Ground settlements induced by tunneling in soft and variable rock formations. Observed performance of SEM tunnelling for a section of Athens Metro - Line 3

Appraisal of ground settlements induced by urban tunneling, provide valuable input for quantifying risks associated with settlement induced damage to buildings and surface facilities. Methods to evaluate such hazards are empirical in nature, and buildup of the empirical database with local data is the best way to gain confidence on such predictions. The motivation of this study is to present and compile tunnel induced ground settlements along a section of Athens metro located in the western suburbs of Athens and associate them with the encountered geology, tunnelling sequence and applied support measures. The study focuses on parameters of settlement trough developed in soft rock formations characterized by great variability and heterogeneity, like the local Athens schist and as well as marly Neogene deposits encountered in the Athens basin. In the studied section max settlement values smax have been controlled well along most of the tunneling length, but specific incidents associated with tunnel face instabilities, resulted to high settlements values, just above tunnel axis. Volume loss VL has been confined in the range VL =0.5%-1.0% while values of settlement trough width parameter i were found in the range of i values reported in the literature for stiff clays/soft rocks.