Synchronous-cross zoned excavation of the oversized basement of Shanghai International Financial Centre by combination of bottom-up and top-down methods: Structural and geotechnical behaviors

Abstract

To date, deep excavations in soft clays were constructed routinely by either the bottom-up (BU) or the top-down (TD) method. Because of its mega size, special underground space arrangements and tight construction schedule, the oversized basement of Shanghai International Finance Centre (SIFC) was divided into five zones for construction, following an innovative synchronous-cross zoned excavation procedure in combination of BU and TD methods and retained by both regular and T-shaped cross-section diaphragm walls (DWs); in addition to cross-lot struts and slabs, DWs were propped by large-diameter circular beam-truss element strutting systems (CB-TESSs). To mitigate adverse influences of dewatering on urban environments, trench cutting and remixing deep soil mix walls (TRD-walls) in place of the conventional overlapped grouting columns were constructed as waterproof curtains. The comprehensive field instrumentation data revealed that on average SIFC exhibited better performance than those local excavations and TRD-walls had excellent watertightness. Since the unsymmetrical plane geometries of CB-TESSs and inconsistent lateral deflections, δh, of the perimeter and inner partition DWs impaired the roundness of circular beams, DWs braced by CB-TESSs developed much greater δh than DWs braced by cross-lot struts. Due to its excessively long excavation duration, the TD zone propped by cast slabs experienced much larger δh than the BU zones propped by struts. DWs played a governing role in restraining time-dependent δh at shallow excavation levels; whereas, strutting systems gradually played a more important role as excavation went deeper. Distinct from δh governed by several major factors, vertical movement of earth retaining structures was dominated by excavation depth. The presence of another excavation nearby reduced lateral earth pressures against the adjacent DW of SIFC and thus it underwent smaller δh. Contrary to the assumption made in the theoretical models for calculating the factor of safety (FOS) against basal heave instability, the strata behind DWs and below excavation levels underwent upward rather than downward movement during excavation, which interprets why δh data were irrelevant of FOS numbers in the excavation databases worldwide. Finally, technical implications of the observed excavation behaviors to the future design and constructions are discussed.