.0% discrete. Dynamic loading tests, however, revealed that for up to 250 loading cycles associated with a cumulative settlement of 50mm, unreinforced soil and lime stabilized soils exhibited higher stiffness than fibre reinforced soil. Beyond 250 loading cycles, the relatively greater capacity of fibre reinforced soil is associated with the ductility of the soil fibre composite. 1.8% discrete fibre reinforcement exhibited minimal cumulative settlement, while fibre-reinforced and fibre-lime composites demonstrated exceptional resistance to dynamic loading. Notably, strand fibres mobilized greater stiffness compared to discrete fibres. The laboratory model tests provide some insight into the capacity and stability of fibre reinforced clay soil under static and dynamic load conditions in relation to other stabilization protocols, thus offering some guidance to field engineers on ground improvements. Indirect Tensile Strength Bearing capacity Plate load test Reinforcement weak subgrade Polypropylene fibre Resilient modulus Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1. Introduction The construction of roads and other earthwork-based infrastructure, like dams, industrial plants, and factories, demands substantial quantities of stone aggregates and conventional binders, which are derived from limited and depleting natural resources. To align with sustainable development goals, it is crucial to prioritize the use of locally obtainable aggregates and explore the potential of waste. A wide range of waste materials, including quarry stones, coal ash, recycled aggregate, plastics, fibres, and polythene bags, can be repurposed to promote natural resource sustainability in road construction. To meet stringent project specifications and enhance the performance of subgrade soils and fill application of compacted residual soil, reinforcement techniques are frequently employed in the construction of high-strength roadbeds [ 1 ]. These measures are particularly crucial for filling out applications where load-bearing capacity and long-term stability are essential. The implementation of such reinforcement strategies not only ensures compliance with project requirements but also contributes to the overall durability and resilience of the road infrastructure. In recent years, interlayer systems have gained significant attention among pavement engineers due to their potential to provide efficient reinforcement solutions. These systems have demonstrated effectiveness in inhibiting crack initiation and propagation within pavement structures [ 2 ]. However, the mechanical performance of these interlayer systems has not yet been comprehensively evaluated, leaving gaps in our understanding of their full potential and limitations. Furthermore, there is ongoing debate within the pavement engineering community regarding the most suitable types of interlayer systems and their optimal placement within pavement structures [ 3 ]. The vast array of commercially available products for interlayer systems has necessitated efforts to categorize this variety into a manageable number of groups, facilitating more structured research and application [ 4 ]. This evolving field presents both opportunities and challenges. While interlayer systems offer promising solutions for pavement reinforcement, more research is needed to fully understand their performance, optimize their use, and integrate them effectively into sustainable road construction practices. Over the past two decades, extensive research has been conducted on interlayer systems in pavement construction. These studies have explored various layer structures, including glass fibre lattices, polypropylene paving grids [ 2 , 5 ], non-woven geotextiles, geogrids [ 6 – 8 ], and steel meshes [ 9 ]. These materials have been employed to reinforce both bound and unbound pavement layers, demonstrating their versatility in pavement engineering. Fibre reinforcement of soil has emerged as an effective method for mechanical soil stabilization. This technique involves mixing soil with loose, non-reactive, tiny fibres [ 10 , 11 ], resulting in a fibre-soil composite with enhanced strength and more elastic stress-strain behaviour [ 12 ]. This improvement is attributed to the synergistic effect of binders and fibres on the soil's stress-strain performance, transforming the inelastic behaviour of the soil into a more ductile one. The inclusion of fibres has been shown to augment the soil's geotechnical properties under both constant and dynamic states, particularly in reinforcing sub-grade roads. Fibres enhance the soil's energy absorption and dissipation capabilities, increasing the damping ratio and resilient modulus [ 13 ]. This discrete fibre reinforcement technique has proven to be a novel and reliable method for improving soil mechanical behaviour [ 14 – 17 ]. Cyclic wetting and drying cycles significantly compromise bridge, highway, and foundation stability for critical infrastructure in the Loess Plateau region [ 18 ]. The implementation of fibre reinforcement technology presents an effective engineering solution to enhance soil stability and structural integrity under these challenging environmental conditions [ 19 , 20 ]. Both natural and synthetic fibres can be used to control soil dehydration cracking, preserving soil integrity for various civil engineering applications and there is effectiveness in their application for the improvement of subgrade soil [ 21 ]. The addition of fibres significantly reduces the shrinkage strain and the crack growth rate [ 12 ] and they are often adopted to improve the mechanical properties of red clay that is used in road subgrade, embankment, or reinforced walls [ 22 , 23 ]. Reinforcement fibres can be sourced from various materials, including nylon, polyester, paper, and metal, each with distinct physical properties [ 24 – 26 ]. There is a growing emphasis on using natural fibres, as non-biodegradable substances like artificial polymers pose critical disposal problems. Soil strengthening using relatively low-modulus fibres has been practiced for years, particularly in developing countries [ 27 – 29 ]. Organic fibres such as cotton, coconut, jute, palm, sisal, barley straw, and bamboo have shown promise in enhancing soil properties. These fibres are often locally available, cost-effective, biodegradable, environmentally friendly, and can form composites with cement or lime [ 30 , 31 ]. However, due to the abundance of waste plastics in emerging economies, the use of synthetic fibres and geogrids in pavement layers has gained general acceptance. This approach not only addresses the issue of plastic waste management but also contributes to the enhancement of pavement performance. Lime stabilization has been extensively studied in geotechnical engineering, with research focusing on lime consumption, stiffness, physicochemical responses, and strength properties for infrastructure applications like railroad embankments and highways [ 32 , 33 ]. The physical-chemical processes induced by lime treatment, including cation exchanges, hydration, and pozzolanic reactions, significantly enhance soil mechanical behavior and workability [ 34 , 35 ]. While fibre-reinforced soil performs better than unreinforced soil, its enhancement is constrained by the constituent fibres' inherent properties [ 36 ]. Although some researchers have investigated fibre reinforcement through plate loading tests on pavement sections, either alone or combined with additives [ 37 – 39 ]. However, there is a notable scarcity of documented research on the performance of fibre and fibre-lime-reinforced subgrade layers in laboratory unpaved model road layers. Current understanding of integrated fibre-soil-lime systems in unpaved roads remains limited, particularly regarding their tensile behavior and load-bearing characteristics under complex loading conditions. While small-scale laboratory studies inadequately represent field performance, this research investigates the mechanical behavior of fibre-reinforced and fibre-lime stabilized unpaved road layers through comprehensive large-scale testing by examining the strength parameters and deformation characteristics, bridging the gap between laboratory investigations and field applications. 2. Materials And Methods 2.1 Soil characteristics and preparation The soil utilized in this study was engineered by mixing various particle sizes obtained through dry sieve analysis, a technique employed to minimize particle size variability and its potential impact on test results. The granular and fine soil components are residual products derived from the weathering of the Park Town Shale of the Witwatersrand Supergroup, which underlies significant portions of central and southern Johannesburg, South Africa. Figure 1 illustrates the grain size distribution curve, while Table 1 summarizes the soil properties. The soil composition was as follows: 13.13% gravel, 83.49% sand, 3.09% silt, and 3.29% clay. Specific gravity was determined to be 2.85, per ASTM standards. The Atterberg limits tests, conducted per ASTM guidelines, yielded a liquid limit of 41.54% and a plastic limit of 34.44%. The average particle size (D50) was less than 0.08 mm. Based on the Unified Soil Classification System (USCS), this soil was classified as well-graded sand with clay (SW-SC). Modified compaction tests, performed according to ASTM standards, resulted in a maximum dry unit weight of 16.70 kN/m³ and an optimum moisture content of 12%. To maintain consistent moisture content, the prepared soil was stored in drums and covered with polyethylene sheets to prevent moisture ingress. Table 1 Soil properties Specific Gravity 2.85 Internal angle of friction 25.1 Cohesion 1.94 Compaction test Maximum dry unit weight (kN/m 3 ) 16.70 Consistency limits Optimum moisture content (%), subgrade 12 Liquid Limit (%) 41.54 Plastic Limit (%) 34.44 Plasticity Index 7.1 USCS SW-SC Well-graded sand with clay 2.2 Fibre Reinforcement and Lime A South African company supplied the lime used for soil layer modification. The polypropylene fibres used for pavement layer reinforcement were supplied by the Chryso Group, South Africa. Two types of fibres were employed and both fibre types have been engineered to exhibit enhanced impact resistance, ductility, robust abrasion resistance, and satisfactory energy absorption capabilities as shown in Fig. 2 . Strand fibres: Pure white in color, soft, and woolly when separated, with a standard length of 12 mm. These fibres have a tensile strength of 240 MPa, a relative density of 0.75, and moisture absorption of 00-0.03%. Discrete fibres: Silver-white in color, thick, and mat-like in appearance, with a standard length of 40 mm. These fibres have a tensile strength of 350 MPa, a relative density of 0.91, and moisture absorption of 00-0.02%. 2.3 California bearing ratio test The CBR test, a widely recognized penetration test, was employed to evaluate the stability of pavement subgrades. This test serves as a fundamental method for measuring and calculating the characteristics of pavement models and their constituent layers, utilizing empirical curves to interpret results. Tests were conducted on both untreated and fibre-treated soil samples under wet and dry conditions, adhering to the ASTM D1883-21 standard. The results were reported as percentages relative to the reference standard material. Load readings were recorded at penetration depths of 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, and 7.0 mm. A graph of dial reading against penetration was generated from these measurements. Loads at 2.54 mm penetration were determined from the resultant curve. These loads were compared with those of the standard material at the same penetration depth. The CBR values for both reinforced and unreinforced soils were evaluated using standard procedures, as outlined in Equations 1 and 2 . These equations provide a standardized method for calculating the CBR, ensuring consistency and comparability of results across different soil samples and treatment conditions. $$\:Penetration\:load,\:piston\:stress\:\left(MPa\right)=\:\frac{Load}{Piston\:cross-sectiona\:Area}$$ 1 $$\:CBR\:value=\frac{penetration\:load}{Standard\:load}\:x\:100$$ 2 2.4 Indirect tensile strength (ITS) test The Indirect Tensile Strength (ITS) test was conducted in triplicate following ASTM D 6931-17 standards to evaluate the tensile properties of fibre-reinforced soil samples. Cylindrical specimens (120 mm diameter, 60 mm thickness, aspect ratio 2) were prepared with varying fibre contents (0%, 0.6%, 1.2%, 1.8%, 2.4%, and 3.0%) using both strand and discrete fibres. Samples were mixed at 13% moisture content and compacted to 95% of Maximum Dry Density in five layers, each receiving 55 blows to ensure uniform density. Figure 3 provides a visual representation of the compacted soil samples, offering insight into their physical characteristics. ITS tests were performed at a constant displacement rate of 1 mm/min, with lateral displacement measured using two digital dial gauges positioned 15 mm from the specimen center on either side, as illustrated in Fig. 4 . This setup enabled precise measurement of sample deformation during loading. This systematic approach to sample preparation and testing methodology, including the use of both unreinforced control and fibre-reinforced samples, ensures data reliability and reproducibility. The rigorous experimental design provides a solid foundation for analyzing the impact of different fibre types and contents on soil tensile strength, contributing valuable insights to geotechnical engineering research on soil reinforcement techniques. 