Civil Engineering Association

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

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.