Board of Audit and Inspection: "Fundamental Measures Needed for SRT Yulhyeon Tunnel Operation Safety"

Up to 189mm Uplift Due to Design and Construction Errors During Construction of Korea's Longest 50km Yulhyeon Tunnel

[Asia Economy Reporter Ryu Jeong-min] The Board of Audit and Inspection has released findings that fundamental measures are needed to ensure operational safety regarding design and construction issues of the Yulhyeon Tunnel, the longest tunnel in South Korea (50.3 km) through which the Suseo-Pyeongtaek High-Speed Railway (SRT) passes.

The Board of Audit and Inspection announced on the 13th the results of their inspection on the appropriateness of design and construction related to the track uplift phenomenon in the Yulhyeon Tunnel, as well as the adequacy of safety management measures such as repair and reinforcement work after the uplift occurred.

The Korea Rail Network Authority contracted construction companies by section (a total of 12 sections) to carry out the SRT construction project (total length 61.1 km), which opened in December 2016. The Yulhyeon Tunnel section of the SRT line passes through the crush zone of a fault where the rock layer has fragmented due to fault activity and its surrounding areas.

[Image source=Yonhap News]

In the Yulhyeon Tunnel, Section 3-1 (the center of the crush zone) experienced an uplift phenomenon (maximum 189 mm) two months after opening, and since November 2018, speed restrictions have been in place (maximum speed reduced from 230 km/h to 90?170 km/h) in this section.

The Board of Audit and Inspection stated, "According to the revised construction standards notified by the Korea Rail Network Authority, if the rock mass rating (RMR) is 20 points or below, evaluations of RMR(★) and Geological Strength Index (GSI) must be conducted to select reinforcement methods. However, although Section 3-1 had an RMR score of 18, evaluations of RMR(★) and GSI were not performed, and the reinforcement method was decided as PD-6A (general reinforcement) based solely on the existing construction standard applying only RMR, which did not include invert construction."

Invert construction is a method where concrete is placed in a reverse arch shape at the tunnel floor to create a circular tunnel structure.

The Board of Audit and Inspection pointed out, "The invert, which is the most effective and reliable method to suppress the uplift phenomenon of the tunnel floor, was not constructed, resulting in inadequate response to the uplift centered around Section 3-1."

It was also found that there were issues with the operation of the rock assessment committee during the excavation of the Yulhyeon Tunnel by the Korea Rail Network Authority.

The Board of Audit and Inspection emphasized, "Fundamental measures are needed to secure the safety of SRT operations, including specific plans to promote invert construction in preparation for additional uplift occurrences," and warned, "Failure to establish fundamental reinforcement measures for SRT track uplift raises concerns about public distrust in the management of social infrastructure and potential disruptions to SRT train operations in the event of further uplift."