Design and Performance of the Deep Excavation of a Substation Constructed by top-down Method in Shanghai Soft Soils

Abstract

The 500kV Hongyang Underground Transmission and Substation was situated in the downtown of Shanghai city in China. The project comprised a 5-storey building above ground and three levels basements. The excavation area and depth of this project were about 10800 m2 and 24.0 m, respectively. The ground soils were mainly thick soft soils comprising Quaternary alluvial and marine deposits. Surrounding environment of this project was quite complex. Considering the excavation area and depth, the soil condition, and the protection requirements of the adjacent facilities, top-down method was adopted to construct the foundation pit of this project. The excavation was supported by 1.2 m thick diaphragm walls. Three levels of underground structures and two additional temporary struts were used to support the diaphragm wall. Steel tubes erected in bored piles were used as vertical support system. An extensive instrumentation program was carried out to monitor the performance of the deep excavation and the adjacent facilities. Monitored data of the retaining structure and the surrounding facilities were analyzed in this paper. Observation results showed that top-down method was effective to control the deformation of the excavation. Design and construction of this project can be used as a reference for similar deep excavations.