Impact of pre-existent Qanats on ground settlements due to mechanized tunneling

Abstract

Abstract Qanats have been used for a long time to extract groundwater in most historic cities of Iran. With the growth of cities and the development of new methods of irrigation and drainage, most of the qanats have been unused and abandoned. These abandoned and obsoleted Qanats are sometimes used for drainage and disposal of urban sewage. Over time, due to environmental and geological factors, parts of these obsoleted Qanats collapses and they become larger sized cavities which often cause problems for other underground structures and infrastructure and urban facilities. In the present study, the ground settlement due to the excavation of East-West lot of Line 7 of the Tehran Metro (EWL7) tunnel and its interaction with the pre-existing Qanats is studied. The monitoring of settlements, as well as the sinkhole in Molavi Street (South of Tehran), indicate that abnormal movements were caused by pre-existent qanat chains, minor branches, and conduits in this area. To investigate more precisely this problem, numerical models using the FLAC3D finite difference software were used. The results show that in the case where the qanat is not turned into a cavity due to environmental factors, it would not cause a significant effect on the ground settlements induced by tunnel excavation. However, in the case where the qanat has turned to a bigger cavity due to environmental factors, its presence in the tunnel route can cause a settlement increase. This process can even lead to a ground surface collapse during the tunnel excavation.