Experimental Study on Pullout Behavior of Coir Geotextiles Based on Transparent Soil

Geosynthetics are widely used due to low cost and favorable mechanical properties, primarily relying on soil–reinforcement interfacial friction. Pullout tests are key to understanding soil–geosynthetic interaction. Existing pullout studies focus largely on synthetic geosynthetics, which may cause pollution and have limited degradability. In the context of low-carbon and sustainability goals, natural fiber geotextiles (e.g., coir) are promising due to renewability and biodegradability. However, the interaction mechanism between coir geotextiles and soil, especially interface pullout behavior, is not well understood. This study uses transparent soil technology—an artificial soil created by matching refractive indices of solid particles and pore fluid—to visualize and quantify the interface pullout mechanism of coir geotextiles. The study aims to analyze the coir geotextile pullout behavior and evaluate reinforcement via interfacial friction coefficient, thereby informing design and application of coir geotextile-reinforced soil systems.

Prior pullout studies have examined soil–geosynthetic interactions across materials and test conditions. Findings include: higher apparent interface friction in dense conditions than loose (Agarwal et al., 2023); increased pullout force with geogrid plate height and distance from load (Abdi et al., 2024); interfacial parameters with sandy vs clayey soils (Abdi and Zandieh, 2014); geogrid-induced rotational movement of soil grains affecting pullout (Cardile et al., 2017); larger-aperture geogrids providing superior reinforcement (Chen et al., 2021); among different reinforcements, geogrids showed highest peak pullout force in granite residual soil (Ferreira et al., 2020); geotextile wrap anchorage angles influence initial and final pullout forces (Xu et al., 2018). Transparent soil-based pullout methods have enabled detailed interaction visualization (Ezzein and Bathurst, 2014; Bathurst and Ezzein, 2016; Derksen et al., 2021). For natural fibers, Table 1 documents applications of PP, PE, PET (synthetic) and jute, water hyacinth, cotton, kenaf (natural). Coir geotextiles have demonstrated improved load-settlement performance (Lal et al., 2017), enhanced footing capacity in loose sand (Vinod et al., 2009), reduced settlements at subsoil–subbase interface (Bhole et al., 2023), tensile properties suitable for road base reinforcement (Sudarsanan et al., 2018), increased subgrade stiffness and CBR with coir content (Harinder and Shankar, 2024), and slope stabilization performance comparable to synthetic geotextiles in simulations (Adajar et al., 2023). Durability improvements for coir fibers include kerosene coating, cashew shell liquid oil, lime, and chemical treatments (Ramasubbarao, 2014; Sumi et al., 2017, 2018; Marques et al., 2014). Despite these, detailed interface pullout behavior of coir geotextiles remains insufficiently characterized, motivating the present transparent-soil-based study.

Materials: Transparent soil was prepared using fused quartz sand as solid particles and a pore fluid composed of No. 15 and No. 3 industrial white oils mixed at a 1.5:1 volume ratio (refractive index 1.4585). Particle grading and fast shear tests confirmed similarity to natural sand: internal friction angles ~37.16° (natural) and ~36.48° (transparent). Coir geotextile: woven coconut fiber with grid-like structure; properties—mesh size 20 mm, linear density 3933 tex, unit weight 401 g/m², thickness 3.6 mm, specimen width 300 mm (set to 3/4 of box width to minimize side-wall friction). Tensile behavior: ultimate tensile strength 8.25 kN/m at 23.31% strain; at 5% strain, strength 0.75 kN/m and stiffness 15 kN/m; at 10% strain, strength 2.63 kN/m and stiffness 26.3 kN/m. Test equipment: Pullout box 1000 × 400 × 300 mm, divided into soil chamber (850 × 400 × 300 mm) and liquid reservoir (150 × 400 × 300 mm), separated by glass with a 10 mm pullout gap. Horizontal pullout via closed-loop stepper motor and ball screw; high-precision tension and displacement sensors used. Normal load applied through an airbag and reaction plate system. Imaging: Canon EOS800D camera, strong incandescent lamp for speckle illumination, tracer particles dyed fused quartz (matching transparent soil properties). Image analysis with PhotoInfor 2009 and PostViewer 2009 for DIC/PIV-based displacement fields. Test plan: Three groups to assess effects of normal stress, anchorage length, and pullout rate. Group 1: L = 500 mm, σv = 10, 20, 40 kPa, rate = 1 mm/min. Group 2: L = 100, 300, 500 mm, σv = 20 kPa, rate = 1 mm/min. Group 3: L = 500 mm, σv = 20 kPa, rates = 1 and 3 mm/min. Procedure: Laboratory temperature controlled; low-light conditions. Transparent soil placed by layering: mineral oil poured, fused quartz spread to keep oil level ~2 mm above particle surface; tracer layer every ~5 mm; compaction at ~50 mm height via rigid plates. Mass per layer calculated from compaction degree K = ρd/ρdmax × 100% to achieve K ≈ 90%. Coir geotextile installed through the sleeve, connected to force transfer rod. Upper layers filled and compacted, airbag placed with rigid reaction plate and I-beams; normal load applied by compressor, stabilized ~300 s. Imaging system set up and pullout conducted at specified rate; each test terminated at 50 mm displacement or unstable curve. After each test, loads released, packing and fabric removed, specimen condition observed. Data reduction: shear stress τ = Td/(2LB), normal stress σv = P/A, and interfacial friction coefficient fgs = τmax/σv computed.

