UP study detects localized ground subsidence in Manila Bay reclaimed areas

A study from the University of the Philippines has detected localized ground subsidence in parts of the Manila Bay Freeport Zone, according to satellite-based measurements from January 2018 to December 2021.

The peer-reviewed research, titled “Ground Deformation Monitoring of Reclaimed Lands Along Manila Bay Freeport Zone Using PS-InSAR Technique,” analyzed reclaimed areas commonly known as Bay City using 119 Sentinel-1 radar satellite images for both ascending and descending tracks.

Researchers from the UP Department of Geodetic Engineering used Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR), a satellite technique capable of detecting millimeter-level vertical land movement.

Across the processed area, the mean vertical velocity rate was -2.06 ± 2.84 mm per year, indicating an overall slight downward movement.

However, the study emphasizes that most of the Manila Bay Freeport Zone remains relatively stable, with subsidence occurring in localized clusters rather than across entire reclaimed islands.

Findings

The study focused on three major reclaimed areas:

1. CCP–Financial Center Area (CCP-FCA)

• 20 subsiding points detected
• Average vertical velocity rate: -0.73 ± 1.82 mm/year
• Seascape Village recorded the highest localized average subsidence rate at -8.22 mm/year

"Upon closer inspection, it was discovered that the southernmost section of the recently built region is comprised of high-rise buildings situated roughly 50 meters away from Seascape Village. This can be attributed to the detected localized subsidence in the nearby area," the study said. 

2. Central Business Park I-A (CBP-IA)

• 31 subsiding points out of 1,634
• Average vertical velocity rate: -1.61 ± 1.64 mm/year
• A cluster in the southwest section recorded subsidence averaging 9.69 mm/year

"Cracks on cemented structures, sidewalks, and restaurant walls were seen in the area where a cluster of subsiding points is located," the researchers said.

3. Central Business Park IB & IC (CBP-IB & IC)

• 35 subsiding points out of 690
• Average vertical velocity rate: -2.53 ± 2.32 mm/year

"A total of 4 subsiding points around Caltex station were detected with the lowest velocity rate of -17.52 mm/yr. It is important to note that the road adjacent to the store is unpaved. Unpaved roads are susceptible to deformation," the study said.

"This kind of road is also prone to erosion by wind or water, which can lead to further deformation. Similarly, a few cracks and gaps were observed on each side of the bridge across the station."

To validate findings, researchers compared satellite results with annual leveling data from benchmarks near the Port of Manila.

The correlation between PS-InSAR measurements and leveling data ranged from 0.81 to 0.96, indicating a strong positive linear relationship and reinforcing the reliability of the satellite technique.

Why subsidence happens in reclaimed areas

The study explains that reclaimed lands are typically composed of unconsolidated marine sediments such as silt and sand. Over time, these materials naturally compact under their weight and under additional load from buildings and infrastructure.

Construction activity can induce localized differential settlement, particularly in newly developed zones.

Researchers warn that subsidence increases the exposure to coastal hazards such as flooding and storm surges.

They also say that ground deformation on reclaimed lands is expected to continue and recommend monitoring to identify areas with significant deformations, detect building and structural deformations, and find solutions to reduce the risk of environmental hazards.

The study can be accessed on the website of The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. —VBL, GMA Integrated News