Stinging nettle (Urtica dioica L.) has been observed to grow spontaneously on metal-contaminated soils marginalised by heavy industrial use, thereby presenting an opportunity for the economic utilisation of such lands. This study explores the potential of nettle as a fibre crop by producing short fibre-reinforced polylactic acid (PLA) composites through compounding and injection moulding. Whole stem segments from three nettle clones (B13, L18, and Roville), along with separated fibre bundles from the L18 clone, were processed. The fibre bundles were separated using a roller breaker unit and a hammer mill. From separation with the hammer mill, not only cleaned fibre bundles but also the uncleaned fibre-shive mixture and the undersieve fraction were processed. The Young's modulus of all composites exceeded that of unreinforced PLA, with mean values ranging from 5.7 to 8.1 GPa. However, the tensile strength of most composites was lower than that of pure PLA, except for the two composites reinforced with cleaned fibre bundles. Of these two, the reinforcement with fibre bundles from separation with the hammer mill led to superior mechanical properties, with a higher Young's modulus (8.1 GPa) and tensile strength (61.8 MPa) compared to those separated using the breaking unit (7.2 GPa and 55.9 MPa). This enhancement is hypothesised to result from reduced fibre damage and lower fibre bundle thickness. The findings suggest that nettle cultivation on marginal lands could be a viable option for producing short-fibre composites, thereby offering a sustainable use of these otherwise underutilised areas.

Fibre reinforcement technology has been widely adopted in soil improvement due to its cost-effectiveness, simplicity, and environmental benefits. In many fibre reinforcement projects, the soil is often in an unsaturated state. However, the numerical simulation mechanisms of fibre-reinforced unsaturated soils remain poorly understood. In this study, a Vangenuchten (VG) model considering fibre incorporating fibres was proposed based on the original VG model. This model considering fibre accurately describes the soil water characteristic curve (SWCC) of fibre-reinforced sand (FRS), as verified by water-holding characteristics tests. Then, unsaturated triaxial tests confirmed the applicability of an unsaturated soil elastoplastic constitutive model and a fully coupled soil-water-air finite element-finite difference (FE-FD) method for simulating the mechanical behaviour of unsaturated FRS. Finally, using the SWCC parameters derived from the VG model considering fibres and mechanical parameters from saturated triaxial tests, slope models were established to analyse the stability of both unreinforced and fibre-reinforced slopes. The results show that the interweaving action of fibres within the soil enhances its strength, reduce permeability, and decreases both saturation and pore water pressure, ultimately increasing slope stability. This study provides valuable insights into the SWCC characteristics and the numerical calculation of FRS under unsaturated conditions.

Cyclic loading may induce changes in the geomechanical behaviour of materials that should be characterised. This work studies the impact of the number of loading cycles on the mechanical behaviour of a fibre-reinforced stabilised soil focusing on its behaviour before failure (yield surface). To this end, an experimental testing program based on triaxial tests was performed on samples not subjected to a cycling loading stage, as well as on samples previously subjected to a cycling loading stage varying the number of loading cycles from 1,000 to 100,000. The results were studied in terms of the accumulated permanent axial strain and the yield surface of the composite material. It was observed that increasing the number of loading cycles led to a rise in the accumulated permanent axial strain and in the undrained resilient modulus. The results also showed an expansion of the yield surface during the first 1,000 loading cycles (the yield occurs later due to the partial mobilization of the tensile strength of the fibres during the cyclic stage) but its shape is maintained. The results also showed a progressive reduction in the yield loci with the increase in the number of loading cycles, reflecting the greater degradation of the solid matrix induced by the accumulated permanent axial strains.

Coastal erosion is a global environmental concern, threatening infrastructure, human livelihoods and ecosystems. Recently, microbial-induced calcite precipitation (MICP) has emerged as a promising ground improvement technique. The present study examined the effects of adding three different fibre reinforcements, namely carbon, basalt and polypropylene, on the physical and mechanical properties of coastal soil through MICP. The fibre content used was 0.20%, 0.40% and 0.60% of soil weight. A comprehensive biotreatment investigation was conducted using Sporosarcina pasteurii (S. pasteurii) in a 0.5 molar cementation solution. The samples prepared for this study had aspect ratios of 2:1 and 1:1. These samples were subjected to biotreatment, consisting of a 24-h cycle for 9 and 18 days. Unconfined compressive strength (UCS), split tensile strength (STS) and ultrasonic pulse velocity (UPV) tests were conducted on the biotreated samples to evaluate the effect of fibre reinforcement on the mechanical properties of the biotreated samples. The amount of calcite precipitation, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to interpret biocementation. Results suggest that adding fibres to the MICP process enhances the mechanical properties of coastal soil. The optimum fibre content for carbon and basalt fibre was 0.40%, whereas, for polypropylene, it stood at 0.20%. The maximum UCS, STS, UPV and average CaCO3 were observed in a basalt fibre-reinforced biotreated sample with a fibre content of 0.40%, subjected to 18-day biotreatment. Conversely, the sample without fibre-reinforcement, biotreated for 9 days, exhibited the lowest values for these parameters. Samples subjected to 18 days of treatment have higher values of UCS, STS, UPV and CaCO3 content than 9-day-treated soil samples. SEM revealed the presence of CaCO3 precipitates on the surfaces of soil grains and their contact points, and the EDS spectrum corroborated this observation.

