Practical Solutions for Concurrent Excavation of Neighboring Mega Basements Closely Surrounded by Utility Tunnels in Shanghai Hongqiao CBD

The design and construction of four 15.3-m-deep neighboring basements in the Hongqiao Central Business District (CBD) in Shanghai, China, were made challenging by the mega excavation sizes (135,491 m2) and the presence of pre-existing utility tunnels nearby. To safeguard the project, save project costs, and reduce construction duration, innovative design and construction solutions were implemented, which included composite earth supporting systems, comprehensive ground treatments, zoned construction methods, special soil removal and disposal procedures, and long-term field monitoring. Benefiting from these composite design and construction solutions, the existing utility tunnels underwent very limited lateral displacement, and their maximum settlements were just one third of the magnitudes estimated before construction. Several key findings were obtained: (1) zoned construction methods, combining zoned soil-removal and strut-casting procedures, proved to be cost effective for the concurrent excavation of mega basement groups; (2) by avoiding side-by-side excavation, adverse superimposing effects deriving from concurrent excavations were remarkably mitigated; (3) the left-in-place earth supporting structures for construction of utility tunnels helped restrain lateral tunnel displacement; and (4) if the soft subgrade below utility tunnels had been improved prior to the adjacent excavations, tunnel settlements could have been reduced significantly. The optimum treatment depth for the soft subgrade below utility tunnels should extend below the elevation at which the maximum lateral displacement of the adjacent multipropped retaining wall occurred.