# Experimental Study on the Bearing Capacity of Gas Oil-Contaminated Coastal Sand
## 4 Results and Discussion
### Physical Modeling Test Results  
Physical model tests have been conducted for two respective subgrade densities as well. Figure 6a,b present the load-settlement relationships at different gas oil contents and relative densities. In these experiments, the ultimate bearing capacity is defined as the displacement at which the applied load peaks; in the absence of a peak, the load corresponding to the tangent intersection was adopted [26]. According to this method, the ultimate bearing capacity of the foundations rested on the loose sand was determined using the tangent-intersection method, and the ultimate bearing capacity of all foundations rested on the dense sand was determined from the peak value, except for the BC15-D test. Figure 6: Applied stress versus footing settlement for (**a**) loose subgrade and (**b**) dense subgrade at different contamination contents. Figure 7 displays the results of the bearing capacity ratio, which represents the ratio of the ultimate bearing capacity under varying gas oil contents to the ultimate bearing capacity under uncontaminated conditions. It can be seen in Figure 7 that, in the tests conducted on the loose subgrade, the bearing capacity of the foundation increases as the gas oil content increases in the BC5-L and BC10-L tests. After adding further gas oil to the mix, in the BC15-L test, the bearing capacity decreases significantly. As discussed earlier, despite a reduction in the friction angle of loose and dense sand, the cohesion of the samples increased significantly when mixed with 5% and 10% gas oil, and then dropped at higher gas oil contents. The appearance of cohesion in the shear strength activates the cohesion term--previously absent in dry sand--in the bearing capacity equation, resulting in an increase in the bearing capacity of the foundation rested on the loose sand at the gas oil content of 5%. In this regard, the negative effects of the internal friction angle reduction and the positive effects of the cohesion increase on the bearing capacity of the foundation cancel one another in a way that results in an increase in the bearing capacity of the foundation. As discussed in the previous section, after passing the peak suction, mobilized in a certain gas oil content, the apparent cohesion reduces, which manifests itself as the reduction in the bearing capacity, as can be seen in Figure 7.  
Based on the results presented in Figures 4b, 5b and 7, it can be seen that, among the different gas oil contents, the greatest ultimate bearing capacity is achieved at the gas oil content that exhibited the highest apparent cohesion in the direct shear test. That is to say, the highest bearing capacity is achieved at 10% and 5% gas oil content for the loose and dense sand, respectively, which is consistent with the results of the direct shear test indicating the highest cohesion at 10 and 5% gas oil content for the loose and dense sand, respectively. The results show that implementing a worst-case scenario and overly conservative approach in the evaluation of the bearing capacity potential of foundations rested on hydrocarbon-contaminated sand might not always be a rational or efficient approach in the design or retrofit of such foundations, which can lead to unnecessarily cautious practices. In this respect, the actual conditions and scenarios in which the foundation bearing capacity could be jeopardized due to hydrocarbon contamination should be assessed before adopting conservative and expensive measures. It should also be noted that the cohesion term contribution to the bearing capacity of the foundation in this study might not be applicable in cases where very large or embedded foundations are designed, as other bearing capacity terms (i.e., the surcharge term and friction term) may become more influential, overshadowing the cohesion term effects.