Forensic study on the collapse of a high-rise building in Shanghai: 3D centrifuge and numerical modelling

This paper reports a forensic investigation on the unexpected collapse of a 13-storey high-rise residential building in Shanghai soft clay. Prior to the building collapse, a shallow excavation was carried out on one side of the building while a slope was filled on the opposite side. Despite the observed breakage of all the piles in the field, a preliminary forensic numerical analysis adopting a typical undrained shear strength (c(u)) profile in Shanghai suggests that the piles are very unlikely to break. The surprising contrast has led to this detailed forensic study on the underlying mechanisms for the building collapse, through a site-specific characterisation of the c(u) profile, three-dimensional centrifuge modelling and numerical back-analysis. Although the typical c(u) profile in Shanghai increases almost linearly with depth, the clay in this site consists of two major layers with distinctive strength - that is, a relatively soft clay layer (average c(u) = 23 kPa, thicknes s= 13.5 m) underlain by a relatively stiff clay layer (average c(u) = 58 kPa, thickness = 18.2 m). By reproducing this c(u) profile in the centrifuge test and numerical analysis, the resulting direction of the building collapse and the depths of pile breakage were broadly consistent with those observed in the field. It was found that the surcharge of the fill slope caused the bearing capacity to be exceeded in the upper soft clay layer. This led to the development of a sliding wedge under the surcharge, which extended horizontally (at a depth of 11 m) across the pile foundations to reach the excavation side, and therefore the breakage of the piles. If a typical strength profile in Shanghai (with c(u) linearly increasing with depth) were adopted, the failure mechanism could be significantly altered from the aforementioned deep slip failure to a shallow passive failure, causing much smaller bending moment in each pile.