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.

In this study, inexpensive dual precursors namely rice husk ash (RHA) and quarry dust (QD) were mixed with an alkali solution (NaOH solution) in different mix proportions. Next, the effect of the dual precursors-alkali solutions (geopolymers) on the strength parameters of the different soil-geopolymer mixes subjected to various curing days were assessed. The result obtained from the assessment showed that the dual precursors-alkali solutions improved the strength properties of all the different soil-geopolymer mixes. The optimum soil-geopolymer mix proportion that resulted in the best improvement of the expansive subgrade soil properties was observed at a combination of 15%RHA+15%QD+3M NaOH. Furthermore, micro-pore analyses (porosity and pore size distribution), which were implemented with scanning electron micrographs and an image segmentation technique, local adaptive thresholding algorithm, showed progressive reduction in the porosity of the natural expansive subgrade soil and that of the optimal soil-geopolymer mix proportion cured for 7 and 28 days.

The differential settlement and excessive deflection in expansive subgrade soils lead to low volumetric stability upon moisture imbalance and cause severe damage to highway and airport field pavement surfaces. The present paper attempts to strengthen the stability behavior of expansive subgrade soil using an envirosafe alkaline binder (AB) with polypropylene fiber (PLF). AB was produced by combining dry aluminosilicate precursors (steel slag [SS] and sugarcane bagasse ash [SBA]) with a liquid activator (mixture of sodium hydroxide [NaOH] and sodium silicate [Na2SiO3]) in 0.4 water to binder (w/b) ratio. California bearing ratio (CBR) based resilient modulus, shear strength ratio (Sr), and consolidation tests were conducted as performance indicators of expansive subgrade soil reinforced with discrete PLF (0-0.5%) in 6% lime and 6% alkaline binder. The study also proposes the optimal sugarcane bagasse ash-steel slag-PLF dosage in the AB-stabilized subgrade layer. Moreover, mineralogical and morphological studies using powder X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were also carried out to understand the crystalline compounds and micro surface texture of the soil-fiber mixture. It was revealed that AB-fiber-soil exhibits higher interfacial frictional bonding and interlocking density than lime-fiber-soil. Also, 2D finite element analyses were carried out to compare the behavior of subgrade under the influence of vehicular load on the surface. PLF-AB soil mixture shows 72% lower vertical deformation than unstabilized expansive soil. Based on the findings of this study, recommendations for the practical application of stabilizing expansive subgrade soils were proposed.