Three-dimensional deformation behaviour of a multi-propped excavation at a “greenfield” site at Shanghai soft clay

Despite the large number of excavation-induced ground deformations reported in the literature, it is still not easy to differentiate ground deformations due to excavation in congested sites (where the ground is strengthened by underground structures) from those due to excavation in “greenfield” sites. To investigate and compare excavation-induced ground deformations in “greenfield” sites and congested sites, in this study a multi-propped excavation at a “greenfield” site in Shanghai soft clay was heavily instrumented and the measured ground deformations were compared with those reported from six excavations conducted under similar conditions but in congested sites in Shanghai. Field measurements from the “greenfield” site show that near the centre of the excavation, the maximum ground settlement (δv-max) ranged from 0.22% to 0.27% of the final excavation depth (He), with a major influence zone extending to 3He behind the wall. The six excavations (near the main station) at congested sites in Shanghai had δv-max values ranging from 0.01% to 0.1% He (with a mean of about 0.05% He) and so were about 20% of that at the “greenfield” site. In addition, the major influence zone of ground settlement at the “greenfield” site extended 33% further than those at the congested sites. On the other hand, the measured maximum lateral wall displacement δh-max (0.24-0.37% He) at the “greenfield” site was comparable to those (0.13-0.43% He) at the congested sites. Due to the corner effect (soil arching around corners) at the “greenfield” site, the average δh-max and δv-max near the corners of the site were 45% and 36% smaller than those near the centre. The maximum tilt of ground perpendicular to the excavation was 1/1064, which was similar to that parallel to the excavation (1/1050).