This study investigates the influence of used motor oil contamination on the geotechnical behavior of soils collected from motor-mechanic workshop areas along Kohat Road and Barra Road in Peshawar, Pakistan. Soil samples were obtained from the ground surface and from a depth of 1 m to assess site-specific and depth-dependent variations in soil behavior. Laboratory tests were conducted to evaluate particle-size distribution, Atterberg limits, moisture-related behavior, unconfined compressive strength, and direct shear strength in comparison with uncontaminated control soils. The results show that oil contamination altered the consistency characteristics of the tested soils, with noticeable changes in liquid limit, plastic limit, plasticity index, and flow behavior. These variations indicate that the influence of contamination depends on soil composition, sampling depth, and local site conditions. The mechanical test results further reveal that oil contamination reduced compressive strength and shear resistance, mainly due to the coating and lubricating effect of oil on soil particles, which weakens interparticle bonding, reduces cohesion, and disturbs the soil structure. The reduction in strength was more evident in some surface samples, although the magnitude of degradation varied between the two sites. Overall, the study demonstrates that used motor-oil contamination can significantly affect the engineering performance of soils and should be carefully considered in geotechnical investigation, foundation design, and construction planning in contaminated urban workshop areas.

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