The Yulhyeon Tunnel () is a railway tunnel in

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

, which opened in 9 December 2016, and is currently the world's fourth longest railway tunnel at . The tunnel, which consists of a double-track tube, is part of the Suseo–Pyeongtaek high-speed railway and GTX-A that connects Suseo station in the southeastern part of

Seoul Seoul, officially Seoul Special Metropolitan City, is the capital city, capital and largest city of South Korea. The broader Seoul Metropolitan Area, encompassing Seoul, Gyeonggi Province and Incheon, emerged as the world's List of cities b ...

with the

Gyeongbu high-speed railway The Gyeongbu high-speed railway, also known as Gyeongbu HSR, is South Korea's first high-speed rail line from Seoul to Busan. KTX high-speed trains operate three sections of the line: on 1 April 2004, the first between a junction near Geumch ...

. The threading into the older high-speed line is in the south of the city of

Pyeongtaek Pyeongtaek (; ) is a city in Gyeonggi Province, South Korea. Located in the southwestern part of the province, Pyeongtaek was founded as a union of two districts in 1940. It was elevated to city status in 1986 and is home to a South Korean naval b ...

. The tunnel itself takes up about 82% of the total new line. The Yulhyeon Tunnel was built using the New Austrian tunnelling method (NATM) and is designed for a maximum speed of . The average cruising speed is about due to the intermediate stop at Dongtan Station in the southern part of the tunnel.

Structural issues

The major section of Yulhyeon Tunnel has been built over the seismic fault called Singal Fault, which triggered an inquest into its structural stability. Korea Rail Network Authority argued that "''the diversion of the line outside of the fault zone was impossible since all those corresponding regions around the line were within the zone''". On the 14th of April, 2021 an audit performed by Board of Audit and Inspection revealed structural and design flaws that have been present virtually since the opening of the tunnel. The audit argued that several factors, including negligent geological surveying and unauthorized construction methods caused the

trackbed The track bed or trackbed is the groundwork onto which a railway track is laid. Trackbeds of disused railways are sometimes used for recreational paths or new light rail links. Background According to Network Rail, the trackbed is the layers of ...

to deform, rendering the track structure uneven which ultimately resulted in excess vibration in the trains, reducing the highest operational speed from 240 km/h to as low as 90 km/h in certain sections. In the wake of these flaws the Korea Rail Network Authority were ordered to prepare a comprehensive reinforcement plan.

References

{{coord missing, South Korea Railway tunnels in South Korea Buildings and structures in Gyeonggi Province Buildings and structures in Seoul Transport in Gyeonggi Province Transport in Seoul