Document Type : Research Article
Authors
1
Ph.D. Student in Geomorphology, Faculty of Geography, University of Tehran, Tehran, Iran
2
Professor in Geomorphology, Faculty of Geography, University of Tehran, Tehran, Iran
3
Associate Professor in Remote Sensing, Faculty of Geography, University of Tehran, Tehran, Iran
4
Associate Professor in Water Resources Engineering, Faculty of Geography, University of Tehran, Tehran, Iran
5
Head of the Precision Leveling and Radar Interferometry Department, National Cartographic Center, Tehran, Iran
Abstract
In this study, using the permanent scatterer radar interferometry technique, Sentinel 1 images were processed in the period of 2022 to investigate the effects of subsidence on urban structures in the southwest of Tehran. For this purpose, GMTSAR and Stamps softwares were used. Also, using groundwater level data from wells in the study area, the standardized cumulative amount of groundwater level changes was calculated. The results of radar processing indicate a displacement rate between +3 and -95 mm per year along the satellite line of sight in the southwest of Tehran. The maximum subsidence is related to the south of region 18 and the western part of region 19 at -70 mm per year. The displacement and lowering of bridge decks due to subsidence was about 2.3 to 4.5 cm per year. Field visits to the studied bridges show that the effects of land subsidence are clearly visible at Bridge No. 3 (intersection of Kazemi highway and Shokofeh Blvd). The results of the study of the relationship between groundwater level changes and elevation changes of permanent scatterer on bridge indicate a high correlation between the two. The high correlation confirms that in addition to the sinking effects of subsidence, hydrological saturation of the soil is effective in the elevation of the ground surface and man-made structures.
Introduction
Land subsidence is the response of the earth's surface to the development, exploitation and evolution of underground space caused by natural and human factors. Due to climate change and reduced rainfall, increasing population growth and increasing water demand, human intrusion into groundwater aquifers has increased, and groundwater abstraction is the most common cause of subsidence. Subsidence-induced subsurface voids can lead to catastrophic consequences for structures, which further increases the need for geosynthetic reinforcement methods to mitigate these effects. Using the radar interferometry technique, suspicious displacements of human-made structures can be monitored on a monthly time scale. Furthermore, there are two main features that make the interferometry technique attractive to the scientific community. The first is that it provides a high-resolution two-dimensional representation of deformation over 10–100 km. The second is the high accuracy (up to 1 mm/year) in measuring deformation. Previous research on Tehran's subsidence has mostly led to the preparation of a subsidence map and its limits, and no research has been conducted to date on the impact of subsidence on urban structures and infrastructure. In this study, using radar interferometry and the permanent scatterer method, the behavior of road bridges in Tehran city has been investigated in relation to the subsidence zone, and we seek to investigate to what extent subsidence has affected bridges located within the subsidence zone.
Material and Methods
In this study, single-view complex radar images (SLC) of the Sentinel 1 satellite in VV polarization were used in the period of 2022 with a time interval of 12 days. The number of selected images is 28 pieces. The images are in the low-pass mode (Descending) and are selected from the path 35. This dataset was processed using the PS-InSAR technique using GMTSAR and Stamps software. The standardized cumulative value for the groundwater level of each well was obtained using data from piezometric wells in the study area from 1381 to 1401.
Results and Discussion
Radar processing results indicate a displacement rate of between +3 and -95 mm/year along the satellite line of sight in 2022. The maximum subsidence is in the south of region 18 and the western part of region 19 at 70 mm/year. The rate of displacement and lowering of bridge decks due to subsidence has been about 2.3 to 4.5 centimeters per year. Field visits to the studied bridges indicate that most bridges are affected by ground subsidence in the form of opening of cracks in the bridge wall and cracking of the bridge deck asphalt. In the meantime, Bridge No. 3 clearly shows the effects of ground subsidence. 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 have curved outwards. The results of the standardized cumulative value of piezometers in the subsidence area all indicate a sharp decline in groundwater since 2013. In order to investigate the relationship between groundwater level changes and bridge elevation 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. The correlation was above the significance level of the data (0.37).
Conclusions
The results of the study of the relationship between groundwater level changes and bridge height changes indicate a high correlation between the height of the piezometer water level and changes in permanent scatterers. From the high correlation between the two, it can be understood that the land subsidence is strongly affected by groundwater level changes, and the rise and fall of the water table will directly affect the land above it. With this in mind, it can be seen that excessive groundwater withdrawal and the continuous drop 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 permanent scatterers selected on the bridges shows that in addition to being affected by the long-term subsidence process caused by the drop in groundwater level, the bridges are also strongly affected by its short-term fluctuations, so that the rise in groundwater level has caused an upward trend in the permanent scatterer diagram.
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