Seoul clears Seongsu Bridge ramps of structural concerns after expert review

Seoul Mayor Oh Se-hoon observes field testing at the south B ramp of Seongsu Bridge in Gangnam-gu, where a step height difference was found. July 29, 2026 [Pool photo. Yonhap]

The Seoul Metropolitan Government said a comprehensive review conducted with outside experts found no structural safety problems with the entry ramps of Seongsu Bridge, where step height differences had been detected. The review found no signs of progressive settlement, voids or ground loosening that could cause further subsidence. Experts determined that the primary cause of the step height differences was the disparity in settlement between the bridge and its ramps, which rest on different types of foundations.

A full survey of 22 Han River bridges — excluding rail bridges — found step height differences at 32 entry ramps across 11 bridges, but the city said Friday that none showed any abnormalities affecting structural safety.

To ensure objectivity and technical rigor, the city formed an 11-member advisory panel of specialists in structural engineering, geotechnical engineering, road design and construction on July 29. The panel held four meetings to review field test results and soil survey data from Seongsu Bridge, along with historical records of step height changes, surrounding environmental factors and the results of the full Han River bridge survey.

Investigators paid particular attention to surface wave exploration tests, dynamic cone penetration tests, borehole surveys and ground-penetrating radar data collected across three lanes — including the south B ramp — at Seongsu Bridge. The analysis showed that the soil beneath the ramps consists mainly of sandy fill and that overall ground conditions were rated "average or better."

A combined review of historical settlement data and current ground conditions concluded that most settlement occurred immediately after construction, and that long-term settlement is now largely complete more than 20 years after the structures were finished, leaving little likelihood of further subsidence.

The main cause of the step height differences at the junction between the bridge and its ramps was identified as the differing amounts of settlement produced by their contrasting foundation types. The bridge sits on pile foundations anchored in bedrock below ground, while the connected entry ramps use shallow direct foundations placed on fill and sedimentary soil layers. The ramps are susceptible to ground settlement under the weight of the structures themselves and vehicle loads, which can create a height difference at the boundary with the bridge, where settlement is minimal.

A diagram illustrating how step height differences form. [Seoul Metropolitan Government]

The advisory panel also examined external factors that could contribute to ramp settlement, including nearby GTX-A construction, underground utility installations, Han River flood levels and groundwater fluctuations. The review found no evidence that nearby construction activity or groundwater changes had caused soil erosion or otherwise contributed to the step height differences.

The city conducted the full survey of 22 Han River bridges to determine whether conditions similar to those at Seongsu Bridge existed elsewhere and to address public safety concerns. Step height differences were confirmed at 32 entry ramps on 11 bridges, but ground-penetrating radar scans found no voids at any of the surveyed locations.

Following the panel's recommendations, the city carried out additional surface wave exploration tests at 13 ramps on eight bridges where step height differences of 5 centimeters or more were recorded — excluding the Seongsu Bridge south B ramp, which had already undergone separate detailed field testing. No signs of voids, ground loosening or other indicators of potential subsidence were found.

Taking the Seongsu Bridge field tests and the full Han River bridge survey together, the panel concluded that the step height differences at bridge entry ramps stem primarily from the difference in settlement between the two foundation types, and that there are no signs of abnormalities affecting the structural safety of the bridges.

The city said it will proceed with preemptive reinforcement and ongoing monitoring to ease public concern and further improve long-term safety, even though the survey confirmed no structural problems. It plans to reinforce the ground beneath the south B ramps of Seongsu Bridge and Olympic Bridge — where the step height differences are relatively large — and to continue measuring ground conditions and tracking changes in step heights after the work is completed.

"This investigation has confirmed that the entry ramps of Seongsu Bridge and other Han River bridges pose no structural safety concerns," said Choi Jin-seok, director of the Seoul Metropolitan Government's Disaster and Safety Management Office. "We will not stop at simply declaring them safe — we will proactively reinforce sections with larger step height differences so that residents can use the bridges with confidence, free from anxiety and driving discomfort."

ikson@heraldcorp.com