2.5 Plate loading test 2.5.1 Testing apparatus The steel-reinforced test box was built in the Kingsway Campus geotechnical laboratory of the University of Johannesburg. The dimensions are 460 mm, width: 410 mm, height: 1000 mm, and steel thickness of 20 mm. Drill bits were used to connect the steel joints. The wooden box was also reinforced on the sides to increase strength and prevent lateral movement, as shown in Fig. 5 . 2.5.2 Experimental setup The experimental setup comprised a plastic-lined test box containing a six-layer pavement structure. The subgrade consisted of five 500 mm-thick sublayers and a 150 mm reinforcement layer, all compacted using a hydraulic compactor. Figure 6 shows the schematic representation of the unpaved road model under static and dynamic loading tests. The subgrade mixture contained 24.4 kg of soil (2.36 mm sieve) and 6.2 kg of soil (0.425 mm sieve), combined with 3.9 L of water to achieve 13% moisture content. The reinforcement layer incorporated polypropylene fibres at concentrations ranging from 0.6–3.0% (275.1-1,375 g), mixed with 46.05 kg of soil and 5.96 L of water. While unreinforced mixtures were placed in single lifts, reinforced mixtures required two-lift placement to ensure uniform fibre distribution. The base layer comprised two compacted aggregate subbase layers, each 100 mm thick, prepared at optimal moisture content and maximum dry density (MDD). This configuration yielded a consistent total base thickness of 200 mm across all reinforced and unreinforced pavement models. While the base layer arrangement mirrored the subgrade layer, it lacked reinforcement. The subbase material was prepared by combining 60.16 kg of granular soil (particle size 2.36 mm) with 9.6 L of water, compacted to achieve 95% of the MDD since the recommended density of the standard pavement construction typically ranges from 90–95% of the MDD [ 40 ]. The load-settlement behavior of unpaved road layers over fibre-reinforced and lime-stabilized soft soil formations, with varying fibre contents and types, was investigated through a series of tests per BS 1377 Part 9:1990, IS 1888–1982, ASTM D1196 (2015), and AASHTO (1993) standards. An INSTRON press with an electronic loading actuator was employed to apply steel plate loads, ensuring a stable assembly. Circular plates were used to simulate wheel impressions on a malleable pavement. An equivalent tyre contact pressure of 550 kPa was determined for a twin wheel load of 10 kN using a 150 mm diameter circular plate, representing one-eighth of an 80 kN axle load - a widely used parameter in modern design standards such as AASHTO (1993). Two LVDTs were installed on the model test box to monitor horizontal pressure on the sidewalls and assess lateral deformation as shown in Fig. 7 . The LVDT measurements, which verified negligible lateral pressure, were not included in this report. Stress evaluation using the approximated pressure bulb stress equation confirmed that plate-induced stresses did not intersect the box sides, precluding lateral box buckling. The pressure bulb reached 187.5 mm (2.5 x 75 mm plate radius), while the box radius was 217.5 mm (half the diagonal of the rectangular box), leaving an excess of 30 mm unaffected by the loading exerted on the model layers. This justified the exclusion of the lateral pressure obtained. Settlement initiation was controlled by a computer-controlled loading actuator. Static plate loading tests were conducted on model road layers reinforced with strand and discrete fibres at 0.6%, 1.2%, 1.8%, 2.4%, and 3.0% content. Dynamic plate loading tests were performed on layers reinforced with 1.8% and 3.0% strand and discrete fibres. An initial dynamic plate loading test for various fibre types and contents determined the optimal fibre content for fibre-lime stabilization of the subgrade layer. Both reinforced and unreinforced subgrade layers were stabilized for 14 days with 8% of the optimal lime requirement before undergoing a dynamic plate loading test. When significant vehicle loads are anticipated on the unpaved road layer, load-bearing capacity failure is possible; thus, static plate load tests were undertaken to establish the bearing capacity and the settlement of the model road layer under static load testing at a 1.2 mm/min rate (ASTM D1196, 2015). The performance of fibre-reinforced and lime-stabilized layers under dynamic loads was also investigated. Scientists in the field and the lab have experimented with various loading increments, i.e., 4 kN, 128 kN, and 40 kN, to replicate real-world conditions [ 41 – 43 ].[ 44 ] looked at the possibility that increasing the machine’s load too quickly would damage the model road layers. In this study, a computer-controlled actuator applied dynamic loads to a 150 mm x 30 mm test plate. The loading protocol began with an initial 1 kN load, considered acceptable for the dynamic sinusoidal wave, and increased by 1 kN per cycle over eight iterations, culminating in an 8 kN increase. For statistical analysis, settlement levels were monitored at predefined intervals of 2.5 mm, 25 mm, and 75 mm. This methodology allowed for a comprehensive assessment of the pavement's performance under dynamic loading conditions, particularly highlighting the benefits of fibre reinforcement in improving pavement structural integrity. 3. Results and analysis 3.1 Effect of fibre content on California Bearing Ratio (CBR) Figure 8 illustrates the impact of strand and discrete fibre reinforcement on CBR under the soaked and unsoaked conditions for samples compacted at OMC and MDD. The CBR values under soaked conditions were tested to simulate poor drainage conditions in typical pavement [ 45 ]. As shown in Fig. 8a and b , a gradual increase in the CBR values for strand and discrete fibre-reinforced soil up to 1.8 % fbre content under soaked and unsoaked conditions was observed. Randomly distributed fibres are typically applied to improve the soil's engineering characteristics and properties, especially shear strength [ 17 ]. However, randomly distributed fibres act as a three-dimensional spatial network that interlocks soil grains, thereby improving the stretching resistance between soil particles and strength behaviour [ 45 , 46 ]. In this current study, further addition of fibre beyond 1.8% resulted in lower strength of the subgrade layer. [ 47 ] reported that the maximum CBR was observed at 0.75% glass fibre for any fibre length and testing condition. However, upon examining the effects of various fibre types and specifically assessing the impact of strand and discrete fibres for subgrade enhancement, it was determined that the optimal fibre content is 1.8% for both strand and discrete fibres under soaked and unsoaked conditions. At higher fibre content (2.4 % ad 3.0 %),the increase in fibre volume in a fixed volume of the soil composite is higher than the soil composite with lower fibre content. Due to the large fibre volume, fibre–fibre interaction exceeds soil-fibre interaction, thereby preventing a homogeneous mixture between the soil and the fibre. This consequently leads to a reduction in the mobilized strength of the compacted soil at higher fibre contents. Figure 8c illustrates the impact of fibre type and content on both unsoaked and soaked CBR. The incorporation and increase in strand fibre content demonstrated no effect on the ratio of unsoaked to soaked CBR, maintaining a constant value of 2.0 for fibre content up to 2.4%. However, for discrete fibres, the ratio increased from 2.0 to an average value of 2.8 at a fibre content of up to 1.8%, followed by an exponential increase with further increase in fibre content. This higher ratio for discrete fibres can be attributed to the mobilised interfacial resistance in the unsoaked state, influenced by the rough texture and length of the fibres. Remarkably, all fibre-reinforced samples at 1.8% content exceeded the minimum subgrade requirement (CBR ≥ 8%) for both soaked and unsoaked conditions when compared with the unreinforced soils with a CBR value of 9.68% and 5.78% for unsoaked and soaked conditions, indicating a poor to fair soil. This CBR value of the untreated compacted soils aligns with established relationships between CBR values and geotechnical quality for pavement applications (Bowles, 1992). At 1.8% fibre content, strand fibres achieved 19.13% and 12.53% of the required subgrade strength in unsoaked and soaked conditions, respectively, while discrete fibres attained 25.05% and 8.63% improvements in unsoaked and soaked conditions, respectively. However, these improvements obtained from strand fibre remain fair for standard specifications for CBR of backfills, but discrete fibre under unsoaked conditions shows a good performance, which is typically between 20–50% for subgrade applications. These findings provide valuable insights for unpaved road construction, particularly in regions experiencing varying moisture conditions, while contributing to the growing body of research on fibre-reinforced soil stabilization techniques. The CBR improvement ratio, the California bearing ratio index (CBRI), which is the ratio of the CBR value of the reinforced samples (CBRr) to the CBR value of the unreinforced samples (CBRu), is expressed as Eq. 3 . \(\:CBRI=\:\frac{{CBR}_{r}}{{CBR}_{u}}\) 3 The CBRI of the reinforced soil under the unsoaked condition increases with the fibre content, irrespective of the fibre type. However, at 1.8% fibre content for both strand and discrete fibre, the CBRI was found to have been improved by 61% and 49%, respectively. Discrete fibre performed better in unsoaked conditions, while strand fibre performed better in soaked conditions due to differential effects of length and texture. For a high fibre content of 3%, the CBRI for both fibre types was very low in the soaked condition. 3.2 Influence of fibre content on peak tensile strength Figure 9 shows the effect of fibre on peak indirect tensile strength. The tensile strength of the samples increased monotonically with the increase in fibre content to 2.4% fibre content for both strand and discrete fibre, beyond which the peak tensile strength decreased. The curves showed that including fibre at concentrations of 0.6–2.4% for strand fibre and 1.2–3.0% for discrete fibre resulted in higher peak tensile strength of the soil composite. It was reported that since fibres in a soil composite are constrained in their ability to move, they can share part of the tensile load, in part, accounting for the strength increase under tension [ 48 – 50 ]. Maximum tensile strength was mobilized at 1.8% strand and 2.4% discrete fibre. Interaction between fibre and soil particles, which may effectively prevent and postpone the development of the tensile failure plane and deformation of soils, may be responsible for the observed increase in tensile strength [ 51 , 52 ]. A decrease in tensile strength was observed when the fibre content increased beyond 1.8 % strad fibre and 2.4 % discete fibres. The fibre content at 0.6 % discete fibre resulted in limited interfacial interaction, which was insufficient to mobilise high tensile strength. It was reported that the tensile strength of soil increased due to the inclusion of up to 0.75% glass fibre content [ 53 ]. Furthermore, similar results were reported on the effect of corn silk fibre on cemented soil's tensile strength [ 48 ]. In this current research, the tensile strength was positively correlated with the fibre contents within a certain range, regardless of the fibre type. 3.3 Influence of fibre type and length on the peak tensile strength Fiber reinforcement enhanced the tensile strength and ductility characteristics of the soil composite, with performance variations observed between fiber types and lengths, as shown in Tables 2 and 3 . The unreinforced soil exhibited a strength of 23.65 kPa, while strand fiber (12 mm) reinforcement