Jiangwei Shi

In 2009, a 13-story residential building founded on lightly reinforced prestressed high-strength concrete (PHC) pipe piles in Shanghai collapsed completely during the excavation of an adjacent underground garage (i.e., on the southern side). In the course of construction, excavated soil had been dumped on the northern side of the building and formed a 10-m-high fill slope before the collapse. In order to identify the principal cause of the sudden building collapse, an extensive field investigation was carried out immediately after the accident. In addition to geotechnical exploration of soil properties, physical evidence was collected and documented including the layout of collapse debris, damage to piles, and induced pile movements. To assist in the investigation and provide insights into the collapse, three-dimensional finite-element analysis was carried out to simulate the process of excavation and dumping. For piles located adjacent to the dumped soil on the northern side of the building, uppermost fractures could be seen near the interface between the reinforced and unreinforced sections, which were located approximately 1–2 m below the original level of the pile head. Most of these failure surfaces were flat, implying that these piles failed under tension. For piles situated on the excavation (southern) side of the collapsed building, the uppermost fracture surfaces were tilted at an angle and the concrete was severely damaged, suggesting that these fractures occurred mainly from compression. Photographs of the pipe interior taken with a high-resolution digital camera and results of a low-strain pile integrity test revealed that most piles examined had generally broken into three or more sections. According to results of the numerical analysis, the largest bending moment was developed in the piles on the excavation side of the building at a depth corresponding to the interface of the soft and firm-stiff clay layers (13.5 m below the ground surface), under the combined effect of excavation and dumping. This suggests that the failure occurred first for piles on the excavation side, and eventually caused the building to fall flat onto the excavation site. A lesson learned from the accident is that geotechnical engineers and contractors should be better trained to improve their understanding of the effects of surcharge loading on PHC piles. Moreover, proper construction procedures and site supervision schemes should be explicitly spelled out and strictly followed by all parties during construction.

A 267-m-long, 54-m-wide, and 14.9–16.4-m-deep multipropped deep excavation with three cross walls in Shanghai soft clay was constructed at a side of an existing metro station. To investigate the performance of this deep excavation and the effect of this deep excavation on adjacent metro station and shield tunnels, the ground deformation and structure responses were extensively monitored. Based on the field observations, the deflections and vertical movements of diaphragm walls, surface settlements, vertical and horizontal displacements of the up track and the down track within the station and nearby shield tunnels, as well as tunnel convergence were extensively analyzed. By adopting cross walls, jet grouting and large dimension concrete struts, the measured maximum lateral wall deflection and ground deformation were much smaller than those at other sites without installing cross walls in Shanghai soft clay. The maximum excavation-induced lateral wall movements and ground settlement behind the reta...

Tunnel excavation inevitably induces stress changes in the surrounding soil leading to significant ground movements and additional stress in nearby pipelines. Most previous studies simplified tunnel–pipe interaction as a plane strain problem and focused on the case in which the pipeline intersects the tunnel perpendicularly. In this study, centrifuge tests were conducted to investigate the three-dimensional response of the ground and pipeline to tunneling, considering different pipeline orientations with respect to the tunneling direction. When the tunnel face was located within ±1.25D (tunnel diameter) away from the monitoring section, the ground settlement and pipe strain induced were more than 95% of the maximum recorded. When the pipeline intersected the tunnel perpendicularly, the maximum pipe strain always occurred above the tunnel centerline. However, when the pipeline intersected the tunnel at an angle of 60°, the location of maximum pipe strain shifted as the tunnel advanced. The maximum ...