Performance of a long, irregular top-down excavation in the centre of Nanjing, China

A 1257 m long special-shaped deep excavation was performed in the city centre of Nanjing, China. Full-width and half-width top-down construction methods were adopted. Based on long-term monitoring data from the project, the evolution characteristics of the diaphragm wall (DW) deformation, the ground settlement and the settlement of adjacent buildings were analysed. The interrelationships among these parameters and their relationships with the excavation process, structural system and geological conditions were also studied. The results showed that the horizontal displacement of the DWs was less than 1% of the excavation depth and the position of the maximum horizontal displacement was generally located from 4 m above the excavation surface to the vicinity of the excavation surface. The influence range of the excavation on the surrounding ground settlement was mostly within three times the excavation depth. The ground settlement varied from 0.1–0.5% of the excavation depth to 0.3–5.0 times the maximum horizontal displacement of the DW. During the construction process, the settlement rate suddenly increased only when the excavation reached the soft soil layer. Building settlement was found to be related to the excavation deformation, structure forms, foundation types and distance to the foundation pit.