Developing robust and stable infrastructure in areas with weak and expansive soils requires creative approaches to improving the subgrade layer. This study aims to determine whether using rice husk ash (RHA) as an environmentally friendly soil stabilizer is feasible. RHA has been shown to have ground improvement potential in recent investigations. An experimental investigation was carried out to support this novel building material's soil stability and geotechnical qualities. To comprehend the impacts of RHA on the geotechnical parameters of subgrade layers, considering different curing periods, sixteen specimen combinations of weak expansive soils with clearly defined subgrade layers of soil-RHA-cement were examined. Each subgrade's admixtures were made by combining soil with 2, 4, 6% RHA, 2, 4, and 6% cement. Three subgrade layers were used: upper, bottom, and subgrade double layers. Our work presents a novel method considering upper, bottom, and double subgrade layers with low cement dosage. We performed SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive X-ray Spectroscopy) investigations and mechanical testing to fully comprehend the stabilized soils’ microstructural and elemental composition characteristics. The link between shear stress, shear displacement, cohesiveness, and internal friction angle was thoroughly examined. The test results showed significant shear strength development for every subgrade layer. The present study adds to the current investigation of sustainable ground improvement techniques by highlighting the possible benefits of RHA-infused subgrade.