Effects of used motor oil and leachate on the geotechnical and microstructural properties of clays with different mineralogy

• Effect of adding water, leachate and UMO on the geotechnical characteristics of CL, CB and SB is evaluated. • Adding leachate to the clays has a more PL, LL, PI, free swelling, MDD and OMC than the UMO. • Adding UMO to clay reduces optimum moisture. • Adding leachate to clay compared to water decreases the free swelling of CL, but the free swelling of SB is higher than CB and CL. In this study, the effect of adding water, leachate, and used motor oil (U.M.O) on the geotechnical and microstructural characteristics of three fine-grained soils with different mineralogy (clay in situ (CL), calcium bentonite and sodium bentonite) is evaluated. Atterberg limits, standard compaction, pH, atomic force microscopy (AFM), Scanning electron microscopy (SEM) and swelling index tests are performed. The results of this study demonstrated that the interaction between pollutant type and soil mineralogy significantly affects soils' geotechnical behavior. The plasticity index (PI) of sodium bentonite, calcium bentonite, and CL soil mixed with leachate increased by 43%, 25%, and 10%, respectively. In contrast, the addition of U.M.O reduced the PI of sodium bentonite, calcium bentonite, and CL soil by 72%, 66%, and 12%, respectively. The addition of leachate, compared to water, decreased the swelling of sodium bentonite, calcium bentonite, and CL soil by 25%, 21%, and 18%, respectively, due to the reduction in the diffuse double layer (DDL) thickness. The U.M.O., compared to water and leachate, caused the least swelling in all soil types due to its non-polar nature and low dielectric constant. The addition of leachate to sodium bentonite, calcium bentonite, and CL soils increased the optimum moisture content (OMC) by approximately 7%, 5.5%, and 3%, respectively. The dry density of CL soil and calcium bentonite decreased by about 8% and 2%, respectively, with the addition of U.M.O. compared to water. AFM images showed that leachate contamination significantly increased surface roughness, degrading clay surfaces and forming a porous structure.