Measured performance of a 26 m deep top-down excavation in downtown Shanghai

Abstract

Via a long-term comprehensive instrumentation program, the performance of a 26 m deep metro station excavation constructed by the top-down method in downtown Shanghai was extensively examined. The measured excavation responses included diaphragm wall deflections, wall settlements, ground settlements, uplifts of interior steel columns, axial forces of propping struts, groundwater table levels, and settlements of adjacent buildings and utility pipelines. Based on the analyses of field data, the following major findings were obtained: (i) the concrete struts along with the floor slabs effectively suppressed later wall movements and consequently reduced the chance that the maximum wall deflections would occur above the excavation surfaces, (ii) with the progress of excavation to a lower depth, the diaphragm walls underwent a serrated settlement pattern over time, (iii) no significant post-excavation wall deflection occurred, (iv) the relationship between the interior column uplifts and the maximum wall deflections can be described by a linear equation, and (v) most system loads due to soil removal were carried by the concrete struts along with the floor slabs. The struts sustained mainly the released earth pressures due to the exposure of the adjacent portions of diaphragm walls, and the soil removal distant from the struts imposed limited effects on the strut axial forces.