The Isu-Gwacheon complex tunnel, which will be built along the section connecting the Isu Intersection in Dongjak District, Seoul, and Gwacheon-daero in Gwacheon, will begin next year. The Seoul Metropolitan Government plans to approve the implementation plan in Jan. next year and break ground in Apr. The complex tunnel is expected to address chronic flooding and traffic congestion at the same time.
According to the Seoul Metropolitan Government and the construction industry on the 31st, the city will approve the implementation plan for the Isu-Gwacheon complex tunnel next month. The project will build a 5.61-kilometer, four-lane bidirectional "road tunnel" from the Isu Intersection to Gwacheon-daero, along with a 3.3-kilometer "stormwater drainage tunnel" with a storage capacity of 424,000 cubic meters. Construction will proceed in sections: areas with only the road tunnel, areas where the road tunnel and the stormwater drainage tunnel are built separately, and a complex tunnel section where the stormwater tunnel is attached beneath the road tunnel (0.5 kilometers from Sadang Station toward Isu Station).
An official at the Seoul Metropolitan Government said, "The project design plan was submitted on the 6th, and based on that we completed consultations with related agencies and autonomous districts," adding, "We plan to approve the implementation plan next month." Once the implementation plan is approved in Jan., after three months of preparation, groundbreaking will take place around Apr.
Completion is targeted for 66 months from the start of construction (70 months for the stormwater tunnel), with work expected to wrap up around Oct. 2031. The project was first conceived in 2017. The Lotte E&C consortium submitted a proposal to the Seoul Metropolitan Government and was selected as the preferred negotiating partner in May 2021. A project agreement was then signed in Dec. 2023. If construction starts next year, it will mark the first shovel nine years after the proposal was submitted.
The lead company in the consortium is Lotte E&C (47% equity). The construction investors participating in the consortium are Daewoo E&C, GS Engineering and Construction, Hanshin Engineering & Construction, Godeok General Construction, Gyeryong Construction, and Shinheung Construction.
The project aims to disperse traffic volume on Dongjak-daero and Gwacheon-daero, where congestion is heavy, and to resolve flooding in the Sadang and Isu areas, which are prone to inundation. The Sadang and Isu areas are in a basin, so when it rains heavily, large volumes of runoff from higher ground flow in and cause flooding.
The core of the work is the complex tunnel section to be built along the 0.5-kilometer stretch from Sadang Station toward Isu Station. Using an ultra-deep construction method that utilizes underground space 40 to more than 60 meters below ground, a large drainage conduit—a stormwater tunnel—that can carry away runoff during downpours will be built, with a road tunnel for vehicles above it. To avoid the subway including Sadang Station, it will be built in the bedrock layer 60 meters underground. The stormwater collected in the complex tunnel will be connected to the stormwater drainage tunnel extending to the vicinity of the Isu Intersection and will discharge into the Han River.
The ultra-deep construction method offers high safety because it is built in solid bedrock. However, it lengthens the construction period and increases costs, lowering business feasibility. As a build-transfer-operate (BTO) project, the private operator that builds the complex tunnel will transfer ownership to the Seoul Metropolitan Government after completion and recoup investment through a 30-year facility management and operation right.
With groundbreaking taking place nine years after the private operator first submitted a proposal to the city, some say the response was too slow. This is because areas near Isu Station have been flooded multiple times, causing damage to residents. In Aug. 2022, the area around Isu Station was completely submerged.
Jeong Hyeok-sang, a professor in the Department of Railway Construction Safety Engineering at Dongyang University, said, "The ultra-deep method is not commonly used, but the city and private operators have examined it extensively and are pursuing it to address both flooding and traffic congestion," adding, "Because drainage facilities in the Gwacheon and Sadang areas are currently at capacity, the complex tunnel project is expected to help resolve these issues."