Observed Performance of an Ultra Large Deep Excavation in Shanghai Soft Clay

This paper conducted a case study of an ultra large deep excavation of approximately 70,500 m² (about 340 m in length and 200 m in width) in soft clay. The excavation depth is mostly located in Shanghai soft clay about 10.3–15.9 m below the ground surface for basements of two or three levels. In the interest of safety and convenience, the construction site was divided into three zones (I, II, III) and excavated individually by the zoned excavation method. The pits at all the three zones were excavated using the bottom-up method with embedded continuous bored pile walls as earth retaining structures. Zone I and III construction started first until the completion of their top slabs, followed by Zone II excavation. In this study, an extensive instrumentation program was carried out to monitor the performance of the deep excavation, including (1) lateral wall deflections, (2) ground surface settlements, (3) axial forces in the struts. The field measurements indicate that at the end of the construction phase, the maximum excavation-induced deflection and ground surface settlement was equal to 0.83% to 0.9% of the excavation depth (He), respectively. These measurements are greater than empirical values for typical size excavations in Shanghai clay. The findings obtained in this case study provide some guidance for the design and investigation of similar excavation projects.