Long-term tunnel settlement mechanisms of Metro Line 2 in Shanghai

Non-uniform excessive tunnel settlements and relatively small tunnel heaves have been observed along Shanghai Metro Lines 1 and 2 over the last 15 years. Tunnels of the Metro lines were constructed using an Earth Pressure Balance (EPB) machine. It is well-known that significant long-term tunnel settlements may pose a threat to the serviceability and safety of the underground metro systems. However, the mechanisms of the observed large long-term tunnel settlements up to 144 mm at some stations are still not fully understood. In this paper, measured tunnel settlements of the 16.4 km long Metro Line 2 from 1999 to 2007 are studied together with groundwater pumping records from 1920s. Four possible causes including the effects of tunnel construction, secondary compression of soft clay, cyclic loading due to running trains, and groundwater pumping for the observed large tunnel settlements at some stations are investigated based on desk study, results of static and cyclic laboratory tests and coupled consolidation finite element analyses. The likely mechanism for the observed long-term tunnel settlement is identified and discussed.