Shield tunnel uplift and deformation characterisation: A case study from Zhengzhou metro

Excavation in close proximity to the existing shield tunnel often leads to impacts on the stability of tunnel. However, interventions committed to mitigate the impacts may not be effective in preventing tunnel movement and/or deformation from occurring. This study presents a case history where two shield tunnels pass between two excavation pits belonging to the east square of the existing Zhengzhou East High Speed Rail (HSR) station. The centre down-top pit retained using a series of bored piles and three rows of tieback is first excavated in each excavation pit, followed by the periphery top-down pit retained by the diaphragm wall. The tunnels measured cumulative uplift and lateral deformation being equal to 22.9 mm and 77.9 mm, respectively, as the two excavations reached the final depth of 19.3 m. The tunnels also observed ballast separation, lining crack, and joint leakage. The contribution from each of the construction stages to the tunnel uplift and deformation is analysed. The effectiveness of the rectifications adopted is assessed and the lessons learnt are summarised preventing future similar incident from reoccurring.