# Geotechnical Evaluation of Diesel Contaminated Clayey Soil
## 3 Results and Discussion
### Consequence of Diesel Contamination on the Shear Strength of the Soil
However, for the 100 and 200 kPa normal stresses, the shear strength of contaminated soil was increased by 18% and 20%, respectively. According to [PERSON] et al. [40], the shear strength of unsaturated soils (\(\tau_{f}\)) can be evaluated in terms of two stress state variables, as shown in Equation (9):  
\[\tau_{f}=c^{\prime}+(\sigma_{n}-u_{a})\ tan\phi^{a}+(u_{a}-u_{w})\ tan\phi^{b}. \tag{9}\]  
The first two terms of Equation (9) correspond to the Mohr-Coulomb criterion, \(c^{\prime}\) is the effective cohesion, \((\sigma_{n}-u_{a})\) denotes the net normal stress, \(u_{a}\) is the pore air pressure, and \(\phi^{a}\) is the angle of frictional resistance related with normal stress. The third term of Equation (9) introduces an additional friction angle (\(\phi^{b}\)) relative to matric suction (\(\psi_{m}=u_{a}-u_{w}\)) contribution to shear strength resistance, where \(u_{w}\) corresponds to the pore water pressure. Total soil suction (\(\psi_{t}\)) considers the effect of the matric suction (\(\psi_{m}\)) and osmotic or solute suction (\(\psi_{o}\)), as expressed in Equation (10). Matric suction was defined as the difference between \(u_{a}\) and \(u_{w}\), and is related to capillary effects. Osmotic suction arises from the presence of dissolved salts or solutes in the soil pore fluid. \[\psi_{t}=\psi_{m}+\psi_{o} \tag{10}\]  
The natural soil had a \(\psi_{m}\) of 4709 \(\pm\) 1 kPa, while the contaminated soil had a \(\psi_{m}\) of 2077 \(\pm\) 12 kPa. As the contaminated soil had a considerably lower matric suction than the natural soil, a decrease in its shear strength could be expected. This is because the tension force developed by the air-water interface or contractile skin on soil particles decreases as the matric suction decreases [41]. In contrast, an increase in shear strength of soil was observed after its contamination. The \(\psi_{t}\) of natural and contaminated soils were 7521 \(\pm\) 1 kPa and 18,484 \(\pm\) 13 kPa, respectively. From Equation (5), the \(\psi_{o}\) can be obtained as the difference between \(\psi_{t}\) and \(\psi_{m}\). As shown in Figure 4a, the \(\psi_{m}\) had a higher effect on the shear strength of natural soil, while the \(\psi_{o}\) had a major role in the shear strength of the contaminated soil (Figure 4b). It is known that \(\psi_{t}\) influences the shear stress behavior of the unsaturated soils. However, in geotechnical engineering applications, the \(\psi_{m}\) component is more frequently considered than the contribution of \(\psi_{o}\)([PERSON], 2017). Nevertheless, in the case of contaminated soils, the impact of \(\psi_{o}\) on the shear strength of soil must be considered ([PERSON], 2017). Therefore, as proposed by ([PERSON], 2017), and considering \(\psi_{o}\) as an independent stress-state variable, Equation (11) could be modified as:  
\[\tau_{f}=c^{\prime}+(\sigma_{n}-u_{a})\;tan\phi^{a}+\psi_{o}\;tan\phi^{c} \tag{11}\]  
where \(\phi^{c}\) corresponds to the fiction angle relative to osmotic suction contribution to shear strength resistance. Figure 5 shows the strength envelop, considering \(\psi_{o}\) as an independent stress-state variable, as well as the equation for shear strength behavior of the contaminated soil, according to the results obtained in this research, as a function of both normal stress and osmotic suction. Figure 4: Influence of diesel contamination: (**a**) matric suction and (**b**) osmotic suction in the shear strength of soil. The symbols filled in black color correspond to natural soil while the symbols in grey color correspond to contaminated soil. Figure 5: Contaminated soil shear strength model based on osmotic suction and normal stress.