demonstrated enhanced performance compared to discrete fiber (40 mm) reinforcement, indicating that fibers can enhance soil failure's tensile strength and ductility [ 48 ]. At 0.6% fiber content, strand fibers increased tensile strength by 7.65%, attributed to enhanced interfacial shear strength and effective bridging mechanisms across tension failure planes [ 54 ]. Maximum tensile strength of 37.45 kPa was achieved at 1.8% strand fiber content, representing a 58% improvement over unreinforced soil. However, performance declined at higher fiber contents, with a minimum tensile strength of 13.2 kPa observed at 3.0% fiber content, indicating an optimal fiber dosage of around 1.8%. Discrete fiber (40 mm) reinforcement exhibited different characteristics, achieving a maximum tensile strength of 30.86 kPa at 2.4% fiber content, representing a 30.5% improvement. The performance trend showed that discrete fibers required higher contents to mobilize peak tensile strength, with a minimum tensile strength of 20.21 kPa occurring at 0.6% content. The enhanced tensile behavior results from interfacial shear strength development between fibers and soil matrix, combined with effective load transfer through fiber bridging mechanisms spanning tension failure planes. The performance of 12 mm strand fibers suggests optimal fiber length-to-diameter ratios for effective stress transfer, while the higher optimal content for 40 mm discrete fibers indicates different mobilization mechanisms governing reinforcement effectiveness. Table 2 Variation of tensile strength with fibre content for strand fibre Fibre content (%) 0% 0.6% 1.2% 1.8% 2.4% 3.0% Peak Indirect Tensile Strength (kPa) 23.65 25.46 32.65 37.45 31.61 13.12 Residual Indirect Tensile Strength (kPa) 0.00 21.76 29.69 22.53 25.22 8.86 Rate of post-peak degradation 1.00 0.45 0.10 0.09 0.14 0.16 Table 3 Variation of tensile strength with fibre content for discrete fibre Fibre content (%) 0.0% 0.6% 1.2% 1.8% 2.4% 3.0% Peak Indirect Tensile Strength (kPa) 23.65 20.21 24.00 30.12 30.86 30.68 Residual Indirect Tensile Strength (kPa) 0.00 11.06 21.67 27.51 26.47 25.70 Rate of post-peak degradation 1.00 0.15 0.09 0.40 0.20 0.32 3.4 Lateral deformation response under indirect tensile loading The lateral deformation characteristics of fiber-reinforced soil specimens under indirect tensile loading revealed distinct patterns attributed to fiber-matrix interaction mechanisms. From Table 4 , unreinforced soil exhibited lateral deformation of 0.050, while strand and discrete fiber reinforcement demonstrated progressive enhancement with increasing fiber content through stress redistribution mechanisms. For strand fibre, initial improvements of 60–62% were observed at 0.6–1.2% fiber content (0.080–0.081 mm), attributed to fiber-matrix interface friction creating lateral deformation constraints that trigger controlled stress concentrations. Further increases to 0.124 at 1.8–2.4% fiber content reflect enhanced plastic deformation zones around fiber interfaces. Maximum lateral deformation of 0.164 at 3.0% fiber content indicates optimal stress redistribution through cumulative fiber-matrix interactions. Discrete fiber reinforcement exhibited immediate and substantial improvements, with consistent lateral deformation of 0.123 (146% increase) across 0.6–1.8% fiber contents. This suggests more effective stress concentration through interface plastic sliding mechanisms. Progressive enhancement to 0.163 at 2.4% content and 0.164 at 3.0% content demonstrates controlled plastic strain concentration that prevents failure. In fiber-reinforced cemented soil systems, horizontally distributed fibers impose lateral deformation constraints on the adjacent soil matrix through interface friction-sliding mechanisms, creating stress concentrations around fiber-matrix boundaries that promote localized plastic deformation and interface sliding, ultimately resulting in plastic strain concentration due to reduced strain capacity at the interface [ 55 ]. The enhanced lateral deformation capacity demonstrates improved ductility through fiber-induced stress redistribution mechanisms critical for pavement applications where the gradual stress redistribution minimizes localized failure and extends pavement service life under repeated loading conditions. This shows that the fibers act as internal confinement elements that are distributed throughout the soil mass and thereby control and redistribute the deformation rather than simply preventing it from sudden deformation. Table 4 Variation of lateral deformation with fibre content for strand and discrete fibre Strand Fibre Discrete Fibre Fibre Content (%) Lateral Deformation Lateral Deformation 0 0.050 0.050 0.6 0.080 0.123 1.2 0.081 0.123 1.8 0.124 0.123 2.4 0.124 0.163 3 0.164 0.164 3.5 Influence of fibre reinforcement on the bearing capacity The bearing capacity of reinforced soil exhibited improvement under increased pressure compared to unreinforced conditions for the unpaved road. Single-layer reinforcement substantially improved the load-settlement characteristics of foundation systems [ 56 ]. Figure 10 illustrates that the disparity between unreinforced and fibre-reinforced models widened as surface pressure increased. At pressures below 320 kPa, all sections showed minimal settlement, with unreinforced sections unexpectedly performing better. However, beyond 320 kPa, fibre-reinforced sections demonstrated superior performance, except for 0.6% discrete and 3.0% strand fibre configurations, which showed improvement at 80mm and 75mm settlements, respectively. The static plate load tests revealed that polypropylene fibre addition at 0.5% enabled the reinforced soil to sustain 135 kN compared to 65 kN for unreinforced soil [ 57 ]. Fibre reinforcement generally enhances soil strength, stability, bearing capacity, and ductility while reducing settlement and lateral deformations [ 58 – 60 ]. The subgrade stiffness was evaluated using the modulus of subgrade reaction (k), defined as the pressure/settlement ratio over the settlement range of 25–75 mm. Figure 11 presents the stiffness characteristics for both strand and discrete fiber-reinforced pavement layers across varying fiber contents. The results demonstrate that maximum stiffness was achieved at 1.2% fiber content for strand fibers and 1.8% for discrete fibers. Stiffness values remained relatively constant up to 2.4% fiber content, beyond which a reduction in stiffness occurred for both fiber types. This performance trend indicates optimal fiber content of 1.2–2.4% for strand fibers and 1.8–2.4% for discrete fibers, with excessive fiber content affecting the stiffness characteristics of the reinforced pavement system. The stiffness behavior reflects the balance between effective fiber-soil interaction at moderate contents and potential interference effects at higher fiber contents, which may compromise load transfer mechanisms within the composite material. The bearing capacity improvement of fiber-reinforced pavement structures was quantified using the Bearing Capacity Ratio (BCR), defined as the ratio of reinforced soil bearing pressure (qr) to unreinforced soil bearing pressure (qu) at equivalent settlements (Eq. 3 ). Results presented in Table 5 and Fig. 12 demonstrate that fiber reinforcement significantly enhanced bearing capacity compared to unreinforced sections, establishing the quantitative effectiveness of fiber reinforcement in improving pavement load-bearing characteristics [ 17 ]. These findings support the potential application of fiber-reinforced soil as fill material for foundation and subgrade applications. $$\:BCR=\:\frac{qr}{qu}$$ 3 At 2.5mm settlement, strand fibre showed a better performance with BCR values of 0.6 and 0.9 at 0.6% and 1.2% content, respectively, while discrete fibre maintained a consistent BCR of 0.1 across all concentrations. The performance distinction became more pronounced at 25mm and 50mm settlements, where 0.6% and 3.0% strand fibre and 0.6% discrete fibre demonstrated enhanced BCR values. Most of the reinforced sections achieved BCR values exceeding 1.0 at 75mm settlement, except for 3.0% strand fibres and 0.6% discrete fibre. However, at 2.4% strand fibre, a maximum BCR of 2.2 at 25mm settlement was obtained, leading to an enhancement in subgrade load-bearing capacity compared to unreinforced sections. This occurred due to the ability of fibres to act like a three-dimensional spatial network that interlocks soil grains, thereby improving the stretching resistance between the soil particles [ 45 , 46 ]. Table 5 BCR values at different settlements for strand and discrete fibre BCR values for strand fibre BCR values for discrete fibre Settlement (mm) Settlement (mm) Fibre content (%) 2.5 25 50 70 2.5 25 50 70 0 0 0 0 0 0.0 0 0 0 0.6 0.6 0.9 1 1.1 0.1 0.8 0.9 1 1.2 0.9 1.1 1.2 1.3 0.1 1.1 1.2 1.5 1.8 0.5 1.3 1.5 1.6 0.1 1 1.1 1.3 2.4 0.2 2.2 2.1 2.1 0.1 1 1.1 1.3 3.0 0.2 0.8 0.9 1 0.1 1.2 1.3 1.4 3.6 Effect of cyclic loading on deformation Cyclic plate loading tests were conducted to evaluate the effects of varying fibre contents, types, and fibre-lime stabilization on unpaved pavement model sections as shown in Fig. 13 . The results demonstrate that 1.8% discrete fibre reinforcement and lime-stabilized soil at the subbase-subgrade interface reduced pavement settlement by approximately 250 loading cycles at 75mm settlement. Initially, unreinforced pavement exhibited marginally better performance compared to sections with 1.8% and 3.0% strand fibre and 3.0% discrete fibre up to 250 loading cycles. The settlement development followed two distinct phases for fibre-reinforced, lime-stabilized and fibre-lime-reinforced models. Phase 1 showed rapid settlement accumulation until reaching specific thresholds (75mm for lime-stabilized soil and 1.8% discrete fibre; 90mm for 3.0% strand fibre and discrete fibre-lime stabilized soil; 120mm for 1.8% strand fibre; 150mm for 3.0% strand fibre lime stabilized soil). Phase 2 demonstrated gradual settlement reduction, stabilizing through 400 loading cycles, consistent with findings by [ 61 ] using geogrid reinforcement. Load diffusion capacity improved with increased loading cycles, supporting previous research [ 26 , 62 – 64 ]. While higher fibre content showed limited advantages during initial loading cycles, attributed to insufficient particle-fibre interaction under low surface pressure, overall performance improved significantly beyond 300 loading cycles regardless of fibre parameters. For the geofibre sand mixture, 0.8% fibre content was shown to have provided adequate structural support for the designed pavement compared with 1.0% fibre content, thereby improving the subgrade layer's strength under military truck traffic [ 63 ]. However, the present study found that using fibre in large quantities offered no advantages during light loading cycles. Low surface pressure may play a role in this phenomenon because it causes soil particles to be less compacted, limiting the relative slide between fibres and soil particles. The impact of fibre reinforcing is not readily apparent since the fibres could not interact appropriately with the soil at this time. 4 Conclusions This research evaluated the influence of polypropylene fibre reinforcement on unpaved road layers, examining CBR, tensile strength properties, and load-bearing behavior through static and dynamic testing. Using a laboratory-scale model, the investigation assessed strength parameters and deformation characteristics of both reinforced and unreinforced pavement sections, analyzing CBR under varied moisture conditions, tensile resistance at different fibre contents, and resilient modulus response under cyclic loading. These comprehensive analyses provide quantitative data for optimized pavement design methodologies. The research yields several significant findings: Fibre-reinforced samples at 1.8% exceeded the minimum subgrade requirement (CBR ≥ 8%) for both soaked and unsoaked conditions. Maximum tensile strength was observed at different optimal contents: 37.45 kPa at 1.8% for strand fibres and 30.8 kPa at 2.4% for discrete fibres. Strand fibres exhibited higher performance compared to discrete fibres at equivalent percentages, attributed to their higher volumetric distribution and more effective load resistance orientation. Discrete fiber reinforcement exhibited immediate and substantial improvements to lateral deformation response under indirect tensile loading, with consistent lateral deformation of 0.123 (146% increase) across 0.6–1.8% fiber contents. Both fibre-reinforced and lime-stabilized pavement systems demonstrated marked performance improvements after 300 loading cycles, suggesting enhanced effectiveness under sustained loading conditions. Settlement