<p>This study examines how varying amounts of diesel oil affect the geotechnical properties of kaolinite clay, focusing on strength, compressibility, and microstructure. Laboratory tests such as Atterberg tests, consolidation, unconfined compression strength, direct shear, X-ray diffraction (XRD), and scanning electron microscopy (SEM), were conducted. The results indicate that diesel contamination reduces the plastic limit, while increasing the liquid limit and plasticity index. A contamination threshold was identified at approximately 15% diesel content, beyond which changes in geotechnical behavior become more pronounced. Consolidation tests revealed that the soil becomes more elastic and compressible as diesel concentration increases, as shown by the compression index (Cc) and rebound index (Cs). XRD results indicates that diesel changes the crystalline structure of kaolinite, reducing peak intensity and increasing disorder. SEM images indicate that the soil structure changes from loosely packed platelets to more amorphous, smoother aggregates as contaminant levels increase. Strength tests display a marked drop in cohesion and unconfined compressive strength at low diesel concentrations, with partial recovery at higher concentrations due to particle re-agglomeration. Overall, diesel contamination drastically alters the structure and engineering characteristics of kaolinite, which may have an impact on the stability and settlement performance of foundations constructed on these soils.</p>