Performance of a long irregular excavation by the top-down technique in a downtown area

A 1257 m-long special-shaped deep excavation was performed in the city center of Nanjing, China. Full-width and half-width top-down construction methods were adopted. Based on the long-term monitoring data of the project, this paper analyzed the evolution characteristics of the diaphragm wall deformation, the ground settlement and the settlement of adjacent buildings and discussed their interrelationships and their relationships with the excavation process, structural system and geological conditions. The results showed that the horizontal displacement of the diaphragm walls was less than 1% of the excavation depth and that 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 excavation on the surrounding ground settlement was mostly within three times the excavation depth, and the ground settlement varied from 0.1%∼0.5% of the excavation depth to 0.3∼5 times the maximum horizontal displacement of the diaphragm wall. During the construction process, the settlement rate suddenly increased only when the excavation reached the soft soil layer. Building settlement was related to the excavation deformation, structure forms, foundation types and distance to the foundation pit.