In this study, the Permanent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technique was applied to Sentinel-1 imagery acquired in 2022 in order to assess the impacts of land subsidence on urban structures in the southwestern part of Tehran. For this purpose, the GMTSAR and StaMPS software environments were employed. In addition, using groundwater level measurements from observation wells within the study area, the standardized cumulative groundwater level variations were calculated. The interferometric processing results indicate displacement rates ranging from +3 to −95 mm per year along the satellite line of sight across the southwestern of Tehran. The maximum subsidence is associated with the southern part of district 18 and the western part of district 19, with an annual rate of approximately −70 mm. The vertical displacement and lowering of bridge decks caused by subsidence were estimated to range between 2.3 and 4.5 cm per year. Field inspections of the studied bridges reveal that at bridge No. 3 (the intersection of Kazemi expressway and Shokoofeh boulevard), the manifestations of ground subsidence are clearly evident. The results of the correlation analysis between groundwater level fluctuations and the elevation changes recorded by bridge selected permanent scatterers demonstrate a strong relationship between the two variables. This high correlation indicates that, in addition to the downward motion associated with regional subsidence, hydrological saturation of the soil contributes to localized elevation changes affecting both the ground surface and human-made structures.
Introduction
Land subsidence is the response of the earth surface to the development, exploitation and evolution of underground spaces caused by natural and human factors. Due to climate change, reduced rainfall, population growth and increasing water demand, human intrusion into groundwater aquifers has intensified and groundwater abstraction is the most common cause of subsidence. Subsidence induced subsurface voids can lead to severe consequences for structures, which highlights the need for geosynthetic reinforcement methods to mitigate these effects. Using radar interferometry, potential displacements of man-made structures can be monitored on a monthly and multi-day time scale. Furthermore, two main features make the interferometry technique attractive to the scientific community. First, it provides a high-resolution two-dimensional representation of deformation over distances of 10–100 km. Second, it allows high accuracy (up to 1 mm/year) in measuring deformation. Previous research on Tehran subsidence has mostly focused on preparing subsidence maps and defining its limits. So far, no study has examined the impact of subsidence on urban structures and infrastructure in Tehran. In this study, radar interferometry and the permanent scatterer method were used to investigate the behavior of road bridges in Tehran in relation to the subsidence zone and to quantify the extent to which bridges have been affected. These bridges are located in the southwest of Tehran, in municipal areas 17, 18, and 19. This area of ​​Tehran corresponds to an alluvial plain with alluvial sediments of the Quaternary period.
Material and Methods
In this study, single look complex (SLC) radar images of the Sentinel-1 satellite in VV polarization were used for the year 2022 with a 12 days revisit interval. A total of 28 images were selected. The images are in descending orbit and were selected from path 35. The dataset was processed using the PS-InSAR technique with GMTSAR and StaMPS software. InSAR processing, including selecting the desired band from the image, initial processing, image cropping, and interferogram formation, was performed using GMTSAR software, and PS processing, including PS selection and retrieval, and preparation of time series graphs of bridge displacements, was performed using StaMPS software.
The main innovation of PSI approaches is the possibility of analyzing specific points on the Earth's surface (PSs) and monitoring their deformation time series. PSs are characterized by a stable amplitude and a coherent phase throughout the entire set of images in a dataset. PSs are usually fixed features on the Earth such as man-made structures and infrastructure (e.g., railways, buildings, bridges, towers), rock outcrops, and any other permanent feature that reflects a stable signal to the satellite.
Standardized cumulative groundwater level values were calculated using data from piezometric wells in the study area for the period 2002 to 2022. Also in order to investigate the relationship between groundwater level changes and bridge height changes, the correlation between the time series graph of PS points and changes in piezometer water levels for the same time period (2022) was determined.
Results and Discussion
Radar processing results indicate displacement rates ranging from +3 to -95 mm per year along the satellite line of sight in 2022. Maximum subsidence was observed in the southern part of district 18 and the western part of district 19, with an annual rate of approximately at -70 mm/year. Vertical displacement and lowering of bridge decks due to subsidence ranged between ±2.3 and ±4.5 cm per year. Field inspections indicated that most bridges were affected by ground subsidence, manifested as crack initiation in abutments and cracking of bridge deck asphalt. Bridge No. 3 (located in the intersection of Kazemi highway and shokoofeh boulevard on the east-west route) clearly showed the most pronounced effects of ground subsidence, including a localized 20 cm deep hole, collapse of the bridge wall facade, sinking of side steps, and outward deflection of the side parapets. Standardized cumulative groundwater level measurements in the subsidence area all indicate a sharp decline since 2013. To investigate the relationship between groundwater level changes and bridge elevation changes, the correlation between PS points time series and piezometer water levels changes for the same period (2022) was calculated. The correlation coefficient was 0.37, which is statically significant. The time series changes of PS points of bridges No. 2 and 5 were compared with the changes of water level of the piezometer closest to each bridge. The correlation rate of permanent scatterers of bridge No. 2 was at least 0.922 and at most 0.957. Also, this rate was at least 0.423 and at most 0.794 for bridge No. 5.
Conclusion
The results of radar processing indicate a displacement rate of between +3 and -95 mm per year along the satellite line of sight. In fact, the center of subsidence is located in the southern plain of Tehran and has spread to the southwest of Tehran. The maximum subsidence is related to the south of area 18 and the western part of area 19 at -70 mm per year, which is spatially consistent with the research of Haghshenas & Moategh (2019) and Babaei et al. (2024). The results of the study of permanent scatterers selected on the bridge decks indicate the impact of ground subsidence in 2022. The displacement and lowering of the bridge decks due to subsidence was about ±2.3 to ±4.5 cm per year. Field visits to the studied bridges indicate that the bridges are mostly affected by ground subsidence in the form of opening of the bridge wall cracks and cracking of the bridge deck asphalt. Meanwhile, Bridge No. 3 clearly shows the effects of land subsidence. This bridge, which is located at the intersection of Kazemi highway and Shokoofeh boulevard, has noticeable subsidence at its western base. In this part, a 20-centimeter-deep hole has been created, the bridge wall facade has collapsed, the side steps of the bridge have sunk, and the side tables of the bridge steps have curved outwards.
The results of the standardized cumulative values ​​of the piezometers in the subsidence area all indicate a sharp drop in groundwater since 2013. The results of investigating the relationship between groundwater level changes and bridge height changes indicate a high correlation between the height of the piezometers and changes in permanent scatterers. From the high correlation between the two, it can be understood that the land surface is strongly affected by changes in groundwater level, and the rise and fall of the water level of the aquifer will directly affect the land above it. With this in mind, it can be seen that excessive groundwater withdrawal and continuous decline in its level have a direct role in land subsidence in southwest Tehran, which can have irreparable effects on man-made structures in this area. The behavior of the selected permanent diffusers on the bridges shows that in addition to being affected by the long-term subsidence process caused by the decline in the groundwater level, the bridges are also severely affected by its short-term fluctuations, so that the rise in the groundwater level has caused an upward trend in the permanent diffuser diagram, which is consistent with the research of Elani et al. (2020), in monitoring the health of the old Aylesford bridge located in Kent, England, using the radar interferometry technique, which has shown that the upward and downward displacements of the bridge correspond to the soil saturation process during the hydrological cycle.