Effect of phytoremediation on the shear strength characteristics of silty clayey sand

Pollution of the environment by crude oil is a global problem that changes the geotechnical properties of soil, including its shear strength. In this study, the effects of phytoremediation of soil contaminated with crude oil have been investigated using Ophiopogon japonicus and Platycladus orientalis. The results of phytoremediation were tested at 30 and 60 days of treatment. The results were compared with samples that had not undergone phytoremediation. The soil shear strength parameters were determined using unconsolidated undrained triaxial compression tests at confining pressures of 50, 100, and 150 kPa. The results showed that an increase in the crude oil content led to a decrease in deviatoric stress at failure associated to an axial strain level of 20%. The deviatoric stress in the treated samples was about 15% higher than the samples without phytoremediation. The internal friction angle and cohesion decreased as the crude oil content increased. This decrease was less in the remediated samples, especially those exposed to P. orientalis. The results of the triaxial compression tests revealed the positive effect of phytoremediation in reducing the negative effects of crude oil on the soil shear strength characteristics. SEM images of the contaminated soil samples showed that soil flocculation initially increased with an increase in the crude oil content and time and then decreased subsequent to the application of phytoremediation.