Deformation of existing underpasses due to pile cutting and shield tunneling: Observations from field monitoring and explanations by analytical model

This paper presents a case study of cutting 47 bored piles using earth pressure balance (EPB) machines during tunneling beneath three existing underpasses in improved clay. The reinforcement columns constructed by the Metro Jet System (MJS) method were adopted to improve the ground. To investigate the impact of tunnel construction on existing underpasses, the vertical displacement of the existing underpass and the hoop stress of the tunnels were monitored. The four-stage deformations of each underpass were consistent with the process of tunnel construction, particularly the heave induced by grouting were observed. Furthermore, the effects of shield tunneling on the underpass were interpreted using a soil-shield interaction model combined with the theory of equivalent thickness. The observed heave of the underpass induced by tail grouting and secondary grouting was also discussed. The results indicate that the MJS columns and favorable tunneling parameters can significantly reduce the vertical displacement of each underpass. The measured results show that secondary grouting has a greater impact on the underpass heave as it causes a higher negative ground loss. The case study can provide valuable insights for the design and operation of cutting piles using shield machine in clay.