## 5 Conclusions  
In this study, we evaluated the engineering properties of an eco-friendly rainwater detention system in terms of the material and the safety of a red clay water-permeable block structure and compared the economic feasibility of the system with that of an existing PE infiltration detention tank. The results of this study can be summarized as follows. * We analyzed the durability of an eco-friendly red clay water-permeable block fabricated by applying non-firing technology to red clay and found that the compressive strength criterion (16.0 MPa) for porous revetment blocks in SPS-KCIC0001-07038 [12] and the flexural strength criterion (5.0 MPa) for water-permeable blocks in KS F 4419 [13] were satisfied at 14 days of age. * During heavy rainfall, the red clay water-permeable block is expected to drain a large amount of water underground while retaining it because its porosity and permeability coefficients are 22.6% and 0.86 mm/s, respectively. * The maximum compressive load of the unit structure, comprising red clay water-permeable block units, was 2030 kN, indicating that it is safe against fracturing caused by the uniformly distributed failure load in the vertical direction. * The computational finite element analysis of the unit structure confirmed its structural safety, with its maximum stress and maximum displacement being 16.6 MPa and 4.75 mm, respectively. * The results of the leaching test conducted to evaluate eco-friendliness showed that there will be no impact on the soil and groundwater environment because no heavy metals, such as cadmium, lead, chromium, arsenic, and mercury, or organic compounds, such as benzene and PCB, were detected. * Furthermore, the economic feasibility of the existing system (PE infiltration detention tank) and the eco-friendly rainwater detention system was compared. The results showed that the eco-friendly rainwater detention system has excellent economic feasibility because the construction cost and LCC are reduced by approximately 30% and 58%, respectively, compared to those of the existing system. Based on the above results, it is expected that improved safety, eco-friendliness, and economic feasibility can be achieved, compared to the existing system, if the eco-friendly rainwater detention system is applied in the field. Conceptualization, H.C. (Hyeonggil Choi) and T.L. ; methodology, H.C. (Hyeonggil Choi); software, H.K. ; validation, H.C. (Hyeonggil Choi), T.L. and H.C. (Hyeonggil Choi); formal analysis, H.K. ; investigation, H.K. ; resources, H.C. (Hyeonggil Choi); data curation, H.K. ; writing--original draft preparation, H.K. ; writing--review and editing, H.C. (Hyeonggil Choi); visu \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Expense Rate**} & \multicolumn{4}{c}{**Cost**} \\ \cline{3-5}  & & \begin{tabular}{c} **Eco-Friendly** \\ **Rainwater** \\ **Detention System** \\ \end{tabular} & \
\begin{tabular}{c} **Existing System** \\ **(PE infiltration** \\ **Facility)** \\ \end{tabular} & **Remark** \\ \hline Direct material & - & 357 & 1944 & \\ cost & - & 84,425 & 109,361 & Expense rate: standard \\ Labor cost & - & 3942 & 5106 & expense rate in 2015 for \\ Expense & - & 3942 & 5106 & calculating civil engineering- \\ General & & & & \\ maintenance & 6\% & 5323 & 6984 &
\begin{tabular}{c} ing/landscape/industrial \\ and environmental \\ facility construction costs \\ \end{tabular} \\ cost & 15\% & 14,053 & 18,217 & facility construction costs \\ Total & - & 108,100 & 141,612 & \\ VAT & 10\% & 10,810 & 14,161 & \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Sediment removal cost analysis. alization, T.L. ; supervision, H.C. (Hyeonggil Choi); project administration, T.L. ; funding acquisition, H.C. (Heeyong Choi). All authors have read and agreed to the published version of the manuscript. This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Korea government (MOLIT), grant number 21 RITD-C162704-02. Not applicable.