• Mechanical response across tests is consistent: pullout force rises rapidly with displacement, then increases slowly to a peak, followed by a fluctuating decline.
• Interface shear strength parameters (from linear fit): cohesion ≈ 5.68 kPa; internal friction angle ≈ 3.43°.
• Interfacial friction coefficient of coir geotextile is variable and decreases with higher normal stress; overall range ≈ 0.2–0.6.
• Normal stress effect (L = 500 mm, rate = 1 mm/min): peak pullout force increases with σv—1.8 kN (10 kPa), 2.2 kN (20 kPa), 2.4 kN (40 kPa). Interface shear band thickness increases—13 mm (10 kPa), 16 mm (20 kPa), 20 mm (40 kPa).
• Anchorage length effect (σv = 20 kPa, rate = 1 mm/min): peak pullout force grows with L—1.3 kN (100 mm), 1.6 kN (300 mm), 2.2 kN (500 mm). Shear band thickness increases—11 mm (100 mm), 14 mm (300 mm), 16 mm (500 mm).
• Pullout rate effect (σv = 20 kPa, L = 500 mm): increasing rate from 1 to 3 mm/min raises peak pullout force from 2.2 kN to 2.4 kN and shear band thickness from 16 mm to 18 mm; higher rates show faster post-peak decline.
• Comparative context: under low σv (≈10–20 kPa), coir interfacial friction coefficient comparable to some synthetic systems; at higher σv, coir shows more deformation/damage leading to reduced coefficient versus geogrid/gabion mesh references.

The study clarifies the interface pullout mechanism of coir geotextiles using transparent soil visualization and quantitative analysis. Increased normal stress enhances interlocking and passive resistance, driving higher peak pullout forces and thicker shear bands; however, coir geotextiles are more susceptible to deformation and local damage at high stresses, reducing interfacial friction coefficients compared to stiffer synthetics. Longer anchorage lengths increase the number of ribs and meshes engaged, expanding the reinforced zone and spatial chain effect, which strengthens soil–fabric interaction and enlarges the shear band. Higher pullout rates hinder particle rearrangement, inducing stress concentration, dilation, and frictional increases, thus raising peak forces while accelerating post-peak softening due to joint damage. The DIC-derived displacement contours substantiate meso-scale mechanisms, showing shear band development and thickness growth with all three variables. These findings address the knowledge gap on natural fiber geotextile interfaces, providing parameters (cohesion, friction angle, friction coefficient range) and performance trends essential for design and application of coir-reinforced soils.

Transparent soil-based pullout tests were conducted to evaluate coir geotextile interface behavior. Key conclusions: (1) Pullout force–displacement response exhibits rapid rise, gradual peak, and fluctuating decline across all conditions. (2) Interface shear strength parameters: cohesion ≈ 5.68 kPa; internal friction angle ≈ 3.43°; interfacial friction coefficient ≈ 0.2–0.6. (3) Higher normal stress increases peak pullout force (1.8 → 2.2 → 2.4 kN for 10 → 20 → 40 kPa) and shear band thickness (13 → 16 → 20 mm). (4) Longer anchorage length raises peak pullout force (1.3 → 1.6 → 2.2 kN for 100 → 300 → 500 mm) and shear band thickness (11 → 14 → 16 mm). (5) Faster pullout rate elevates peak force (2.2 → 2.4 kN for 1 → 3 mm/min) and shear band thickness (16 → 18 mm). Coir geotextiles, as renewable and biodegradable materials, show promising application in ecological slope protection, roadbed reinforcement, and foundation reinforcement. Future research directions include material performance optimization (weaving, physical/chemical modification) and long-term field evaluations under varying soil types and environments.

The study considers only three factors (normal stress, anchorage length, pullout rate); other influential parameters such as geotextile morphology and structural variations may alter pullout behavior. Measurement tool sensitivity may introduce uncertainty. Tests were laboratory-based using transparent soil, which, while enabling visualization, differs from natural soils in cost and setup and may not capture long-term environmental effects (e.g., biodegradation, moisture, temperature). Boundary effects were minimized but not wholly eliminated. Future work will implement stricter standardization and explore morphology effects and broader conditions.