Shear strength of a silt at various oil/water contents

It is important to understand the shear behavior of oil-contaminated soils in many geotechnical problems, such as the analysis of pipelines and storage tanks affected by oil leakage. In this study, the shear behavior of a silt permeated with silicone oil/water (denoted by OS and WS, respectively) was investigated through direct shear tests. The residual shear strength was measured at various degrees of liquid saturation (65–100%) and net normal stresses (50 to 400 kPa). The oil and water retention curves were measured and used to explain the shear behavior. The results show that the residual friction angle of OS is 30% larger than that of WS. This is likely because OS and WS show aggregated and matrix structures, respectively, as evidenced by the Scanning Electron Microscope (SEM) test. The former structure includes more particle contacts than the latter one. These structural differences can be attributed to varying fluid properties, such as wettability, viscosity, and dielectric constant. When the volumetric degree of water saturation decreases, the total cohesion of WS increases substantially because of the strengthening effects of air-water interfaces on the soil skeleton. In contrast, the degree of oil saturation has minuscule effects on these variables, mainly because OS has a low oil retention ability and the air-oil suction remains very low (i.e., less than 2 kPa) during the desaturation process.