# Geotechnical Evaluation of Diesel Contaminated Clayey Soil
## 3 Results and Discussion
### Consequence of Diesel Contamination on the Shear Strength of the Soil  
Figure 3 shows the changes in shear stress resistance of both natural and contaminated soils. Cohesion and internal friction angle were obtained from the failure envelop plotted for each soil condition. The cohesion of the contaminated soil decreased by 20% (from 36.7 \(\pm\) 0.2 kPa to 29.2 \(\pm\) 0.1 kPa), and its internal friction angle increased by 30% (from 26.9\({}^{\circ}\)\(\pm\) 0.2\({}^{\circ}\) to 34.8\({}^{\circ}\)\(\pm\) 0.1\({}^{\circ}\)) after soil contamination. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**Limit of Detection (\%)**} & \multicolumn{2}{c}{**Concentration (\%)**} \\ \cline{3-4}  & & **Natural Soil** & **Contaminated Soil** \\ \hline Si & 0.033 & 50.94 \(\pm\) 3.10 & 54.01 \(\pm\) 1.97 \\ Fe & 0.001 & 15.97 \(\pm\) 1.15 & 14.28 \(\pm\) 2.20 \\ Al & 0.097 & 14.80 \(\pm\) 0.77 & 17.06 \(\pm\) 1.61 \\ Ca & 0.001 & 7.42 \(\pm\) 0.16 & 5.40 \(\pm\) 0.47 \\ Na & 0.71 & 5.04 \(\pm\) 3.61 & 3.79 \(\pm\) 0.88 \\ K & 0.002 & 2.17 \(\pm\) 0.13 & 2.43 \(\pm\) 0.43 \\ Ti & 0.001 & 2.13 \(\pm\) 0.28 & 1.97 \(\pm\) 0.23 \\ Mg & 0.165 & 0.96 \(\pm\) 0.86 & 0.59 \(\pm\) 0.14 \\ Mn & 0.001 & 0.29 \(\pm\) 0.06 & 0.25 \(\pm\) 0.07 \\ Zn & 0.001 & 0.28 \(\pm\) 0.09 & 0.22 \(\pm\) 0.04 \\  & **Ratios** & & \\ Ca/Si & & 0.15 \(\pm\) 0.01 & 0.10 \(\pm\) 0.01 \\ Si/Al & & 3.44 \(\pm\) 0.04 & 3.18 \(\pm\) 0.18 \\ Mg/Al & & 0.06 \(\pm\) 0.05 & 0.03 \(\pm\) 0.01 \\ \hline \end{tabular} Values in the table: mean value \(\pm\) standard deviation (\(n\) = 3).  
\end{table} 
Table 6: Impact of diesel contamination on the concentration of the chemical elements of the soil. Figure 3: Effect of diesel contamination on the direct shear behavior of soil. The cohesion of soil depends on the material type as well as the intermolecular bonds between clay particles and the absorbed water. As the natural soil particles got coated by diesel, clay particles agglomerated, formed silt-like size grains, and reduced the clay-like size grains content in the contaminated soil. Although both soils had nearly the same equivalent water content (\(E_{wc-natural}=36.94\%\)\(\pm\) 0.19% vs. \(E_{wc-contaminated}=35.13\%\)\(\pm\) 0.22%) and saturation degree (\(s_{r-natural}=70.82\)\(\pm\) 0.18 vs. \(s_{r-contaminated}=71.28\%\)\(\pm\) 0.21%), the chemical characteristics of the diesel and soil water content reduction contributed to the decrease in the contaminated soil's cohesion. The reduction of the soil's cohesion has been reported by other authors [5; 12; 13]. They attributed this decrease to reducing the specific surface area (SSA) of soil particles due to hydrocarbon deposition over the soil grains. The reduction of the SSA decreased the CEC of the soil and reduced the contact between the soil particles and water molecules [5; 12; 13]. Therefore, the cohesion of the soils was reduced since soil particles absorbed a lower quantity of water. The agglomeration effect of diesel over the soil particles increased the friction angle of the contaminated soil. Thus, it can be inferred that under normal stresses between 100 and 200 kPa, contaminated soil will exhibit a higher shear resistance than natural soil. The quality of the soil is not only defined by its shear strength but also by its consolidation behavior. So, before considering that the contaminated soil quality was enhanced due to its higher shear strength, it is necessary to consider its permeability and compressibility changes. As has already been reported, diesel deposition over soil particles reduced the soil's hydraulic conductivity [9] and SSA [12; 13]. Hence, higher settlements may be observed compared to the ones obtained for the natural soil [7; 12]. Furthermore, the slope stability of the trenches located over contaminated soils must be verified because of the decrease in its cohesion. Peak shear stresses were plotted for each soil condition (Figure 3). For a normal stress of 50 kPa, natural and contaminated soils showed almost the same shear strength.