## 4 Conclusions  
The feasibility of the electrical resistance heating method developed in this study was evaluated for the thermal desorption and vitrification of multi-contaminated silty sand by heavy oils and heavy metals in terms of environmental and geotechnical aspects. The target temperature of electrical resistance heating was designed at 700 \({}^{\circ}\)C in order to remediate contaminated soil with heavy metals and volatile organic compounds. The power consumption was set to 6 kW/h, and purification was performed for 24 and 48 h. The treated soil was analyzed through environmental and geotechnical tests in terms of TPH, leaching of heavy metals, shear strength, shear modulus and modified CBR:  
1. Based on the Korean Ministry of Environment standards [5], contaminants, such as heavy oil, organic matter and heavy metal, were not detected; the removal efficiency of TPH was estimated as 99.99% and heavy metals, as some of the contaminants, were isolated as a crystal in a treated soil without any harmful leakage. 2. It was also geotechnically found that decontamination processes, such as the thermal treatment of heavy metals and heavy oils, had an effect on the increase in the internal friction angle, shear modulus and modified CBR of treated silty sand. The obtained results are attributed to the reduction in the lubrication effect of oil occupied between soil particles. 3. The physical properties (particle size and Atterberg limits) and improved mechanical properties (internal friction angle, shear modulus and modified CBR) of remediated silty sand satisfied the criteria for construction fill materials. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Category** & **Unit** & **Criteria** & **Test Result** & **Remarks** \\ \hline \multirow{6}{*}{Fill material} & Maximum aggregate size & mm & less than 100 & 4.75 & satisfied \\  & 5 mm Sieve pass rate & \% & 25–100 & 99.1 & satisfied \\  & 0.08 mm Sieve pass rate & \% & 0–25 & 21.7 & satisfied \\  & Liquid limit \({}^{\dagger}\) & \% & less than 50 & N.P. & satisfied \\  & Plastic limit \({}^{\ddagger}\) & \% & less than 25 & N.P. & satisfied \\  & Plasticity index \({}^{\dagger}\) & & less than 10 & N.P. & satisfied \\  & Modified CBR & & more than 10 & 10.4 & satisfied \\ \hline \hline \end{tabular} \({}^{\dagger}\) Afterberg limits: liquid limit, plastic limit and plasticity index.  
\end{table} 
Table 6: Assessment of recyclability of remediated soil. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Technology** & **Removal** & **Treatment** & **Contaminants** & **Cost (USD per Metric** \\  & **Mechanisms** & **Conditions** & **Targeted** & **Ton)** \\ \hline Developed electrical resistance heating & Entrapment in molten glass, desorption, pyrolysis, oxidation & 500–1300 \({}^{\circ}\)C & Full range of hydrocarbons & USD 35–USD 70 \\  & & 500–1300 \({}^{\circ}\)C & hydrocarbons & (2021) \\  & & Low temp.: & Volatile and & \\ Ex situ thermal & Desorption & 100–300 \({}^{\circ}\)C & semi-volatile & USD 46–USD 99 \\  & & High temp.: & hydrocarbons, & (2016) \\  & & 300–550 \({}^{\circ}\)C & creosote, and TPH & \\ Incineration & Oxidation & 600–1600 \({}^{\circ}\)C & Full range of & USD 150–USD 2900 \\  & & & hydrocarbons & (2016) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of remediation costs with thermal treatment methods (based on [18]). * In conclusion, it was shown that the electrical resistance heating method developed in this study is an environmentally and geotechnically effective technology for the recovery of clean construction fill material from hazardous-waste-contaminated silty sand. C.K. : Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing--original draft; T.K. : Conceptualization, Resources, Writing--review & editing, Funding acquisition, Supervision, Project administration; D.L. : Conceptualization, Resources, Writing--review & editing, Methodology, Visualization; D.P. : Conceptualization, Writing--review & editing, Supervision, Project administration.