The cause of successive ground subsidence incidents at underpasses in downtown Busan has been narrowed down to "inadequate backfilling." Analysts determined that insufficient soil compaction after deep tunnel construction, combined with the spring thaw season and vehicle loads, caused the ground to sink.
The Busan Metropolitan Government held a background briefing at City Hall on Sunday and stated that the subsidence incidents at the Suyeonggang Riverside Underpass and Naeseong Underpass approach roads the previous day appeared to be caused by "inadequate backfilling procedures following the Mandeok-Centum deep tunnel construction."
The city explained that a GPR (Ground Penetrating Radar) inspection was conducted at the Suyeonggang Riverside Underpass and a direct excavation survey at the Naeseong Underpass immediately after the incidents. No structural anomalies such as cavities were found. However, the city estimated that the ground subsided approximately 5 centimeters as backfilled sections weakened during the spring thaw after winter construction, compounded by vehicle traffic loads.
Regarding the concentration of subsidence at underpass entry and exit points, the city suggested that in addition to these sections being boundary points between underground utilities and structures, recent rainfall may have also contributed by allowing water infiltration that reduced soil cohesion. The affected sections were confirmed to be neither soft ground nor located directly above the deep tunnel mainline.
A total of five subsidence points were detected at the two underpasses the previous day, prompting full closure of traffic in both directions. Normal traffic was restored within a maximum of 14 hours after repair work.
The city plans to conduct section-by-section precision monitoring and additional GPR surveys over the next two weeks to check for further signs of abnormality.