Covered Semi-Top-Down Excavation of Subway Station Surrounded by Closely Spaced Buildings in Downtown Shanghai: Building Response

Based on field data, this study investigates responses of six pre-existing buildings to an adjacent 24.8–25.2 m deep subway station excavated by the covered semi-top-down method. During excavation of 1 m wide and 50.2–50.5 m deep slurry trenches for diaphragm wall panels (i.e., diaphragm walling), buildings on both shallow and deep foundations developed noticeable settlements up to 15 mm. Subsequent excavation of the upper 15.9–17.7 m thick soils inside the pit only incurred limited building settlements less than 10 mm, which contrasted sharply with remarkable displacements of the diaphragm wall and ground nearby. As excavation continued to the final level, buildings on strip footing, stiffened raft foundation, strip footing atop long piles, and short piled raft foundation settled dramatically up to 40 mm, but those on long piled raft foundation still were hardly displaced. Like diaphragm wall and ground, all buildings developed significant postexcavation settlements up to 34.5 mm. Eventually, intolerable settlements culminated in damage to buildings. Generally, diaphragm walling-induced building settlement was mainly affected by the distance between building and slurry trench, building width perpendicular to slurry trench, and building weight rather than foundation type; excavation-induced building settlement was predominantly governed by foundation type and the distance between building and pit. Rapid casting of rigid underground structures after completion of excavation was of paramount importance for controlling time-dependent building settlement. Inconsistent with assumptions in literature, building settlement development was not concurrent with ground settlement development. Final building settlements were comparable to ground settlements for shallow foundations but much smaller for short piled foundations; in spite of this inconsistency, both of them conformed to underlying ground settlement troughs. Numerical simulations disclose that progressive extension of the ground displacement zone below the building undermined foundation bearing capacity, and consequently, building settlement took place. Underpinning was effective to copy with an adverse effect on buildings from adjacent excavation; however, excessively extending pile length below the excavation base could only result in limited additional reduction in building settlement. Compared to underpinning, the way of minimizing the ground displacement zone by modifying the lateral deflection profile of the retaining wall could be a more cost-effective alternative without sacrificing pit performance.

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  • English

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  • Accession Number: 01601987
  • Record Type: Publication
  • Files: TRIS, ASCE
  • Created Date: Apr 18 2016 3:03PM