Various materials have conventionally been utilized to improve the geotechnical properties of soils. Recently, utilization of microorganisms to this end has attracted much attention because of its high capabilities. Various oil-degrading bacteria have proven to be effective in degrading crude oil pollution from the soil. However, their effect on geotechnical properties of the soil has not been thoroughly investigated. In this research, various oil-degrading bacteria were employed to degrade crude oil contamination from crude oil–contaminated sandy soil. Then alterations on geotechnical properties of bio-treated sand were ascertained. The results showed that by employing these microorganisms, more than 50 and 80 percent of the crude oil was degraded after 10 and 30 days of bio-treatment, respectively. Moreover, bio-treated samples, which are slightly crude oil contaminated, acquire larger values of maximum dry density and cohesion in comparison to untreated contaminated samples. As treatment time increases, the value of the internal friction angle remains almost constant, and unconfined compression strength slightly increases. Generally, bio-treatment alters the geotechnical properties of the contaminated soil, and the application of bio-treated soil as a road basement in road construction and erosion controller is suggested.

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