Busan’s Naeseong Road Deformation Raises New Questions Over Backfill

BUSAN — The road near Naeseong Underpass did not simply become uneven again on May 17. It deformed after Busan had already diagnosed the earlier April subsidence as ground weakening caused by residual settlement after soil backfilling.

That distinction matters because it changes the question. The case is not best understood as a vague fear of a hidden sinkhole beneath Busan’s new deep underground expressway, nor should it be inflated into evidence that the main Mandeok–Centum Urban Expressway tunnel is structurally failing. The available record does not support that conclusion. The more precise issue is narrower and more difficult for the city: whether the repair, testing and monitoring after the first incident matched the cause Busan itself had identified.

On April 6, Busan said that excavation at Naeseong Underpass confirmed ground weakening due to residual settlement after soil backfilling. The city also said Suyeonggang Riverside Underpass had reopened after a GPR survey and restoration work, with no additional risk factors such as cavities found in the preliminary analysis. Busan’s own statement therefore pointed to two different categories of evidence: cavity detection at one location, and backfill-related settlement at another. 

The May 17 recurrence put that distinction under pressure. According to Yonhap, a pavement level difference appeared near the Myeongnyun-bound access road at Naeseong Underpass, prompting Busan to control two Myeongnyun-bound lanes and one Gyodae-bound exit lane from 9:18 a.m. The same report said the incident occurred at the location where large-scale repair work had been carried out after the April 5 subsidence. 

That sequence is the core of the story. A negative GPR finding may be meaningful. It may reduce the likelihood that the affected road section is sitting above a detectable cavity. But if Busan had already identified residual settlement after backfilling as the mechanism at Naeseong, then the central question after May 17 is not only whether a void existed. It is whether the backfilled ground was tested, stabilized and monitored well enough before traffic returned.

The April–May Sequence

The Mandeok–Centum Urban Expressway opened on February 10 as a major mobility project for Busan. The city described it as Korea’s first deep underground tunnel open to all vehicles, connecting Mandeok-dong in Buk-gu to Jaesong-dong in Haeundae-gu, roughly 40 meters below ground. Busan promoted the route as the final missing link in its inner ring road network and said travel time between Mandeok and Centum would fall from about 42 minutes to 11 minutes. 

Less than two months later, on April 5, subsidence occurred on access roads around Naeseong Underpass and Suyeonggang Riverside Underpass. Busan said it implemented emergency safety measures, held an emergency assessment meeting, conducted a GPR survey at Suyeonggang Riverside Underpass and began excavation work at Naeseong Underpass to identify risk factors. 

On April 6, the city said Suyeonggang Riverside Underpass had reopened after restoration work and that Naeseong Underpass fully reopened before the morning rush hour. The same release said the cause at Naeseong had been confirmed as ground weakening due to residual settlement after soil backfilling, and Busan said it would conduct two weeks of GPR monitoring before repaving while strengthening monitoring of subsidence-prone construction sections. 

On April 20, Busan announced that a GPR survey and detailed investigation conducted from April 5 to 7 with the Korea Authority of Land and Infrastructure Safety found no cavities in the recently subsided pavement section, which the city described as about 5 to 7 centimeters, or in nearby roads around the Mandeok–Centum Urban Expressway. The city also said three minor cavities, about 7 to 15 centimeters deep, were found outside the construction impact zone and would be restored separately. 

Then came May 17. Newsis reported that a complaint about road subsidence was received at 8:28 a.m., traffic control began at 9:18 a.m., road paving started at 12:30 p.m., the Myeongnyun-bound direction reopened at 5 p.m., and the Gyodae-bound control was lifted at 7 p.m. The same report said Busan completed emergency work on a 730-square-meter section that had impaired road flatness, and that the location was the same area where excavation checks and road-surface maintenance had been carried out after residual settlement on April 5. 

This timeline does not prove that Busan’s April 20 cavity finding was wrong. It shows something more specific: the city’s public explanation answered one question while leaving another unresolved. Once the road deformed again at a location previously tied to residual settlement, the burden shifted from cavity detection to repair verification.

What GPR Could Not Answer

GPR is not irrelevant. Ground-penetrating radar is a necessary tool in urban subsurface management, especially where cavities beneath roads can develop before they become visible at the surface. Seoul’s own GPR program, for example, treats radar surveys as a preventive measure against ground subsidence and publicly discloses detected cavity locations through a survey map. 

But GPR is a detection tool, not a complete performance test of a repaired roadbed. A 2022 study of cavities beneath flexible pavement in Seoul used vehicle-type multichannel GPR to scan for underground cavities, while the effectiveness of rehabilitation was examined separately with a lightweight deflectometer. The study also found that rehabilitation improved elastic modulus above repaired cavities but did not fully restore it to the level of normal pavement nearby. 

That distinction is directly relevant to Naeseong. A radar survey can help determine whether a detectable empty space lies under the road. It does not by itself show whether the backfilled soil reached required density, whether moisture conditions were controlled, whether drainage weakened the support layer, or whether the repaired roadbed could carry repeated traffic loads.

Pavement engineering uses other tools to answer those questions. The U.S. Federal Highway Administration describes the falling weight deflectometer as a nondestructive testing device used to evaluate pavement structural condition and assess a pavement’s ability to handle future traffic loads. The same FHWA page notes that lightweight deflectometers can be used to evaluate unbound bases. 

