Seoul City Concludes Seongsu Bridge Unevenness Safe, Plans Proactive Reinforcement

The Seoul Metropolitan Government announced that an external expert review of the uneven step found on the entry ramp of Seongsu Bridge concluded there are no safety issues.

However, the city decided to reinforce this section to alleviate public anxiety and driving discomfort.

Additionally, following a comprehensive inspection of bridges, uneven steps were identified at 32 entry ramps across 11 bridges, including Seongsu Bridge, but officials determined there are no signs of ground subsidence.

In a press release issued today (August 21), the city revealed the results of the Bridge Connection Point Safety Advisory Group, which was formed on July 29 and consists of 11 experts in structures, geotechnical engineering, roads, and construction.

The advisory group was established after it was revealed last month that there was an uneven step on the Seongsu Bridge ramp.

Led by Yonsei University Professor Emeritus Kim Sang-hyo, all 11 members were appointed from outside the government, comprising 3 structural experts, 3 geotechnical experts, 3 road experts, and 1 construction expert.

Through four meetings, the advisory group comprehensively reviewed on-site tests and ground surveys at Seongsu Bridge, past changes in the uneven step, surrounding influences, and the results of the comprehensive inspection of Han River bridges.

In particular, the group closely analyzed data from spectral-analysis-of-surface-waves (SASW) tests, dynamic cone penetration tests, core drilling surveys, and ground-penetrating radar (GPR) surveys for the three lanes left and right, including the southern B ramp of Seongsu Bridge, which had raised concerns due to a large step.

The analysis showed that the ground beneath the ramp mainly consists of sandy soil fill layers, and the overall ground condition was rated as "normal or better" and sound.

There were no factors likely to induce additional settlement, such as underground cavities or soil erosion caused by water currents.

The cause of the uneven step was identified as the difference in settlement amounts due to the different foundation types of the two structures.

Unlike the main bridge, which features pile foundations supported by the underground bedrock layer, the ramp connected to the bridge uses direct foundations installed on soil such as fill and sedimentary layers.

It is understood that the ramp was subject to potential ground settlement, while the bridge experienced almost no settlement, resulting in a height difference at the boundary.

The city explained, "Such uneven steps are a phenomenon that can appear on many bridges at home and abroad due to differences in the structural characteristics of bridges and earthwork sections."

Based on the advisory group's comprehensive review of past changes in the step and current ground conditions, most of the settlement occurred in the early stages of construction, and with 20 years having passed since completion, long-term settlement has also concluded, making the possibility of additional settlement minimal.

Furthermore, the advisory group reviewed nearby construction for the Great Train Express (GTX) Line A, underground utilities, and fluctuations in Han River flood levels and groundwater levels, but found no factors that would affect the step.

In addition, the city conducted a comprehensive inspection of 22 Han River bridges, excluding railroad bridges, to check for uneven steps on other Han River bridges and alleviate safety concerns.

As a result, uneven steps were identified at 32 entry ramps across 11 bridges, but no cavities were found in GPR surveys.

The identified steps were categorized as follows: ▲ 1 location with 15 cm or more, ▲ 4 locations with 10 to 15 cm, ▲ 9 locations with 5 to 10 cm, and ▲ 18 locations with less than 5 cm.

The largest step was found at the Olympic-daero entry ramp, measuring approximately 15 cm.

The uneven step on the Seongsu Bridge ramp, initially reported as 9 cm, was measured at 11 cm during the investigation.

Additionally, steps of 10.5 cm were found on the Gayang Bridge ramp, and 10 cm on the Wonhyo Bridge and Dongjak Bridge ramps, respectively.

Following the advisory group's recommendation, the city additionally conducted spectral-analysis-of-surface-waves tests on bridges with steps of 5 cm or greater, but no signs suggesting ground subsidence, such as cavities or loosened soil, were revealed.

Advisory Group Chairperson Kim Sang-hyo explained, "Experts in the fields of structures, geotechnical engineering, roads, and construction comprehensively reviewed the on-site tests at Seongsu Bridge, past changes in the step, surrounding influencing factors, and the results of the comprehensive inspection of Han River bridges."

He added, "The uneven steps on the entry ramps identified this time, including Seongsu Bridge, are judged to be mainly caused by differences in settlement amounts due to different foundation types, and no significant progressive settlement or abnormal signs affecting structural safety were confirmed."

Although the city confirmed there are no problems with structural safety, it decided to implement preventive reinforcement and management to further alleviate public concerns and enhance safety.

The city emphasized, "This measure is not repair work due to structural defects, but preemptive safety management to further enhance ground integrity and reduce public anxiety and vehicle driving inconvenience."

The city plans to preemptively reinforce the ground at the southern B ramp of Seongsu Bridge and the Olympic Bridge B ramp, where the steps are relatively large, and continue monitoring changes in the ground and steps through ongoing measurements even after reinforcement.

Choi Jin-seok, head of the Seoul Metropolitan Government's Disaster Safety Office, stated, "Through this investigation, we confirmed that there are no problems with the structural safety of the Han River bridge entry ramps, including Seongsu Bridge."

He added, "Seoul will not stop at a safety determination; we will preemptively reinforce sections with large steps, conduct continuous measurements, and meticulously address the anxiety and driving discomfort felt by citizens so they can use the bridges with peace of mind."

(Photo: Yonhap News)
※ Please note: This article was translated by AI and may contain errors.