<p>Design and construction of low-volume roads or flexible pavements on high-plasticity clay (HPC) presents significant challenges for engineers due to its swelling and shrinkage characteristics. This study investigated the strength characteristics of Indo-Gangetic (IG) HPC soil in terms of unconfined compressive strength (UCS), California bearing ratio (CBR), and volumetric shrinkage characteristics using industrial waste ashes, such as fly ash (FA) and bottom ash (BA). The results showed that the UCS values increased by 58.8 and 77.04% when 30% FA and 20% BA were added to HPC, respectively. In both cases, the CBR value exceeded 5. Statistical analysis also confirms that the most critical factor for the strength gains is the higher proportion of FA and BA. Further, microstructural analyses, such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR), were performed to explore the mechanisms responsible for the improvement in high-plasticity clayey soil. It also revealed that the formation of cementitious material in S-FA30 and S-BA20 mixtures makes high-plasticity clay a more stable, denser soil matrix. Further, the stability was also confirmed by volumetric shrinkage strain (VSS) of IG high-plasticity clay, which was reduced from 17.6 to 7.78% with 30% FA, and to 6.62% with 20% BA. Further, the microstructural analysis revealed that the formation of cementitious materials, such as C-S-H and C-A-S-H gels, simultaneously enhances particle-to-particle contact in the S-FA30 and S-BA20 mixtures, leading to a more stable, uniform, and denser soil structure. Overall, the utilization of 30% FA and 20% BA can be recommended for ground engineering applications to improve the strength characteristics of HPC.</p>