Effects of oil contamination and bioremediation on geotechnical properties of highly plastic clayey soil

Abstract

Leakage of oil and its derivatives into the soil can change the engineering behavior of soil as well as cause environmental disasters. Also, recovering the contaminated sites into their natural condition and making contaminated materials as both environmentally and geotechnically suitable construction materials need the employment of remediation techniques. Bioremediation, as an efficient, low cost and environmental-friendly approach, was used in the case of highly plastic clayey soils. To better understand the change in geotechnical properties of highly plastic fine-grained soil due to crude oil contamination and bioremediation, Atterberg limits, compaction, unconfined compression, direct shear, and consolidation tests were conducted on natural, contaminated, and bioremediated soil samples to investigate the effects of contamination and remediation on fine-grained soil properties. Oil contamination reduced maximum dry density (MDD), optimum moisture content (OMC), unconfined compressive strength (UCS), shear strength, swelling pressure, and coefficient of consolidation of soil. In addition, contamination increased the compression and swelling indices and compressibility of soil. Bioremediation reduced soil contamination by about 50%. Moreover, in comparison with contaminated soil, bioremediation reduced the MDD, UCS, swelling index, free swelling and swelling pressure of soil, and also increased OMC, shear strength, cohesion, internal friction angle, failure strain, porosity, compression index, and settlement. Microstructural analyses showed that oil contamination does not alter the soil structure in terms of chemical compounds, elements, and constituent minerals. While it decreased the specific surface area of the soil, and the bioremediation significantly increased the mentioned parameters. Bioremediation resulted in the formation of quasi-fibrous textures and porous and agglomerated structures. As a result, oil contamination affected the mechanical properties of soil negatively, but bioremediation improved these properties.