<p>This research used three fine-grained clay soils with different mineralogy to investigate the behavior of different soils used in engineering landfills. Due to the chemical reactions between the contaminants in landfills and surrounding soil, it is necessary to continuously monitor landfill behavior during the operational period and post-closure to identify potential remediation requirements. This study is the first time use UPV tests for estimating hydro-mechanical parameters in contaminated soils. The proposed framework and the relationships developed in this research can be utilized to assess landfill behavior. This soil consists of clay with low plasticity clay (CL) found in the landfill of Sari City, sodium bentonite (SB), and calcium bentonite (CB). A comprehensive experimental program was implemented to investigate the effect of various pore fluids, specifically leachate, water, and used motor oil (UMO), on the engineering behavior of the studied soils. This program evaluated mechanical strength, compaction characteristics, permeability, and microstructural properties. The laboratory tests involved standard Proctor compaction, Atterberg limit determination, unconfined compressive strength (UCS), indirect tensile strength (ITS), direct shear testing, and permeability measurements. Additionally, ultrasonic pulse velocity (UPV), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were utilized to analyze internal structure and mineralogical changes. Examining the results shows that the non-destructive testing of the UPV can be used to estimate the mechanical, shear, and hydraulic parameters of contaminated soils. The passing of leachate causes a significant decrease in strength compared to adding the amount corresponding to the optimum moisture. Also, adding UMO and leachate to the soil reduces the internal friction angle and cohesion. Scanning electron microscope images of soil mixed with UMO showed the flocculated internal structure.</p>