Swelling clayey soils present significant challenges to civil engineers, especially geotechnical engineers. These soils possess the capacity to expand, contract, and experience excessive settling or even fail under relatively low-stress conditions and pose a substantial threat to infrastructure by considerable damage. These negative properties of such soils can be resolved by making the soil suitable for engineering works by stabilizing it with various materials like cement, lime, rice husk ash, etc. This study focuses on the suitability of thermocol with petrol to enhance the strength characteristics of swelling black clay. Thermocol is used extensively as single-use plastic for safeguarding various electronic or home needs gadgets and other products while packing for transport. Due to its non-biodegradable nature, thermocol has emerged as a significant contributor to environmental pollution, primarily due to its extensive utilization as a disposable plastic material. It is difficult for solid waste management to dispose of thermocol, and incineration leads to the release of harmful gases into the ecosystem. Thermocol tends to change its physical state from solid to liquid by dissolving in petrol. Based on this property of thermocol, it is utilized to stabilize clayey soil. In this study, the suitability of thermocol for stabilization of clayey soil is studied at different ratios: 0.30%, 0.50%, 1.0%, and 2.0%, in the presence of 4.0% petrol by dry weight of soil. Subsequently, the engineering properties of the stabilized soil are subjected to tests such as the standard Proctor compaction test, unconfined compressive strength test, and California bearing ratio test; additionally, FESEM is used for analyzing the microstructure, and phase composition is analyzed by XRD test. The test results revealed notable changes that the California bearing ratio value, unconfined compressive strength, and undrained shear strength values demonstrate an increase for the treated soil when 1% thermocol is used in conjunction with petrol. Employing thermocol for soil modification enhances strength properties while simultaneously mitigating environmental pollution.