Large long-term tunnel settlements are generally observed after the completion of shield tunnel construction in soft clay. It is well-recognized that significant tunnel settlements may pose a threat to the serviceability and safety of underground metro systems. However

the mechanisms of the observed large long-term tunnel settlement in soft clay are still not fully understood. Measured tunnel settlements of a metro line in Shanghai from 1999 to 2007 are studied. Several possible causes

especially secondary compressions of clay and sand layers due to groundwater mining

are investigated based on laboratory test and field monitoring results. Noticeable secondary compression of Aquifer IV was found in some areas in Shanghai. The compression of Aquifer IV

especially the secondary compression due to groundwater mining is one of the principal reasons for long-term tunnel settlement. Controlling groundwater mining in Aquifer IV is vital to minimize large tunnel settlement in Shanghai.