under initial dynamic loading (0-250 cycles) reached approximately 75 mm for lime-stabilized soil with discrete fibre, 90 mm for combined strand-discrete fibre systems, 120 mm for strand fibre, and 150 mm for strand fibre-lime-stabilized soil. This study advances sustainable pavement through innovative fibre reinforcement and lime stabilization techniques. The research demonstrates the effectiveness of optimized fibre-soil composites in enhancing pavement performance while reducing material consumption and maintenance requirements. The documented synergistic effects between fibre types and lime stabilization present opportunities for developing more resilient and resource-efficient pavement systems. Future research directions should focus on long-term durability under diverse environmental conditions and the potential integration of hybrid fibre systems. Field-scale implementation studies are essential to validate these findings and develop practical guidelines for sustainable pavement infrastructure. This research establishes a foundation for next-generation pavement solutions that balance engineering performance with environmental sustainability. Declarations Acknowledgements The authors would like to acknowledge the generous financial support provided by the University of Johannesburg, South Africa, through the Fourth Industrial Revolution Scholarship (GES 4IR), which facilitated the completion of this research. Additionally, the authors extend their gratitude to Chryso Group, South Africa, for supplying the polypropylene fibres utilized for soil reinforcement. The authors express their appreciation to Afrisam, a cement-producing company in South Africa, for their donation of road construction lime, which was instrumental in conducting this research. The authors would like to extend their gratitude to Motloung Tebello for his valuable technical support in machining and assembling the experimental test setups utilized in this study. Author contributions Adesola Habeeb Adegoke: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Sinenkosi Nxumalo: Writing – review & editing. Okonta Felix: Writing – review & editing, Supervision, Resources, Investigation, Funding acquisition. Funding The author sincerely expresses their gratitude to the generous financial support provided by the University of Johannesburg, South Africa, through the Fourth Industrial Revolution Scholarship (GES 4IR), which facilitated the completion of this research. Data Availability This is an experimental study, and the data are present in the manuscript itself. Ethics approval and consent to participate Not applicable. Consent to publication Not applicable. 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Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 16 Jun, 2025 Reviewers agreed at journal 13 Jun, 2025 Reviewers invited by journal 13 Jun, 2025 Editor invited by journal 12 Jun, 2025 Editor assigned by journal 10 Jun, 2025 Submission checks completed at journal 10 Jun, 2025 First submitted to journal 07 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6845001","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":472163045,"identity":"eea8ffab-6af4-40b6-aedc-c779513307c1","order_by":0,"name":"Adesola Habeeb Adegoke","email":"","orcid":"","institution":"Arizona State University","correspondingAuthor":false,"prefix":"","firstName":"Adesola","middleName":"Habeeb","lastName":"Adegoke","suffix":""},{"id":472163046,"identity":"5b194045-3344-4811-82d7-7cf7bbb58485","order_by":1,"name":"Sinenkosi Nxumalo","email":"","orcid":"","institution":"University of Johannesburg","correspondingAuthor":false,"prefix":"","firstName":"Sinenkosi","middleName":"","lastName":"Nxumalo","suffix":""},{"id":472163048,"identity":"27a64f2b-cd23-4600-b185-4830f2a5ce9a","order_by":2,"name":"Okonta Felix","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyUlEQVRIiWNgGAWjYBAC9gYGZjCDn4EhgTgtPAfAWgwYJBtI1mJwgFiH8Ug3Hzb4uOOPvPGNhAcMP2oY7PkbCGmROZacOPOMgeG2GwkJjD3HGJglCFlnL5FjfJi3zYARpIWBt4GBjYGQFh6oFvvNM4C2/G1g4JEnRksyUEviBomEBGagLRIEw4FHIi3ZcGabcfKMMw8SDssckzAwJKwl+bDExzY52/72nMSHb2ps7OUIaUHWnQBULEG8eiBgJ8H4UTAKRsEoGFEAABqFO2Bex6ZiAAAAAElFTkSuQmCC","orcid":"","institution":"University of Johannesburg","correspondingAuthor":true,"prefix":"","firstName":"Okonta","middleName":"","lastName":"Felix","suffix":""}],"badges":[],"createdAt":"2025-06-08 01:38:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6845001/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6845001/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":84845214,"identity":"b8835545-d50d-424c-92df-7e27ebf5ee85","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":19622,"visible":true,"origin":"","legend":"\u003cp\u003eThe grading curve of the soil\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/737c5862deebb296ced27ba0.png"},{"id":84845215,"identity":"b6dd275b-576e-4e57-b92c-f38215a0b92f","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":279686,"visible":true,"origin":"","legend":"\u003cp\u003ePolypropylene fibre (a) discrete fibre of 40 mm long, (b) strand fibre of 12 mm long, and (c) wool-like appearance of 12mm strand fibre upon remoulding.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/f272eac095b6e43f2ad0f573.png"},{"id":84845222,"identity":"553ffb1c-e562-4f29-812c-f2625e999487","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":286456,"visible":true,"origin":"","legend":"\u003cp\u003ePreparation stages of soil samples for ITS (a) Strand fibre-reinforced soil samples and (b) Discrete fibre-reinforced soil samples\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/b6989a14b9f007609714ab7f.png"},{"id":84845748,"identity":"2f21a38f-2d82-45ec-9744-7c8e9d073014","added_by":"auto","created_at":"2025-06-18 03:04:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":463035,"visible":true,"origin":"","legend":"\u003cp\u003eITS test setup for fibre-reinforced soil samples. (a) Specimen positioned in the testing apparatus before load application. (b) Specimen after completion of the ITS test, demonstrating typical failure mechanisms.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/4988b7d7e3fcf0ed74e73306.png"},{"id":84845744,"identity":"36e4b5b7-9669-4ff2-81ee-c2cfcfbfb8b1","added_by":"auto","created_at":"2025-06-18 03:04:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":57830,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 6.\u003c/strong\u003e Schematic representation of the unpaved road model under static and dynamic loading tests\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/ae488064d6b90fd8c9edc33a.png"},{"id":84846577,"identity":"10a4bce7-93d2-4ca7-a3bf-a681cfdd122c","added_by":"auto","created_at":"2025-06-18 03:12:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":254687,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 7.\u003c/strong\u003e Model test box and load actuator for plate loading test\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/ffd7ec512308c8330e77968d.png"},{"id":84846576,"identity":"e3023b8a-d4de-4e74-8217-3d5dbcfe23fb","added_by":"auto","created_at":"2025-06-18 03:12:10","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":21702,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 8: Effects of fibre on CBR variation for strand and discrete fibre under (a) soaked condition and (b) unsoaked condition. (c): Ratio of CBRomc/CBRsoaked (ICBR) for different fibre type and content\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/f1c47b979ff85f69d1333716.png"},{"id":84845219,"identity":"66948695-feba-4a70-a901-af58d3c8b2bd","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":18337,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 9. Variation of peak tensile strength with different fibre content and fibre type\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/716030e37fa0178c1f2191ae.png"},{"id":84845216,"identity":"8ca9387d-4158-49b4-8ad2-400da9cc4adc","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":30235,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 10. Applied pressure–settlement curves for model plate load tests with (a) strand fibre and (b) discrete fibre\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/cd8508bdbc608ededa9d82eb.png"},{"id":84845226,"identity":"465f9258-32eb-47eb-bd52-93635f3286e7","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":10467,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 11: Pressure /Settlement ratio for settlement margin from 25mm to 75mm for strand and discrete fibre-reinforced pavement layer\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/5e326316ae9d5ed942dc1154.png"},{"id":84845223,"identity":"5e79ff75-9f27-4ee3-ab51-3aeeb1a117bc","added_by":"auto","created_at":"2025-06-18 02:56:10","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":34504,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 12.\u003c/strong\u003eRelationship between the BCR and settlement for (a) Strand fibre and (b) discrete fibre\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/cbace2d81ccb5aa6e104bef2.png"},{"id":84846578,"identity":"1dd68268-17b0-4b7c-ab9a-28c21529c911","added_by":"auto","created_at":"2025-06-18 03:12:10","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":25751,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 13: Variation of settlement to loading cycles for (a) fibre-reinforced and unreinforced pavement model, (b) fibre-reinforced, lime-stabilised, and unreinforced pavement models.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/a35e3f5e1b4f71e7c4ecdeba.png"},{"id":84846895,"identity":"b0d6a2fe-c885-4812-81c4-1d003b05714d","added_by":"auto","created_at":"2025-06-18 03:20:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2945844,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6845001/v1/a83bad45-b1ce-46fa-9e30-ae942dafd7cf.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe construction of roads and other earthwork-based infrastructure, like dams, industrial plants, and factories, demands substantial quantities of stone aggregates and conventional binders, which are derived from limited and depleting natural resources. To align with sustainable development goals, it is crucial to prioritize the use of locally obtainable aggregates and explore the potential of waste.\u003c/p\u003e \u003cp\u003eA wide range of waste materials, including quarry stones, coal ash, recycled aggregate, plastics, fibres, and polythene bags, can be repurposed to promote natural resource sustainability in road construction. To meet stringent project specifications and enhance the performance of subgrade soils and fill application of compacted residual soil, reinforcement techniques are frequently employed in the construction of high-strength roadbeds [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. These measures are particularly crucial for filling out applications where load-bearing capacity and long-term stability are essential. The implementation of such reinforcement strategies not only ensures compliance with project requirements but also contributes to the overall durability and resilience of the road infrastructure.\u003c/p\u003e \u003cp\u003eIn recent years, interlayer systems have gained significant attention among pavement engineers due to their potential to provide efficient reinforcement solutions. These systems have demonstrated effectiveness in inhibiting crack initiation and propagation within pavement structures [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, the mechanical performance of these interlayer systems has not yet been comprehensively evaluated, leaving gaps in our understanding of their full potential and limitations. Furthermore, there is ongoing debate within the pavement engineering community regarding the most suitable types of interlayer systems and their optimal placement within pavement structures [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The vast array of commercially available products for interlayer systems has necessitated efforts to categorize this variety into a manageable number of groups, facilitating more structured research and application [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. This evolving field presents both opportunities and challenges. While interlayer systems offer promising solutions for pavement reinforcement, more research is needed to fully understand their performance, optimize their use, and integrate them effectively into sustainable road construction practices.