Lessons Learned from Construction of Shanghai Metro Stations: Importance of Quick Excavation, Prompt Propping, Timely Casting, and Segmented Construction

Four metro stations with similar supporting systems and pit geometries in Shanghai soft clay were excavated using different soil removal procedures, which led to distinctly different pit behaviors. Through analyzing and comparing the well-documented field data from these four bottom-up excavations, the significances of quick excavation, promptly propping, timely casting of floor slabs, and segmented construction in control of pit deformations were identified and their influences were comprehensively quantified. As to these four pits, the measured time-dependent wall deflections and wall heaves in 20–35 days at relatively deep excavation levels were up to 47–94% the magnitudes at completion of excavation to the final depths. The measured postexcavation wall deflections in about 20–30 days were up to 36–45% the maximum wall deflections at completion of excavation and the corresponding postexcavation deflection rates were up to twice those during excavation. The poorly excavated pits, which featured long excavation duration, long wall exposure lengths, or delay in propping struts or casting floor slabs, experienced wall deflections and ground settlements $1–4$ times greater than the appropriately excavated pit. The wall deflection rates of the poorly excavated pits were up to $2–3\times$ and $4–6\times$ those of the appropriately excavated pit during and after excavation, respectively. Compared with the countermeasure of increasing supporting system stiffness by casting rigid concrete struts and/or bracing double steel pipe struts at one level, the approach of quick excavation, promptly propping, and timely casting of rigid floor slabs in combination with a segmented construction procedure was demonstrated to be much more efficient in controlling excavation-induced deformations.