Long-term settlement behaviour of metro tunnels in the soft deposits of Shanghai

• We presented the results of field measurements on the long-term deformation behaviour of the metro tunnels in Shanghai.
• We presented the geological and geotechnical conditions of Shanghai and the soil properties.
• We explained the mechanism of the long-term settlement of the metro tunnels in different aspects.

The Quaternary soft deposits of Shanghai form a multi-aquifer–aquitard system (MAAS) with aquitards of high compressibility and aquifers of high groundwater table. The metro tunnels in Shanghai are generally constructed at a depth of 9–15 m in a very soft clay layer with high water content, high compressibility and low permeability. According to the field monitoring results since completion of construction, significant settlement and substantial differential settlement of tunnels have occurred after more than 10 years’ operations. The long-term settlement rate was greater in the first few years and afterwards reduced gradually. Non-uniform settlement was observed in the following situations: (a) under variable soil conditions; (b) between station and tunnel; (c) at cross passages and the ramp section; (d) at tunnel sections crossing below river. The long-term settlement and differential settlement of tunnels has led to serious longitudinal deformation. The deformation pattern of tunnels is step between rings rather than by beam/cylindrical shell bending. Most of the lining rings distort into the shape of a horizontal ellipse (tunnel squat) but a few acquire the shape of a vertical ellipse. Large tunnel deformation has caused groundwater infiltration and the separation of ballastless track bed and lining. Further analysis shows that long-term tunnel settlement is mainly due to urbanisation-induced land subsidence in Shanghai. The magnitude of tunnel settlement is correlated to sublayer settlement rather than ground surface settlement. In the early operational years, post-construction settlement induced by tunnelling and the cyclic loading of trains may contribute significantly to the tunnel settlement. However, at some special places, nearby construction and groundwater infiltration are responsible for the long-term settlement of tunnel.