\u003c/p\u003e \u003cp\u003eOver the past two decades, extensive research has been conducted on interlayer systems in pavement construction. These studies have explored various layer structures, including glass fibre lattices, polypropylene paving grids [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], non-woven geotextiles, geogrids [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], and steel meshes [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. These materials have been employed to reinforce both bound and unbound pavement layers, demonstrating their versatility in pavement engineering. Fibre reinforcement of soil has emerged as an effective method for mechanical soil stabilization. This technique involves mixing soil with loose, non-reactive, tiny fibres [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], resulting in a fibre-soil composite with enhanced strength and more elastic stress-strain behaviour [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. This improvement is attributed to the synergistic effect of binders and fibres on the soil's stress-strain performance, transforming the inelastic behaviour of the soil into a more ductile one. The inclusion of fibres has been shown to augment the soil's geotechnical properties under both constant and dynamic states, particularly in reinforcing sub-grade roads. Fibres enhance the soil's energy absorption and dissipation capabilities, increasing the damping ratio and resilient modulus [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. This discrete fibre reinforcement technique has proven to be a novel and reliable method for improving soil mechanical behaviour [\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCyclic wetting and drying cycles significantly compromise bridge, highway, and foundation stability for critical infrastructure in the Loess Plateau region [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The implementation of fibre reinforcement technology presents an effective engineering solution to enhance soil stability and structural integrity under these challenging environmental conditions [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Both natural and synthetic fibres can be used to control soil dehydration cracking, preserving soil integrity for various civil engineering applications and there is effectiveness in their application for the improvement of subgrade soil [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The addition of fibres significantly reduces the shrinkage strain and the crack growth rate [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] and they are often adopted to improve the mechanical properties of red clay that is used in road subgrade, embankment, or reinforced walls [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Reinforcement fibres can be sourced from various materials, including nylon, polyester, paper, and metal, each with distinct physical properties [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThere is a growing emphasis on using natural fibres, as non-biodegradable substances like artificial polymers pose critical disposal problems. Soil strengthening using relatively low-modulus fibres has been practiced for years, particularly in developing countries [\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Organic fibres such as cotton, coconut, jute, palm, sisal, barley straw, and bamboo have shown promise in enhancing soil properties. These fibres are often locally available, cost-effective, biodegradable, environmentally friendly, and can form composites with cement or lime [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. However, due to the abundance of waste plastics in emerging economies, the use of synthetic fibres and geogrids in pavement layers has gained general acceptance. This approach not only addresses the issue of plastic waste management but also contributes to the enhancement of pavement performance.\u003c/p\u003e \u003cp\u003eLime stabilization has been extensively studied in geotechnical engineering, with research focusing on lime consumption, stiffness, physicochemical responses, and strength properties for infrastructure applications like railroad embankments and highways [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The physical-chemical processes induced by lime treatment, including cation exchanges, hydration, and pozzolanic reactions, significantly enhance soil mechanical behavior and workability [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. While fibre-reinforced soil performs better than unreinforced soil, its enhancement is constrained by the constituent fibres' inherent properties [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Although some researchers have investigated fibre reinforcement through plate loading tests on pavement sections, either alone or combined with additives [\u003cspan additionalcitationids=\"CR38\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. However, there is a notable scarcity of documented research on the performance of fibre and fibre-lime-reinforced subgrade layers in laboratory unpaved model road layers.\u003c/p\u003e \u003cp\u003eCurrent understanding of integrated fibre-soil-lime systems in unpaved roads remains limited, particularly regarding their tensile behavior and load-bearing characteristics under complex loading conditions. While small-scale laboratory studies inadequately represent field performance, this research investigates the mechanical behavior of fibre-reinforced and fibre-lime stabilized unpaved road layers through comprehensive large-scale testing by examining the strength parameters and deformation characteristics, bridging the gap between laboratory investigations and field applications.\u003c/p\u003e"},{"header":"2. Materials And Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Soil characteristics and preparation\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe soil utilized in this study was engineered by mixing various particle sizes obtained through dry sieve analysis, a technique employed to minimize particle size variability and its potential impact on test results. The granular and fine soil components are residual products derived from the weathering of the Park Town Shale of the Witwatersrand Supergroup, which underlies significant portions of central and southern Johannesburg, South Africa. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the grain size distribution curve, while Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the soil properties.\u003c/p\u003e \u003cp\u003eThe soil composition was as follows: 13.13% gravel, 83.49% sand, 3.09% silt, and 3.29% clay. Specific gravity was determined to be 2.85, per ASTM standards. The Atterberg limits tests, conducted per ASTM guidelines, yielded a liquid limit of 41.54% and a plastic limit of 34.44%. The average particle size (D50) was less than 0.08 mm. Based on the Unified Soil Classification System (USCS), this soil was classified as well-graded sand with clay (SW-SC). Modified compaction tests, performed according to ASTM standards, resulted in a maximum dry unit weight of 16.70 kN/m\u0026sup3; and an optimum moisture content of 12%. To maintain consistent moisture content, the prepared soil was stored in drums and covered with polyethylene sheets to prevent moisture ingress.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSoil properties\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecific Gravity\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.85\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInternal angle of friction\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCohesion\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.94\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eCompaction test\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaximum dry unit weight (kN/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.70\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eConsistency limits\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOptimum moisture content (%), subgrade\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLiquid Limit (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e41.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlastic Limit (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e34.44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlasticity Index\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUSCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSW-SC\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWell-graded sand with clay\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Fibre Reinforcement and Lime\u003c/h2\u003e \u003cp\u003eA South African company supplied the lime used for soil layer modification. The polypropylene fibres used for pavement layer reinforcement were supplied by the Chryso Group, South Africa. Two types of fibres were employed and both fibre types have been engineered to exhibit enhanced impact resistance, ductility, robust abrasion resistance, and satisfactory energy absorption capabilities as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eStrand fibres: Pure white in color, soft, and woolly when separated, with a standard length of 12 mm. These fibres have a tensile strength of 240 MPa, a relative density of 0.75, and moisture absorption of 00-0.03%.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eDiscrete fibres: Silver-white in color, thick, and mat-like in appearance, with a standard length of 40 mm. These fibres have a tensile strength of 350 MPa, a relative density of 0.91, and moisture absorption of 00-0.02%.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 California bearing ratio test\u003c/h2\u003e \u003cp\u003eThe CBR test, a widely recognized penetration test, was employed to evaluate the stability of pavement subgrades. This test serves as a fundamental method for measuring and calculating the characteristics of pavement models and their constituent layers, utilizing empirical curves to interpret results. Tests were conducted on both untreated and fibre-treated soil samples under wet and dry conditions, adhering to the ASTM D1883-21 standard. The results were reported as percentages relative to the reference standard material.\u003c/p\u003e \u003cp\u003eLoad readings were recorded at penetration depths of 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, and 7.0 mm. A graph of dial reading against penetration was generated from these measurements. Loads at 2.54 mm penetration were determined from the resultant curve. These loads were compared with those of the standard material at the same penetration depth. The CBR values for both reinforced and unreinforced soils were evaluated using standard procedures, as outlined in Equations \u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. These equations provide a standardized method for calculating the CBR, ensuring consistency and comparability of results across different soil samples and treatment conditions.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:Penetration\\:load,\\:piston\\:stress\\:\\left(MPa\\right)=\\:\\frac{Load}{Piston\\:cross-sectiona\\:Area}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:CBR\\:value=\\frac{penetration\\:load}{Standard\\:load}\\:x\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Indirect tensile strength (ITS) test\u003c/h2\u003e \u003cp\u003eThe Indirect Tensile Strength (ITS) test was conducted in triplicate following ASTM D 6931-17 standards to evaluate the tensile properties of fibre-reinforced soil samples. Cylindrical specimens (120 mm diameter, 60 mm thickness, aspect ratio 2) were prepared with varying fibre contents (0%, 0.6%, 1.2%, 1.8%, 2.4%, and 3.0%) using both strand and discrete fibres. Samples were mixed at 13% moisture content and compacted to 95% of Maximum Dry Density in five layers, each receiving 55 blows to ensure uniform density. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e provides a visual representation of the compacted soil samples, offering insight into their physical characteristics.\u003c/p\u003e \u003cp\u003eITS tests were performed at a constant displacement rate of 1 mm/min, with lateral displacement measured using two digital dial gauges positioned 15 mm from the specimen center on either side, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. This setup enabled precise measurement of sample deformation during loading. This systematic approach to sample preparation and testing methodology, including the use of both unreinforced control and fibre-reinforced samples, ensures data reliability and reproducibility. The rigorous experimental design provides a solid foundation for analyzing the impact of different fibre types and contents on soil tensile strength, contributing valuable insights to geotechnical engineering research on soil reinforcement techniques.