Characterizing the Shear Modulus Variations of Crude Oil-Contaminated Clay on Small-Strain Range

The environment is constantly exposed to various pollutants. Petroleum products may contaminate soils located next to industrial areas and other facilities. Oil pollution is one of the pollutions that can cause irreparable damage to the environment. Every day, a large amount of petroleum products enters the environment in various ways. Oil pollution affects the mechanical, chemical and dynamic properties of the soil. Changing the geotechnical properties of the soil is an important issue for structures adjacent to or on oil contaminated soil that can cause collapsing or variation in soil resistance. Since the behavior of many structures and foundations during dynamic loads is in the range of small strains, investigating and evaluating the velocity of waves in the soil skeleton can provide researchers and engineers with useful and significant information about the small-strain behavior of the soil. The importance and value of oil industry structures in Iran, as one of the active countries among oil exporting countries and as a country with a high level of seismicity, has made the research a vital way to improve the design level and accuracy of the behavior of structures exposed to pollution. Despite the wide range of oil industry structures in Iran and the other countries, there is limited literatures on oil-contaminated soil behaviors. Heretofore, the effects of diverse kinds of hydrocarbon contaminants on majority of geotechnical properties of clay soils such as grain size, hydraulic conductivity, plasticity, compressibility, internal friction, cohesion, and shear strength have been investigated. However, there has not been a concentrated research study examining shear wave velocity of hydrocarbon-contaminated clay soils as an important geotechnical property of soil due to the fact that, in small/very small strain levels, the maximum shear modulus of soils can be determined using shear wave velocity. This study aimed to measure the shear wave velocity and consequently, identify the shear modulus of clay soils in oil-contaminated condition with different percentages of contamination, and to compare them with non-oil-contaminated clay soils on small-strain range, using a Bender Element system. In order to prepare comparable clean and contaminated samples (containing 2, 4, 6, 8, 10 and 12 weight percent (wt%) of crude oil, respectively), it was performed similar to the density test method. In this regard, all the clean and contaminated clay samples were tested with a minimum moisture content equal to the optimal moisture content. Bender element tests were conducted on the identically prepared clean and contaminated clay samples at various amounts of frequency (5–20 kHz) and under various confining pressure (50–500 kPa) to find the best method for accurately determining shear wave travel time in the Bender Elements tests. Thereafter, Bender Elements placed in triaxial cell in Razi university laboratory. Bender Elements test conducted to examine shear wave velocity in the clean and contaminated specimens. As a contribution to the literature and potential engineering application, the experimental test results indicated that whilst adding 10wt% of crude oil in clay samples with an increase in the confining pressure, it correspondingly increase the shear wave velocity and consequently increase the shear modulus among of all samples. In addition, at each level of confining pressure, the shear modulus of clean clay was lower than the other contaminated samples. Moreover, under all confining pressures and excitation frequencies, the addition of 10wt% of crude oil caused significant changes in the maximum shear modulus, which was due to the effects of hydrocarbons on the behavior of clay particles. The degree of change due to hydrocarbons is highly dependent on the amount of confining pressure, so that the more the confining pressure is increased; the shear waves velocity an then the shear modulus is increased.