# Effect of Heat-Treatment Remediation on the Mechanical Behavior of Oil-Contaminated Soil
## 4 Conclusions  
Heat treatment is widely used in the remediation of oil-contaminated soil. To re-use the treated soil as a construction material in civil engineering, such as in backfill and road base materials, the changes to its physical properties must be understood. Therefore, this investigation studied the removal percentages of total petroleum hydrocarbons (TPHs) from petroleum-contaminated soils by heat treatment as well as changes in their physical properties. It also discussed the effect of heat-treatment operating conditions (treatment temperature and time) on soil properties. The experimental results reveal a rightward shift of the particle size distribution curve of the soil upon heat treatment, indicating a decrease in the overall particle size of the soil, mainly due to thermal stress during high-temperature treatment, which breaks down soil particles.  
Under various operating conditions, if the contaminant concentration is 5000 ppm, a treatment temperature of approximately 600 \({}^{\circ}\)C can reduce the pollution concentration to below regulatory levels. However, if the concentration of gasoline or diesel is 10,000 ppm, only heat treatment at 900 \({}^{\circ}\)C can meet regulatory standards. The friction angle increases with the removal percentage at different pollution concentrations and under various treatment conditions. Comparing the friction angles of the soil with added gasoline or diesel at the same pollution concentration after heat treatment indicates that the \(\phi\) value of the diesel-contaminated soil is greater than that of the gasoline-contaminated soil. The increase in the friction angle (\(\phi\) value) represents an increase in the strength of the soil, indicating a tendency for the engineering properties of the soil to improve as the treatment efficiency increases. Heat treatment slightly increased the permeability coefficient of the original soil specimen, but it significantly increased that of the soil with added gasoline or diesel. As the removal percentage increased, the permeability coefficient tended to rise. In particular, the permeability coefficient of the contaminated soil with added diesel increased more. This study clarified the effect of heat-treatment operating conditions on soil properties to provide a reference for future site remediation. Last but not least, some limitations of this study should be noted. For the amount of heat-treated soil, this study only deals with volumes used for laboratory experiments. For a larger area of contaminated soil, further study is necessary. In addition, weathering may affect the type of hydrocarbons that remain in the soil and enhance the sorption of hydrophobic organic contaminants (HOCs) to the soil matrix.  
Figure 7: Comparation of the permeabilities of gasoline and diesel.  
Freshly spiked soil is different in terms of behavior when compared to old soil. The weathering effect of pollutants could be investigated in the future.  
Data curation, M.-C.W. and C.-H.L; Formal analysis, M.-C.W., C.-H.L. and C.-L.L.; Funding acquisition, M.-C.W.; Investigation, C.-H.L; Methodology, M.-C.W. and C.-L.L.; Project administration, M.-C.W., Resources, M.-C.W. and C.-L.L.; Supervision, M.-C.W. and C.-L.L.; Writing - original draft, M.-C.W. and C.-L.L.; Writing - review & editing, M.-C.W. and C.-L.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Ministry of Science and Technology, Taiwan, grant number MOST 107-2811-M-009-030 and MOST 107-2625-M-009-010.  
The authors declare no conflict of interest.