When a material is subjected to cyclic loading, there are changes in the material's geomechanical behaviour that need to be characterized for a safe design. For unbounded granular materials, the shakedown theory is used to explain the soil's behaviour under cyclic loading. However, it is not clear yet if such theory is extendable to unreinforced and fibre-reinforced stabilized soils. To this end, a series of unconfined compression cycling loading tests were performed, to study the effect of the number of cycles and initial deviatoric stress level on the behaviour of an unreinforced and reinforced stabilized soil. The results were analysed in terms of shakedown theory, elastic and plastic deformation energy and damping ratio. It was observed that shakedown theory seems to represent the behaviour of the stabilized unreinforced and fibre-reinforced soils under cyclic loading, with threshold between the plastic shakedown and the plastic creep shakedown behaviour at around an absolute axial strain 1 x 10-3. The effect of increasing binder content (from 12 to 39%), comparable to reducing the initial deviatoric stress level (from 85 to 15%), promoted a reduction in plastic deformation (from 2.09 to 0.19% without fibres, and 2.21 to 0.24% with fibres) and damping ratio (from 25.17 to 10.01% without fibres, and 29.18 to 15.95% with fibres) due to the lower degradation of the solid matrix. It also promoted an increase in the difference between elastic and plastic energy (from - 1.04 to 13.92 kJ/m(3) without fibres, and - 1.68 to 10.19 kJ/m(3) for the first cycle).

The application of chemical stabilizers and fibres for the stabilization of weak soil subgrades can mitigate the cost and CO2 emissions associated with pavement construction. The present study evaluates the feasibility of improving clay and sand subgrades using a calcium-based stabilizer (CBS)-commercial name: RBI Grade 81-and synthetic fibre-polyester fibre for building economic and sustainable pavements. Two soils, i.e. silty clay of low plasticity and silty sand, were stabilized and reinforced with independent and combined proportions of the CBS and polyester fibre. The test program included plasticity, compaction, advanced cyclic triaxial (ACT), and California bearing ratio (CBR) tests. The experimental and theoretical resilient moduli were determined using ACT and CBR tests, respectively. Subsequently, scanning electron microscopy and X-ray diffraction tests were then conducted to assess the microstructural and mineralogical changes in the soils due to the stabilization and reinforcement. Flexible pavements were designed with experimental and theoretical resilient modulus (MR). A good correlation was developed between the CBR and experimental MR. The results of the study demonstrate a significant overestimation of MR by the theoretical method. It was seen that with up to 186% higher CBR, 228% higher experimental MR, 96% higher theoretical MR, 230% higher traffic benefit ratio, 22% savings in construction cost, and 24% reduction in greenhouse gas emissions, the stabilized soils exhibited superior performance. The study thus demonstrates that the CBS combined with polyester fibre can be used for economical and sustainable pavement construction.

Landfill mining is emerging as an effective strategy towards mitigation of the problems associated with legacy landfills. About 40-80% of the landfill mined waste is soil-like fraction (landfill mined soil like fraction, LMSF) that is obtained after removing the combustibles and recyclables. However, due to its lower calorific value and presence of organic content, LMSF has not found many applications except landfill cover material and filling of low-lying areas. In this context, the present study explores the strength properties of LMSF with and without fibres (coir and polypropylene fibres) through unconfined compression, direct shear, UU triaxial, cyclic UU triaxial, indirect tension and bending strength tests. Overall, the shear strength properties of LMSF are similar to sand with some cohesion. Further, the performance of LMSF with optimum fibre proportion (2% for coir fibres and 1.5% for polypropylene fibres) is quite superior under monotonic and cyclic compressive loading conditions, as well as indirect tension and flexural loading conditions. An assessment of 1 m replacement of soft soil with LMSF with/without fibre reinforcement infers substantial increase in the allowable bearing pressure of LMSF. Overall, though LMSF may need assessment/pre-treatment (as required) due to presence of organic content and any possible heavy metal concentration, utilization of LMSF would encourage the adoption of landfill mining process. The current research insights contribute in the direction of achieving sustainable infrastructure development by reduction in the landfill area that would reduce the depletion of conventionally utilized natural resources for fill applications such as sand and gravel, and contribute positively towards environment.