ABSTRACT: Fine- and coarse-grained soils at waste-handling, fuelling, and industrial spill sites are frequently exposed to petroleum hydrocarbons from diesel and waste engine oil. Despite shared petroleum origins, these contaminants differ substantially in composition, volatility, viscosity, and chemical reactivity, which can lead to complex and time-dependent changes in soil behaviour. At the field scale, the effects of combined diesel and waste engine oil contamination may not be predictable from data developed for single contaminants, particularly when soils vary widely in grain size distribution, mineralogy, and fabric. Current geotechnical assessment practices often rely on simplified assumptions about contaminant loading and soil response, leaving key gaps in understanding the coupled mechanisms driving changes in engineering properties. These mechanisms can vary between fine-grained soils (e.g., via pore structure and adsorption effects) and coarse-grained soils. Therefore, there is a need to develop a scientifically grounded understanding of how combined diesel and waste engine oil contamination impacts the geotechnical response of fine- and coarse-grained soils across relevant contamination concentrations and exposure durations. This study evaluated the combined effect of diesel and waste engine oil on the geotechnical properties of both fine-grained (clay) and coarse-grained (sand) soils. A controlled laboratory experiment was performed by contaminating soil samples with a standardized petroleum product mixture (80% diesel, 20% waste engine oil) at varying concentrations (0% to 16% by weight). Key geotechnical parameters, including Atterberg limits, compaction characteristics, shear strength, California Bearing Ratio (CBR), and permeability, were determined in accordance with ASTM and BS standards. A two-way ANOVA was employed to analyse the influence of soil type, contamination level, and their interaction. The results demonstrated that combined contamination significantly degraded the engineering properties of both soil types, with fine-grained soils exhibiting greater sensitivity. The principal findings revealed a decrease in maximum dry density (MDD) of up to 16.9% in fine-grained soil and 13.5% in coarse-grained soil, coupled with a severe reduction in soaked CBR by 26.5% and 38.6%, respectively. The angle of internal friction declined markedly, while permeability decreased in both soils due to pore clogging by hydrocarbons. The study concludes that even low levels of mixed hydrocarbon contamination can render both fine- and coarse-grained soils unsuitable for structural applications without prior remediation or stabilization, highlighting a critical consideration for geotechnical practice in contaminated environments.Keywords: Combined petroleum contamination, Diesel, Waste engine oil, Fine-grained soil, Coarse-grained soil, Geotechnical properties