This study introduces a novel method for stabilising expansive subgrade soils by integrating microbially induced calcite precipitation (MICP) process with a synergistic combination of waste sugarcane bagasse and recycled polyester fibres. This innovative approach aims to enhance strength properties and reduce volume susceptibility. The study demonstrates increases in Unconfined Compressive Strength (UCS), Split Tensile Strength (STS), and California Bearing Ratio (CBR), while substantially decreasing linear shrinkage, swell strains and pressures, indicating improved soil stability. The study also investigates the microstructural and chemical transformations through SEM-EDS, FTIR, and DSC-TGA, further corroborated by 16S metagenomic sequencing to understand microbial dynamics. Optimal stabilisation results were obtained with 0.5% fibre content and a four-day mellowing period, enhancing soil structure and durability by calcite precipitation and leveraging the combined benefits of natural and synthetic fibres. These fibres strengthen the soil structure and facilitate calcite nucleation, ensuring lasting stability, particularly valuable for stabilising expansive subgrade soils.

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.