Advantages and potential challenges of applying semi-rigid elements in an immersed tunnel: A case study of the Hong Kong-Zhuhai-Macao Bridge

The immersed tunnel of the Hong Kong-Zhuhai-Macao Bridge (HZMB) is the first of its kind constructed using semi-rigid elements and making a breakthrough in the structural theory. This paper explains the reason for applying the semi-rigid elements in an immersed tunnel and provides the design details using the example of the HZMB Tunnel. A theoretical analysis method based on the body-spring model is proposed for segmental joints of the semi-rigid elements. Potential challenges in the application of the semi-rigid elements are identified. Compared with the rigid (monolithic) elements and flexible (segmental) elements, the semi-rigid elements can improve the safety of segmental joints by increasing the flexural rigidity and joint friction. In addition, applying the semi-rigid elements can reduce the construction cost by retaining prestressed cables. Despite the advantages of the semi-rigid elements, some uncertainties arise due to the functional change of prestressed cables from a temporary structure to a permanent one. The location, number, and prestress level of the remaining prestressed cables need to be studied in accordance with the mechanical and waterproofing properties. The long-term prestress loss of prestressed cables should also be taken into consideration, especially the loss of asymmetric prestress in a marine environment. With the development of economy in costal regions, more immersed tunnels will face the similar engineering geological problems as the HZMB Tunnel. Discussing the technology of semi-rigid element has both engineering and theoretical significance to the development of immersed tunnels.