The point is not that Busan should have ignored GPR. The point is that a negative GPR finding was not enough to verify the mechanism the city had already identified at Naeseong. If the April cause was residual settlement after soil backfilling, the relevant technical record should show whether the repair addressed backfill density, roadbed support, water movement and post-repair settlement.

The Technical Record That Matters

The missing evidence is not another assurance that no cavity was found. It is the repair record that would show whether the diagnosed mechanism was actually corrected.

Field-density tests would show whether the backfilled soil reached the required compaction level. Moisture-content records would show whether the material was placed under acceptable conditions. Compaction logs would show how thick each layer was, what equipment was used and whether rework was required. Plate-load, FWD or LWD tests would help show whether the repaired roadbed had enough support to carry traffic. Drainage inspections would clarify whether rainwater, groundwater or utility-related leakage could have weakened the support layer.

The monitoring record is just as important. Busan said in April that it would continue monitoring subsidence-prone construction sections. After the May recurrence, the question is whether the monitoring between April 6 and May 17 detected gradual movement, whether complaints or field observations came in before traffic control became necessary, and whether the May work had already been planned as part of pre-rainy-season maintenance. Newsis reported that Busan said it had planned maintenance before the rainy season but carried out flatness work first after the May incident. 

The records should also distinguish between emergency restoration and final correction. If the April work was intended only to reopen traffic and stabilize the surface temporarily, Busan should say so. If it was treated as a complete repair, the city should show what tests supported that decision. If some residual settlement was expected, the city should explain the expected range, the intervention threshold and why the May deformation required another emergency response.

Without those records, the public receives a conclusion rather than an explanation. “No cavity” may be an accurate finding within the scope of a GPR survey. It does not explain why a road section already linked to residual settlement after backfilling required another intervention weeks later.

From One Underpass to Busan’s Subsurface Risk

The Naeseong case should not be stretched into a claim that the main Mandeok–Centum tunnel is failing. It should be read as a test of how Busan manages localized subsurface risks around major infrastructure.

That is an important distinction. A surface deformation near an access road or backfilled interface is not the same as structural distress in the main tunnel. Busan’s earlier public explanations and related reporting pointed instead to access roads, residual settlement, possible backfilling weakness and monitoring around sections vulnerable to subsidence. 

But localized does not mean minor. Access roads, cut-and-cover sections, buried utilities, drainage structures and backfilled zones are exactly where large infrastructure meets older urban ground. For drivers, the project is not experienced as a legal boundary between a main tunnel, an approach road and a contractor’s work zone. It is experienced as one public road that either remains reliable or does not.

Busan promoted the Mandeok–Centum route as a solution to chronic east-west congestion and as part of a broader inner ring road network. That gives the city’s explanation a higher burden. A megaproject is not judged only by the day it opens or by travel-time estimates. It is judged in the months afterward, when repairs, drainage, interfaces and monitoring reveal whether the system performs under ordinary traffic. 

This is also why Busan’s press releases cannot be treated as the final technical record. They provide the city’s official position. They do not, at least in the public releases reviewed, provide the underlying density tests, drainage inspection results, support-capacity measurements, settlement logs or reopening approval chain needed to verify that position.

Responsibility Cannot Remain Diffuse

A complex infrastructure project can make responsibility easy to scatter. The city owns the public explanation. Contractors hold construction and repair records. Inspectors and supervisors hold quality-control evidence. Operators may manage routine monitoring. Utility or drainage managers may control systems that interact with the roadbed.

That complexity is precisely why the public explanation has to be structured.

Busan does not have to prove negligence to explain responsibility. It has to show who performed the repair, who tested it, who approved reopening, who monitored the section after traffic resumed and who is accountable if the same mechanism recurs. Those are not bureaucratic details. They are the chain through which engineering judgment becomes public accountability.

Emergency response and accountability also need to be separated. Busan may have acted quickly to restrict traffic and restore the road surface. That matters. But a fast repair does not answer whether the earlier repair was technically sufficient. A reopened road does not prove that the roadbed below it was verified. A repaved surface does not establish that water movement, compaction or residual settlement was corrected.

A stronger public account would not need to be dramatic. It would need to be specific. It would identify the affected section, distinguish it from the main tunnel, describe the suspected mechanism, list the tests performed, state whether backfill quality, drainage and support capacity were verified, and explain what obligations now fall on the city, contractor, operator or inspector.

The institutional risk is not only that a road surface may deform again. It is that official reassurance loses force when it is not backed by documents that can survive the next incident.

The Road Reopened. The Explanation Has Not.

The road near Naeseong Underpass reopened. That should not be mistaken for the end of the case.

The available record does not support turning Naeseong into a tunnel-collapse story. It should not be written that way. A localized road-performance problem near an access road or backfilled section is not the same as a tunnel-wide structural failure.

But proportionality cuts both ways. If the case should not be exaggerated, it should also not be reduced to a routine paving matter. Busan had already identified residual settlement after soil backfilling as the cause of the April problem at Naeseong. When the same area deformed again in May, the question became whether the city had verified that mechanism after the first repair.

That question cannot be answered by GPR alone. The scientific and engineering record points to a broader standard: cavity detection, backfill quality, moisture control, drainage inspection, support-capacity testing and post-repair monitoring. Seoul’s cavity-management research separates GPR detection from rehabilitation performance checks, while pavement engineering practice uses tools such as FWD and LWD to evaluate structural condition and support capacity. 

Busan may be right that no hidden cavity was found. But after identifying residual settlement after backfilling as the cause, it still has to show that the repair matched the diagnosis.