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Plate loading test\u003c/h2\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.5.1 Testing apparatus\u003c/h2\u003e \u003cp\u003eThe steel-reinforced test box was built in the Kingsway Campus geotechnical laboratory of the University of Johannesburg. The dimensions are 460 mm, width: 410 mm, height: 1000 mm, and steel thickness of 20 mm. Drill bits were used to connect the steel joints. The wooden box was also reinforced on the sides to increase strength and prevent lateral movement, as shown in \u003cb\u003eFig.\u0026nbsp;5\u003c/b\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.5.2 Experimental setup\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe experimental setup comprised a plastic-lined test box containing a six-layer pavement structure. The subgrade consisted of five 500 mm-thick sublayers and a 150 mm reinforcement layer, all compacted using a hydraulic compactor. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the schematic representation of the unpaved road model under static and dynamic loading tests. The subgrade mixture contained 24.4 kg of soil (2.36 mm sieve) and 6.2 kg of soil (0.425 mm sieve), combined with 3.9 L of water to achieve 13% moisture content.\u003c/p\u003e \u003cp\u003eThe reinforcement layer incorporated polypropylene fibres at concentrations ranging from 0.6\u0026ndash;3.0% (275.1-1,375 g), mixed with 46.05 kg of soil and 5.96 L of water. While unreinforced mixtures were placed in single lifts, reinforced mixtures required two-lift placement to ensure uniform fibre distribution. The base layer comprised two compacted aggregate subbase layers, each 100 mm thick, prepared at optimal moisture content and maximum dry density (MDD). This configuration yielded a consistent total base thickness of 200 mm across all reinforced and unreinforced pavement models. While the base layer arrangement mirrored the subgrade layer, it lacked reinforcement. The subbase material was prepared by combining 60.16 kg of granular soil (particle size 2.36 mm) with 9.6 L of water, compacted to achieve 95% of the MDD since the recommended density of the standard pavement construction typically ranges from 90\u0026ndash;95% of the MDD [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe load-settlement behavior of unpaved road layers over fibre-reinforced and lime-stabilized soft soil formations, with varying fibre contents and types, was investigated through a series of tests per BS 1377 Part 9:1990, IS 1888\u0026ndash;1982, ASTM D1196 (2015), and AASHTO (1993) standards. An INSTRON press with an electronic loading actuator was employed to apply steel plate loads, ensuring a stable assembly. Circular plates were used to simulate wheel impressions on a malleable pavement. An equivalent tyre contact pressure of 550 kPa was determined for a twin wheel load of 10 kN using a 150 mm diameter circular plate, representing one-eighth of an 80 kN axle load - a widely used parameter in modern design standards such as AASHTO (1993).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003eTwo LVDTs were installed on the model test box to monitor horizontal pressure on the sidewalls and assess lateral deformation as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The LVDT measurements, which verified negligible lateral pressure, were not included in this report. Stress evaluation using the approximated pressure bulb stress equation confirmed that plate-induced stresses did not intersect the box sides, precluding lateral box buckling. The pressure bulb reached 187.5 mm (2.5 x 75 mm plate radius), while the box radius was 217.5 mm (half the diagonal of the rectangular box), leaving an excess of 30 mm unaffected by the loading exerted on the model layers. This justified the exclusion of the lateral pressure obtained.\u003c/p\u003e \u003cp\u003eSettlement initiation was controlled by a computer-controlled loading actuator. Static plate loading tests were conducted on model road layers reinforced with strand and discrete fibres at 0.6%, 1.2%, 1.8%, 2.4%, and 3.0% content. Dynamic plate loading tests were performed on layers reinforced with 1.8% and 3.0% strand and discrete fibres. An initial dynamic plate loading test for various fibre types and contents determined the optimal fibre content for fibre-lime stabilization of the subgrade layer. Both reinforced and unreinforced subgrade layers were stabilized for 14 days with 8% of the optimal lime requirement before undergoing a dynamic plate loading test.\u003c/p\u003e \u003cp\u003eWhen significant vehicle loads are anticipated on the unpaved road layer, load-bearing capacity failure is possible; thus, static plate load tests were undertaken to establish the bearing capacity and the settlement of the model road layer under static load testing at a 1.2 mm/min rate (ASTM D1196, 2015). The performance of fibre-reinforced and lime-stabilized layers under dynamic loads was also investigated. Scientists in the field and the lab have experimented with various loading increments, i.e., 4 kN, 128 kN, and 40 kN, to replicate real-world conditions [\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e] looked at the possibility that increasing the machine\u0026rsquo;s load too quickly would damage the model road layers.\u003c/p\u003e \u003cp\u003eIn this study, a computer-controlled actuator applied dynamic loads to a 150 mm x 30 mm test plate. The loading protocol began with an initial 1 kN load, considered acceptable for the dynamic sinusoidal wave, and increased by 1 kN per cycle over eight iterations, culminating in an 8 kN increase. For statistical analysis, settlement levels were monitored at predefined intervals of 2.5 mm, 25 mm, and 75 mm. This methodology allowed for a comprehensive assessment of the pavement's performance under dynamic loading conditions, particularly highlighting the benefits of fibre reinforcement in improving pavement structural integrity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results and analysis","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Effect of fibre content on California Bearing Ratio (CBR)\u003c/h2\u003e \u003cp\u003e \u003cb\u003eFigure 8\u003c/b\u003e illustrates the impact of strand and discrete fibre reinforcement on CBR under the soaked and unsoaked conditions for samples compacted at OMC and MDD. The CBR values under soaked conditions were tested to simulate poor drainage conditions in typical pavement [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. As shown in \u003cb\u003eFig.\u0026nbsp;8a and b\u003c/b\u003e, a gradual increase in the CBR values for strand and discrete fibre-reinforced soil up to 1.8 % fbre content under soaked and unsoaked conditions was observed. Randomly distributed fibres are typically applied to improve the soil's engineering characteristics and properties, especially shear strength [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. However, randomly distributed fibres act as a three-dimensional spatial network that interlocks soil grains, thereby improving the stretching resistance between soil particles and strength behaviour [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this current study, further addition of fibre beyond 1.8% resulted in lower strength of the subgrade layer. [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] reported that the maximum CBR was observed at 0.75% glass fibre for any fibre length and testing condition. However, upon examining the effects of various fibre types and specifically assessing the impact of strand and discrete fibres for subgrade enhancement, it was determined that the optimal fibre content is 1.8% for both strand and discrete fibres under soaked and unsoaked conditions. At higher fibre content (2.4 % ad 3.0 %),the increase in fibre volume in a fixed volume of the soil composite is higher than the soil composite with lower fibre content. Due to the large fibre volume, fibre\u0026ndash;fibre interaction exceeds soil-fibre interaction, thereby preventing a homogeneous mixture between the soil and the fibre. This consequently leads to a reduction in the mobilized strength of the compacted soil at higher fibre contents.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure 8c\u003c/b\u003e illustrates the impact of fibre type and content on both unsoaked and soaked CBR. The incorporation and increase in strand fibre content demonstrated no effect on the ratio of unsoaked to soaked CBR, maintaining a constant value of 2.0 for fibre content up to 2.4%. However, for discrete fibres, the ratio increased from 2.0 to an average value of 2.8 at a fibre content of up to 1.8%, followed by an exponential increase with further increase in fibre content. This higher ratio for discrete fibres can be attributed to the mobilised interfacial resistance in the unsoaked state, influenced by the rough texture and length of the fibres.\u003c/p\u003e \u003cp\u003eRemarkably, all fibre-reinforced samples at 1.8% content exceeded the minimum subgrade requirement (CBR\u0026thinsp;\u0026ge;\u0026thinsp;8%) for both soaked and unsoaked conditions when compared with the unreinforced soils with a CBR value of 9.68% and 5.78% for unsoaked and soaked conditions, indicating a poor to fair soil. This CBR value of the untreated compacted soils aligns with established relationships between CBR values and geotechnical quality for pavement applications (Bowles, 1992).\u003c/p\u003e \u003cp\u003eAt 1.8% fibre content, strand fibres achieved 19.13% and 12.53% of the required subgrade strength in unsoaked and soaked conditions, respectively, while discrete fibres attained 25.05% and 8.63% improvements in unsoaked and soaked conditions, respectively. However, these improvements obtained from strand fibre remain fair for standard specifications for CBR of backfills, but discrete fibre under unsoaked conditions shows a good performance, which is typically between 20\u0026ndash;50% for subgrade applications. These findings provide valuable insights for unpaved road construction, particularly in regions experiencing varying moisture conditions, while contributing to the growing body of research on fibre-reinforced soil stabilization techniques.\u003c/p\u003e \u003cp\u003eThe CBR improvement ratio, the California bearing ratio index (CBRI), which is the ratio of the CBR value of the reinforced samples (CBRr) to the CBR value of the unreinforced samples (CBRu), is expressed as Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:CBRI=\\:\\frac{{CBR}_{r}}{{CBR}_{u}}\\)\u003c/span\u003e \u003c/span\u003e 3\u003c/p\u003e \u003cp\u003eThe CBRI of the reinforced soil under the unsoaked condition increases with the fibre content, irrespective of the fibre type. However, at 1.8% fibre content for both strand and discrete fibre, the CBRI was found to have been improved by 61% and 49%, respectively. Discrete fibre performed better in unsoaked conditions, while strand fibre performed better in soaked conditions due to differential effects of length and texture. For a high fibre content of 3%, the CBRI for both fibre types was very low in the soaked condition.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Influence of fibre content on peak tensile strength\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the effect of fibre on peak indirect tensile strength. The tensile strength of the samples increased monotonically with the increase in fibre content to 2.4% fibre content for both strand and discrete fibre, beyond which the peak tensile strength decreased. The curves showed that including fibre at concentrations of 0.6\u0026ndash;2.4% for strand fibre and 1.2\u0026ndash;3.0% for discrete fibre resulted in higher peak tensile strength of the soil composite. It was reported that since fibres in a soil composite are constrained in their ability to move, they can share part of the tensile load, in part, accounting for the strength increase under tension [\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMaximum tensile strength was mobilized at 1.8% strand and 2.4% discrete fibre. Interaction between fibre and soil particles, which may effectively prevent and postpone the development of the tensile failure plane and deformation of soils, may be responsible for the observed increase in tensile strength [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. A decrease in tensile strength was observed when the fibre content increased beyond 1.8 % strad fibre and 2.4 % discete fibres. The fibre content at 0.6 % discete fibre resulted in limited interfacial interaction, which was insufficient to mobilise high tensile strength.\u003c/p\u003e \u003cp\u003eIt was reported that the tensile strength of soil increased due to the inclusion of up to 0.75% glass fibre content [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Furthermore, similar results were reported on the effect of corn silk fibre on cemented soil's tensile strength [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. In this current research, the tensile strength was positively correlated with the fibre contents within a certain range, regardless of the fibre type.