Surface Displacement Monitoring for Urban Underground Transportation Infrastructure Construction Using Satellite SAR Data

Rapid population concentration and infrastructure overload have intensified traffic congestion in urban areas. In response, South Korea has promoted underground transportation systems such as the Great Train Express (GTX) in the Seoul metropolitan area and large-scale underground roads in Busan. However, underground construction poses significant risks to surface stability, particularly ground subsidence, highlighting the need for monitoring systems that provide wide-area coverage with high accuracy at reasonable cost.

This study applies time-series interferometric synthetic aperture radar (InSAR) techniques to monitor surface displacement associated with construction of the Mandeok–Centum underground road in Busan, which connects the eastern and western parts of the city. A total of 165 Sentinel-1 A/B SAR images acquired between May 2015 and January 2021 were analyzed using both Small Baseline Subset (SBAS) and Persistent Scatterer InSAR (PSInSAR) approaches.

The study area spans mountainous terrain and densely urbanized subsurface zones, underlain primarily by Cretaceous andesite and granodiorite, with alluvial deposits in low-lying urban areas. Major geological structures include the Yangsan Fault in the western section and the Dongnae Fault in the central section, both trending predominantly NNE–SSW. Using descending orbit path 61 imagery, 705 interferograms were generated for SBAS analysis with temporal baselines limited to 60 days and perpendicular baselines constrained to 2% of the critical baseline.

Ground Control Points (GCPs) were established at 68 locations sufficiently distant from the tunnel alignment and assumed to be stable. Most GCPs exhibited displacement within ±10 mm, consistent with typical Sentinel-1 DInSAR accuracy, while three GCP clusters showed variations up to ±20 mm, suggesting possible excavation-related effects. Points of Interest (POIs) were selected along a 500 m-wide corridor centered on the tunnel route to assess excavation influence.

Results indicate that most POIs exhibited near-linear displacement trends with magnitudes up to ±50 mm. Localized anomalies were detected at vegetated and construction-affected sites, with abrupt displacement changes observed in 2016 and 2018. PSInSAR results were generally consistent with SBAS-derived trends, though spatial coverage was limited in vegetated and water-covered areas. Notable subsidence of up to ±20 mm was identified near the Minam Intersection and along both banks of the Suyeong River.

Although fault-related displacement was not clearly detected in the urban environment, time-series InSAR effectively captured temporal surface deformation patterns at intervals of several weeks. The results demonstrate that satellite SAR-based monitoring is well suited for preliminary site investigation, design evaluation, construction-phase monitoring, and operational surveillance of underground transportation infrastructure. Areas exhibiting cumulative displacement of several centimeters or deformation rates exceeding several millimeters per year should be prioritized for complementary ground-based monitoring.

This study contributes to the development of cost-effective wide-area surface displacement monitoring techniques for the safe construction and management of underground transportation infrastructure in complex urban environments.(KICT project No. 20250285-001, second year).