The micaceous weathered granitic soil (WGS) is frequently encountered in civil engineering worldwide, unfortunately little information is available regarding how mica affects the physico-mechanical behaviors of WGS. This study prepares reconstituted WGS with different mica contents by removing natural mica in the WGS, and then mixes it with commercial mica powders. The geotechnical behavior as well as the microstructures of the mixtures are characterized. The addition of mica enables the physical indices of WGS to be specific combinations of coarser gradation and high permeability but high Atterberg limits. However, high mica content in WGS was found to be associated with undesirable mechanical properties, including increased compressibility, disintegration, and swelling potential, as well as poor compactability and low effective frictional angle. Microstructural analysis indicates that the influence of mica on the responses of mixtures originates from the intrinsic nature of mica as well as the particle packing being formed within WGS. Mica exists in the mixture as stacks of plates that form a spongy structure with high compressibility and swelling potential. Pores among the plates give the soil high water retention and high Atterberg limits. Large pores are also generated by soil particles with bridging packing, which enhances the permeability and water-soil interactions upon immersion. This study provides a microlevel understanding of how mica dominates the behavior of WGS and provides new insights into the effective stabilization and improvement of micaceous soils. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Micaceous weathered granitic soil (MWGS) is prevalent in the tropical regions of southern China, characterized by high compressibility, low compactability, and inadequate strength properties due to the presence of mica. Stabilization is crucial for transforming MWGS into a sustainable construction material for geotechnical engineering. This study focuses on enhancing the mechanical properties of MWGS by using coir fibers and fly ash, both locally available agricultural and industrial byproducts. Unconfined compression tests and consolidated drained triaxial compression tests were conducted on fiber-reinforced and fly-ash-treated MWGS under different stabilization conditions, considering the effects of fiber content, fly-ash content, and curing age. Scanning electron microscopy and energy-dispersive spectroscopy were also used to trace the microstructural evolution of the soil fabric in response to fiber reinforcement and fly-ash treatment. The experimental results indicate that higher fly ash content and longer curing give significantly improved soil strength and stiffness but poorer ductility. Incorporating coir fibers into the cemented soil matrix not only enhances the composite's strength, stiffness, and toughness but also shifts the shear response from brittle to ductile. For example, the compressive strength of MWGS could be improved by 49.1% with the inclusion of 1% content of coir fiber. Under the optimal dosage of fiber and fly ash, the soil compressive strength increased significantly from 114 kPa to 725 kPa. Microstructural analysis reveals that the bonding, friction, and interlocking among fibers, hydration products, and soil particles are the main contributors to the stable and strong microstructure and consequently the enhanced mechanical behavior of MWGS. This study provides an innovative and effective method for utilizing waste byproducts in stabilizing MWGS for practical geotechnical engineering applications.