Abstract:It has been observed that in the urban center of Shanghai, land subsidence has accelerated, and the groundwater level has continued to drop even though the net withdrawn volume (NWV) of groundwater has remained unchanged since 1980. An analysis of monitoring data shows that drawdown of the groundwater level is one of the factors that have influenced land subsidence since 1980. The NWV of groundwater in urban areas, however, is not the critical factor controlling the drawdown of the groundwater level. Since the 1980s, there have been many underground works constructed in the unique strata of Shanghai, which has an interlayered structure known as a multi-aquifer-aquitard system (MAAS). Investigation into land subsidence caused by urban construction is now receiving much attention. Based on the principle of a strategic environmental assessment (SEA) for sustainable urban development, this paper presents a discussion and analysis of the factors which can influence the development of land subsidence during continued urbanization in Shanghai. The main factors include the additional loading caused by the construction of structures, the cut-off effect due to construction in aquifers, the drawdown of groundwater level caused by leakage into underground structures, and the decrease of groundwater recharge from neighboring zones. SEA is recommended for the future development of Shanghai.

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This paper investigates the influence of moving trajectories on ground loss ratio (GRL) due to the double-O-tube (DOT) tunnelling method. DOT tunnelling has three moving trajectories: pitching, yawing, and rolling, which have different behaviours during tunnel construction compared with those from single circular shield tunnelling. These moving trajectories cause overexcavation during tunnelling. The calculation method of gap area between the DOT shield machine and linings is evaluated in this research. Based on the superposition concept, the modification equation of GLR is proposed, which takes both moving trajectory and grouting volume into consideration. A field DOT tunnelling case is analysed to determine the correlation between moving trajectories and ground settlement. The influence of tail grouting is discussed by adjusting the grouting volume in different periods. The finite element method is also employed by setting the modified ground loss ratio (GLR′) as the contraction increment of linings. Results from both the measured and simulated settlements verify the reasonability of the proposed equation and the effect of moving trajectories on ground loss.