# Experimental Study on the Bearing Capacity of Gas Oil-Contaminated Coastal Sand
## 4 Results and Discussion  
### Direct Shear Test Results  
Direct shear tests were performed on the clean and contaminated sand according to the test program presented in Table 4. Failure envelopes for the relative densities of 30 and 70% at different gas oil contents are presented in Figures 4a and 5a, respectively. Soil properties at the mentioned relative densities, as well as the values of the friction angle and the apparent cohesion for the different soil conditions, are presented in Table 5. In these experiments, failure was defined as the displacement at which the shear strength peaked; in the absence of a peak, the shear stress corresponding to the tangent intersection was adopted. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test ID** & \begin{tabular}{c} **Subgrade Relative** \\ **Density (\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Gas Oil Content (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Degree of Saturation** \\ **(\%)** \\ \end{tabular} \\ \hline \multicolumn{4}{c}{**Bearing Capacity Tests**} \\ BC0-L & 30\% & 0\% & 0\% \\ BC5-L & 30\% & 5\% & 15.5\% \\ BC10-L & 30\% & 10\% & 31\% \\ BC15-L & 30\% & 15\% & 46\% \\ BC0-D & 70\% & 0\% & 0\% \\ BC5-D & 70\% & 5\% & 18\% \\ BC10-D & 70\% & 10\% & 36\% \\ BC15-D & 70\% & 15\% & 54\% \\ \multicolumn{4}{c}{**Direct Shear Tests**} \\ DS0-L & 30\% & 0\% & 0\% \\ DS5-L & 30\% & 5\% & 15.5\% \\ DS10-L & 30\% & 10\% & 31\% \\ DS15-L & 30\% & 15\% & 46\% \\ DS0-D & 70\% & 0\% & 0\% \\ DS5-D & 70\% & 5\% & 18\% \\ DS10-D & 70\% & 10\% & 36\% \\ DS15-D & 70\% & 15\% & 54\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of the conducted tests with respect to the sand condition. Figure 4: (**a**) Shear stress vs. normal stress; (**b**) Variation in the friction angle and apparent cohesion for loose samples at different gas oil contents. Friction angle and cohesion values corresponding to each gas oil content in loose and dense soil conditions are depicted in Figures 4b and 5b, respectively. Results indicate a generally decreasing trend in the sand's friction angle as the gas oil content increases, with the loose and dense sands experiencing a maximum of 7 and 8% decrease. The decrease in the friction angle is primarily due to the lubricating effect of the hydrocarbon coating on the sand particles, which reduces the surface roughness, facilitates interparticle sliding and slipping, and consequently lowers the friction angle. The behavior of coarse-grained soils is greatly dependent on the physical interaction of the soil particles. [PERSON] and [PERSON] [31] utilized scanning electron microscopy (SEM) images to examine sand contaminated with light crude oil, revealing significant effects on the sand's microstructure. These effects include the formation of a viscous hydrocarbon coating on sand particles and the infiltration of contaminants into the contact areas between the particles and the empty spaces within them. This coating notably influences the surface characteristics of sand particles, leading to a reduction in the friction angle. Moreover, the thickness of this coating increases with a higher hydrocarbon content, resulting in a more pronounced decrease in the friction angle. Additionally, Figures 4b and 5b show that, as expected, the friction angle increases with the increase in the soil's relative density in all gas oil contents tested. Apparent cohesion, on the other hand, increased with an increase in the gas oil content to a certain point, and then decreased. As can be seen from Figures 4b and 5b, the cohesion of loose and dense sand reached 3.05 and 5 kPa at gas oil contents of 10 and 5%, respectively, and by further increasing the gas oil content to 15%, the cohesion dropped in both loose and dense samples. The increase in the cohesion of sandy soils contaminated with hydrocarbons has been frequently reported in previous studies [13; 14; 15; 20]. They attributed this increase to the high dynamic viscosity of oil and its inherent cohesiveness.