Earth subsidence analysis using radar interferometry technique, geotechnical and piezometric wells (case study: Urban region 18 Tehran )

A subsidence is the collapse of land surface that may have a small horizontal displacement vector. This movement is not limited by magnitude, scope and extent of the areas involved. The Urban region 18 in South West of Tehran megalopolis has been developed. This study is conducted on radar interferometry, geotechnical analysis, piezometer wells analysis, and geomorphological effects. In the first step, images taken by Sentinel-1 data related to the years 2015 and 2017 were collected. size of subsidence obtained from radar interferometry was about 430 mm around Yaftabad and showed an increase from West to East and from North to South. Maps showing the distribution of gravel, sand were prepared using the geotechnical data within the depths of 5, 10, 15, 20, 25, and 30m. Study results show that the Particle size gets smaller in the region under study from North to South. in the Continuation, Examination of geomorphological effects revealed that as the urban region 18 is located on young alluvial fan of Kan, which contains high gravel distribution and porosity in lower depths, and as the depth of wells in the area indicate thick alluvium, water table decline, emptied pore space and as a result increased alluvium weight along the direction of gravity may increase the compression force and justify the subsidence phenomenon. next a map of phreatic surface was first prepared and then water table of area under study was compared over a period of 14 years. The phreatic surface map indicated that its level has been decreased from North to South. It also showed that the  water level of piezometric wells has been declined from West to East. Comparison of the water table over a period of 14 years revealed an average decline of 13 meters. The maximum water table decline in the area under study was related to the Northern part with 33 meters fall. The Eastern part of the urban region 18 is highly influenced by the subsidence phenomenon. Study results showed a relationship between the size of subsidence and areas with fine-grained soil. About 30 Km of main roads in the region are highly influenced by the subsidence phenomenon. railway of Tehran-Tabriz and the subway line 3 are important elements of transportation in region 18 which Length more than 12 Km over zones influenced by the subsidence phenomenon.