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Influence of fibre type and length on the peak tensile strength\u003c/h2\u003e \u003cp\u003eFiber reinforcement enhanced the tensile strength and ductility characteristics of the soil composite, with performance variations observed between fiber types and lengths, as shown in Tables\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The unreinforced soil exhibited a strength of 23.65 kPa, while strand fiber (12 mm) reinforcement demonstrated enhanced performance compared to discrete fiber (40 mm) reinforcement, indicating that fibers can enhance soil failure's tensile strength and ductility [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. At 0.6% fiber content, strand fibers increased tensile strength by 7.65%, attributed to enhanced interfacial shear strength and effective bridging mechanisms across tension failure planes [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. Maximum tensile strength of 37.45 kPa was achieved at 1.8% strand fiber content, representing a 58% improvement over unreinforced soil. However, performance declined at higher fiber contents, with a minimum tensile strength of 13.2 kPa observed at 3.0% fiber content, indicating an optimal fiber dosage of around 1.8%.\u003c/p\u003e \u003cp\u003eDiscrete fiber (40 mm) reinforcement exhibited different characteristics, achieving a maximum tensile strength of 30.86 kPa at 2.4% fiber content, representing a 30.5% improvement. The performance trend showed that discrete fibers required higher contents to mobilize peak tensile strength, with a minimum tensile strength of 20.21 kPa occurring at 0.6% content. The enhanced tensile behavior results from interfacial shear strength development between fibers and soil matrix, combined with effective load transfer through fiber bridging mechanisms spanning tension failure planes. The performance of 12 mm strand fibers suggests optimal fiber length-to-diameter ratios for effective stress transfer, while the higher optimal content for 40 mm discrete fibers indicates different mobilization mechanisms governing reinforcement effectiveness.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eVariation of tensile strength with fibre content for strand fibre\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e \u003cp\u003eFibre content (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.2%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.8%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.4%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e3.0%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePeak Indirect Tensile Strength (kPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e23.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e25.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e32.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e31.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e13.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eResidual Indirect Tensile Strength (kPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e21.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e29.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e22.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8.86\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRate of post-peak degradation\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eVariation of tensile strength with fibre content for discrete fibre\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e \u003cp\u003eFibre content (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.2%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.8%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.4%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e3.0%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePeak Indirect Tensile Strength (kPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e23.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e20.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e30.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e30.68\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eResidual Indirect Tensile Strength (kPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e11.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e21.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e27.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e26.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e25.70\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRate of post-peak degradation\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Lateral deformation response under indirect tensile loading\u003c/h2\u003e \u003cp\u003eThe lateral deformation characteristics of fiber-reinforced soil specimens under indirect tensile loading revealed distinct patterns attributed to fiber-matrix interaction mechanisms. From Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, unreinforced soil exhibited lateral deformation of 0.050, while strand and discrete fiber reinforcement demonstrated progressive enhancement with increasing fiber content through stress redistribution mechanisms. For strand fibre, initial improvements of 60\u0026ndash;62% were observed at 0.6\u0026ndash;1.2% fiber content (0.080\u0026ndash;0.081 mm), attributed to fiber-matrix interface friction creating lateral deformation constraints that trigger controlled stress concentrations. Further increases to 0.124 at 1.8\u0026ndash;2.4% fiber content reflect enhanced plastic deformation zones around fiber interfaces. Maximum lateral deformation of 0.164 at 3.0% fiber content indicates optimal stress redistribution through cumulative fiber-matrix interactions.\u003c/p\u003e \u003cp\u003eDiscrete fiber reinforcement exhibited immediate and substantial improvements, with consistent lateral deformation of 0.123 (146% increase) across 0.6\u0026ndash;1.8% fiber contents. This suggests more effective stress concentration through interface plastic sliding mechanisms. Progressive enhancement to 0.163 at 2.4% content and 0.164 at 3.0% content demonstrates controlled plastic strain concentration that prevents failure. In fiber-reinforced cemented soil systems, horizontally distributed fibers impose lateral deformation constraints on the adjacent soil matrix through interface friction-sliding mechanisms, creating stress concentrations around fiber-matrix boundaries that promote localized plastic deformation and interface sliding, ultimately resulting in plastic strain concentration due to reduced strain capacity at the interface [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe enhanced lateral deformation capacity demonstrates improved ductility through fiber-induced stress redistribution mechanisms critical for pavement applications where the gradual stress redistribution minimizes localized failure and extends pavement service life under repeated loading conditions. This shows that the fibers act as internal confinement elements that are distributed throughout the soil mass and thereby control and redistribute the deformation rather than simply preventing it from sudden deformation.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eVariation of lateral deformation with fibre content for strand and discrete fibre\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eStrand Fibre\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete Fibre\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFibre Content (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLateral Deformation\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLateral Deformation\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.050\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.050\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.080\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.123\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.081\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.123\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.124\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.123\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.124\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.163\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.164\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.164\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Influence of fibre reinforcement on the bearing capacity\u003c/h2\u003e \u003cp\u003eThe bearing capacity of reinforced soil exhibited improvement under increased pressure compared to unreinforced conditions for the unpaved road. Single-layer reinforcement substantially improved the load-settlement characteristics of foundation systems [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Figure\u0026nbsp;10 illustrates that the disparity between unreinforced and fibre-reinforced models widened as surface pressure increased. At pressures below 320 kPa, all sections showed minimal settlement, with unreinforced sections unexpectedly performing better.\u003c/p\u003e \u003cp\u003eHowever, beyond 320 kPa, fibre-reinforced sections demonstrated superior performance, except for 0.6% discrete and 3.0% strand fibre configurations, which showed improvement at 80mm and 75mm settlements, respectively. The static plate load tests revealed that polypropylene fibre addition at 0.5% enabled the reinforced soil to sustain 135 kN compared to 65 kN for unreinforced soil [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Fibre reinforcement generally enhances soil strength, stability, bearing capacity, and ductility while reducing settlement and lateral deformations [\u003cspan additionalcitationids=\"CR59\" citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe subgrade stiffness was evaluated using the modulus of subgrade reaction (k), defined as the pressure/settlement ratio over the settlement range of 25\u0026ndash;75 mm. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e11\u003c/span\u003e presents the stiffness characteristics for both strand and discrete fiber-reinforced pavement layers across varying fiber contents.\u003c/p\u003e \u003cp\u003eThe results demonstrate that maximum stiffness was achieved at 1.2% fiber content for strand fibers and 1.8% for discrete fibers. Stiffness values remained relatively constant up to 2.4% fiber content, beyond which a reduction in stiffness occurred for both fiber types. This performance trend indicates optimal fiber content of 1.2\u0026ndash;2.4% for strand fibers and 1.8\u0026ndash;2.4% for discrete fibers, with excessive fiber content affecting the stiffness characteristics of the reinforced pavement system. The stiffness behavior reflects the balance between effective fiber-soil interaction at moderate contents and potential interference effects at higher fiber contents, which may compromise load transfer mechanisms within the composite material.\u003c/p\u003e\u003cp\u003eThe bearing capacity improvement of fiber-reinforced pavement structures was quantified using the Bearing Capacity Ratio (BCR), defined as the ratio of reinforced soil bearing pressure (qr) to unreinforced soil bearing pressure (qu) at equivalent settlements (Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Results presented in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cb\u003eFig.\u0026nbsp;12\u003c/b\u003e demonstrate that fiber reinforcement significantly enhanced bearing capacity compared to unreinforced sections, establishing the quantitative effectiveness of fiber reinforcement in improving pavement load-bearing characteristics [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. These findings support the potential application of fiber-reinforced soil as fill material for foundation and subgrade applications.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:BCR=\\:\\frac{qr}{qu}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAt 2.5mm settlement, strand fibre showed a better performance with BCR values of 0.6 and 0.9 at 0.6% and 1.2% content, respectively, while discrete fibre maintained a consistent BCR of 0.1 across all concentrations. The performance distinction became more pronounced at 25mm and 50mm settlements, where 0.6% and 3.0% strand fibre and 0.6% discrete fibre demonstrated enhanced BCR values.