Construction of a large-section long pedestrian underpass using pipe jacking in muddy silty clay: A case study

This paper presents a case study of constructing a large-section long pedestrian underpass using pipe jacking method in Nanjing, China. The underpass, having a width of 7 m and a height of 4.3 m, was jacked 94.5 m in muddy silty clay under a busy roadway with 6.2 m overburden soil, meanwhile it traverses above the existed shield metro tunnels with just 4.5 m from the underpass bottom to tunnel vault. This paper introduced the design and construction schemes of this project in detail. A pre-construction three dimensional numerical simulation was conducted to investigate the responses of the roadway and metro tunnels to pipe jacking construction. Based on the simulation results, the field monitoring program was proposed, and the tunnels deformation and ground settlement were constantly monitored. The field performances of the metro tunnels and roadway were analyzed according to the monitoring data. In the jacking process, the micro-underbreak method was adopted. In order to decrease the tunnels uplift and ground settlement, the actual volume of soil conveyed out from soil chamber to ground surface was kept 95-98% of theoretical soil volume cut by cutter head. In general, this project is completed successfully without taking any additional time and money-consuming deformation control measures. The ground traffic and underneath metro runs well during the whole construction process.