\u003c/p\u003e \u003cp\u003eMost of the reinforced sections achieved BCR values exceeding 1.0 at 75mm settlement, except for 3.0% strand fibres and 0.6% discrete fibre. However, at 2.4% strand fibre, a maximum BCR of 2.2 at 25mm settlement was obtained, leading to an enhancement in subgrade load-bearing capacity compared to unreinforced sections. This occurred due to the ability of fibres to act like a three-dimensional spatial network that interlocks soil grains, thereby improving the stretching resistance between the soil particles [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBCR values at different settlements for strand and discrete fibre\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eBCR values for strand fibre\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e \u003cp\u003eBCR values for discrete fibre\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eSettlement (mm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e \u003cp\u003eSettlement (mm)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFibre content (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Effect of cyclic loading on deformation\u003c/h2\u003e \u003cp\u003eCyclic plate loading tests were conducted to evaluate the effects of varying fibre contents, types, and fibre-lime stabilization on unpaved pavement model sections as shown in \u003cb\u003eFig.\u0026nbsp;13\u003c/b\u003e. The results demonstrate that 1.8% discrete fibre reinforcement and lime-stabilized soil at the subbase-subgrade interface reduced pavement settlement by approximately 250 loading cycles at 75mm settlement. Initially, unreinforced pavement exhibited marginally better performance compared to sections with 1.8% and 3.0% strand fibre and 3.0% discrete fibre up to 250 loading cycles.\u003c/p\u003e \u003cp\u003eThe settlement development followed two distinct phases for fibre-reinforced, lime-stabilized and fibre-lime-reinforced models. Phase 1 showed rapid settlement accumulation until reaching specific thresholds (75mm for lime-stabilized soil and 1.8% discrete fibre; 90mm for 3.0% strand fibre and discrete fibre-lime stabilized soil; 120mm for 1.8% strand fibre; 150mm for 3.0% strand fibre lime stabilized soil). Phase 2 demonstrated gradual settlement reduction, stabilizing through 400 loading cycles, consistent with findings by [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e] using geogrid reinforcement. Load diffusion capacity improved with increased loading cycles, supporting previous research [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan additionalcitationids=\"CR63\" citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile higher fibre content showed limited advantages during initial loading cycles, attributed to insufficient particle-fibre interaction under low surface pressure, overall performance improved significantly beyond 300 loading cycles regardless of fibre parameters. For the geofibre sand mixture, 0.8% fibre content was shown to have provided adequate structural support for the designed pavement compared with 1.0% fibre content, thereby improving the subgrade layer's strength under military truck traffic [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e]. However, the present study found that using fibre in large quantities offered no advantages during light loading cycles. Low surface pressure may play a role in this phenomenon because it causes soil particles to be less compacted, limiting the relative slide between fibres and soil particles. The impact of fibre reinforcing is not readily apparent since the fibres could not interact appropriately with the soil at this time.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eThis research evaluated the influence of polypropylene fibre reinforcement on unpaved road layers, examining CBR, tensile strength properties, and load-bearing behavior through static and dynamic testing. Using a laboratory-scale model, the investigation assessed strength parameters and deformation characteristics of both reinforced and unreinforced pavement sections, analyzing CBR under varied moisture conditions, tensile resistance at different fibre contents, and resilient modulus response under cyclic loading. These comprehensive analyses provide quantitative data for optimized pavement design methodologies. The research yields several significant findings:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eFibre-reinforced samples at 1.8% exceeded the minimum subgrade requirement (CBR\u0026thinsp;\u0026ge;\u0026thinsp;8%) for both soaked and unsoaked conditions.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eMaximum tensile strength was observed at different optimal contents: 37.45 kPa at 1.8% for strand fibres and 30.8 kPa at 2.4% for discrete fibres.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eStrand fibres exhibited higher performance compared to discrete fibres at equivalent percentages, attributed to their higher volumetric distribution and more effective load resistance orientation.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eDiscrete fiber reinforcement exhibited immediate and substantial improvements to lateral deformation response under indirect tensile loading, with consistent lateral deformation of 0.123 (146% increase) across 0.6\u0026ndash;1.8% fiber contents.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eBoth fibre-reinforced and lime-stabilized pavement systems demonstrated marked performance improvements after 300 loading cycles, suggesting enhanced effectiveness under sustained loading conditions.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eSettlement under initial dynamic loading (0-250 cycles) reached approximately 75 mm for lime-stabilized soil with discrete fibre, 90 mm for combined strand-discrete fibre systems, 120 mm for strand fibre, and 150 mm for strand fibre-lime-stabilized soil.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThis study advances sustainable pavement through innovative fibre reinforcement and lime stabilization techniques. The research demonstrates the effectiveness of optimized fibre-soil composites in enhancing pavement performance while reducing material consumption and maintenance requirements. The documented synergistic effects between fibre types and lime stabilization present opportunities for developing more resilient and resource-efficient pavement systems. Future research directions should focus on long-term durability under diverse environmental conditions and the potential integration of hybrid fibre systems. Field-scale implementation studies are essential to validate these findings and develop practical guidelines for sustainable pavement infrastructure. This research establishes a foundation for next-generation pavement solutions that balance engineering performance with environmental sustainability.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003eThe authors would like to acknowledge the generous financial support provided by the University of Johannesburg, South Africa, through the Fourth Industrial Revolution Scholarship (GES 4IR), which facilitated the completion of this research. Additionally, the authors extend their gratitude to Chryso Group, South Africa, for supplying the polypropylene fibres utilized for soil reinforcement. The authors express their appreciation to Afrisam, a cement-producing company in South Africa, for their donation of road construction lime, which was instrumental in conducting this research. The authors would like to extend their gratitude to Motloung Tebello for his valuable technical support in machining and assembling the experimental test setups utilized in this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions Adesola Habeeb Adegoke:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing, Writing \u0026ndash; original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. \u003cstrong\u003eSinenkosi Nxumalo:\u0026nbsp;\u003c/strong\u003eWriting \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eOkonta Felix:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing, Supervision, Resources, Investigation, Funding acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003eThe author sincerely expresses their gratitude to\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003ethe generous financial support provided by the University of Johannesburg, South Africa, through the Fourth Industrial Revolution Scholarship (GES 4IR), which facilitated the completion of this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u0026nbsp;\u003c/strong\u003eThis is an experimental study, and the data are present in the manuscript itself.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to publication\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interest\u0026nbsp;\u003c/strong\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eS. 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Ash, Fiber-Reinforced Fly Ash Subbases in Rural Roads, 134 (2008) 171\u0026ndash;180. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1061/(ASCE)0733-947X(2008)134\u003c/span\u003e\u003cspan address=\"10.1061/(ASCE)0733-947X(2008)134\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-civil-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Civil Engineering](https://www.springer.com/journal/44290)","snPcode":"44290","submissionUrl":"https://submission.nature.com/new-submission/44290","title":"Discover Civil Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Indirect Tensile Strength, Bearing capacity, Plate load test, Reinforcement, weak subgrade, Polypropylene fibre, Resilient modulus","lastPublishedDoi":"10.21203/rs.3.rs-6845001/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6845001/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe use of fibre for the improvement of residual clay soils for road backfill and infrastructure rehabilitation applications was investigated through a series of laboratory model tests. A series of standard tests to evaluate the California Bearing Ratio (CBR) and Indirect Tensile Strength (ITS) of residual clayey soil that was reinforced with strand and discrete fibres (0.6-3.0%) was conducted. The results show improvement in CBR of 25.05% and 19.13% for discrete and strand fibres, respectively, under unsoaked conditions, as well as an increase in ITS with a maximum tensile strength of 37.45 kPa at 1.8% for strand fibres and 30.8 kPa at 2.4% for discrete fibres. A series of laboratory model testing was conducted on fibre reinforced samples that were prepared in a steel reinforced box model (460 mm x 410 mm x 1.0 m). The result revealed that the static bearing capacity of the soil, associated with settlement of 25mm \u0026ndash; 50mm, can be improved by the incorporation of 1.8\u0026ndash;2.4% strand and 3.0% discrete. Dynamic loading tests, however, revealed that for up to 250 loading cycles associated with a cumulative settlement of 50mm, unreinforced soil and lime stabilized soils exhibited higher stiffness than fibre reinforced soil. Beyond 250 loading cycles, the relatively greater capacity of fibre reinforced soil is associated with the ductility of the soil fibre composite. 1.8% discrete fibre reinforcement exhibited minimal cumulative settlement, while fibre-reinforced and fibre-lime composites demonstrated exceptional resistance to dynamic loading. Notably, strand fibres mobilized greater stiffness compared to discrete fibres. The laboratory model tests provide some insight into the capacity and stability of fibre reinforced clay soil under static and dynamic load conditions in relation to other stabilization protocols, thus offering some guidance to field engineers on ground improvements.\u003c/p\u003e","manuscriptTitle":"The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-18 02:56:06","doi":"10.21203/rs.3.rs-6845001/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2025-06-16T18:56:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"56945654348089327659404433628108723312","date":"2025-06-13T15:44:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-13T13:53:43+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-06-12T14:14:39+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-10T07:04:47+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-10T07:03:16+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Civil Engineering","date":"2025-06-08T01:26:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-civil-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Discover Civil Engineering](https://www.springer.com/journal/44290)","snPcode":"44290","submissionUrl":"https://submission.nature.com/new-submission/44290","title":"Discover Civil Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"03226828-a182-44c8-99bf-7d8be70b52e5","owner":[],"postedDate":"June 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-09-22T11:38:12+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-18 02:56:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6845001","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6845001","identity":"rs-6845001","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}